In This Article
Interior Vs. Exterior
Type of Foam
Installation
Subslab Insulation
Subslab Vapor Barriers View all Foundation & Sitework articles
Basement wall insulation is recommended in most of the U.S. except for the deep South. Energy savings are about the same whether you insulate on the interior or exterior. The best insulation to use is rigid foam, which is largely unaffected by moisture. There are some small theoretical advantages to insulating on the exterior having to do with thermal mass, but these are far outweighed by practical concerns.
INTERIOR VS. EXTERIOR FOAM
In practice, insulating on the interior is usually a better option. The main advantages are:
• Exterior foam insulation is often damaged during backfilling, and is difficult to protect above ground. There are a number of protection systems, but most get damaged over time.
• Exterior foam provides a perfect entry point for termites and carpenter ants, as well as a comfy home for carpenter ants who like to hollow out the foam for nesting. (In termite-prone areas, you may be required to leave an “inspection gap” at the top of the wall with either interior or exterior foam.)
• Interior insulation is easier to install and usually results in a better overall job. In retrofits, it is often the only option.
TYPE OF FOAM
Use either extruded polystyrene (XPS) such as Styrofoam or Foamular or expanded polystyrene/EPS, sometimes called “beadboard.” EPS is less expensive and rates about R-4 per inch vs. R-5 for EPS. Use unfaced boards to allow for some drying to occur inward. Both types of foam need to be covered by drywall or another code-approved fire barrier.
If you’re not planning to finish the basement, another option is foil-faced polyisocyanurate, such as Thermax, with an R-value of about 6.5 per inch. You may be allowed to leave foil-faced insulation uncovered – check with your local building department. If you are planning to finish the basement, however, insulation faced with foil facing is not a good choice as it can trap moisture.
Spray foam is also an option, but usually a lot more expensive than foam boards.
When using foam insulation on basement walls, the building code (IRC) requires R-5 for southern states (excluding the deep South) and R-10 for the rest of the US. Many energy experts recommend R-15 to R-20 for far northern states. Estimated savings for R-10 insulation range from $250 in Washington DC to $400 in Minneapolis. You’ll also have a much warmer and dryer space, which may be the more important reason to insulate.
FOAM INSTALLATION
Place the foam board directly again the concrete or block and secure it with adhesive or concrete fasteners and washers. Make sure you use an adhesive formulated for use with foam board. For mechanical fastening, Plasti-Grip PMS fasteners work well for both concrete and block.
Seal the foam boards together with canned foam or a high quality building tape such as Dow Weathermate Construction tape or Dupont Tyvek tape. Also seal the foam at top and bottom seams with a small bead of canned foam. For a job this large, buy the large cans and a professional quality applicator gun – you’ll be happy you did.
If you are planning to finish the basement, you can use fewer fasteners and help hold the insulation in place with furring strips nailed through the foam or 2×4 studs, which provide nailing for the drywall and a space for wiring.
If you are adding studs, you can place unfaced fiberglass or rock-wool batts between the studs for added insulation. Because the added cost is modest, this a good idea in cold climates — as long as you have a very dry basement. If you’re not sure, best to leave out the fiberglass. If but could contribute to moisture or mold problems long-term.
No Vapor Barrier in Wall. Whatever approach you take, do not use a vapor barrier anywhere in the wall as it will do more harm than good, potentially trapping moisture and promoting mold and mildew growth.
SUBSLAB INSULATION
If you are planning to finish the basement space, you should also consider adding a layer of foam insulation below the slab. The insulation may not save much money in energy bills due to moderate earth temperatures at that depth, but it will warm the floor surface enough to reduce condensation.
Without insulation, water will condense on the cool surface of the slab in warm, humid weather. The damp slab causes musty odors and can damage floor coverings, finishes, and materials stored in the basement. This is especially problematic with carpeting on the slab. A warm, dry floor will greatly improve the quality of the finished space.
Two-inch extruded polystyrene foam (XPS) is typically used as one-inch foam is too fragile to place under a slab. Expanded foam (EPS) is a less expensive option, and considered better for the environment, but you would need to use high-density Type II EPS for this application. Type II foam has a nominal density of about 1.5 lbs/cu ft.
It is possible to place the insulation above the slab in a retrofit. There are two main approaches:
- Place 2x4s flat on the slab and insulate between them. The 2x4s can “float” or be secured with concrete nails or Tapcons. Then nail subflooring to the 2x4s.
- Place 1 to 2 in. of rigid foam insulation on the slab and float the subflooring above. Float two layers of min. 1/2 in. insulation above the foam, staggering all joints. Secure the top layer of plywood to the lower layer with nails or screws.
If there is no vapor barrier below the slab, or you’re not sure, it’s best to add a vapor barrier on top of the slab, under the insulation.
