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Basement Insulation / Water

Posted by WiscoJoe (My Page) on
Fri, Apr 12, 13 at 8:25

I am making plans to insulate / finish our basement located in Wisconsin. I've only been in the house for less than 2 yrs and haven't noticed any water or musty smells but would like to assume the worst...even if I don't have a water problem now I could see a 100 yr flood as soon as the drywall is up. Current set up is half finished, with drywall (no insulation and furring strips against concrete blocks) and carpet. I had the rim joist spray foamed about a year ago.

I think I've read almost all worthy's posts (thanks!) and have a general direction in mind:
1) demo everything
2) inspect for water / efflorescence behind drywall
3) seal cracks with polyurethane, apply sealer to blocks
4) attach 1.5" of xps to wall with adhesive and frame stud wall to press it against concrete block. Shim where necessary to keep tension between studs and xps.
5) tape seams and great stuff around perimeter
6) add roxul to stud bays for to bring up r value and still allow for inward drying.

Here are a couple questions that come to mind:

1) I'm thinking about avoiding stud wall and just use tongue and groove xps with furring strips to attach to wall. The cons I see are making a bunch of holes in the block. It seems counterproductive after meticulously repairing cracks and sealing the wall. Also, I loose the potential for added insulation such as roxul.

2) Leave 4 or 5 inches of the bottom of the wall open (drywall and insulation stop short) and cover with easily removeable baseboard (using magnets?) to allow for periodic inspection of concrete wall and allow a path for water to exit if it ever occurs.

3) Remove carpet, use something like the paint radonseal sells to cover floor and leave as is with area rugs. I like the idea of being able to see the floor for possible seepage and I don't think it will add that much to heat loss to justify finishing. Right now the basement is 56 degrees in winter and unusable for kids to watch TV so I think wall insulation will go a long way.

Any thoughts / comments on my questions above are appreciated.


Follow-Up Postings:

RE: Basement Insulation / Water

Wisconsin spans cold and very cold hygro thermal zones, Zones 6 & 7.

Under the 2009 and 2012 International Energy Conservation Codes, basement wall insulation should be R15 continuous or R 19 for a framed wall. Your proposed R7.5 is half the requirement in those two most recent IEC Codes, even the 2006 IECC, and the Wisconsin State Code. However, what you're proposing should be more than enough to prevent moisture from forming on the foundation wall, even on the exposed strip near the floor.

While I have concerns about putting holes in block walls, too, using concrete screws of the correct length or percussive charge tools (Ramset etc.) will prevent that.

If you have concerns about eventual minor flooding, simply put in high baseboards over moisture resistant insulation-- XPS, EPS, polyiso.

International Energy Code, 2012

This post was edited by worthy on Fri, Apr 12, 13 at 16:25

RE: Basement Insulation / Water

Thanks, worthy.

Just making sure we’re on the same page on r value. I’m thinking I have a total of 22.5 with 1.5” xps (7.5) and 3.5” stud bay with roxul (15)? How were you thinking R9?

RE: Basement Insulation / Water

I was responding to your second set of ideas--that is, just use tongue and groove xps with furring strips to attach to wall.

The first proposed method exceeds Code and is much preferable if you're planning to keep your home awhile.

How were you thinking R9?

Thanks. Corrected.

This post was edited by worthy on Fri, Apr 12, 13 at 16:27

RE: Basement Insulation / Water

Great...two more questions came to mind:

1) Any tips for insulating around things close to the concrete wall, like sewer stacks, drains, conduit, electrical box, water lines, etc? About 40% of my basement is unfinished and has these obstacles. I'm wondering if it is worthy cobbling xps pieces together or just accept that I will lose a little heat on those sections?

2) What is the counter argument to people who say that there is a greater risk of foundation damage when insulating above the frost line because the wall is no longer kept warm and undergoes more thermal shock, heaving, etc.?

RE: Basement Insulation / Water

1) Looks don't count. So cobble together what you can in those hard to reach spots and seal with polyurethane spray foam such as Great Stuff.

2) You are referring to adfreezing, that is the condition of soil freezing to the exterior of the foundation wall and then exerting upward pressure on the wall, leading to heaving and damage. The risk of adfreezing increases the more the foundation is insulated, which prevents heat from keeping moisture in the surrounding soil from freezing. The solution is not to stop insulating, but to: 1) use granular material as backfill, or 2) provide a slippery material on the exterior of the foundation, such as a bubble plastic membrane. (I even use it on the interior of attached garage foundations.) or 3) use a horizontal insulation to keep the soil from freezing. (See link, which recognized this problem more than 40 years ago, for further details.)

RE: Basement Insulation / Water

thanks're a wealth of resources. Looks like I have some more reading to do.

RE: Basement Insulation / Water

So now that I've read a bit more about adfreezing you got me paranoid. Again, I'm not aware of any water / seepage issues now but it does seem possible that by insulating I could do more harm than good by reducing the heat loss to the outside soil and encouraging freezing conditions that didn't exist before. And with everything covered up I would have no way to monitor it.

What about just insulating the above grade portion of the concrete block wall? It would seem I'd get the biggest bang for the buck by insulating the top portion and also avoid the risk of causing cracks in the foundation. The total height of the basement is about 7 feet and I could insulate just the upper 2 feet which are above grade. That would give me about 30% coverage but perhaps a 50% reduction in heat loss? Paranoid?

RE: Basement Insulation / Water

The biggest risk from significant adfreezing is with unheated basements or garages.

Theory and practice confirms that as long as there is some movement of heat from a basement, the adfreezing strength will not affect the structure.

Even, "[s]uper insulated basements, which exert a very minor influence on the soil thermal regime, still experience weak adfreeze bond strengths because the direction of the moisture movement in response to smaller temperature gradients is still away from the bonding area, or at least not toward it," according to J. Timusk and K.D. Pressnail of the Department of Civil Engineering at the University of Toronto writing in the Canadian Journal of Civil Engineering, Vol. 14 at p. 710.

Dr. Joe Lstiburek is, typically, blunter: "Adhesion freezing just does not happen with heated basements, regardless of insulation." * (PDF)

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