HowTo Foundation And Slab Floor Insulation

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    Since about 1995 there has been a strong movement to increase insulation in all areas of our homes to reduce heat loss and lower energy costs. For most of us this has meant increasing the amount of insulation in our attics and adding insulation to exterior walls if the home is old enough that it was not built with insulated walls.

    New construction has seen a variety of different products and methods that take insulation to another level. Homes are now being designed tighter with increased use of vapor barriers, fiberglass and foam insulation and double wall insulated windows with argon and other gases to reduce energy transfer.

    The final step in a whole house system has been to insulate the foundation walls and floor slabs with foam insulation boards and coatings or membranes that will stop heat loss and water penetration.

    Now although the use of watter barriers is commonly accepted and has been tested for decades the new practice of using foam board insulation below slabs is causing some people to question if it will result in ultimate failure of the home’s foundation.

    Here you can see a detail for exterior wall foam board insulation used on the first floor and on the foundation wall. This method is very acceptable because it does not place at risk any structural components of the foundation or wall and floor systems.


    The added benefit of foam insulation is minimal a 2 inch thick board may only provide an R value of less then 10 while in our attics fiberglass insulation can provide from R20 to R60

    Exterior foundation walls are insulated with a foam board which is fastened with concrete nails or applied directly to the wet sealer that acts like an adhesive and then backfilled with dirt. If failure occurred due to the breakdown of the material then the resulting effect would be compression of the foam and slight movement of the backfill dirt around the foundation. It can be expected that the breakdown of the material will not cause failure of the water barrier and it should not result in clogging of the drain tile and sump pump lines which remove water from the foundation and footing areas.

    Insulation foam board that is located under the slab of the home does have a potential to break down and cause problems. Foam Board is not a structural material and as it breaks down under pressure and moisture due to direct contact with the soil the potential is apparent that there could be eventual cracking and heaving of slab floors. If the material breaks down in such a way to cause a void under the slab this will increase the risk that water runoff or naturally occurring water from a spring could enter the area and cause a washout effect of the supporting earth.

    Additionally the system details for slab construction recommends the use of a floating slab which is separated and not supported by the foundations footings. If the slab was to move or settle then any wall structures built in the basement would also be damaged.

    If the slab is not supported by the soil below it then it will fail and crack, split or heave because slabs only contain a rebar wire mesh to allow the support of minimal weight (people and furnishings) and they are not built to be structural members that contain #5 rebar that is found in footings, beams and foundation walls.

    The risk of using insulation under a slab is factored against the life of the home and the potential payback of lower heating costs. The amount of and type of insulation used may not bring such a large beneifit that could outweigh any future damage that may result from using the product.

    Earth or soil is itself an insulator. We can feel the insulating factors by entering our basements on a hot summer day. You can always expect a lower temperature in your basement because the soil surrounding your foundation walls is acting as an insulator.  In winter months although we do not feel the same benefit the insulation of the soil maintains heat in our basements. The normal temperature below ground is about 50F so even if it is cold in your basement due to loss of heating services the loss of heat will be faster above ground then below. 

    For this reason although there may be some factors that support the use of insulation below ground and its benefits on perimeter walls the cost of repairing any future problems warns against its use.

    If you would like to reduce your homes energy costs we recommend that you add additional insulation in your attic area and if cooling is a concern you should add a whole house attic fan which will reduce temperatures in your attic and your home.

    If your home builder wants to install foam board under your slab request an engineer’s report and an insurance rating that will last 10 to 20 years to cover full repair costs not just the single year warrantee most home builders provide.

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