Q: I’m tearing the house down to bear studs inside and outside. I’m putting 1-inch rigid foam board on the outside. Do you suggest foil-faced or unfaced foam board? Where and how should I use moisture barriers in the construction of the interior walls? I live in zone 5, should I use rain screen? What do you think of cement lap board for siding? – Ron B.
A: Foam sheathing can help protect walls from moisture problems, but only if it is thick enough and installed properly. There are three key issues with foam sheathing:
1) Use a thick enough layer of foam to minimize condensation on its interior face. Interior moisture can enter the wall from air leaks or diffusion through the wall finish. The goal is to keep the inside face of the foam sheathing above the dew point of the interior air.
2) Don’t use a vapor barrier, such as poly, on the interior. If the wall cavity get wet for any reason, then the wall will need to dry primarily to the interior. It’s best to use only a Class III vapor retarder such as painted drywall. If you want a continuous vapor retarder, consider one of the “smart” (and expensive) vapor retarders such as MemBrain or Intello. These have low permeance when humidity is low (in the winter) and high permeance when humidity is high (during the summer or if the wall cavity is wet).
3) Use an effective interior air barrier to keep warm, moist interior air out of the wall cavities. This could be sealed drywall or sealed sheathing such as the Zip system.
For Zone 5, the IRC and IECC recommend minimum R-5 foam sheathing for 2×4 walls and R-7.5 for 2×6 walls. Remember that these are minimums – the greater the R-value of the foam the better, as this creates a warmer wall cavity with less condensation. These numbers assume that the wall cavity is filled with fiberglass or cellulose insulation. The minimum thickness of foam sheathing for US Climate Zones is shown the table below.
For Zones 1 through 4 (except Marine 4), any thickness of foam is suitable as the sheathing is warm enough to avoid condensation problems as long as there is an adequate indoor air barrier and vapor retarder. But foam sheathing provides benefits in warm and mixed climates as well.
You can use either foil-faced, EPS, or extruded foam. Of the three, EPS is the most permeable, allowing some drying the exterior if the wall cavity ever gets wet – for example, from a flashing leak. Extruded polystyrene (XPS) is about alf as permeable as EPS, and foil-faced foam is completely impermeable.
EPS has a perm rating (permeance) of about 1 for a 1-inch-thick board, making it a Class II vapor retarder similar to the Kraft-paper backing on fiberglass batts. The drying potential through EPS or XPS is low, but better than nothing. For that reason, I prefer unfaced foam on the exterior, although many people use foil-faced polyiso foam without problems.
EPS and polyiso board are the most environmentally friendly choice as they typically use pentane as its blowing agent, which is fairly benign to global warming. XPS is the worst as it uses HFCs (hydrofluorocarbons) which are very potent contributors to global warming. XPS manufacturers could solve that problem completely by switching to HFO (hydrofluoroolefin), but have been reluctant to change due to the higher cost.
In answer to your other questions, the water-resistive barrier (WRB), such as asphalt felt or housewrap (Tyvek, Typar, etc.), can go on either side of exterior foam sheathing. The important thing is to properly integrate the WRB with the window and door flashing and other wall flashings. The placement of the windows in the wall will determine the best place for the housewrap. For manageable details, you want the housewrap in the same plane as your window flanges, usually on the outside of the foam sheathing.
However, some people like to recess the windows so they sit against the plywood sheathing (“innies” vs. “outies”). This provides some modest energy benefits in terms of wind penetration, but introduces a lot of detailing problems with wide exterior sills and jamb extensions. Because window flashing is the number 1 place for exterior water leakage, I like to keep things simple here, so I prefer outies. Both innies and outies are simplified by installing plywood boxes in the window openings. With outies, this solves the problem of nailing your window flanges through thick layers of mushy foam.
Rains screens are always a good idea, but not a necessity with fiber-cement siding, which is highly resistant to moisture and holsd paint very well. If you were installing wood siding over foam sheathing, I’d recommend some type of rain screen. If you want to longest lasting siding and exterior paint, then go ahead with the rain screen. But it does add cost and complexity to your wall system.
Read More on: Exterior Foam vs. Dense-Pack Foam Sheathing in Cathedral Ceilings
S.A. says
How To Insulate a High R Wall?
We are remodeling a half bath (no shower). Its a exterior wall.
One of the walls is a double stud wall two x 2×4 and the other exterior wall is just a 2×4 framed wall.
