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KatieAK 11-16-2009 01:07 AM

fiberglass over rigid foam board in cantilever?
I'm trying to address a poorly insulated and air sealed cantilever, and will have the adjacent downstairs ceiling opened up (so I'll be accessing the joist cavities from the side, rather than by removing the soffit from outside). I'm hoping to get some feedback/advice on my current plan, as this is the first time I've done this sort of project!

There's currently aluminum unvented soffit on the bottom of the cantilever. My current plan is to slide rigid foam board (cut-to-size) in and put it on the bottom of each floor joist cavity, using spray foam to keep in place and seal edges. Then, I will fill the remaining space with fiberglass, trying to fill the entire remaining space (the joist cavities are 15" wide, 9 1/2" tall, and ~40" deep). By the way, we live in Alaska, so it's COLD (currently single digits).

* First off, does this sound like a good plan, to achieve both air sealing (w/ the foamed in rigid board down below, resting on top of the metal soffit) and thermal insulation?
* Should I use faced fiberglass, with facing up to the subfloor? Would I be able to install that from the side, without great reach (i.e., do I need to affix that/seal the facing to the subfloor to be effective)? Or, could/should I use unfaced, since the foam board will be sealed on the lower/cold side? Would foil-faced foam board (polyiso, like R-max Thermasheath) serve as a vapor barrier as well, so that I can use unfaced above it?
* What thickness foam board would be recommended? With 9 1/2" height to work with, we're trying to get the best insulation balanced with ease of installation and reasonable cost. [I have considered spray-in foam, but we're trying to do it cheaper.]
* I've got to consider what thickness fiberglass roll/batt is available (once I figure out if I will be using faced or unfaced), as I know I shouldn't compress the insulation. Do I need to have an exact match height-wise (meaning, if I have 7 1/2" of open space above the foam board, I should only use a 7 1/2" height fiberglass product) -- or, can I use a height of fiberglass slightly taller than my space if needed, knowing that it may compromise listed R-value some, but still will be good insulation for that space (e.g., if I use 8" tall batt in that 7 1/2" high space)?

Thanks much for your help/feedback -- any advice would be appreciated!

Anchorage, AK

Gary in WA 11-16-2009 11:01 AM

Air sealing and zip code (4th para.) for your location:

Vapor barriers:

Similar situation for ends of wood joists:

Wood joists R-value (why you need the foam board under bottoms):
And.............. Welcome to the forum!

Be safe, Gary

KatieAK 11-17-2009 01:45 AM

Thanks, Gary, for taking the time to share those links. I've looked them over, and have a few follow-up questions.

Re: your link --[Similar situation for ends of wood joists:]

I see that Figure 7 shows something like what I have in mind, and I do take note that the foam board is installed on the underside of the joists (vs. my plan of putting near the bottom between each joist, due to access available from the side). Good to know I'm not alone in thinking this might work! :) But, I want to clarify on these points:
1) Am I correct in reading that the foil-facing should face DOWN (toward cold below cantilever) in my situation?
2) Why the airspace above the fiberglass (below the subfloor)? How much space should I allow?
3) I assume using unfaced batt (vs. faced) and leaving plywood subfloor w/out vapor barrier would be best, according to this article, to allow moisture to 'breathe' upward? (We've got carpet above, FYI - at least not vinyl!)

Also, on your last point/link, [Wood joists R-value (why you need the foam board under bottoms):] -- am I reading correctly that a 2x6 has nearing R7, and so perhaps our 2x10 might be almost R9? Hmm, I see your point, in the preferred method of applying the rigid board below the joists. However, I'm really hoping to not have to remove the soffit (b/c I don't know how and am afraid I'd mess it up and create a lot more work, and it's really cold here now and we need to do this project now!). Any resources/guides on removing and reusing alum. soffit that might boost my confidence in attempting to do so? Removing the soffit and doing the job from below would certainly make the actual insulation/air sealing work a lot easier, so I'd love to hear that it's not that hard a job (but I want the truth!).... :)

Thanks again for your help -- and for the warm welcome!


Gary in WA 11-17-2009 07:48 PM

"Foam board insulation is commonly placed between the exterior finish (i.e., siding, brick) and the studs of exterior walls. To prevent air infiltration, you should place rigid insulation boards tightly together and seal the seams with tape or caulk. However, this practice may worry some builders in cold climates since the foam board may act as a second vapor diffusion retarder. Studies have shown, however, that condensation rarely occurs in these areas unless something else is seriously wrong with the wall assembly (i.e., massive uncontrolled air leakage into the walls from the house). If the assembly is constructed correctly, the inside surface of the foam board stays warm enough to keep water vapor in its gaseous state long enough for it to escape." From:

Radiant barriers perform a function that is similar to that of conventional insulation, in that they reduce the amount of heat that is transferred from the attic into the house. They differ in the way they reduce the heat flow. A radiant barrier reduces the amount of heat radiated across an air space that is adjacent to the radiant barrier. The primary function of conventional insulation is to trap still air within the insulation, and hence reduce heat transfer by air movement (convection). The insulation fibers or particles also partially block radiation heat transfer through the space occupied by the insulation.

Figure 6: Vapor Barrier—Installing impermeable rigid insulation keeps the wood framing warm and provides a low perm layer that addresses the upward vapor drive. How impermeable? Less than 0.1 perms. Foil faced rigid insulations are the ticket here. This approach allows any type of flooring to be used above. Even better—exposing the shiny side of the foil facing (face it down into the crawlspace) almost eliminates radiative coupling and means that the surface of the insulation approaches the temperature of the ventilation air reducing condensation. From previous article stated. High-lights are mine.

Be safe, Gary

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