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Community and Q&A

Keep the basement as finished or convert to unfinished?

marcinn | Posted in Energy Efficiency and Durability on


We own a house build in 1976. There is musty smell in the basement/basement feels dump during summer. The house is located in Central Massachusetts (Zone 6). The basement has been finished at least 30 years ago, or perhaps was always finished – since the house was build. There is a utility room, full bathroom and sauna there. Unfortunately, it’s wooden framing directly against the concrete, no gasket at the bottom of the basement floor, stud bays are filled with fiberglass. I have removed the carpet and padding from the floor, but I am worried about mold growth in the walls and the quality of the air in the basement/house – the house is located near wetlands. The basement slab is not insulated, there is a sump pump that works during heavy rains. There is some efflorescence on the foundation walls (sections that are visible).

I plan on leaving the floor as unfinished concrete. In terms of walls, I am considering the following options:
1) Remove existing walls, insulate the basement according to best practices (closed cell foam/XPS against the foundation + steal studs framing).
2) Convert the space to unfinished basement.

I am leading towards 1), but I am worried about the lack of gasket between the seal plate and the foundation. There are a few locations around the house where the foundation is exposed only 6 – 10 inches above the ground. Is there any way to mitigate this risk?

Am I making a mistake trying to keep this space as “finished”? Another option would be to start running dehumidifier in summer and heat the space in winter (baseboard heaters are installed in the basement), but’s not solving the root cause of the problem.

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    It sounds as if you have a good understanding of the problem. Most homeowners would just install a dehumidifier. It's the easiest route, although clearly not the best route from a building science perspective.

    If you decide to remove the existing wall framing and fiberglass batts to start from scratch, I certainly advise you to insulate the walls properly. Building codes require basement walls to be insulated. The insulation will make your basement more comfortable and will lower your energy bills. It will also reduce the rate of evaporation from the concrete, potentially lowering the humidity level in the basement.

    If you haven't seen them yet, I urge you to read these two articles:

    Fixing a Wet Basement

    How to Insulate a Basement Wall

    Q. "I am worried about the lack of gasket between the sill plate and the foundation. There are a few locations around the house where the foundation is exposed only 6 - 10 inches above the ground. Is there any way to mitigate this risk?"

    A. You didn't mention whether the sill plate (mudsill) is made of ordinary untreated lumber or pressure-treated lumber. If the mudsill is pressure-treated, I wouldn't worry. (That said, it's important to caulk the crack between the top of the foundation wall and the mudsill.)

    If the mudsill is untreated, here's what you can do to mitigate the risk: remove any bushes that shade the exterior of the foundation wall. You want sunshine and air flow in this area, to keep the top of the concrete as dry was possible. If there is mulch in this area, remove the mulch and lower the grade, consistent with the need to keep the grade sloping away from the foundation.

  2. marcinn | | #2

    Thank you, Martin!

    Unfortunately, the sill plate is not pressure treated. The land is fairly flat on one side of the house, so there are not too many options with grading. There is also mulch in multiple areas. After reading a few articles (thank you for the links), I am thinking about creating a slightly sloped 3 - 4 feet wide walkway with pavers around the perimeter of the house to redirect the water away. Although, I am not sure if they will not be trapping moisture under them.

    The most problematic section of the foundation (close to the ground level - 6 - 10 inches) has a stone facade (standard wall, followed by approx. 1 - 2 inches gap, followed by 3-4 inches granite blocks). There are small ventilation holes on both sides at the bottom and ventilation at the top of this wall). Would exposing the foundation, removing mulch and assuring air circulation allow for sufficient drying of the sill plate in this case?


  3. Expert Member
    Dana Dorsett | | #3

    Like all of MA, central MA is in US climate zone 5, not zone 6.

    Both XPS and (most) closed cell polyurethane come with a hefty environmental hit due to the very high global warming potential blowing agents used. EPS & polyisocyanurate are blown with pentane, with less than 1% of the global warming hit. Either are fine to use against the foundation, but the cut edge of polyiso has to be kept off the floor to prevent moisture wicking from the (clearly damp, in your case) slab.

    IRC 2015 code min for foundations in zone 5 is R15 continuous insulation. That can be had with 3" of polyiso or 4" of EPS trapped to the foundation with your steel framed studwall.

    In central MA there are multiple companies trading in reclaimed & factory seconds foam. Typical pricing on used-but near-perfect 3" fiber faced roofing polyiso is $15-25 per 4'x8' sheet, and it's performance will be between R17-R20, depending on age, density, etc. Some of the bigger outfits also deal in factory blemished virgin-stock goods as well. The two largest in the area are Green Insulation Group in Worcester, and Nationwide Foam in Framingham:

    There's a few others, one mid-sized operator in Winchendon, but inventory with the smaller outfits is spotty (though the pricing often favorable.) Many can be found advertising here (but keep checking back- the smaller guys only seem to advertise when they run out of storage space):

    The performance of XPS drops to that of EPS as it's blowing agents leak out over time. While it might be labeled R5/inch, don't count on on more than R4.2/inch performance, especially when using reclaimed/reused goods.

    When gutting the walls, take note of the high-water mark on the framing from any prior basement flooding. If using polyiso, stop the foam at least 5-6" above high-tide, but it's fine to run EPS/XPS all the way to the slab, and it's fine to transition to 3" EPS below that mark if insulating with 3' polyiso for most of it. The greatest amount of heat loss is at the above-grade and near-grade portion of the foundation wall, so the lower performance below the polyiso is "in the noise", especially if you won't be insulating the slab.

  4. marcinn | | #4

    Dana, thank you for the links to reclaimed foam suppliers. I will definitely check this out.

    One more thought. Would it help to install a thiner layer of foam (2 inches for example) and add 4 inches of mineral wool to let foundation to dry towards the interior a little bit? (Considering how close to the ground my foundation on the outside is).

    Thank you again for your feedback!


  5. Expert Member
    Dana Dorsett | | #5

    The foam is going to dramatically lower the drying rate of the foundation, no matter where you place it in the stackup. If the place has EVER flooded, don't use fiber insulation of any type below the high water mark. Using rock wool or fiberglass wall insulation in a basement that needs a sump pump (like mine), is a lousy idea.

    Digging a foot wide foot deep trench of the foundation a foot or so, laying in some landscape fabric and backfilling with washed 3/4" stone screenings, wrapping it with the landscape fabric will improve drying of the concrete toward the exterior. (A 2" layer of washed stone over the top to hide the landscape fabric makes it more aesthetic.)

    Alternatively, jacking up the house 1/4" and slipping in sheet EPDM under the sill as a capillary break would also relieve the wood of moisture wicking up from the footing. (That sounds like a more radical idea than it actually is.)

  6. marcinn | | #6

    Thank you, Dana - very helpful information.

    Is is necessary to spay the rim joist area, or will it be sufficient to fill them with mineral wool? My walls on the outside (except the one finished with stone) have the plywood taped to the foundation with permeable Siga tape (I taped all seams, filled nail holes when reinstalling the siding) - the wall also has a rainscreen.

    Also, is there any disadvantage of installing a cement board on the interior side over the steal studs (or hardi architectural siding panels). If the basement gets flooded for some reason, the walls would not have to be taken down.


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