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Insulating exposed rim joists

We are building a home in Maine where the basement won't be finished for some time. Besides loose fill insulation, what can I use on the rim joists that is fire resistant when left exposed.

Asked by tom ruben
Posted Sat, 12/14/2013 - 22:49

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12 Answers

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1.
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Tom,
Thermax is a brand of rigid foam insulation (it's a brand of polyisocyanurate) that has a facing that has passed fire safety tests. Most local building inspectors allow Thermax to remain exposed in basements and rim-joist areas.

I'm not sure what you mean by "loose fill insulation," but fiberglass batts are a bad choice for rim-joist insulation, especially in Maine, because fiberglass is air-permeable. For more information, see Insulating rim joists.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 12/15/2013 - 08:54

2.
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Any product that can be applied as a spray, so that it can cover pipes and wires? How about cellulose?

Answered by tom ruben
Posted Sun, 12/15/2013 - 19:28

3.
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Tom,
Cellulose insulation is air-permeable, so it is not recommended at this location -- especially in a cold climate like you have in Maine.

If you read the article I suggested (Insulating Rim Joists), you'll discover that the answer to your question is "Yes, there is an insulation that can be installed as a spray": closed-cell spray foam.

You can buy two-component spray foam kits at a lumberyard. Here is a link to an article describing the work: Basement Insulation — Part 2.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/16/2013 - 10:24

4.
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Okay, but now I'm back to the original problem of having to cover the spray foam because of its flammability.

Maybe a commercial product that is applied over the foam exists? Exposed steel frame buildings come to mind. Surely they have similar insulating issues.

Answered by tom ruben
Posted Mon, 12/16/2013 - 10:56

5.
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Tom,
As with your previous question, the answer can be found in the article I linked to. I suggest that you read the article. Here, once again, is the link: Insulating Rim Joists.

The article notes, "As long as your cured spray foam is no thicker than 3 1⁄4 inch, the International Residential Code (IRC) allows spray foam at the rim-joist area to be left exposed, without any protective drywall. The relevant provisions can be found in section R314.5.11 of the 2006 IRC and in section R316.5.11 of the 2009 IRC."

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/16/2013 - 11:26

6.
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We need to do more with our kids on reading comprehension. I did miss that note on the 3 1/4" max thickness in the article. Thanks for your patience.

Answered by tom ruben
Posted Mon, 12/16/2013 - 11:45

7.
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Tom,
Don't worry -- such lapses happen to all of us. Glad to help -- and good luck with your rim joists.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/16/2013 - 11:54

8.
Helpful? 0

With as little as a flash-inch of closed cell foam you can use R15 rock wool batts on the interior side as a thermal barrier, with effectively zero risk of damaging condensation in any ME location, even without an interior side air barrier. During the cool temperature extremes there may be a modest amount of condensing haze on the foam/fiber interface, bit it won't wick toward the wood, and will dry quickly when conditions warm. Without the flash-foam there would be at least some risk, though before the house is occupied that risk is pretty low.

At 2" of foam (or foam thicker than the 3-1/4"), with R15 rock wool you wouldn't need an interior air barrier to prevent even the occasional condensation haze events, and it would still be a code-compliant thermal barrier for the foam.

Answered by Dana Dorsett
Posted Mon, 12/16/2013 - 12:52

9.
Helpful? 0

Martin,
I see that your article mentions that the code views spray foam different from rigid foam. If I were to use spray foam to seal the gaps around pieces of 4" thick rigid foam in the rim joist area, and then spray an additional 1" over the rigid foam, does the code now view it as all spray foam?
If so, would mineral wool qualify as a "thermal barrier"?
Thank you very much.

Answered by Pete Marthaler - Zone 7
Posted Mon, 12/16/2013 - 13:01

10.
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If I may, to Dana's detail: Getting the foam to cover the pipes and wiring reduces some of the difficulty of insulating with batts but the irregularities of the foam then create a new challenge of closing the air gap between the two insulations. That is, air gap for effective R not air gap causing condensation issue.

Answered by tom ruben
Posted Mon, 12/16/2013 - 13:17

11.
Helpful? 0

Pete,
I hesitate to make a code interpretation; I suggest that you talk to your local building inspector to determine what is acceptable in your local jurisdiction.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/16/2013 - 13:47

12.
Helpful? 0

Dana and I must have been working on our posts at the same time. I believe Dana's comments may have answered my questions re: use of mineral wool as a thermal barrier. Thank you.

Answered by Pete Marthaler - Zone 7
Posted Mon, 12/16/2013 - 14:07

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