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From a building science standpoint, everything I’ve learned over the past 20 years…

o2826KxmUb | Posted in Energy Efficiency and Durability on

… 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.

Dave Borski
Madison Gas and Electric Company
Madison, WI
[email protected]

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  1. GBA Editor
    Martin Holladay | | #1

    I don't think you'll have any condensation problems. Your Thermax should work fine.

    But I'm mystified as to why anyone would build a new house with a full basement and forget to apply dampproofing or waterproofing to the exterior of your basement walls.

  2. J99aAMQzYo | | #2

    "I'm mystified as to why anyone would build a new house with a full basement and forget to apply damp proofing or waterproofing to the exterior"

    ...or worse yet, actually make a conscious decision to intentionally omit it??

    The other possible concern with essentially zero drying potential is for the build-up of hydrostatic head against the foundation eventually creating the cracks and gaps that are "theoretically" non-existent in the original question.

    This may not be a specific concern in Madison based on your local water table levels, but here in the mid-Atlantic, that can be a fatal flaw (structurally speaking) in system design. I inspected a house this past year, after 100 year record snowfalls, which was at the bottom of their subdivision. All of the snow melt ran down hill to them, fully saturated the ground, and their basement wall was cracked and bowed in almost 3" when we pulled a string-line across it. It won't even get in to the time & money required to fix it once the building dept. and a PE got done with their evaluations.

    With such an extreme occurrence, who knows if anything could have saved them, but certainly a well-designed, active perimeter drainage system would be bound to have given them a fighting chance. (They have one of those now by the way :-)

  3. Riversong | | #3


    I'm also mystified why anyone would build a new house without foundation waterproofing (not damproofing, which is next to worthless) and why anyone would build in a northern climate without exterior foundation insulation - which puts the thermal mass in the conditioned space, allows drying to the interior and creates a an additional capillary break and drainage plane on the outside.

    Basement insulation slows heat loss into the soil and increases the frost depth, silty soils are the most frost-active if they are saturated, and clay deposits can prevent or restrict drainage thereby increasing the moisture content of the soil. I hope you at least have well-drained gravel backfill and a footing drain to daylight or a sump.

  4. Steve El | | #4

    Dave, I have a side bar to add in case you haven't thought of it already. You noted the landscaping is graded properly.... bear in mind I'm just a very interested homeowner and like to read so when I saw your signature line (about your being with a utility company) some reading came to mind that I thought I would mention... the short side bar article said that sometimes when the soil is clay-ey, porous back fill used in utility trenches can be a hidden conduit for bringing bulk water up to a foundation wall. The sidebar article suggested tying a short vertical drain from the utility trench down to the footing drain so any such water can easily escape. Good luck figuring out your main question,

    Steve El

  5. Riversong | | #5

    Since, ideally, a footing drain is for evacuating rising ground water, not surface water which should be dealt with through overhangs, gutters & leaders, grading and French drains - and since utility trenches typically become conduits for surface water or shallow ground flows - I would never attach any drain pipe to the perimeter drain or you risk putting water exactly where it shouldn't be. And, if the trench drain carries dissolved soil particles, it can quickly silt up a perimeter drain and render it useless.

    A utility trench can include a perforated drain pipe that is daylighted away from the house, with the trench bottom sloped away as well, and it can be capped with loam to prevent surface water intrusion.

  6. Steve El | | #6

    Thanks Robert, you remind me of an important point I left out... the sidebar article I read was about EXISTING basements, and curing seepage under the service entrance, where the basement is already in place. For new construction your idea makes good sense to this layman.

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