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Community and Q&A

Help with drafty home/basement insulation

TomTomorrow | Posted in General Questions on

Hoping to get some advice on the right strategy for insulating/air-sealing a basement. Briefly: Have a two-story home in New York State (Zone 6A) built circa the 1920s. For the past few years, I’ve noticed an increasing draft–not moving cold air, but a cool feeling within moments of the gas forced-air furnace shutting off. Decided to top off some ancient attic insulation with 15 inches of R-49 but didn’t notice any real difference. Windows are new and doors are weather-stripped. Not sure about insulation in the walls.

I suspect I’m getting some cold air infiltration in the basement, which has a concrete block foundation. A couple of things stick out: The foundation seems to extend well above grade, past the basement windows and well over the first floor. The cells of the concrete blocks that are exposed over the four windows are filled with batt insulation, which is dirty and moving air. Taking some of the insulation out to inspect the cavities, I could feel cool air coming in. These voids seem to be at least two or three feet deep, putting them well above the floor overhead. I’m wondering if I should remove the ancient batting and fill these voids with foam, or if expanding foam might affect the structural integrity of the foundation by “pushing” against the concrete blocks.

The other problem is the rim joist–or what I think is the rim joist. Along the left and rides sides of the house, it’s wood that “floats” slightly over the foundation. Someone stuffed batt insulation in here, too, and I’m again wondering if I would be better served pulling it out and putting in spray foam–but also concerned the foam could push the rim joist away from the foundation.

I don’t seem to have a conventional sill plate to speak of. The joist looks more like photos I see online in the front and back of the house, but there’s still no “bottom” that would hold a piece of rigid foam insulation.

Basically, I think air is getting in via the concrete block cells, between the blocks and the sill plate, and I’d like to stop it if possible. Thanks in advance.

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  1. GBA Editor
    Brian Pontolilo | | #1

    HI Tom.

    When you did the insulation upgrade in the attic, did you do air sealing work first? If you house is leaking a lot of air through the roof during the heating season, it may be pulling air in through leaks in the basement.

    I'm not sure I understand what's going on in your basement from your description or photos, but fiberglass insulation is not doing you any favors stuffed inside the cells of concrete blocks or at the rim joist. You probably should replace the fiberglass with any combination of spray foam and/or rigid foam sealed in place with canned spray foam or caulk. I doubt that you will cause any structural issues doing this work.

  2. TomTomorrow | | #2

    Thanks, Brian. Yes, the contractor air-sealed the attic.

    Would it be possible to push the batting behind the joist and in the blocks further back and then seal with spray foam, or is it better to remove it all and use foam? The problem with the rim joist is that I'm not sure the foam would get all the way back in there. I'm thinking of having someone come out with some closed-cell foam to spray the entire perimeter, but I still feel the area behind the joist would be a problem if they don't get back behind it. Thanks.

  3. Expert Member
    Dana Dorsett | | #3

    Cold air entering the basement is usually due to warm air leaving the top of the house somewhere. the attic floor is usually pretty leaky on 1920s vintage houses, but there might also be balloon framing or partition walls sans top plates, flue chases, plumbing stack chases behaving as stack-effect flues depressurizing the fully conditioned space or basement, drawing cold air in at lower levels.

    Foundation sill beams & sill plates typically have MASSIVE air leakage between the top of the foundation and the beam, representing a bigger net hole than all the window & door crackage in the house combined. That can usually be sealed with polyurethane caulk anywhere the crack is a half-inch or less, expanding foam where it's larger than that.

    Don't be concerned about expanding foam moving the rim joist off the foundation- that would take a hydraulic jack, at pressures many orders of magnitude higher than would be seen with expanding foam (any type.)

    Any visible cracks in the cinder block (both above and below grade) can be air sealed with polyurethane caulk as well, using a version formulated for concrete, just not the soupy "self-leveling" type. Use self-leveling polyurethane caulk to seal any cracks in the slab (which can move a surprising amount of not-very-clean air), as well as the seam between the slab & foundation wall.

    The exposed above grade foundation represents a large amount of heat loss in your climate, even if the basement is not fully heated. An 8" CMU wall with empty cores runs about U0.4 to U0.5 (See Table 2: ). Taking the mid-point call it U0.45. At an average wintertime outdoor temp of +20F and a basement temp of only 50F that's a 30F difference, and every square foot of wall is losing 30F x U0.45= 13.5 BTU/hr. If there is 3' of exposure and a perimeter of 150' that's 450 square feet x 13.5 BTU/hr= 6075 BTU/hr of conducted heat loss, not counting your infiltration issues. Over ~2000 hours of winter that's 12,150,000 BTU, which would take ~130 therms of natural gas in a condensing burner to cover, and that's only if the basement idles along at 50F. If it's warmer than that the heat losses are higher.

    After air sealing the foundation and band joists to the extent possible it's "worth it" to insulate the basement to the current IRC code minimum of R15 continuous insulation, on both a comfort and efficiency basis. The greenest (and often cheapest) way to do that is using RECLAIMED roofing foam board. That would be ~2.75" or more of 2lb polyiso, or 3.5" or more of Type VIII EPS, glued to the foundation with foam board construction adhesive, then strapped with 1 x 4 furring through-screwed to the wall with masonry screws onto which a layer of half-inch gypsum (painted or not) as thermal barrier against ignition. Tape the seams of the foam with house wrap tape, and seal the seam at the top with foam board construction adhesive, polyurethane caulk, or expanding foam. It's often best to leave the bottom seam unsealed for drainage of any bulk water that may find it's way through the CMU.

    There are several sources of reclaimed roofing foam in or near 6A NY, many of which can be found via this search:

    Newly made foam board has a fairly hefty greenhouse gas footprint, whereas reclaimed goods does not. Re-use is only piling on to the benefit side of the environmental cost/benefit balance of a hit already taken. This is far better than virgin stock goods, and WAY better than using closed cell spray foam (even the lesser-impact HFO blown goods):

    (Virgin stock 2lb density roofing polyiso has an impact roughly equivalent to HFO blown closed cell foam, reclaimed goods practically zero.)

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