Pierre writes: My basement wall is insulated with a 2 inches of Styrofoam on the interior and I built a 2×4 wall and added Roxul insulation. I planned to install vapor barrier on top. Is it OK to add this vapor barrier? It is for a bathroom with a tiled shower. I live in central Ontario, Canada.
Steve Bliss, of BuildingAdvisor.com, writes: The short answer is: Don’t use a vapor barrier in a finished basement. This is asking for trouble with a framed wall as the vapor barrier will trap moisture within the framed wall and can lead to mold in the fiber insulation and mold and rot in the wood framing. Some people call this detail a “diaper” wall for obvious reasons.
Rigid foam board on the interior face of the basement walls, like you have, is the ideal insulation for concrete or block wall walls. XPS or high-density EPS foam are my top picks for foamboard in the basement. R-10 foam is recommended for the central and northern U.S.
It is also fine to add unfaced fiberglass or Roxul (mineral wool) between the studs if you already have 2 in. of foam board against the concrete. This is a good way to add inexpensive R-value where you have a stud wall against the foam.
The problem with a vapor barrier on the interior is that moisture tends to condense on the cool concrete walls. The water vapor can come through the concrete from the soil or from indoor air that comes in contact with the concrete. A vapor barrier will tend to trap moisture in the fiber insulation leading to moldy insulation and decay in the wood framing.
The foam layer, placed directly against the concrete/masonry wall, should be continuous and sealed at joints to keep indoor air away from the concrete and prevent exterior moisture from migrating inward.
Also pay attention to the band joist area in the floor framing above to avoid condensation and moisture build-up here during cold weather. Insulate the band joist with foam blocks that are 2 in. or thicker, and seal the foam tightly in place with spray foam. Avoid foam faced with foil or plastic sheeting.
It gets a little more complicated since you are adding a tiled shower, where the recommended method is to place a vapor barrier behind the backerboard and a waterproofing membrane between the backerboard and tile (thinset is applied over the membrane). This detail should be fine here since the waterproofing membrane and tile have already created an effective vapor barrier. However, I’d avoid a vapor barrier in the non-tiled areas of the bathroom.
I would also recommend an airtight air/vapor barrier on the bathroom ceiling to keep any moisture out of the floor system above. Also install a good bath fan and use it when showering.
All of this assumes that you have a dry basement with good site drainage, working foundation drains, and foundation waterproofing on the exterior. If you have existing basement moisture problems or leakage, make sure you solve the problems before insulating and finishing.
Read more on Basement Insulation Protecting Foundation Insulation
Aaron says
How Much Foamboard is Needed On Basement Wall In Cold Climate?
I’ve recently decided to finish the basement in my home and am currently planning out a materials list. When it comes to the construction of my exterior walls I have a few questions. I’ve searched the forum and haven’t found anything specifically related to my questions, so hopefully this isn’t a duplicate post.
A few years back as I was renovating the rest of my house I slapped up some 1″ thick XPS foamboard onto the unfinished exterior walls in order to get a little bit of insulation value. This layer is continuous from the main level subfloor down to the concrete slab. All edges and gaps were sealed with blue tuck tape and spray foam. For a little more context on the situation.. I live in quite a cold climate. I also very recently dug up and water proofed my foundation.
Now my original plan was the finish the basement using the existing fiberboard as a moisture barrier, 2×4 framing, R12 fiberglass insulation in the stud bays, and a 6mil poly vapor barrier to top it all off. I am also planning on using DMX 1-Step underlayment ran up to the exterior walls, and then placing my framing on top of it to allow for ample airflow underneath the finished floor.
There are a few potential problems I see with this plan.. First off I absolutely detest the idea of sealing poly in between every single joist cavity with acoustic sealant. Secondly, if this 1″ foamboard acts as a vapor barrier I could be trapping moisture inside of the wall.
So my questions are:
Would this 1″ foamboard be a sufficient thermal break to stop moisture from condensing on the inside if I was to forego installing a poly vapour barrier?
If not, would an additional 1″ foamboard glued onto the face of the existing one act in a sufficient manner for me to avoid using a poly vapor barrier at all?
As moisture slowly comes up through the slab underneath the DMX underlayment, would it be detrimental to have foamboard acting as a vapor barrier directly on the exterior wall so as to not allow the moisture to dry to the foundation?
Please feel free to ask me for any more details and let me know what you think.
buildingadvisor says
Where approx. are you located? Are you in Canada, where poly vapor barriers are still common. (Also you mentioned acoustical sealant…) Poly vapor barriers are used very little in the US and not recommended in basements. Vapor barriers in the basement do more harm than good, trapping moisture in the frame wall, leading to mold and possible wood decay.
Please describe your basement walls starting on the exterior and working your way in.
If you already have 1″ foamboard (XPS or EPS?) on the interior side of your basement walls, and the foam board is sealed with spray foam and construction tape, then you do not need to worry about moisture condensing on the concrete walls. Unless you have extreme amounts of condensation dripping down to the floor, condensation on concrete or masonry is not a problem.
However, the interior surface of the foam may be cold enough for condensation to occur here, especially on the colder, above-grade portions of the wall.
In a cold climate, R-10 foam is recommended, so adding another inch of foam would be a good idea. It will keep the interior surface of the foamboard above the dew point of the interior air, preventing excessive condensation on the foam.
The foam board also keeps ground moisture on the concrete side of the foam where it does no harm. Concrete and block are unaffected by moisture.
You mention “existing fiberboard”. Did you mean foamboard? Most types of fiberboard are a very poor moisture barrier.
I don’t think the DMX board will create any problems. It mainly creates a capillary break under the flooring and small amount of insulation between the slab and the finish flooring. It makes the flooring warmer, drier, and possibly a little squishy in spots.
Aar0n says
I am currently in the middle of finishing my basement. I live in Canada, so we see hot summers and cold winters. I have 1″ XPS foamboard on the walls already from a few years ago. This foam runs from the main level subfloor all the way down to the concrete slab, and the edges are completely sealed with blue tuck tape and spray foam. I framed one of the walls years ago so it would be difficult, but not completely impossible to get another sheet of foamboard behind it. I won’t be able to count on painted drywall as a vapour retarder in this renovation.
Will this 1″ XPS foamboard be sufficient as a vapour barrier behind my fiberglass batt insulation? If not, will gluing another layer of 1″ XPS to the existing one act in the same manner as a 2″ sheet? I am worried that R5 insulation will not be enough to prevent vapour from inside the basement from condensing on the foamboard behind the fiberglass insulation on very cold days.
buildingadvisor says
Do you enjoy math?
