# Rim Joist / Cantilever Insulation and exposed foam board



## DiyGuy3000 (Jan 2, 2017)

I am working on re-doing the insulation in the rim joists/cantilever's on my raised ranch. Right now there is fiberglass batts stuffed in there. I have already purchased Greengaurd XPS 2 inch foam board from lowes to air seal this area.

From my readings I keep seeing that you cannot leave the foam board exposed to the interior in the rim joist area due to fire reasons. They say you have to drywall over it. The area I am doing this in is the laundry/storage/mechanical area of the house and it is an unfinished space so I'd rather not have to drywall it. Is there another option I can take to do this? Thanks!


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## Bud9051 (Nov 11, 2015)

Not getting a good picture of where this is. Is the cantilever hanging out over open air or over the utility room? 

Guidelines will vary by local code authority so ultimately they have the answer. Try again with the description, or picture, or sketch, 
Thanks,
Bud


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## jlhaslip (Dec 31, 2009)

In the Jurisdiction I work under, foam MUST always be fire protected.
If the fire gets to the foam board, toxic fumes from the burning foam can kill you.

It is a safety issue, not an appearance issue.


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## DiyGuy3000 (Jan 2, 2017)

Are there any other alternatives to covering the foam board with drywall? 

This is what I'm trying to accomplish.


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## ront02769 (Nov 28, 2008)

Yes, Flammabe foam must be protected. Which can be Sheetrock, plywood, Roxul, or possibly polyethylene sheeting. Check to see what your inspector will be happy with. Ron


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## Bud9051 (Nov 11, 2015)

Can you cover the inside piece with drywall or plywood? The horizontal piece up under the floor should be fine with the batt insulation and plywood below it, but as ront said check your local inspector. Seal as shown as air leakage defeats much of the improvement.

Bud


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## DiyGuy3000 (Jan 2, 2017)

Appreciate the replies, I will definitely call my township building department and see what they have to say. 

I was just hoping to find an easier solution than cutting up a bunch of drywall squares and covering the rim joists in an area where there is not even any drywall, it would look a little weird. Roxul may be an easier alternative and look half way decent.


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## Bud9051 (Nov 11, 2015)

I'm not sure how accurate your sketch is, but the floor joists usually rest on a sill plate. If you position your end piece 1/2" in you would have a lip to set the drywall on. Then caulk and done, no screws needed.

As for covering it with exposed Roxul, any mice that find a way in would love that. Seen tunnels right through the Roxul. 

Bud


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## jlhaslip (Dec 31, 2009)

Vent under the cantilever.


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## DiyGuy3000 (Jan 2, 2017)

That's a good point. I guess drywall or plywood maybe the only option then. I will still call my local building dept and see what they have to say. Thanks for the replies.


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## DiyGuy3000 (Jan 2, 2017)

Drywall will be a pain in some areas, particularly where there are pipes (duct work, dryer vent, etc) going through the rim joist area and in to the cantilever. Would it be acceptable to just cover the foam board with fiberglass batts in those areas instead of drywall?


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## Bud9051 (Nov 11, 2015)

That would be up to your local code official.

I know you already purchased your rigid foam board, but there is one brand that has been certified to remain exposed, Dow Thermax. It has a foil facing that has passed the required testing. But it still needs approval from local officials.

There is also an intumescent paint that can be applied to the rigid foam to meet the requirements. I looked at it years ago and know it has changed a lot and come down in price. And you might not need much. Just an option.

Bud


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## jlhaslip (Dec 31, 2009)

Bud9051 said:


> That would be up to your local code official.


Bud, 

I have suggested that the bottom of the cantilever should be vented.
You are more experienced in this area than I am. 
Would you suggest venting?

Thanks.


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## Bud9051 (Nov 11, 2015)

Hi jl,
Actually the bottom should be sealed to eliminate any air leakage. That cavity should be able to dry to the outside through the batt insulation and the plywood below. Even the rim joist or foundation will provide a path if it were needed, but there should be very little moisture to be concerned about. Air leakage from the inside would be the biggest source of moisture to be concerned about.

Bud


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## Gary in WA (Mar 11, 2009)

--------Welcome to the forums!--------------------------

Just switch the locations of the foam board with the cavity insulation and you will be all set, Fig.7; https://buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

If you have a gap above the cavity fill next to the subfloor, no problem; https://buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance

To also stop thermal bridging of the floor joists add the FB *under *them, extending down some new trim board around cantilever 3 sides to hide, use caulking between FB and plywood to air seal. Can foam any holes in subfloor to wall above first. IMO, use some thin plywood (check locally per code) on the room side of FB, as said- seal with caulk (doesn't pull away after curing- leaving an air gap of 1/16" or bigger). Many floors have plywood subfloor or underlayment with the glue layers being a vapor barrier. Seal around the HVAC duct/register, if existing.

Gary


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