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nadsab 04-22-2011 01:32 PM

Dow Foamular 150 on crawlspace foundation walls?

I live in Michigan and am about to re insulate my craw space for our addition. Half the house is on crawl space, and the other half has a full basement.

The previous owner has 2 inch thick old style white Styrofoam insulation glued to the foundation walls in my crawlspace.

I read somewhere that the older style beaded type white sheet Styrofoam is a no-no for crawl spacewalls and that it would be better to use something like EPS or XPS rigid foam board – the kind they sell at Home Depot – the Dow Foamular 150 due to the white Styrofoam absorbing and holding moisture.

Is it OK to use foamular 150 along the crawl space walls – which is the foundation wall?

The reason why I thought foamular would be good was because I read somewhere that it is semi permeable – vapor retarder not barrier, and allows air to pass through, yet still is moisture resistant, and this would allow the foundation walls in the crawl space to breathe or dry out enough during our rainy season yet the foam would not hold water, such that the floor joists sitting on top of the foundation wall would not by capillary action draw too much moisture from the foundation walls and thus would not rot out.

I understand that if I use foamular in a crawl space, that I will need to cover the foamular 150 with gypsum board for fire protection to meet code. Would adding drywall negate the semi permeability of the formular?

We had a problem with the sill plate and floor joist ends rotting out and I just replaced all the rotten wood. I realize that much of this moisture is from bad drainage around the house which I also plan to take care of this summer - but want to make sure I don't do anything wrong with insulation that would trap moisture along the foundation wall.

I also plan on putting down a 6 mil plastic sheet vapor barrier on the dirt ground in the crawl space and staple a radiant barrier on the underside of my floor joists.

Thanks for any advice, info.

Gary in WA 04-26-2011 12:19 AM

is not quite thick enough;

Find your Zone on map or City below map;

Compare your Zone for insulation required;

See if you are in a termite area requiring leaving 6” above grade exposed with no foam board for inspection; bottom of page;


asnigro 05-16-2015 09:55 AM

Had pretty much the same situation as above. Sadly it appears the links are out of service in the reply.

I'm also deciding between the Formular 150 or 250 for the below grade crawl space walls above the dirt floor. About 4' of exposure.

Gary in WA 05-16-2015 11:39 AM

Sadly, a LOT of my links are out of date... lol.

Welcome to the forum, and where are you located!?


asnigro 05-17-2015 12:19 PM

Thanks Gary, I'm in WI.

Just bought 24 sheets of 2" 4'x8' formular 150 sheets and plan to use a foam board specific adhesive to attach them to the cinder block crawl space exterior walls before putting down a 6 mil poly liner.

Gary in WA 05-17-2015 03:20 PM

That will work. If the crawlspace is accessed regularly, you may need a thicker poly on the dirt... If you cover any vents to exterior, be sure there is air movement everywhere to prevent rot in framing. being coupled to the basement should work, just don't stop any any air exchange between the two spaces. Check locally if need top 2" of foundation exposed for termite inspection... foam board the rims with leftover rips, don't forget the top of concrete wall, Figs. 2,3;

Another option; FB the joist bottoms (need an ignition barrier- ply, drywall, etc.);
And let the insulation touch?sag the FB for warmer toes;


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