It’s hard to answer your question with precision because there are so many variables. The most important are:
• The temperature and relative humidity of the air in your finished basement
• The average outside temperature in winter
• The amount of insulation in the stud wall inside the foam.
• The airtightness of your basement wall finish. Is it drywall? Moist air passing through cracks and gaps is transports much more moisture than than water vapor “diffusing” through permeable materials.
Also, are you conditioning the air in your finished basement. Let’s say the winter air is 70 degrees F and 40% RH. Then the dewpoint would be 43 degrees F.
Let’s say you have R-11 in the stud cavities and R-5 foam board. Then, when it’s 0 degrees outside, the (theoretical) temperature on the inside face of the foam board (on the upper, above-grade portion of the wall, would be 22 degrees. With R-10 foam board the temperature on the foam surface would be 33 degrees F, both below the dew point of the interior air at 70 degrees and40% RH.
It is a simple proportion: foam R ÷ total R x temp. difference = wall temp. So for R-5 foam, 5/16 x 70= 22 degrees. For R-10 foam, 10 ÷ 21 x 70 = 33 degrees.
With no insulation in the stud cavities, the temperature on the inside face of the foam would be closer to the temperature of the interior air. The calculation would include the R-value of the air film, which is about 1. So the calculations would put the inside foam temperature at 58 degrees for R-5 foam and 64 for R-10, both safely above the dew point.
These calculations are for the above-grade portion of the foundation wall. Below-grade, conditions will be get gradually warmer with the depth of soil, so the rest of the wall will be warmer.
Building scientists use the average outdoor temperature for the coldest three months to determine the condensation potential. Walls and ceilings are designed so the interior face of the foam sheathing is above the dew point for those winter conditions.
With an air-tight air barrier at the finished wall surface, the potential for condensation is minimal because very little interior air will come in contact with the surface of the foam. Drywall with well-finished seams and sealant on the perimeter joints can work well as the air barrier.
Finally, with foam sheathing on walls or ceilings, experts (and most codes) recommend that the interior vapor retarder be moderately permeable (Class 3) to allow drying of the wall cavity toward the interior. Standard latex paint on drywall meets these requirements.
The table in the article How to Install Foam Sheathing articles provides helpful guidelines. Although developed for above-grade foam sheathing, the same principles and calculations apply to foundation insulation.
In the “Minimum R-value” table, you will see that R-10 foam is sufficient for a 2×4 insulated wall in U.S. climate zones 7 and 8, including the coldest parts of the U.S.
Bottom line, R-10 foam will give you much better condensation protection than R-5 if you want to insulate the stud wall. If you leave the stud space uninsulated, R-5 foam should be sufficient. A little bit of condensation is not a problem as it most likely evaporate before causing any harm
Aaron says
Hey Steve,
I want to start by thanking you for such a detailed response. I do enjoy math and building science more than the next guy, so this read is greatly appreciated. I knew that I didn’t provide you with enough details, but you’ve still managed to sum everything up.
So if I understand correctly, in my current situation there will be some condensation on the inside of my foamboard whether I keep it 1″ or go up to 2″. Seeing as how I live in the Canadian building zone of 7A. We see an average winter seasonal temperature of between -5°F to 15°F. The coldest days can get down to around -25°F to -30°F without the windchill. It’s these days that worry me the most.
I plan to condition the air in my basement. It will most likely sit around 68°F in the winter and around 40% RH. 2×4 studs with R12 fiberglass batts on the inside of the foamboard is my preferred construction. There will be spots where the fiberglass touches the foamboard and I worry about the fiberglass wicking moisture into the wall cavity. I was not planning on meticulously making sure that drywall edges were sealed, as my ceiling will be left painted and open.
Furthermore, I wasn’t planning on drywalling the walls in my mechanical room at all, let alone finishing and painting it. So I can assume there would be a good amount of air being moved between the finished room and the wall cavity. This will both bring vapour into the cavity, but would also help dry it out, right?
With all this being said, am I worrying too much about condensation on the foamboard? What do you think about just leaving the existing 1″ board up as is, and stapling some housewrap to the exterior side of the wall framing to prevent any moisture wicking into the batt? Any moisture that condenses and beads would just run down the foamboard or housewrap and end up on the concrete slab. I’ve laid down dimpled underlayment and I plan on placing the walls directly on top of this layer, so any moisture that would run down would not sit against any wood.
If I’m to opt out of the second layer of foamboard, it drastically reduces my renovation price and time spent putting it up. Now I’m not one to cut corners, but I also don’t like wasting money. If I’m going to have to deal with moisture condensing on the foamboard whether I put the time and money into it or not, it seems as though it may be wasteful.
Thanks,
Aaron
buildingadvisor says
People smarter than you or I have run lots of simulations and, hopefully looked at performance in the field, as well. And they strongly recommend al least R-10 foam against the masonry walls. You can leave it at R-5, but you are certainly increasing the likelihood of excessive condensation – especially without any attempt at building an airtight air barrier on the interior side of the wall.
The area with no inside finish (open studs) should not be a problem. Any condensation within those walls should dry quickly to the interior when conditions are favorable.
On the walls with drywall, you are considering adding a layer of Tyvek in the hope that it will keep any condensation away from the fiberglass. If the Tyvek were meticulously sealed to form an air barrier, it might keep interior moisture from hitting the foam. However, the Tyvek would the become the first plane of condensation – so you might find water beading up on the Tyvek on super cold nights – just speculation.
Bottom line is that I can’t say what will happen when you don’t follow codes and standards and try to come up with a novel solution. Experimenting is great — especially on your own house — but predicting the outcome is never easy. Temperature and moisture flows are notoriously hard to model below grade. In the past, a lot of people who built insulated stud walls against the concrete or block foundation walls , followed by a poly vapor barrier and drywall ended up with a mess of mold behind the poly.
So I would encourage you to follow the current recommendations — that is, 2 inches of foam if you plan to insulate the 2×4 stud wall, a good air barrier, and no poly — just painted drywall or similar on the interior. If you want to experiment with novel solutions, I strongly recommend leaving an observation port (like a removable panel) in the drywall so you can periodically take a look at what’s going on behind the drywall in the middle of the winter.
Kyle says
Should I Put Poly On Basement Wall Before Foamboard?
