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Community and Q&A

Polyiso foamboard (foil-faced on both sides) used in unconditioned space

user-6434635 | Posted in GBA Pro Help on

Hello All,

I live in Portland,OR climate zone 4 & 5 marine.
I consider it mainly a heating climate and lots of rain.

I have a cape cod style house with 13/12 pitch roof on the main roof.
I have three gable dormers on the front, two new ones and one existing all with 20/12 roof pitch.
I just had ridge vents installed on the main roof and all three gable dormers.

I have managed to keep my new HVAC ducts in the floor joists and the are only exposed in the knee wall area before to going down into the (conditioned) floor/ceiling joists on the way to each room.

As a consequence, code requires R9/R10 insulation for ducts in unconditioned space. So, my builder and I put in some 2″ polyiso foil faced between existing 2×4 rafters (3.5″ deep).
Soffit vents to be installed next to connect with the ridge vents, created about 1.5″ vent baffle.

I have read that ideally I should put the polyiso on the exterior but it may be outside of my budget. So, I am trying to stick with insulating on the interior of the walls (2×6) . Our currently plan just calls for R21 unfaced fiberglass in the knee wall and exterior walls.

I have a short section of catherdral ceiling which I am trying to maintain my air path, but also meet an even higher Rvalue requirement (R30). I was thinking of going with either 4″ or 4.5″ polyiso foil is on both sides from Homedepot Rmax Thermasheath-3.

My foam board serves two purposes:

1. baffle path for ridge vent
2. insulate the rigid duct from unconditioned space.

So, my question is should I be using XPS both areas as my site built baffles of the polysio ?

The area is partially in the knee wall area, but then becomes a sloped catheral ceiling it the upper portion before it hits the flat ceiling part of the room.

Thank You,


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  1. GBA Editor
    Martin Holladay | | #1

    Johnny / Chalin,
    If you have ducts behind your kneewalls, then the triangular attics behind the kneewalls should be detailed as interior space, so that your ducts are inside, not outside. It sounds like you understand this concept -- sort of. But you need to follow through with the implications of this fact.

    Portland, Oregon is in Climate Zone 4C. According to the 2012 IRC, your roofs should be insulated to R-49 or more.

    It's perfectly OK to detail your insulated sloped roof assembly as a vented assembly, but you need to consider that you'll need deep rafter bays for this to work.

    All of the sloped roof assembly, from the soffits to the area above the small flat ceiling above your second floor, needs to be insulated to R-49. That includes the sloped roof assembly above the small triangular attics behind your kneewalls.

    You can use 2-inch-thick rigid foam as a ventilation baffle if you want. Make sure that this ventilation baffle is installed in an airtight manner. For more information on this aspect of your work, see Site-Built Ventilation Baffles for Roofs.

    Two inches of polyiso will have an R-value of R-10 or R-12, depending on how much you want to de-rate the R-value to acount for polyiso's poor performance in cold weather. That means that you still need to install about R-39 of insulation on the interior side of your ventilation baffle.

    You can achieve this additional R-39 with about 11 inches of fluffy insulation (fiberglass, mineral wool, or cellulose), or with about 7 inches of polyiso if you use the "stack of pancakes" variation of the cut-and-cobble method. For more information on the cut-and-cobble method, see Cut-and-Cobble Insulation.

    For an overview of the challenges you face with this work, see “Two Ways to Insulate Attic Kneewalls.”

  2. Dana1 | | #2

    In Portland OR there is no need to de-rate polyiso to as low as R5/inch. Even December's & January's binned hourly mean temperature is over 40F. Even in an R49 stackup the mean temperature in the foam layer will be well above 45F over the course of a winter, performing at or slightly ahead of it's labled R-value (depending on how much you want to up-rate it for the performance of the low-E foil facing the 1.5" cavity.)

    With a soffit to ridge fully vented roof deck you can get pretty close to code-min performance with 4-4.5" of continuous polyiso (fiber or foil faced) on the underside of the rafters. You can probably do that reasonably cheaply with 3.5-4" of reclaimed roofing polyiso covered with a layer of half-inch Thermax fire rated polyiso facing the interior. That is, provided you can get big sheets of foam in behind the knee wall (which can sometimes be a problem.)

  3. user-6434635 | | #3

    Hello All,

    I do appreciate that input, I do understand pushing the conditioned space to the roof would be ideal.

    I have poured through multiple articles on vapor barriers, roof ventilation, knee wall insulation
    and cathedral ceilings. What is difficult to determine is if polyiso with foil facers is going to really restrict moisture condensation if I move my conditioned space to the cathedral ceiling area.

    Sounds like air sealing is going to be the real key to avoiding moisture problems. I have talked to the drywall contractors and they are not that familiar with gluing the drywall to the studs to get a tight air seal. So, my confidence is not high in getting an air tight seal.

    I was considering switching to XPS with the cut and cobble style, but after reading the post on cathedral ceiling gone bad has me wondering about a sprayfoam application in this complex area that has gable dormers and HVAC ducts.

    So, we already got our roof installed and we got ridge vents put in on each our our three gable dormers and also 32 feet on our main roof line.

    Would it be better to just spray closed-cell foam this area and make it un-vented? We have not cut in any sofits but it appears I may be stuck with my the ridge vents. I am wondering how I can go to an un-vented roof in the front of my house and then maybe do a vented roof in the back of the house where my rafter bays are more simple? Any thoughts or suggestions ?

  4. Dana1 | | #4

    Cut'n'cobbled unvented roofs are a disaster waiting to happen. Doing it wih XPS is it's own environmental disaster, due to it's high impact blowing agents, and the fact that as the blowing agent leaks out it drops to the same performance as EPS- there is little if any performance advantage that you're buying with those HFCs.

