Adding rigid foam insulation to attic side of knee walls

[thenewguy]

In the next month I'd like to make a project of covering the attic side of several attic knee walls (actually almost all are full height walls, but same principle - currently fiberglass batts/studs open to attic) with rigid foam board in order to air seal and add a little additional R-value.

What I'm not clear on, and I've seen conflicting information, is whether or not an additional fire barrier is required.  I've seen several references to adding a layer of drywall on top, but I think (hope?) this applies only when the rigid foam is used on the interior side of a wall.

Looking at something like this:
https://www.lowes.com/pd/Rmax-R-Matte-Plus-3-R5-Faced-Polyisocyanurate-Foam-Board-Insulation-Common-0-75-in-x-4-ft-x-8-ft-Actual-0-75-in-x-4-ft-x-8-ft/3014174

The description says "This is not a structural panel or approved for exposed application." In my case, the material would be exposed to attic only - not interior and not truly exposed to the elements. I understand the foam material is combustible, but it's faced on both sides - so I'd tend to think that's not a concern.

Would this be something where requirements vary based on local code? I'm in central Texas if that's at all relevant.

Thanks!

[bacchi]

IRC requires a thermal barrier if the attic is accessible and used as storage. See IRC 316.4 (2009).

http://media.iccsafe.org/news/eNews/2013v10n4/2009_irc_qandaseries_p95-6.pdf

[thenewguy]

Thanks for the link!

That's frustrating - it seems that just about anything that's acceptable as a barrier costs almost as much as the insulation itself, which more or less doubles the cost of the project.

I came across this, which references that there are insulating products out there that meet the thermal barrier requirements themselves, but they all seem to be pretty difficult to get ahold of:
https://www.greenbuildingadvisor.com/question/is-rmax-thermasheath-xp-an-acceptable-alternative-to-dow-thermax

I might check Lowes and HD to see if they can special order one of those, if the cost is reasonable.

[bacchi]

Yeah, it does suck.

I'm using some leftover fiber cement panels as my fire barrier. Maybe you can find some drywall or fiber cement remnants on craigslist? It doesn't even have to be whole pieces -- you just need 16" strips, probably, though it'd seriously increase labor time.

[zolotiyeruki]

I'm having a bit of trouble understanding where the new insulation is going--are you attaching it from inside the conditioned space to existing walls?  Or are you planning to crawl around in an unfinished attic, attaching the foam board to the outside of the walls?  Can you post a picture or two to help understand?

[thenewguy]

New insulation would be on vertical walls in the unfinished attic.

Not sure how clearly the attached pic shows it, but that's one of the walls I'm talking about from the attic side. If I had to guess, I'd say 60% of the walls upstairs are like this (some more easily accessible than others). Seems like a poor design... lots of wasted space and energy.

The idea is that in a conventional exterior wall, the fiberglass batts are sandwiched in between drywall (interior side) and sheathing (exterior side), so the fiberglass is exposed to very little air movement. In the type of wall I've got upstairs, the existing batts are open to the attic, and convection currents result in air flowing through the fiberglass and significantly reduce its effectiveness, not to mention that attic air in the summer is often hotter than outside air. Sealing them off reportedly has a major improvement, and any additional R-value is an added bonus.

https://www.energyvanguard.com/blog/52426/How-to-Sheathe-an-Attic-Kneewall-And-How-Not-to

[zolotiyeruki]

In that case, I don't see why you'd be required to add any fire barrier.  I'd be surprised if you'd even need a permit (much like blowing more insulation into your attic.

I say go nuts and have fun.  The trickiest part would be cutting the sheets to fit through the opening into the attic.

[DoNorth]

I just did it for the kneewalls in my bonus room over garage.  R7.5 + R15 batts gave me almost R-22; there shouldn't be any other code requirement if its on the exterior of the wall.  It also serves as a backer for the bats.

[thenewguy]

Thanks for the feedback everyone.

I think I'm going to give it a shot in the next couple weeks. My walls are 2x4, so existing insulation is R-13. The stuff I linked above will add an additional R-5, so R-18 total. But from what I understand the greatest benefit is in the air sealing... will make sure to tape all the seams and seal any gaps with spray foam.

If an inspector has an issue with it, I can deal with that when we get there.

Next phase will be to air seal and then add some blown in insulation to the horizontal ceiling surfaces. My hope is that when it's said and done I can run the heat downstairs (heat pump) and not need the upstairs unit (resistance heat) to kick in at all.