Hi – Would love some advice for what to do if you can’t make a continuous vapor barrier with the foam board. Specifically, I have a drainage pipe (3″ pipe) that runs essentially the entire length of a side wall and the front wall in my basement. The pipe is about halfway up the wall and hung from metal strips that are nailed directly into the concrete wall. Because of this, I can’t get 2″ foam boards to run continuously up and down the entire length of the wall. I can put the foam boards above and below the pipe, but I can’t get it behind it. Basement is dry but does get humid especially in summer. Would love to hear feedback on my plan:
1. To create a continuous vapor barrier, I was planning to add poly directly against the basement wall and run it behind the drainage pipe. I would have to tape around the metal hangers. Would start on the sill plate and run down the side of the concrete to the slab and then have it go under the wall I plan to frame.
2. I would then add the foam board above and below the pipe directly against the poly, and spray foam around the pipe and around edges. Taping/sealing as you described above.
3. Insulate sill plate/rim joists with the foam boards as discussed above.
4. I would then frame a wall directly up against pipe (there would be about 1″ of space between the wall and the foam boards as the pipe is wider than 2″). Then insulate that with unfaced batt insulation.
5. There are a few other obstructions in various places (water pipes, electrical panel, etc.). I figured I would just run poly up to these items and seal just best I could.
6. On walls without obstructions would not do the poly and just use the boards as discussed.
Really appreciate the help if this would work and any other pointers.
buildingadvisor says
Your approach sounds OK. It’s possible that you could get moisture trapped between the poly and foamboard, but it probably won’t cause any harm there. Water generally needs wood, drywall, paper-backed insulation, or other organic materials to get a foothold. However, I have seen mildew grow on inert materials such as glass window panes in extreme conditions.
If feasible, I would forget about the poly and cut the foam board to fit around the drainage pipe and other obstacles. Then use canned spray foam to seal the foamboard to the pipe, etc. This seems cleaner to me and doesn’t create a double vapor barrier. Around the electrical service panel, you could use a non-expanding foam or other sealant. You want to be careful about any insulation material getting into the electrical box, so spray foam may not be the best choice.
Also, you could eliminate the one-inch air space by notching the 2×4’s around the pipe if you want a little extra finished space in the room.
Erik says
Is Vapor Barrier Needed With Batt Insulation on Foundation Wall?
I’m finishing my basement. It’s a raised bungalow with a walk out – 1970’s vintage. Poured concrete foundation, electric heat. Location is Ontario, Canada. We get hot (+30°C) sticky humid summers and frigid cold (-30°C) dry winters. We plan to build the stud walls an inch or so away from the concrete, and intend on using batt insulation due to budget.
My understanding is that the vapour barrier should be on the inside face of the stud wall in winters, and on the backside of the wall in the summers. It seems there’s no good side to put it on in our climate because we’d be wrong half the year. For this reason I’ve been thinking of eliminating the vapor barrier all together.
– What happens if we don’t use any sort of vapor barrier?
– What about putting vapor barrier against the concrete wall, away from the stud wall & insulation?
I appreciate any advice you can provide. I’m finding lots of contradictory information from professionals, but I’m not educated enough on the subject to sift through it all and pick out the correct information.
buildingadvisor says
You’re not the only one confused by this issue. It’s a complex topic, on which experts may disagree. As you point out, vapor flow is reversed during summer and winter (at least on the above-grade portion of the wall). But this is also true for the rest of your house. The big difference is that below grade, vapor is moving inward year-round, since vapor pressure in the soil is almost always higher than inside the finished basement.
The other big difference is that the walls are masonry, which is unaffected by moisture. So you do not need to worry about moisture being trapped within the block or concrete wall. In many ways, this make below-grade spaces easy to insulate. Just make sure you keep the cold and wet conditions on the masonry side of the insulation.
The best way to achieve this is to use foam insulation directly on the foundation walls and seal it well at joints and around the perimeter. This reduces air leakage and keeps moisture away from the interior space. You can put studs or strapping on the interior side of the foam for attaching the drywall – and add fiberglass or mineral wool if you want additional insulation. Do not use a vapor barrier on the interior side of the wall. This would create a double vapor barrier and now allow drying to the interior.
If you wish to use fiberglass batts to save money, you are risking moisture problems within the framing and fiberglass. You definitely don’t want a poly vapor barrier on the interior. There are differing opinions about whether you need to put a vapor barrier or moisture barrier such as Thoroseal on the interior side of the foundation wall to limit moisture migration into the wall. It may not be necessary, but doesn’t hurt. You generally want to stop the moisture barrier just above grade level so the top portion of the wall can allow some drying to the exterior.
In its energy efficiency guidelines, Natural Resources Canada, does suggest using a moisture barrier against the masonry when insulating a basement with batt insulation. However, they recommend house wrap, such as Tyvek or Typar, rather than poly from under the bottom plate to just above the grade line. According to the report “ Plastic was once the material of choice, but in some cases where air and moisture entered the insulated wall, condensation formed on the plastic and caused wetting and mold issues within the wall.”
Read more on Basement Vapor Barriers and Basement Insulation Strategies
Jim Sidfara says
Should I Apply Drylok Before Insulating Basement?
Hello,
I am refinishing a relatively dry concrete basement and will follow all your advice you’ve laid out here (1/2″ XPS laid first on the floor, 2″ XPS glued directly to the concrete walls, 2’x4 framing with PT bottom plates floated on top of the 1/2″ XPS, unfaced batts, drywall, and ordinary latex paint). My question concerns drylock: Is sealing the floor and/or concrete walls with drylock first (before any of the above) preferable to leaving the concrete floor and/or walls bare? Again, it is a fairly dry basement with concrete that gets occasionally damp but no cracks or leaks.
buildingadvisor says
I’m not a big fan of waterproofing concrete on the interior with products like Drylok. If you have a serious leakage problems, this needs to be addressed on the exterior with improved site drainage and basement drainage. I have used Thoroseal a couple of times, which is a more durable coating, but still not a long term solution to basement leakage.
If you have a dry foundation wall (and floor), there’s no need for an interior coating. If there is minor seepage through porous block or concrete, Drylok might provide a temporary fix, but is likely to flake or peel over time if the water is exerting any hydrostatic pressure. Also, Drylok is highly permeable to water vapor, so it will not keep moisture out of the basement.
Basements are often damp in the summer due to condensation of humid air allowed in to ventilate the space – a double-edged sword. And they are damp year-round due to water vapor in the soil migrating into the house.
Insulating the walls with 2 inches of XPS foam, taped or foamed at edges and seams will keep ground the moisture out of the living space. The concrete is unaffected by the moisture (and can even be used underwater as in bridges).