We are in climate zone 5, which I believe is R-50. How can we maximize the insulation? Try to get as close to R-50 as possible?
I was thinking to use:
1. RMAX
Pro Select R-Matte Plus-3, 2 in. x 4 ft. x 8 ft. R-13.1 Foam Insulation Board between the studs closest to the exterior wall
2. Tape all foam-to-foam joints with SIKA SikaSeal-148 – Insulation Seam Tape – Clear – Multipurpose, Hand-tearable Acrylic Adhesive – Indoor and Outdoor use
3. Foam the outside edges where the foam board meets the wood framing with ”
GREAT STUFF PRO 26.5 oz Construction Adhesive”
4. In the second 2×4 stud wall add
ROCKWOOL
R-15 Comfortbatt 3-1/2 in. x 15 in. x 47 in. Fire Resistant Stone Wool Insulation Batt
5. Then apply a Class III vapor retarder (1-10 perms).
Vinyl-It 4-1/2 ft. x 75 ft. Clear ‘8 mil’ Plastic Sheeting (I’m assuming 8mil equals 8 perms)
6. Spray foam around electrical outlets
7. Paint drywall with latex or enamel paint for the vapor retarder.
8. Caulk perimeter of drywall
9. Put shiplap over drywall
But that only gets us to R. 28. Could we stagger the foam board on top of each other in the first 2×4 stud bay? If so what is the best protocol to stagger it?
THANK YOU VERY MUCH!
buildingadvisor says
Not sure where you are getting your recommendation for an R-50 wall.
In Zone 5, the minimum requirements from the 2021 IECC (International Energy Conservation Code), which is referenced in most U.S. building codes, is as follows:
Walls: min. R-30 cavity insulation or R-20+R-5 sheathing or R13 + R-13 sheathing or R-20 foam.
Attic/Ceiling: R-60 (up from R-49 from 2012-2018)
Bear in mind that, compared with an R-13 wall, an R-30 wall has cut your heating loss (and heating bill) for heat lost through that wall by 63%. Going to R-50 adds increases the savings to 78% so you are only saving an additional 15% based on heat loss in that one wall. This is because of the “law of diminishing returns” — that is, each time you double the R-value, you get only half the benefit. So the incremental savings will be modest.
You might be better off spending extra money on air-sealing, which may result in greater savings.
That said, if you want to reach R-50, you will need to use foam insulation – either entirely or in combination with fiber insulation like you are doing. Depending on the wall thickness, you might be able to reach R-50 with only fiber insulation on the double wall.
A couple of points:
If you combine fiber insulation in the cavity with foam on the exterior, you do not want an impermeable vapor barrier on the interior — as the wall needs some ability to dry toward the interior. The exterior foam needs to meet minimum requirements based on the climate and amount of fiber insulation, as described in the article and table above. Based the table, that works out to a min. R-6 exterior foam with R-15 fiber batts in Zone 5.
Because your foam is between the studs, the effective R-value of the foam will be reduced somewhat – and that’s assuming you get a good seal between the foam and the framing, not an easy thing to achieve. Sealing individual pieces of foam board between the studs is sometimes called “cut and cobble”. You can more about cut-and-cobble insulation in this article on cathedral ceiling insulation. The issues are the identical.
An 8-mil vapor barrier is 8/100 inch thick – not a perm of 8. In fact, the permeance is effectively 0. With the right amount of exterior foam, you want a tight interior air barrier and a mid-range Class 2 vapor retarder, such as painted drywall. A “smart” vapor retarder is ideal and I would highly recommend this in a humid space such as a bathroom.
Deyan says
Best Way To Insulation Wood-Frame Portion of Basement wall?
Hello,
I started a project insulating the basement. The basement walls are block on the lower half and wood-frame on the top half, which is above grade. At this point, I am just insulating the above-grade portion of the walls, which are wood-frame. I have already glued ¾ inch XPS foam boards against the sheathing between the studs and between the rim joists, using foam board adhesive. The edges will be sealed with spray foam.
My idea was for the EPS to be used as a vapor/ air barrier. I was planning to fill the rest of the cavity with paper-faced fiberglass (R-13) batts, which is usually recommended for wall insulation. I thought that this combination will improve the entire R-value of the wall.
I noticed that year-round the air temperature in the basement never goes below 59 degrees F, so there will not be condensation between the foam board and the fiberglass layers. Should I continue, or do I need to think of another way to finish this project? I live in Upstate NY, climate zone 6.