    If you add continuous foil faced polyiso on the interior side you MUST vent the roof deck. If unvented it's a moisture trap, and even though you may get away with it for years, it's almost certain to become a problem at some point in the future. All roofs leak minor amounts of water, and only vented roof can dry toward the exterior. With a sheet of aluminum on the interior it will not be able to dry toward the interior.

    To do an IRC2015 code-min unvented roof with plenty of dew point margin in zone 4C takes 2" of closed cell foam on the underside of the roof deck, with R36-R38 of fluff, which DOES fit in a 2x12 rafter depth.

    You really only need R38 "whole assembly" performance to meet code on a U-factor basis, with all the materials in the stackup and thermal bridging is accounted for. A 2x4/R13 roof only comes in at about R10 "whole assembly", but from there it only takes R25 (about 4") of continuous polyiso to meet code on U-factor basis. Ideally that foam would be on the exterior, but if you used fiber faced polyiso, and used R13 open cell foam (=air impermeable) in the rafter bays you would be fine. Unlike foil facers, most fiber faced polyiso is a Class-II vapor retarder, with about 5x the drying capacity of a #30 felt + asphalt shingle layup on the roof (not that you have enough drying hours in Portland OR for the vapor permeance of the shingles to allow any significant drying to the exterior no matter what the permeance was.)

  5. user-6434635 | | #5

    Hello All,

    OK, does anyone know where I can get polyiso foam boards with just fiber facers?
    If not, then it seems I am back to where I started trying to use the polyiso foil facers.

    Please confirm that stacking foil faced polyiso is the same as using one layer of polyiso .

    Since I will be venting the roof no matter what to allow for drying towards he exterior
    would this set up sound like an acceptable knee wall / roof stack:
    asphalt shingles, tar paper, 1/2" plywood, 1.5" air gap, 2" polyiso foil facer both sides between rafters (spray foam sealed), then continuous layer of 4.5" polyiso foil facers nailed directly to top of rafters(spay foam sealed)

    In the cathedral ceiling area: asphalt shingles, tar paper, 1/2" plywood, 1.5" air gap, then 4.5" polyiso foil facers cut and cobble between rafters and foam sealed.

    Would it be worth the effort to add .5" or 1" continuous polyiso layer just to get rid of the rafter thermal bridging ? It would be a minimal loss in head room and pehaps complaints from the drywall installers.

    My thought on using a final top layer of continuous polyiso, is to help get that much needed air sealing for the tricky ceiling locations.

  6. GBA Editor
    Martin Holladay | | #6

    As long as you have a vented roof assembly, and it sounds like that's what you are planning to do, the cut-and-cobble approach is fine. (My only comment is that one of your sentences apparently includes an erroneous description. When you wrote, "a continuous layer of 4.5-inch polyiso, foil faced, nailed directly to top of rafters," I think you meant to write "nailed directly to the underside of the rafters.")

    Q. "Please confirm that stacking foil faced polyiso is the same as using one layer of polyiso."

    A. It's the same from the perspective of vapor permeance. One layer of polyiso with foil facing has a vapor permeance that is close to zero. Two layers of polyiso (or three layers) with foil facing have the same vapor permance -- that is, close to zero. No change. However, these different assemblies don't have the same R-value. If one layer of polyiso is R-12, then two layers are R-24, and three layers are R-36.

    Q. "Would it be worth the effort to add 0.5 inch or 1 inch continuous polyiso layer [on the interior side of the rafters] just to get rid of the rafter thermal bridging?"

    A. Sure.

  7. user-6434635 | | #7

    Hello All,

    Seems like the polyiso is going to also act as a vapor barrier,
    Is this a correct assumption?

    So, in the flat ceiling area It seems
    I should slso have a vapor barrier too?
    What is usually used?

    I was only going to use 1/2" drywall and insulate with loose fill fiberglass
    Filled to R49 in attic floor area. Been trying to figure out how to air seal all the odd angled
    Connections of the vaulted ceiling area.

  8. Dana1 | | #8

    Polyiso with foil facers is indeed a vapor barrier. Fiber faced polyiso is usually a Class-II vapor retarder, not a true vapor barrier, but not very vapor open.

    A vented attic space or any vented roof in a zone 4C climate does not need a vapor retarder- an air-tight ceiling with standard latex paint has sufficient vapor retardency. I for some unexplained reason you feel the need for some broadsheet product as a vapor barrier, use 2-mil nylon (Certainteed MemBrain) which is very forgiving, since it become vapor open if the assembly reaches mold-supporting levels of moisture. It runs about 12-15 cents per square foot- not free, but not super-expensive either.

    Fiber faced polyiso is available through building supply distributors catering to commercial construction folks. Rigid polyiso and EPS are the most common materials used for large commercial building roof insulation.

  9. GBA Editor
    Martin Holladay | | #9

    Just because you won't have any problems if you install foil-faced polyios on the interior side of your vented cathedral ceiling, doesn't mean that an interior vapor barrier is required. It isn't. Your horizontal ceiling doesn't need a vapor barrier. All is needs is a vapor retarder -- a less stringent layer than a vapor barrier -- and you can use vapor-retarder paint to achieve that.

    In all cases, it makes sense to strive for airtightness when installing your ceiling.

    For more information on vapor retarders, see these articles:

    Do I Need a Vapor Retarder?

    Vapor Retarders and Vapor Barriers

  10. Expert Member
    Dana Dorsett | | #10

    In Portland OR you don't need to use a special vapor retardent paint as the interior side vapor retarder. Any standard latex primer with standard latex ceiling paint is sufficiently low vapor permeance (3-5 perms) to be protective, without limiting the drying capacity the way half-perm "vapor barrier latex" primer does.

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