Bottom line: Drylok won’t hurt anything, but is not necessary for the type of wall you are planning.
Karl says
Question regarding basement fiberglass batt insulation:
We are finishing our basement in Minnesota. The foundation is CMU blocks, and we have drain tile and radon treatment installed.
Our contractor is planning to insulate as follows:
Block wall–1-inch gap, studs, R13 faced batt insulation, then drywall. He assures me this is 100% fine. In fact, he said that this is how his basement is insulated.
I spoke with the city inspector and explained the plan. Although there are no insulation code requirements for older houses, he said this approach will be do the job just fine.
Adding complexity to the situation is that the walls are already framed, so adding 2-in foam behind the studs is not an option.
Do you have any recommendations on the best approach here: let it ride with the gap and faced batt insulation (plus drain tile and radon system to remove moisture), adding some 1-in foam board then batt, closed-cell spray foam (Spraytite) or something else? Thanks!
buildingadvisor says
The risk in your approach is that water vapor will condense on the cold block surface in winter and run down the wall, collect at the footing, and potentially wet the framing and insulation. Many basements are insulated this way. Some are fine; others end up with wet insulation and moldy wood – especially if they use an impervious vapor barrier like poly sheeting, under the drywall.
Since you can’t put foam board against the block (my first choice), I would focus my efforts on creating a very tight air barrier at the finished wall. This could be the drywall, itself, installed in a very airtight manner with gaskets and canned foam — or the addition of a “smart” vapor barrier, installed in an air-tight fashion, under the drywall.
I would use unfaced batts, since the painted drywall will provide a sufficient vapor retarder. An insulated basement wall should be able to dry inward (which a smart vapor barrier allows).
Not sure if your drain tile is on the interior or exterior and whether/how it is connected to the radon system. However, both of these systems will help keep the basement dry. In addition to venting radon, a radon fan removes moist air from under the slab resulting in a drier basement year-round.
If you want to spend the money, it would help to spray a couple of inches of closed-cell foam again the block. This would help protect against future moisture problems. If you don’t do this, you will probably be OK as long as the foundation is very dry and you follow the approach outlined above.
Dan says
Hi Karl,
I am facing the same predicament in Wisconsin. Code guidance suggests just using R19 faced batts in the basement framed walls with small air gap from my wall frame. What did you do, and what has happened?
thanks!
karl says
Hi Dan,
I empathize.with your situation. There seems to be a significant disconnect between the current best practices we read about online and the actual execution by (some) GCs.
This misalignment certainly generated a lot of mental anxiety on my part that was not pleasant (e.g.. “Is this whole project ruined because of the insulation?” “Why/how does the GC think batt is OK given all the info/recommendations online?”),
In my situation, the framing was already completed, so adding foam board would have been expensive in time and money. Additionally, I was on a tight timeline and needed to maintain a copesetic relation with the GC to ensure work progressed. Thus, my basement has batt insulation, and will remain that way until it’s re-remodeled or otherwise needs to be replaced (hopefully in the far future).
So far, I have not seen any negative impacts in terms of moisture/condensation but that would take time to develop.
Overall, this was my first time working with a GC on a project of this scale and I learned a lot. The next time I do a project like this I’ll be way more direct in level-setting best practice expectations.
Good luck with your project and let the group know how it goes. And try not to worry yourself sick like I did.
–Karl
Jon says
Should I Insulate Block Walls With Fiberglass?
Hello. I just had a French drain system installed in my basement. Home is about 100 years old with a block foundation with only 4ft of basement below grade. The installer installed a vapor barrier as part of this system directly to block foundation running about 4 ft H to align to height of exterior grade. Basement had been finished by previous owner. Can I close up the walls with the following (going interior to exterior): mold resistant drywall, unfaced batt, 2-4″ air gap, vapor barrier, concrete block foundation? Or should I just remove the vapor barrier entirely? Climate zone 4
buildingadvisor says
If the foundation is exceptionally dry, you may be able to get way with fiberglass batts, but this widely used approach has a bad track record in terms of moisture problems. Do not use a vapor barrier under the drywall with this or any finished basement wall.
In nearly all cases, I recommend 1-2 inches of foam board directly against the block wall, more in colder climates. XPS (extruded polystyrene) foam board is my first choice. The board should be taped or foamed at seams and foamed at all perimeter joints to make a tightly sealed air and moisture barrier. If you want additional insulation, you can add fiberglass after the foam
The foam accomplishes two things: it keeps the moisture on the outside of the finished space and it presents a warm surface to the indoor air, so there should be minimal condensation in cold weather — not a big concern in your mild climate.
With any approach, you don’t want a vapor barrier on the warm (interior) side of the finished wall, as you want to wall to be able to dry inward if it ever gets wet. Simple painted drywall will provide enough of a vapor retarder. Also an interior vapor barrier could collect condensation on its exterior face if you air condition in summer.
You can leave the 4 ft. high vapor barrier along the interior face of the block wall. It is not necessary, but shouldn’t cause any harm — unless you put a vapor barrier on the interior and create a “moisture trap.” If you are required by code to put a vapor retarder on the interior, use one of the “smart” vapor barriers such as MemBrain.
Read more on Basement Insulation.
Brian says
Best Way To Keep Basement Wall Framing Dry?
Hello, thank you for all the information and helpful responses to comments. I’m getting ready to finish my basement and had a couple questions I didn’t see the answer to in the article and comments. I live in Ohio which is zone 5 I believe.
My basement consists of 8″ concrete block, some poured solid others with air cavities. I redid my French drains a couple years ago and coated the outside of the foundation with a rubber coat waterproofing product and then filled the ditch to the surface with river gravel. Since then I’ve never had any liquid moisture come in my basement but will occasionally get “dampness” visible on the walls after a hard rain soaking the exposed block from the outside in but never enough to liquefy and run down the wall.
What would be my best course of action to eliminate a possibility of mold on the future? I would like to install stud walls to hang drywall on but didn’t know which scenario would be best.
1. I could install 6mil poly directly on the block wall and let it run down the wall and under the wall plate of my stud wall trapping and funneling any moisture on the interior block surface down and under the stud wall. I would then insulate the stud wall with unfaced fiberglass insulation and install purple drywall. My thoughts were if I trap the moisture behind the fiberglass and under the wall plate there shouldn’t be any chance of the fiberglass ever contacting the block to soak in moisture there fore eliminating the possibility of mold growth.
