Insulating basement walls of old home in a cold climate
I live in a 1911 home in Duluth, Minnesota (Climate Zone 7A). I have started to insulate my basement walls to R-15 using 3 inches of XPS foam board. Because of the age of the home, our cold climate, and the presence of clay soils, the building inspector recommend that I remove the bottom two feet of insulation in the basement so that the walls are exposed near the basement floor to keep the concrete warm during the winter. I’ve see conflicting reports about the potential for adfreezing on basement walls, such as this article concluding it is not a problem (http://buildingscience.com/documents/insights/bsi-045-double-rubble-toil-trouble#f01). I’m not sure if there is a perimeter drain or sand backfill in the basement, but with a dehumidifier running, the basement stays dry. Is this a concern if I insulate the full basement wall, or should I take the inspectors advice and not look into it further?
Brad
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Replies
Brad,
It's best to extend your wall insulation all the way down to the floor. For more information, see How to Insulate a Basement Wall.
How deep is the bottom of your basement below grade? If it was very shallow--a semi-basement--there might be reason to consider this carefully, but if the floor of the basement is 6 feet below grade there's nothing to worry about at all.
Minnesota has inserted many modifications to the IRC regarding foundation insulation (including R10 max for foundations on previous iterations of the IRC), and it would be a code violation to go all the way to the floor with R15 under current rules. Read all the particulars, including R402.2.8 (and the exceptions- R11 max unless spray foam?):
http://codes.iccsafe.org/app/book/content/2015_Minnesota/Residential/Chapter%204%20%5BRE%5D.html
See also, from prior years:
http://www.ci.chisholm.mn.us/vertical/sites/%7B89A03091-259E-44A3-B82B-4ACB22D45D19%7D/uploads/%7B2C45A225-793C-4E6B-8B5A-FAC85865B3B2%7D.PDF
The frost-heave potential is effectively zero even if the footing isn't below the frost line, and that's doubly true if the slab isn't insulated. It's not clear what history or rationale the code-makers in MN have for these changes, but it's been the case for several iterations of the IRC.
Are we looking at poured concrete, cinder-block, field stone, or quarried stone? (Any o f these are possible for a 1911 house.)
Thanks for the replies and links. It is a poured concrete basement with the slab ~5 feet below ground surface, although I would think the footing is another foot lower. I went with R-15 since that was recommended for this climate zone, but I didn't realize until later that R-10 is maximum for interior installation.
Just to clarify a point (and hopefully I am reading this correctly), it looks like the old Minnesota energy code requirements limited the insulation of basement walls to R-10, but it appears the current code does not apply this limit, unless it is an unconditioned basement and the ceiling (1st floor of the home) is insulated, then R402.2.8a and R402.2.8b apply.
http://codes.iccsafe.org/app/book/content/2015_Minnesota/Residential/Chapter%204%20%5BRE%5D.html
That's rigth- new MN code requires R15 c.i, on foundations, but no more than R11 of it can be on the interior unless it is spray foam, and there must be at least R10 on the exterior. Huh?
It's nuts, but that at least makes a minimalist insulated concrete form code-legal, whereas under previous code it was a technical violation.
This is all a MN specific fetish around foundation insulation, and if you can convince the local inspectors that a variance should be allowed (as it appears you have), its safe to violate it. The requirement to stop the insulation 2' above the slab is a bit odd bizarre, but if you don't have a capillary break at the foundation sill and don't have sufficient above grade exposure to the exterior for the foundation to dry that would be a reasonable suggestion, since it allows moisture wicking from the footing to dry toward the interior, lowering the moisture content of the concrete where it meets the foundation sill. From an energy use point of view if the insulation extends at least 4-5' below grade it has taken care of the lions-share of the problem.
After looking at the code a few times I am interpreting the R-10 exterior requirement only if the basement is unconditioned and the floor overhead is insulated, since then very little heat would be making it into the basement. It would seem odd (and difficult/expensive to apply) if this R-10 exterior requirement is true for conditioned basements or where the floor above is not insulated.
The inspector suggested that I take the bottom two feet of insulation off after I asked about frost damage. He did mention the building code requires insulation down to the basement floor. I think I am going to hand auger near the foundation and see what type of backfill is there and then talk with him once more. After reading the responses, I am not too worried about adfreezing or frost heave on the foundation.