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Spray foam ignition barrier

When one applies spray foam on the rim joist of above an unfinished basement is an "ignition barrier required? Will a 3 1/2" bat of mineral wool meet the requirement?

Asked by Jerry Liebler
Posted Jun 11, 2014 9:38 AM ET


4 Answers

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When spray foam is installed on the interior of a rim joists, there is no need for an ignition barrier or an thermal barrier, as long as the spray foam thickness is 3 1/4 inches or less.

If you have sprayed more than 3 1/4 inches of spray foam on your rim joist, then the foam needs to be protected by thermal barrier. For more information on acceptable thermal barriers, see Thermal Barriers and Ignition Barriers for Spray Foam.

Answered by Martin Holladay, GBA Advisor
Posted Jun 11, 2014 10:39 AM ET
Edited Jun 11, 2014 10:41 AM ET.


To answer the broader question of whether 3.5" of rock wool qualifies as a thermal barrier for foam in the cases where it's required...

...yes it does, 3" minimum. (Most spray applied cellulose qualifies at 3" or more too, but not all- you need to read the product specifications.)

Answered by Dana Dorsett
Posted Jun 11, 2014 1:05 PM ET


Do you have a link to a document that shows that 3.5 inches of mineral wool passes the 15-minute fire test and is therefore considered equivalent to 1/2-inch drywall when used as a thermal barrier? If such a test has been conducted, is it specific to a certain brand of mineral wool?

Answered by Martin Holladay, GBA Advisor
Posted Jun 11, 2014 1:24 PM ET
Edited Jun 11, 2014 1:25 PM ET.


IIRC it takes 1.5" of generic mineral wool or fiberglass to meet the ignition barrier requirement, and 3" of generic mineral wool to meet the thermal barrier specs, but I'd have to dig to find it.

In a quick google search it seems some foam vendors think generic mineral wool qualifies at 2", but you can't always consider the words of foam guys (or me!) as gospel, eh? ;-)

http://www.foam-tech.com/theory/firebarriers.htm (see the table)

I remember seen test documentation for spray-applied rock wool that was formally tested to NFPA 275 at even less than 3" under the trade name "FireStop TB" , but couldn't find the test data page quickly on the web. It's still being advertised, but the manufacturer's website seems to be missing:



Answered by Dana Dorsett
Posted Jun 11, 2014 5:30 PM ET

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