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Crawl space insulation questions

I’m going to be converting my crawl space to a sealed, conditioned space. The concrete block walls will be insulated with rigid foam. I’ll be using TherMax, as I understand that unfaced rigid foam does not meet the ignition barriers of the IRC. I am also planning to use spray foam to air seal and insulate the rim joists (per Fine Homebuilding No. 189 , September 2007). Interestingly, I’ve never seen any discussions about ignition barrier for the foam for the rim joists. What’s the story here? Also, living in a moderate-to-high termite risk zone, I recognize the need to leave the top 2-3” of the block wall uninsulated and exposed, as a termite inspection strip. On the other hand , leaving that strip, as well as the 2-1/2” strip along the top of the block wall (I have a 2x6 sill plate on an 8” block wall) seems like an incomplete insulation job. Since termite inspection would typically occur no more than once a year, wouldn’t it make sense to make some sort of removable rigid foam strips to fully insulate the wall?

Asked by Michael Hansen
Posted Nov 24, 2012 5:52 PM ET
Edited Nov 24, 2012 5:55 PM ET


3 Answers

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The IRC allows the use of exposed spray polyurethane foam to insulate the rim joist area -- without the protection of a thermal barrier or an ignition barrier -- under certain circumstances, as spelled out in R316.5.11 (Sill plates and headers): "Foam plastic shall be permitted to be spray applied to a sill plate and header without the thermal barrier specified in Section R316.4 subject to all of the following:
1. The maximum thickness of the foam plastic shall be 3 1/4 inches (83 mm).
2. The density of the foam plastic shall be in the range of 0.5 to 2.0 pounds per cubic foot (8 to 32 kg/m3).
3. The foam plastic shall have a flame spread index of 25 or less and an accompanying smoke-developed index of 450 or less."

Of course, you should always check with your local code official before proceeding with any such plan to be sure that your method complies with your local code.

You are correct that the need for a termite inspection area reduces the effectiveness of crawl space insulation. Check with your local code official to find out whether your proposed detail (a removable strip of rigid foam) complies with your local code.
when tested in accordance with ASTM E 84 or UL 723.

Answered by Martin Holladay, GBA Advisor
Posted Nov 25, 2012 6:38 AM ET
Edited Nov 25, 2012 6:39 AM ET.


Thanks, Martin.

I'm curious about one thing, though. Do you know what the reasoning for the 3 1/4 thickness limitation is?

Answered by Michael Hansen
Posted Nov 25, 2012 4:08 PM ET


Q. "Do you know what the reasoning for the 3 1/4 inch thickness limitation is?"

A. As far as I know, the answer is simple: the thicker the foam, the greater the fire hazard. Cured foam burns intensely when it gets hot enough to ignite. I imagine the logic is: 3 1/4 inches of foam provides less fuel than 6 inches of foam.

Answered by Martin Holladay, GBA Advisor
Posted Nov 25, 2012 5:40 PM ET

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