Reading other posts here suggest that my plan does not sound right — the board insulation is not thick enough, and the added fiberglass needs to be unfaced, The latter can be easily fixed- I can rip off the facing of the fiberglass rolls-already purchased. And there will be 1/2 drywall installed. Would that be sufficient?
Another approach would be to add an additional layer of foam board to the board already installed to reach R-10 and cover it with drywall. I live in Upstate NY, climate zone 6.
If I have 2” of foam board and want to add fiberglass on top, it looks like it will go over the surface of the wood studs. That is why I may finish the rest of the wood frame wall with no board, just faced fiberglass and drywall.
Does this sound reasonable? Thank you.
buildingadvisor says
If you are combining foam and fiberglass, the important issue is for the foam to be thick enough to avoid condensation on the interior face of the foam for most of the winter. In Zone 6, that means that the foam should provide at least 36% of the total R-value (see Table above). Your 3/4-inch XPS foam has an R-value of about R-3. So with R-13 batts, the foam accounts for about 7% of the total R-value –well below the recommended level.
One option would be to build out your studs a bit to accommodate thicker foam. Then you can get closer to the recommended level. Another option is to use all foam and no fiberglass. You can get R-5 per inch with EPS.
One caution is that the foam-thickness rules are intended for above-grade, conditioned interior spaces with moderate levels of indoor humidity. If the space you are insulating has high moisture levels, or moisture from the block wall is getting into the stud spaces, then you may need to take extra precautions.
If the basement is damp, consider improving site and foundation drainage, or dehumidifying the space and using only foam insulation. The fact that the basement stays above 59 degrees has little bearing on the condensation potential. Condensation is a function of the indoor humidity level and the temperature on the inside surface of the foam insulation.
Also, you are correct that you should avoid polyethylene or other impermeable vapor barriers on the inside of the wall. Ordinary latex paint combined with unfaced batts is preferable as it allows more drying to the interior. Kraft paper has a fairly low perm rating of 0.3. More important is to focus on air infiltration from the interior space into the walls. Seal all drywall edges well as well as openings for electrical outlets, etc. to keep interior air from leaking into the wall cavities.
While you are at it, you should also consider insulating the block portion of the wall with min. R-10 foam board. Since this section is not buried very deep, it will have significant heat loss and tend to make the space cool and damp.
Joe says
Best Wall Detail With Foam Sheathing?
Thanks for this site. You said don’t use a vapor barrier, such as poly, on the interior with foam on the exterior. If the wall cavity get wet for any reason, then the wall will need to dry primarily to the interior. It’s best to use only a Class III vapor retarder such as painted drywall…….So are you saying in the wall bays to use unfaced fiberglass insulation when using exterior foam insulation. Even more important when using polyiso due to it being impermeable? So the order of materials from the inside to outside is: Drywall, studs, unfaced insulation, foam board, and WRB. Do I have that right?
buildingadvisor says
Yes, seems like you got it straight and seem to have a good handle on the issues involved. Although, the housewrap or can either go inside our outside the foam, but generally is best in the same plane as the window flanges and flashing for good integration between the window flashings and WRB.
It’s also important to use thick enough foam to minimize condensation on the inside surface. The minimum foam thickness for each climate zone is shown in the table in the article above.
If you want maximum protection against moisture problems, you can also add a rain screen on the outside of the foam using vertical furring or one of the rains-creen housewraps on the market.
Larry Boone says
How To Insulate Shed With Foam Board?
I am insulating a 14′ X 36 ‘ shed 2″ x 4″ studs with T11 siding. I want to insulate with 2 1/2″ foil faced Polyisocyanurate Foam Board Insulation both sides into the studs then finish with OSB board.
What issues if any will this cause?
buildingadvisor says
The answer depends on your climate and the usage of the building — especially whether it is heated/cooled and the potential sources of moisture. These are usually related to occupancy — cooking, bathing, drying firewood, houseplants — and the foundation type and drainage. A wet basement or crawl space, or uncovered soil, is the worst.
If you have none of these, you’re probably fine.
In any case, you want to keep moist household air out of the wall cavities. That means sealing the inside surface of the insulation, or the OSB, against air leakage. You can seal the foam with contractor’s sheathing tape at seams and canned foam at the edges of the panels.
With 2-1/2 in. of foam, you’ll get R-15 to R-18, depending on the specific product. You could save a lot of money by using R-15 batts. For better performance, add a layer of foam board over the studs (and seal them) before installing the OSB.