2. Paint the interior of the wall with the same rubber coat product install the poly over it and run it down and under the wall and insulate and finish it as described in option 1
3. Install xps foam directly to the block and then build a wall in front of it with studs using a rot resistant plate for the bottom. My concern with this theory is what happens to the inevitable moisture and vapor that gets stuck in the wall will it ever dry out or should I leave a gap between the stud wall and the foam board.
Thanks so much for the help I greatly appreciate it.
buildingadvisor says
In most cases, poly anywhere on the interior of a basement wall does more harm than good. While it may keep moisture from the block wall away from the wood framing, it also creates a cold surface where moisture from the interior air will tend to condense during the heating season.
This condensed water wets the framing and collects in the fiberglass insulation resulting in mold, musty odors, and possible wood decay. If you feel compelled to run poly on the wall, stop it at grade level, so some drying to the exterior may occur.
The best approach is your third — put 1-2 inches of extruded polystyrene directly against the block wall. If well-sealed with tape and spray foam, this creates an effective moisture barrier and prevents condensation by keeping the indoor moisture away from the cold concrete block. Moisture on the block side of the foam is not a problem as concrete and block are unaffected by moisture.
With foam against the concrete, you can safely put additional unfaced fiberglass insulation in the framed walls. In Zone 5, you would want at least R-5 foam to prevent condensation. The minimum recommended thickness for foam sheathing would apply here as the conditions are very similar.
Brian says
Thank you for your quick response. One other wrinkle that I forgot to mention is that when I backfilled the exterior of my foundation I used 1″ thick xps foam board and installed it using the foundation coating as “glue” to hold it up. I’ve read on various parts of your site that a vapor/moisture barrier on both sides of the foundation could possibly trap moisture on the inside of the foundation. I don’t particularly see this as a problem as the possible moisture could run down inside the block wall ((if any) and leach out under the slab into the footer area since the block wall is sitting on the footer without any barrier (block sitting directly on concrete)
Also would it be advisable to use metal studs to frame the walls or are wood studs acceptable if I take proper precautions such as using a treated wood plate installed over a piece of composite decking or similar product to prevent wicking of moisture?
Do I need to glue the foam board to the foundation or should it be attached mechanically with fasteners?
buildingadvisor says
The double vapor barrier problem is caused by trapping a stud wall with fiberglass insulation between poly (or similar) on both sides. This is inviting a moldy disaster — and has been dubbed a “diaper wall” by some.
However, trapping a block or concrete wall between two moisture barriers is not a problem. Concrete and block are unaffected by water — in fact, they are often used underwater.
Make sure that your foam board is well-sealed along the bottom with spray foam. Also, using a PT bottom plate with a capillary break underneath is a good idea. One option is to raise the plate a half inch or so with plastic or PT spacers.
Most people use wood studs in basements, and I prefer to use PT just in case. But steel is certainly a good option, since mold can grow on PT wood.
Finally, either glue or mechanical fasteners are fine. With the stud wall in place, the insulation is not going anywhere.
Brian says
I’ve started on the project gluing the 2″ xps foam board directly to the basement block walls, and also filling in each rim joist bay with a rectangle of foam board attached to the rim joist, and a second piece laying down on the sill plate to ensure proper air sealing and insulation of all parts of the block wall and the rim joist. I then ran spray foam around all the edges for an air seal.
Before completing the job I started to wonder if I might be promoting moisture damage in the ends of each floor joist, where they are sandwiched between the rectangular foam blocks used against the rim joist, or in the sills where covered by pieces of foam (problem areas marked red in (Illustration). ). Could the the sandwiched portion of the floor joist harbor mold and rot from condensation since the floor joist runs all the way to the rim joist without any break. Am I thinking about this right or am I overthinking it. Thanks again for your insight.
buildingadvisor says
Not overthinking. Good questions and, with moisture issues, the answers are rarely clear cut. It often comes down to the wetting potential of a piece of wood vs. its drying potential. Wood can hold a lot of moisture safely as long as it has the opportunity to dry. Over time wood will gain or lose moisture depending on the relative humidity of the surrounding air. That’s why wood floors tend to expand in summer and shrink in winter.
The ideal detail is to put two inches of foam on the exterior side of the rim joist and sill, keeping the wood warm and able to dry to the interior.
However, builders frequently insulate the band joist/sill area on the interior and don’t encounter problems. The wood in the rim joist and sill stay pretty dry and can dry to the exterior if need be.
The joists in your detail should be fine as the interior portions are warm enough to have minimal condensation. The exposed portions of the joists are protected by about R-5 of wood (for 3-1/2 in. of softwood) compared to R-3 to R-4 for a good low-e widow. If you don’t have condensation on the window, you won’t have it on the joists.
Even if you did get a little condensation on the exposed portions of the floor joists, the wood would absorb the moisture and lose it at a later date. For wood decay to occur, it must remain at saturation levels for extended periods in warm weather.
The sill is a different case because of its direct contact with masonry. I have seen rotten sills a number of times in older homes. (The only time I saw rotten joist ends were where they sat directly on the foundation.) The rotted sills were uninsulated, but wet and poorly ventilated, causing wood decay in warm weather.
If you have a wet foundation and no capillary break under the sill, moisture can wick into the sill by capillary action. If that’s the case, then insulating the sill with foam would make things worse by making the wood colder (and wetter), and reducing its drying potential. If you are concerned, I’d suggest that you seal all sill joints, but not insulate the top or edge of the sill.
The reduction in overall R-value would be pretty small, but you can sleep at night and not worry about rotting sills. If you decide to insulate the sill, you could leave an access panel in the basement ceiling and a removable piece of foam. Take a look a couple of times a year – with a moisture meter if you really want to know what’s going on behind the drywall.
Mark says
Should I Install Poly Against Block Wall Before Studs?
I’m finishing a basement in Chicago and the walls are cement block and it’s below grade. No cracks, completely dry basement that is 30 years old. The temperature is very consistent so they do not want to spend money on insulation. Should I use a vapor barrier between the studs and block – Studs and drywall – or no barrier at all?
Thanks
buildingadvisor says
Most basements are damp, so foam insulation on the walls accomplishes two things:
1) It helps keep soil moisture from entering the finished space.
2) It makes the interior surface of the basement wall warm enough to prevent condensation of interior air. Especially in warm weather, the interior face of the block or concrete wall is often cool enough for water vapor in the indoor air to condense.
In addition to saving energy, the foam insulation will result is more comfortable and less musty basement living space.
