Basement Insulation R-Value

jarvinenDecember 5, 2010

I'm going to be insulating and framing the walls of our poured basement. We live in Michigan's Upper Peninsula on top of a well-drained bluff so our basement is very dry and the climate is very cold in the winter.

Any recommendations for R-value? I've read that an R value of 25 would be recommended for our region.

The plan is to use 2" foam board against the concrete, then frame against that and roll in insulation between the studs.

Any recommendations or advice for our project would be appreciated. Thank you.

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R25 is on the high end.

Building in Aspen, Col. Building Science has gone with R13 closed cell spray foam (ccspf).

But keeping in mind aged R values and rising heating costs, it may not be unreasonable. In merely a cold climate (Toronto) I aim for R20.

By foam board, I take it you mean extruded polystyrene (XPS). You can use EPS, but it must be much thicker.

Don't forget the space between the joists. You can either foam that--open or closed cell--or laboriously place in cut foam board.

Other tips:

*tongue and groove seals tighter.

*use building tape and/or suitable
caulking on all seams.

*rest the baseplate on min 6mm poly or
one-inch strips of XPS.

Here is a link that might be useful: Building Science Corp. on Basements

    Bookmark   December 5, 2010 at 12:43PM
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What you describe might achieve R-21 to 23 but not 25.

Heat loss from an in-ground concrete basement is far less than from a typical above-grade wall. R-10 is the usual code requirement for your area and the additional cost for R-25 would probably be better spent elsewhere.

Why wasn't insulation installed on the outside of the basement wall?

    Bookmark   December 6, 2010 at 7:26AM
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Our home was built in 2006 and the basement wasn't insulated outside. I was unaware that that was the best option so we're doing it inside now.

If R-20 or higher is too much, should I go with 1" XPS instead? The main reason for using the XPS was to act as a vapor barrier as well as insulator.

    Bookmark   December 6, 2010 at 2:04PM
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Dr. Lstiburek's rule of thumb is a minimum R20 in basement walls north of the Mason-Dixon Line. (Of course, he doesn't always follow his own rules.)

I've used R13 dense fg batts and 1" of XPS in basements plus spf in the rim joist area. That yields a nominal R18. However, in your colder climate (very cold on climate maps) I would be concerned that the 1" board (R5) would not be enough to keep the wall free from condensation on the above grade portion.

What's more important than the nominal R value is the tightness of the wall to air penetration. That's why the tape, t&g, caulking and polyurethane spray is so important.

BTW, since I live in my own homes, I can say the basement comfort in homes with the XPS/fg combo is noticeably greater than the previous standard fg. Plus there's less need for supplementary heating.

    Bookmark   December 6, 2010 at 6:35PM
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DH used the book linked below when we owner-built our house. I don't know if Joe adds info for your very cold climate, but there's TONS of great info & drawings in this book--definitely worth the money!


Here is a link that might be useful: Joe Lstiburek's

    Bookmark   December 6, 2010 at 8:11PM
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Just reread your post and see that you already built the house! So I guess my post is for any future builders in ANY of the climates Joe has written books for.


    Bookmark   December 6, 2010 at 8:13PM
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I am wondering the benefit of adding R20 or 25. Our basement (not daylight, 4 standard basement windows, bulkhead behind outside-rated door) gets down to 53-55 (at the coldest) during the winter with no heat at all. The only insulation is on the rim joist (foam board and then fiberglass). We are in Maine. I can't imagine that insulating to R20 would make much of a difference. Or would it?

    Bookmark   December 6, 2010 at 8:53PM
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Would 1.5" XPS do the job? Or, if it were you, would you go with the 2". It would save a few bucks to go with the 1.5", but I'd pay the extra if its not overkill.

We do have larger windows and about 3' of concrete exposed on the north side of our basement. Other than that, it's mainly underground.

    Bookmark   December 7, 2010 at 8:49AM
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Any concrete exposed above actual grade can have high heat loss since now you have outside ambient temperatures and wind.

Keep in mind that the loss through walls down to the frost depth is can also be significant.
You have a solid 32F temperature sinking heat (and possibly even lower as you get near the surface).

    Bookmark   December 7, 2010 at 9:22AM
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am wondering the benefit of adding R20 or 25.

The US government's EERE recommends R25-30 as cost efficient for floors. Presuming this applies too, to basement walls, it seems an upgrade would makes sense. At least for the first four feet down.

Here is a link that might be useful: Insulation Recommendations

    Bookmark   December 7, 2010 at 11:21AM
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I wish I knew the math! If the inside temp with no insulation is 55 degrees, what would the inside temp be with the added R25 of insulation? Then determine the payback period on that insulation.

This is all conjecture...I am sure that when we get around to finishing the basement we will do some type of insulation, but it won't be R25 for sure (we have R21 in the walls upstairs). That would require spray foam which is just too expensive here.

    Bookmark   December 7, 2010 at 12:15PM
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