From a building science standpoint, everything I’ve learned over the past 20 years…
… points me to the conclusion that installing foil faced insulation from the top of the foundation to the floor on the interior of a below grade concrete wall is …
The particular house in question is a newly constructed 1,000 sqft ranch with a full basement with less than 1 foot of exposed 8” concrete foundation wall. There is no water- or damp-proofing, or insulation on the exterior foundation wall.
1 ½” of Thermax was installed, using an adhesive, directly against the inside of the concrete wall from the floor to the top of the foundation. It was sealed along the top and on the seams with foil tape. The 1 mil foil facing has a perm rating of less than .03 (ASTM E96). This is a northern climate (Madison, WI) with about 7,600 degree days and about 300 to 400 cooling degree days (base 65 degrees). The soil is a silt loam with sporadic clay deposits and the landscaping is sloped away from the foundation.
Let’s assume, for the sake of discussion, that the concrete wall is perfect, with no cracks so no bulk water from rain or melting snow can seep into the basement.
My question concerns the ability of the wall to dry. My understanding is that below grade walls should be designed to dry to the interior and that can’t happen if foil is installed on the inside foundation wall.
Given these parameters, do you think there’s a good possibility for moisture problems (vapor condensing on cold surfaces) during the summer and the shoulder seasons because the vapor pressure during those times is generally toward the inside? Or do you think the possibility insignificant?
I’m not too concerned with condensation occurring during the winter months because the vapor pressure is generally towards the outside and the insulating quality of the Thermax should warm the wall enough so that the dew point is not reached. Although, I have seen the sun warm the concrete wall enough during the winter so that moisture was “pushed” through the concrete into the interior.
Thank you and I appreciate any insight you could give.
Madison Gas and Electric Company
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