Since you are NOT insulating with foam (or fiberglass), you may be tempted to place a layer of 6-mil poly on the basement wall as a moisture barrier. While this will help keep soil moisture out of the basement, it will cause greater problems by trapping condensed moisture against the frame wall — potentially leading to mold and wood decay.
With fiberglass in the stud spaces, the moisture problems are even worse as the poly is now colder (more condensation) and the fiberglass will tend to hold the moisture and remain soggy.
The key problem is that moisture from the indoor moisture will tend to condense on the interior face of cold poly, run down, and collect on the floor and the footing of the stud wall.
Without poly, the block or concrete wall tend to absorb the condensation without causing any problems. So poly in basements usually does more harm than good.
So whether or not you insulate, I would not recommend a poly moisture barrier against the concrete. Whatever system you use, you definitely do NOT want a vapor retarder on the finished side of the wall other than ordinary latex paint and primer. This is to allow the wall system to dry to the interior.
Also it’s important to caulk all drywall edge joints (top, bottom, corners) that are not sealed with mud to create a tight air barrier — keeping indoor air out of the stud cavities.
For the reasons outlined above, the best strategy is to insulate with 1-2 inches of foamboard directly against the block well and to seal it well at all joints and seams.
Also, use PT lumber for the bottom plate — and for the rest of the wall, as well, if it’s in the budget.
Read more on Basement Insulation
Carson Bruess says
Will I Trap Moisture Between Foam Layers Inside Basement Wall?
I’m planning to add a furred out wall to my basement perimeter. It’s a walkout basement with the block foundation running about 3.5’ above the basement slab. Plan is to attach 2” xps directly to foundation wall (seams taped). Then I’ll frame up a 2×2 wall in front of xps.
I wanted to use unfaced batts in the stud cavities but can’t find anything to fit a 2×2 wall. We are using 2×2’s as opposed to 2×4’s to avoid shrinking the finished space even further. My thought was to use 1.5 xps between the studs (cut loose and spray foamed around edges to hold in place). NO vapor barrier on framed wall. Do you see any issue with trapping moisture here between the two layers of xps? bearing in mind the layer in the stud wall is not continuous (broken up by wall studs). We live in Minnesota.
Thanks!
buildingadvisor says
I don’t see a problem here, but you might consider using EPS (expanded polystyrene) between the studs rather than XPS (extruded polystyrene) as the EPS is much more permeable to moisture. It has a perm rating or about 2-6 per inch vs. about 1.0 for XPS. The EPS will allow some drying to the interior should any moisture ever find its way into the wall system.
Since most of the moisture migration in basements is from the outside in, using XPS against the wall will help keep the wall cavities and interior space dry.
I wouldn’t be concerned about trapping moisture between the two layers of foam for a couple of reasons. First, the outer foam layer accounts for more than 50% of the total wall R-value, so the interior face of outer foam layer will be well above the dew point of the interior basement air. So even if basement air leaks into the insulated wall, it will not condense and cause problems.
You can read more here about the minimum recommended thickness of foam sheathing. The numbers are for above-grade walls with foam and fiberglass, but apply equally to your detail with two layers of foam.
Second, foam board can tolerate of lot of moisture without suffering any damage. So even if excess moisture collected between the foam layers, it would not harm the foam. As an extra precautions, it wouldn’t hurt to use pressure-treated lumber for the framing — always a good idea in basements and other damp spaces.
Finally, make sure the outer layer of foam is well sealed at seams and perimeter joints as an air and moisture barrier. For best thermal performance, it would also help to seal the inner pieces of foam board to the studs with spray foam. This will eliminate gaps in the insulation and create a continuous air barrier to block entry of interior air into the wall system.
Read more about best practices Basement Insulation
dave says
Should I Use Unfaced Batts in Basement?
I’m going to finish basement perimeter walls per your previous advice, by putting 1.5 in rigid foam directly to waterproofed block walls, then 2×4 framing.
You had recommended unfaced insulation next , prior to drywall, but am having a hard time finding unfaced batts. Would it be detrimental to use R-11 kraft-faced under 1/2″ drywall
please advise, thanks.
buildingadvisor says
By using kraft-paper or other vapor retarders under the drywall, you increase the risk of moisture problems in the stud cavities by trapping moisture between two vapor retarders — the foam insulation and the kraft paper. Concrete and block are very porous to water vapor, so even a waterproofed wall or the floor slab, can still add a significant amount of moisture to the stud cavity.
To minimize future problems, I would stick with unfaced batts. If you can’t find them locally or by special order, you can easily peel the Kraft paper off faced batts and get the same result.
Dave G says
Should I Add Vapor Barrier Over Drylok?
I am finishing a basement ( Maryland on the Delaware border ) with concrete block foundation walls, coated with Drylok, per their specs. Perimeter 2×4 framed walls will be inches from the block, and was going to install R-13 Kraft faced insulation then 1/2 in. drywall. Do you recommend
A) gluing 6 mil plastic vapor barrier to the block before wall construction, then proceeding as stated above, or
OR
B) 2-in. rigid foam to block, then proceeding as stated
Or do you have a better system that i could follow (Drylok is already completed and keeping cost in mind ) Also was going to put hardboard (beadboard) panel on ceiling, is that acceptable?
Your response and input will be truly appreciated
buildingadvisor says
Drylock provides a good vapor seal and can solve some minor moisture problems, but if there is a significant cracking, leakage, or build up of water (hydrostatic pressure) on the outside of a foundation wall, Drylock will not prevent moisture intrusion over the long run. Only proper site drainage and foundation drains on the exterior will keep a concrete or block wall dry on the interior. Waterproofing or damproofing on the exterior is far more effective than any interior wall treatment.
Assuming you have dry basement walls and a coating of Drylock, you do not need poly or any additional vapor barrier on the interior. The best approach is to install two inches of rigid foam directly to the block, sealing all joints, corners, and edges with high-quality construction tape or canned foam. Also cut foam blocks to fit against the band joists and foam them in place.
The stud wall can go right against the 2-in. foam. Use unfaced batts in the wall as you do not want a second vapor barrier trapping moisture in the stud spaces. Also use pressure-treated lumber at least for the bottom plate, and the rest of the framing if you can afford it.
It’s also a good idea to use mold-resistant drywall, as the paper in ordinary drywall is a choice food for mold and mildew.
Read more about basement insulation .
