Spray Foam in Rim Joists


Old 05-01-15, 07:29 PM
Thread Starter
Join Date: Jan 2015
Location: USA
Posts: 20
Spray Foam in Rim Joists

Alright so I have given in and am having a company come spray foam my 500 sq ft basement finish. I have a couple different quotes and my question is about the rim joists (I am in climate zone 4). One company is suggesting closed cell foam followed by fiberglass to reach the needed R-19 value. The other company is swearing by open cell foam at 5-6" which will obtain R-19.05 at 5" (no fiberglass needed).

Closed cell product- Demilec Heatlok SOY-200
Open cell- Demilec Sealection 500

Any clear choice to anyone who has experience with spray foam? Also, I thought spray foam needed an "approved thermal barrier" to cover it, but they would be spraying the joists in the storage/unfinished area as well.

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Old 05-01-15, 09:16 PM
Join Date: Oct 2008
Location: New England
Posts: 9,994
A wall assembly, basement or otherwise consists of 3 elements, a thermal barrier, an air barrier, and a moisture barrier. In some cases the materials serve more than one function. But moisture, be it air or permeability, must always be isolated from cold surfaces. In your rim joist, the open cell would in theory permit moisture to pass through and reach the cold rim joist.

There are guidelines where some will choose to use 2" of closed cell and then fill the rest with open cell.

As for the fiberglass (mouse nest material), leaving it exposed is not ideal. Some people will object to the air contamination factor.

As for an ignition barrier, yes the foam material should have one, but local authority has control. they do make a very expensive spray on Intumescent Paint the serves th ignition barrier purpose. Otherwise they cover it with drywall.

Another approach is to close the cavity with a layer of approved foil faced rigid foam. The foil on the approved brand meets the requirements, IF the local authority accepts it.

Old 05-02-15, 04:34 AM
Join Date: Jan 2014
Location: usa
Posts: 466
Bud is correct on the vapor issues and the paint coating. I don't think the paint is that expensive though, considering what it is doing for you. I think you will find it to be something around $. 60.00/gal. In 5 gal. pails.

Fire Retardant, Intumescent Paint | Contego International Inc.
Manufactures intumescent latex fire resistant paint for residential and commercial buildings. Includes material data safety sheets and detailed specifications.

The other consideration you may want to address is that if your area is subject to termite or carpenter ant infestation then spraying foam may be looking for a problem as it can conceal the potential damage caused by the insects. If you use a pest control company check with them for how your actions may impact their warranty.

I also tend to be cautious about using foam where the exterior grade level of the home is very close to the top of the foundation. If you have wet soil and a high grade I am of the opinion that you can be forcing moisture into the sill and rim. If it is an older home and you have no pressure treated wood or moisture barrier at the sill/foundation interface this would be a potential for issues.

I would, if I felt the above issues were considered safe, use the closed cell foam.

Like all construction, an observant eye can help avert disasters by routinely taking a look at the home to see what might be going on. It is my experience that there are no infallible products and homeowners tend to rely on the "once and done" approach to home improvements.
Old 05-02-15, 04:45 AM
Join Date: Oct 2008
Location: New England
Posts: 9,994
Good post calvert,
My only search for the special paint was early in its introduction so I agree my price concerns are probably overstated.
I do know that in heavy termite areas some codes require access for inspection, thus they use a batt insulation that can be removed and replaced.


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