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. Both types of foam need to be covered by drywall or another code-approved fire barrier.
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. Spray foam is also an option, but usually a lot more expensive.
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.
Some people add additional fiberglass insulation between the studs, which is probably fine in a very dry basement, but could lead to moisture or mold problems long-term. Best to leave out the fiberglass.
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
It’s important to use heavy plastic sheeting 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.
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 same much money in energy bills, but it will warm the floor enough to reduce condensation in warm weather. This will reduce the likelihood of musty odors and ruined floor coverings and greatly improved 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.
SUBSLAB VAPOR BARRIERS
It’s important to add a minimum 10-mil 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 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 over below the slab.
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?
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.