Mikez says
In the same situation you describe above, Drylock’d sealed open block, can Rockwool SafenSound be used between the studs instead of foam? Or should I just pull the studs, put foam in, and go from there. The vapor and moisture barrier issue was driving me crazy till I came across this site. Again, 20 years, no major water issues, old wood paneling with foil insulation with i removed it (circa 1960), think this Drylock will do the basic moisture job?
Studs are already there and am redoing the walls with purple because of mildew/mold along the plate over the summer. Never had an issue with water coming in, but condensation created some mildew over the summer, as I’. now learning. Science is fun.
The den is always on the cool side, but this past summer, a window and door were out, so more hot air got in there than usual.
buildingadvisor says
To prevent condensation in warm weather, you want to keep warm, moist indoor air from reaching a cold surface like the block wall (think condensation on a cold beer can on a hot summer day).
Rockwood insulation board is highly permeable to moisture, so it will not accomplish this. You are much better of with extruded polystyrene (XPS). Install it well-sealed with construction tape at seams and canned foam at edges.
With the foam board in place, you can put additional fiber insulation between the studs. To determine the minimum foam thickness, you can use the table for insulated sheathing, which also applies here.
Karl says
Should I Use Faced Batts For Basement Insulation?
Question regarding basement fiberglass batt insulation:
We are finishing our basement in Minnesota. The foundation is CMU blocks, and we have drain tile and radon treatment installed.
Our contractor is planning to insulate as follows:
Block wall–1-inch gap, studs, R15 faced batt insulation, then drywall. He assures me this is 100% fine. In fact, he said that this is how his basement is insulated.
I spoke with the city inspector and explained the plan. Although there are no insulation code requirements for older houses, he said this approach will be do the job just fine.
Adding complexity to the situation is that the walls are already framed, so adding 2-in foam behind the studs is not an option.
Do you have any recommendations on the best approach here: let it ride with the gap and faced batt insulation (plus drain tile and radon system to remove moisture), adding some 1-in foam board then batt, closed-cell spray foam (Spraytite) or something else?
Thanks!
buildingadvisor says
The risk in your approach is that water vapor will condense on the cold block surface in winter and run down the wall, collect at the footing, and potentially wet the framing and insulation. Many basements are insulated this way. Some are fine; others end up with wet insulation and moldy wood – especially if they use an impervious vapor barrier like poly sheeting, under the drywall.
Since you can’t put foam board against the block (my first choice), I would focus my efforts on creating a very tight air barrier at the finished wall. This could be the drywall, itself, installed in an airtight manner with gaskets and canned foam — or the addition of a “smart” vapor barrier, installed in an air-tight fashion, under the drywall.
I would use unfaced batts, since the painted drywall will provide a sufficient vapor retarder. An insulated basement wall should be able to dry inward (which a smart vapor barrier allows).
Not sure if your drain tile is on the interior or exterior and whether/how it is connected to the radon system. However, both of these systems will help keep the basement dry. In addition to venting radon, a radon fan removes moist air from under the slab resulting in a drier basement year-round.
If you want to spend the money, it would help to spray a couple of inches of closed-cell foam again the block. This would help protect against future moisture problems. If you don’t do this, you will probably be OK as long as the foundation is very dry and you follow the approach outlined above.
Richard Siderko says
Is Gap OK Between Basement XPS and Studwall?
Great Thread. I intend to attach 2″ XPS foam board to my concrete block basement walls.
1. Is it ok to to have the bottom of the foam board rest on the concrete floor?
2. Because of several French drain cleanouts my wall framing will be 6 inches from the foundation wall leaving a four inch air gap between the foam board and wall studs. I will also be inserting 2 inch XPS foam board in the rim joists sealed with fire retardant spray foam. I will be finishing with drywall without adding a vapor barrier for fear of trapping condensation. The aforementioned 4 inch air gap is my cause for concern. Do you think I should be worried with this approach? Thank you.
buildingadvisor says
Assuming that you are attaching the XPS directly to the concrete block, your detail sounds fine and follows all the recommended practices. A gap between the XPS is not a problem as long as you do not place a vapor retarder under the drywall. Also, paint with ordinary latex paint and primer, not a vapor-barrier paint.
Make sure all joints in the XPS are sealed with high-quality construction tape or canned foam.
As with any basement finishing project, you want to make sure that the wall is free of moisture problems before finishing. With a wet or dripping basement wall, you are likely to get mold and mildew growth whether the framing is tight to the XPS or spaced away.
Hardboard or MDF paneling is an OK product, but it does outgas formaldehyde for the first few years. This is usually not an issue unless someone in the home is sensitive to the gas. Outgassing is highest in high-moisture environments, so if the space has high humidity, another product may be preferable.
Read More on Basement Insulation
Robert V says
Faced Vs. Unfaced Foam in Basement?
I’m a little confused.
I have recently installed 1 inch foam board on my basement walls, and have removed the plastic off both sides of the foam board before installation. These boards are tongue and grooved correctly, but if we are supposed to have both sides unfaced, why are we supposed to tape the seams?
If the goal is to have the slight airflow going through the foam, wouldn’t seams that were not taped also give slight air flow? Or is that air flow too great at that point?
buildingadvisor says
Sorry about the confusing advice. The usual recommendation is to use unfaced foam on the interior of a basement wall to allow for some minimal drying toward the interior. The advice to tape the seams is to maintain a good thermal barrier with square-edged panels. With T&G panels, properly installed, taping would not provide much benefit.
In a basement, the primary source of moisture is the soil and concrete. Concrete is very porous to water vapor and also acts like a sponge sucking moisture from the soil. In most cases, the soil has a higher vapor pressure than the basement, so the moisture is moving inward.
The worst thing you can do is to put framing and fiber insulation against the concrete or block and then install a vapor barrier on the interior. This traps moisture in the cavity and will lead to moldy framing and insulation.
With foam board directly against the concrete, there is less concern about moisture buildup. The concrete is unaffected by moisture and most types of foam board are highly moisture-resistant, but can absorb moisture over time. While this is unlikely to harm the foam, it would degrade the R-value. Using unfaced foam helps protect against this type of moisture buildup. I’m not aware of any studies looking at this issue with interior basement insulation, so it’s based mainly on studies of foam buried in the soil (exterior basement insulation).
On the other hand, people have successfully used foil-faced iso board on the interior of basement walls. There may be some water absorption around the edges, but the facing keeps the rest of board dry. No moisture in, no moisture out.