SUBSLAB VAPOR BARRIERS
It’s important to add a heavy-duty vapor barrier under the slab to create a barrier to water vapor, which can readily pass through the concrete. The water vapor will harm any paints or floor coverings placed on the slab and can add significantly to household moisture levels.
The plastic sheeting use for the vapor barrier should be at least 10-mil thick unless you are using one of the reinforced plastics approved for this application. It is easy to damage a thin layer of standard poly while prepping for and placing the concrete.
The placement of underslab vapor barrier has been debated heavily in the concrete industry for two decades. The current consensus within in the concrete industry is that, when a vapor barrier is used, the best location is directly under the slab.
The problem is that the vapor barrier slows the drying of the wet concrete, extending the bleed time and finishing time, and sometimes leading to curling of the slab edges. Curling is a result of the top of the slab drying more rapidly than the bottom.
For years, many industry experts recommended placing a layer of sand or gravel above the vapor barrier to absorb bleed water and, thereby, reduce the finishing time and curling. However, this was found to cause more problems than it solved. Water was trapped in the sand or gravel “blotter” for extended periods, leading to high moisture levels in the slab and basement — causing the very problems the vapor barrier was intended to solve. Placing the vapor barrier below the insulation creates similar problems by trapping water.
Concrete contractors have adjusted to the use of vapor barriers by adjusting their mixes, adding a little more reinforcement at the bottom of the slab, and using well-graded, compacted aggregate below the slab. In general lowering the water content and water-cement ratio will solve most of the finishing problems.
One last thing: Before insulating any basement, make sure there are no basement water problems. In many cases, water problems can be solved by better management of rainwater with gutters and downspouts that direct water away from the foundation. In new construction, it’s also important to place a durable vapor barrier directly beneath the slab.
Read More on Basement Vapor Barriers Foundation Insulation Coatings
Marlowe says
Basement Insulation For A Tight Space
Building a basement in Colorado. The space it really compact and I would like to add some rigid foam to help with the cold weather but i don’t want to lose any interior square footage i don’t have to. Can i use 1/2″ rigid foam and some unfaced batts? or is this a terrible idea?
buildingadvisor says
The best way to preserve square footage on the interior is to use exterior insulation. The main problems are that bugs like to burrow in and under the foam, and the portion of the foam above grade needs protection from the elements.
Mineral wool boards such as Roxul Comfortboard is a great option for exterior foundation insulation as it fire-, bug-, and mold-resistant. It has an R-value of R-4 per in. – in between EPS foam and XPS foam, the other main alternatives.
If you’d rather insulate on the interior, and want to save space, why use fiberglass insulation, which only provides about R-2.5 per in.? Foil-faced polyiso board (Polyisocyanurate) will give the highest R-value at about R-6 per inch. In some areas, code allows to leave the foil-facing exposed – not the case with polystyrene, which must be covered by a fire-rated material. However, since you intend to finish the space, this shouldn’t be a problem either way.
I’d recommend two inches of foam board, with vertical or horizontal strapping on the interior as nailers for the interior finish. The 3/4-in. air space will get you a little extra R-value with the foil facing as well.
Avraham says
Should I Put Vapor Barrier on Basement Wall?
I am finishing a basement in Brooklyn, new york. I am confused as to whether I need a vapor barrier, a moisture barrier, or no barrier at all? Also how much insulation is enough, if needed at all? Thank you
Also, can I tile directly to the concrete floor? Is there any way to insulate the floor?
buildingadvisor says
The short answer is no, you generally do not want a vapor barrier on the walls of a finished basement.
The place to control vapor migration is on the exterior of the basement wall, in the form of waterproofing or damproofing.
On the interior, the best insulation to place against the concrete or block is rigid foam without any facing, such as foil or plastic sheeting. This allows for some drying to take place as foam board is somewhat permeable.
The recommended minimum is R-10 for your area, which you can achieve with two inches of extruded polystyrene (such as Dow Blueboard) or 2-1/2 inches of beadboard. Seal all joints with foam sealant to keep moist interior air from reaching the cool concrete wall where water will condense.
If you are placing a stud wall against the foam, it’s cost-effective to add unfaced fiberglass batts between the studs. You can also use flat 1×3 furring instead of studs to save space, but without the added insulation. Finally, you install the drywall and paint with a standard latex paint.
In addition to saving energy, the space will be more comfortable and dryer when insulated and finished.
The goal is to create a wall that is somewhat permeable to water vapor, so drying to the interior can take place when needed. Moisture can collect on the basement wall either from the interior (condensation) or exterior (moisture transmission from the soil). Allowing the moisture to escape helps protect the wall system from mold and moisture damage.
If your basement has any moisture problems, these need to be solved before insulating. Otherwise, you’re asking for trouble by trapping excess moisture inside the wall system.
Read more on Basement Vapor Barriers
Regarding your second question about installing tiles over the concrete slab, there are several ways to do this. You can apply tile directly to a concrete slab with thinset, as long as the slab is level, clean and stable. If the slab is uneven or out of level, you can add a lightweight cement leveling compound to the surface before tiling.
