Insulating basement in MN
Hello,
We are trying to finish a room in our basement, and in the process insulate the basement wall in that room from inside (house built in the 50s, Zone 6). We have lived in the house for only two years but have not observed water seepage in the area except a drop of water on the floor once (could not tell where it came from). We have not observed any condensation on the walls. However, since there was the drop of water on the floor, and also because in MN there is always a chance of a severe water event (heavy rain during spring thaw), although rare, we decided to not seal the interior wall with xps or other boards. We are currently intending to have the framing (2×4) one inch from the wall and have fiber glass R15 put in. Does this plan sound OK – meaning in terms of the potential water even that we may be possible, this idea is better than foam board? Or should we not expect to do insulation at all (just dry wall)? Not finishing the area is not an option.
Also, the contractor currently plans to put R11 on the walls but I am hoping to negotiate changing it to R15. Does that make sense or there is not really a big difference between R11 and R15?
Thank you.
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Replies
SGTM,
Don't do it! You don't want to use fiberglass to insulate the interior side of a basement wall. Read these two articles:
"How to Insulate a Basement Wall"
"Three Ways to Insulate a Basement Wall"
Thank you for your reply! We cannot do spray foam due to budget (and our contractor not offering it). At first I thought I should not have fiberglass, but based on how little water there is in our basement (at least so far), thought fiber glass 1 inch from the wall may be OK. If we are talking about a more extreme water event, it seems like nothing would be a good idea. Foam board will seal water between the wall and the board and water will not be able to escape. Even not having insulation and just dry wall, it would rot/mold. Not sure if spray foam would be OK if there is water seeping. What do you recommend instead of fiber glass in this kind of situation? As I said, the wall seems dry, but in MN we cannot rule out the possibility of a water event. The other areas of the house has drain tiles/sump pump but not this area, so I imagine this was not an area to target for drain tiles/sump pump for previous owners.
Respectfully.
SGTM,
Did you read the articles? Fiberglass batts allow indoor humidity to reach the cold concrete via air flow and vapor diffusion. You'll get water forming on the cold concrete, flowing down the wall, and puddling near the bottom plate of the wall. You'll also get mold.
Seems like there is not good way to insulate from inside in MN -- unless the basement is completely water proofed from outside. If there was no risk of water intrusion we would do the foam board. But it seems like the foam board traps water, so in our case where there is at least some small chance of water intrusion, we cannot use it. Is not doing any insulation and having just dry wall OK or it has the same issue as fiber glass?
SG_TM
Why would it matter if the water couldn't get through the foam board?
Oh, if it does not matter I'd like to know. I thought the water being trapped between the wall and the foam board would not be a good thing - perhaps causing mildew growth?
I don't know if you read the articles Martin posted but the majority of water you will get with a conditioned space and no rigid is warm moist interior air condensing on concrete. You mainly want to stop that air from getting to concrete. If you are worried about soil water intrusion, I would seal with a paint on vapor retarder, like red guard or other fluid applied barrier designed for that. You could then place a dimple mat that drains down to an interior French drain that is then pumped out. But you would still want the rigid foam air sealed to not allow interior warm moist air from touching the cold condensing surfaces. You should read those articles.
Don't do the fiberglass! I pulled apart a basement wall (about 40 feet, 70's vintage work) with exactly that setup in 7A. All of the insulation was wet. Someone in the past had applied sealant to the interior concrete surface and that had failed. The concrete walls had to be treated for mold. This is one of those cases where spray foam makes a lot more sense. The cost difference is not much after you consider that the insulation and vapour sealing costs after spraying are zero. You can have the contractor frame the basement and do all plumbing and wiring, then have a spray foam outfit do closed cell foam at that point on exterior walls.
The difference a few years later is night and day with respect to warmth, dryness, mold growth on lower wall sections etc.