The permeability of foam board varies from 0 for foil-faced iso board to about 2 for 2 in. of EPS. Two inches of XPS has a perm rating of about 0.5. By comparison, Kraft paper has a perm rating of 1. So very little moisture is passing through the foam whether it is faced or unfaced.
Bottom line: with foam insulation on the interior of a basement wall, directly against the concrete, don’t worry too much about the use of a facing or vapor barrier. But to err on the side of caution, unfaced foam is probably a better choice.
Robert V says
How To Fireproof the Foamboard
So the Styrofoam board is installed against the foundation wall.
My question now is, how does this work in regards to fireproofing my walls? Do you know the best way to fireproof at this point? The foamboard says it is flammable, which makes me think that I’ve potentially made a mistake, but then again, I’m using this foamboard for it’s designed purpose.
I look forward to your response.
buildingadvisor says
It’s not particularly easy to ignite Styrofoam (XPS), but it can melt or burn at high temperatures and, like most building materials and home furnishings, will give off toxic fumes if ignited. In a house fire, most deaths are caused by smoke inhalation rather than burns.
Most residential building codes require that foamboard be covered with min. ½ drywall or an equivalent 30-minute fire barrier. (Foil-faced iso board is sometimes allowed to remain uncovered as it is somewhat less flammable.)
If you want additional protection, you can use a double layer of drywall or switch to fire-rated (Type X) drywall, commonly used in multifamily dwellings and commercial buildings.
Since it’s virtually impossible to build a fireproof home, it’s critical to have working smoke detectors on every floor so everyone is out of the building before the fumes from a fire become lethal. By the time the fire penetrates the drywall and reaches the foam insulation, you don’t want anyone to remain in the building.
buildingadvisor says
Insulating above the slab is fine. This is more for comfort underfoot than energy savings as the ground temps below the slab are pretty warm and stable. While the usual recommendation for cold climates is R-10, R-3 will still dramatically raise the interior temperature of the floor surface. By warming the interior face of the flooring, it will also reduce the likelihood of condensation on the floor – especially problematic with carpeting.
For comparison, you rarely see condensation on the interior of low-E window which typically ranges from R-3 to R-4 (and it gets a lot colder on the other side).
If you seal the joints and seams of the EXP on the floor, it will provide an effective vapor barrier. If possible run the floor on floor all the way to the foam on the wall and seal the corner for a continuous thermal break and moisture barrier. The bottom plate of the wall framing can sit on the foam.
I wouldn’t worry about moisture building up below the XPS, unless you have an especially wet basement or high water table. As mentioned earlier, a sub-slab gravel layer and vapor barrier will help keep things dry, but you may not know what’s under your slab.
A test used by flooring contractors is to tape a square of poly to the basement floor for 24 hours and see if water droplets collect under the poly. This is used to tell if the concrete is sufficiently cured, but can also detect high levels of moisture in an existing slab.
In any case, the XPS and concrete are unaffected by moisture. If you have a leaky basement or a seasonally high water table, you’ll need to deal with that with proper waterproofing, drainage, and a sump pump if needed.
Best of luck!
Derek says
Thanks, Steve, for the quick and thorough response. I have done a lot of reading on this topic and totally agree with you that the vapor needs to be able to dry in one or both directions. I also plan to run a dehumidifier for the damp months to also try and keep things dry.
Since I do not have the headroom for a 2”+ floor, do you think laying 1/2” XPS (R3) with a floating floor directly on top would cause moisture issues below the XPS? My other option would be
Plastic Waterproofing Tile Membrane then tile with no insulation, but the floor would be cold then.
Derek says
Should I Put Vapor Barrier on Interior Basement Wall?
I live in Massachusetts and I’m finishing 3/4 of my basement. On the exterior concrete walls I glued 2-in. XPS foam (extruded polystyrene) before building the interior walls to create a thermal break/vapor barrier.
On the interior walls between the finished and unfinished areas, I was going to use Rockwool insulation with 1/2 Sheetrock on both sides. The rockwool is unfaced.
My question are:
1) Should I be adding a vapor barrier on the inside walls on the warm finished side directly underneath the interior Sheetrock?
2) Should I be adding a vapor barrier on the cold side of these interior walls as well?
3) On the concrete walls with 2in XPS, should I also be adding Rockwool insulation and NO vapor barrier?
My main objective is to avoid mold growth.
buildingadvisor says
You are right to be concerned about mold growth on the drywall as the paper facing is a good food for mold. Mold-resistant drywall (Gold Bond XP or GP Mold Guard) offers some protection, but can still support mold and mildew growth. Another, more expensive option is paperless drywall such as GP DensArmor Plus, which has a treated gypsum core with a fiberglass facing. Nothing is 100% immune to mold growth (I’ve seen it grow on glass), but this should offer pretty good protection.
The tricky thing in a basement is predicting the direction of moisture flow. It is always from areas of higher vapor pressure to lower vapor pressure. Vapor pressure is a function of air temperature and relative humidity. Since the air in the soil is typically close to 100% relative humidity (RH), water vapor is often heading inward, even though the soil is usually cooler than the interior air.
For example, if the soil is 50°F, moisture movement would be inward if the air temperature inside the basement is less than 70°F at 50% or lower RH. Basement air can be fairly dry in winter if air going up a flue or to a leaky house above by the stack effect. There are a lot of variables.
There may also be moisture moving through the concrete – slab and walls – by capillary action or leakage if any liquid water is in contact with the concrete. If the slab is built over a layer of gravel with a sub-slab vapor barrier, then the basement space will be a lot dryer. A capillary break under the bottom plate of the frame wall, such as 6-mill poly, is also a good idea to help protect the framing.
In answer to your questions:
1) On the interior wall between the heated and unheated portion of the basement, I would not use with a vapor barrier. The main concerns are condensation and drying potential. In winter, the dew point of the interior air is typically about 50°F, assuming an air temperature of 70°F and a relative humidity of 50%. Since the temperature inside the wall is will rarely be below 50°F, condensation here should not be an issue. In summer, you could get condensation on the back of the drywall if the interior space is air-conditioned – another reason to avoid vapor retarders in the wall. You want the wall to be able to dry out in either direction.
2) No, same reason. If you put a vapor barrier on both sides of the interior wall (between the conditioned and unconditioned space), you are potentially trapping moisture within the wall – a surefire route to mold and wood decay.
3) Yes, you can add rockwood between the studs if you want the added insulation. For the same reasons discussed above, do not use a vapor barrier here. However, you want to seal all foam board seams and edges with spray foam or high quality tape to limit moisture migration into the conditioned space.
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