Use a leveling compound approved as a tile substrate. An “isolation membrane” is highly recommended directly under to tile to protect it from any cracking or motion in the underlying concrete slab.
If you want to insulate the floor, you can use 1 to 2 inches of extruded polystyrene, fit between pressure-treated 2×4 sleepers. Screw ¾ inch plywood subflooring to the sleepers, and then screw another layer of plywood underlayment (min. ½ in.) or tile backerboard to the first layer of plywood. If you use two layers of plywood, adding waterproof carpenters glue between the two layers of plywood will provide a very strong and stable tile base.
Offset the joints between the two layers of plywood (or plywood and tile backer) and screw the top layer only to the subflooring, not into the sleepers. Next install your isolation membrane (which can also serve as a waterproof membrane as needed). Finally install the tile with thinset mortar.
I would not recommend the tile work as a DIY job unless you’ve got a fair amount of experience with tile.
Beth says
Insulation & Vapor Barrier Under Basement Floor?
We are in the process of building in Maine. The walls of the foundation were poured last week and they will backfill on Monday, 9/17/18. We plan to insulate the inside walls of the basement with at least the R10 foam board. They put the black tar stuff painted on the outside. We plan to have gutters and they are digging trenches to divert water away from the house that the gutters will work with.
The one thing that our builder has not mentioned is putting any type of foam board in the floor of our basement. Also, you said above, during new construction its important “to place a durable vapor barrier below the slab.” What is recommended?
buildingadvisor says
Because deep-earth temperatures remain pretty constant, at about 50-60ºF, insulating under the basement floor will not save you much money, unless you plan to heat the space and use it as living area. However, insulation under the basement slab will help reduce condensation on the basement floor in warm weather, reducing musty odors and damage to floor finishes. You can read more here on Subslab Insulation.
Whether or not you insulate under the basement slab, a sub-slab vapor barrier is a good idea. It needs to be “durable” to survive the process of placing and finishing the concrete. Special reinforced polyethylene, designed for this application, is available from several manufacturers. Products include Stego Wrap, Perminator, and Viper. In general, sub-slab vapor barriers should be at least 10-mils thick. Read more here on Subslab Vapor Barriers.
The “black tar stuff” they brushed onto the outside of you foundation is called “dampproofing”. This cuts down on water vapor transmission from the soil through the foundation wall into the basement. It is not waterproofing, however. If you are considering finishing areas below grade to use as living space, you should consider a full waterproofing system, if it’s not too late (before they backfill). You can read more here on Basement Waterproofing.
Mike says
Underslab Insulation Down South?
I’m building a house in Burnet, TX, in the Texas Hill coutry. I like the idea of putting 2 inches of foam board under the slab to keep cold from coming up from the ground in winter months. Is this a good idea to thwart thermal bridging to the ground?
buildingadvisor says
Most people in your part of the world do not insulate slabs-on-grade and are not required to by code. Your county is in DOE Climate Zone 3, just over the line from the warmer Climate Zone 2. Slab foundations in the colder Zones 4, 5, and 6 are required to have insulation around the slab perimeter under the IRC. Additional subslab insulation is recommended in those locations.
by energy experts.
In your area, you would get some benefit from slab insulation around the perimeter of the slab where a slab’s heat loss is greatest. Two inches of vertical perimeter insulation will keep the outer portions of the floor warmer in winter, increasing your comfort and modestly reducing heating bills.
You could also add 2 inches of horizontal underslab insulation in your area. This would keep the slab a little warmer in summer, reducing condensation on the cool floor, but also raising the air conditioning load a little, so it is a trade-off. Because the energy benefits are questionable, most people do not install underslab insulation in warm and hot climates.
You can see the energy benefits of these configurations in Table 4.1 from the US DOE Builder’s Foundation Handbook.
In a monolithic slab, the perimeter insulation needs to go on the exterior of the foundation, and will need protection from UV and physical damage (see pic). In a slab with a stem wall, the insulation can go on either the interior or exterior of the stem wall. While it is possible to add horizontal insulation under a monolithic slab, it is tricky to accomplish and not commonly done.
With an insulated slab, you need to consider termite entry. In addition to soil treatment, you may need a termite shield so you don’t provide a hidden entry path behind the foam and into your structure. Requirements vary with state and local codes so check first before proceeding.
For insulation, the most common choice is XPS (extruded polystyrene), but high-density EPS rated for ground contact can also be used. You can expect to lose 30% to 40% of the insulation value over time from water absorption by the insulation. A newer option is Roxul insulation board made from mineral wool. It is a very good option for vertical applications as it is self-draining, and may also be suitable for underslab insulation – but check with your local code official before using it under a slab.
You can view these and many more foundation insualtion details at the new, online version of the DOE Builder’s Foundation Handbook at this link.