Glass Reinforced Facer (GRF) Polyiso for ceiling cavity?
As part of my porch-to-room conversion, I am planning to install ISO boards between the rafters of the porch ceiling. On the used market, I am running into listings for C1289 Polyiso (GRF), instead of the usual Aluminum Foil Facer (AFF) one. GRF Polyiso, it’s my understanding, is typically used for commercial metal roofs. Is it ever used in cavities? What would the drawbacks of doing so be? And does anyone know what is the R value of such boards? is it different than that of “regular,” aluminum-foil-faced ISO rigid insulation? (An additional worry I have is that the fiberglass facer might be harder and more annoying to cut.)
Thanks!
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You can email this company and ask.
https://roof.atlasrwi.com/support/
https://roof.atlasrwi.com/products/acfoam-polyiso-roof-insulation/acfoam-iii/
Item 10 on this page also contains a tiny bit of info, but does not answer your question
https://www.atlasrwi.com/polyiso-insulation/
10. WHAT ARE THE MOST COMMON TYPES OF FACERS FOR POLYISO INSULATION AND WHAT DO THEY DO?
Polyiso is an incredibly durable, strong and chemical resistant material. But one of the unique features of polyiso insulation is that it comes sandwiched between layers of facing. These thin organic and inorganic sheets offer a number of additional benefits that make polyiso an even more attractive option to designers and builders. Facers include:
Non-asphaltic, fiber-reinforced organic felt facers, like those on ACFoam-II. These are the standard facers for roofing, but they are not used in wall products. They provide economical and lasting performance and are compatible with a wide variety of roof membrane assemblies. Inorganic coated glass facers, like those on ACFoam-III, ACFoam HD Cover Board, EnergyShield CGF and EnergyShield CGF Pro, are significantly more durable and are chemical and mold resistant.
Foil facers, like those in EnergyShield, EnergyShield Pro and ACFoam Supreme, provide increased dimensional stability as well as decreased potential for water absorption and water vapor transmission.
Thanks! Indeed, emailing the company might very well answer some of my questions. One issue I read about is that the fiberglass/cellulose facer might attract moisture; so, perhaps departing from foil-faced might not be a good idea.
Depending on the thickness, LTRR of fiber face polyiso is around R5.9.
The facer is easy to cut, about the only issue with it is that it itches, make sure to wear protective equipment.
It sounds like you are planning a cut and cobble install. If the roof is unvented this type of assembly is prone to moisture issues no matter how well you seal it. It can work with a vented roof but way more work than batts for about the same R value. The issue is the thermal bridging from the rafters, even though the foam is about r6/inch, the rafter is only R1. Because of this, if you look at the overall assembly R value, it barely changes from using standard batts.
Assuming you can build a vented roof, the better use of iso board is under the rafters with batts between the rafters and vent space above. This would get you a much higher assembly R value and be less work. Drywall can be installed directly over the iso with long drywall screws.
If you can't went the roof, your two options are spray foam under the roof deck or rigid over the roof deck.
Follow-up on the roof, with a picture:
The current rood is not vented. Enclosed is a picture taken when the roof was being replaced. The roof has a one-inch (I think) gap between the boards above the beams and the roof sheathing. Such a gap used to be filled with some rigid foam, which turned up being soaked with water and home to carpenter ants.
So, I had the whole thing removed, and the gap is empty now. From the attached picture, you can see that, above the boards there are 2"x4" boards that run PARALLEL to the house, instead of running from top to bottom of this sloped, shed-style roof. That's unfortunate because, had those 2x4's been perpendicular to the house, I could have replaced the soffit with a vented soffit, and added a wall-2-roof vent at the edge near the house. Instead, if we want to vent this roof, I am afraid the shingles and sheathing and 2x4's will all have to be removed, and ventilation created anew.
From you comment about close-cell foam, you seem to think that that would prevent condensation even without a vented roof. Is that correct?
In any event, I am going to inquire with some roofers, to see how costly it would be to remove the existing roof down to the boards that are nailed to the beams, and rebuild the roof assembly with rigid insulation above the roof's deck. If I were to add two inches of polyiso abobe the roof's deck, I could probably just add mineral wool between the rafters, and no additional board to the beams, hence not reducing the ceiling's height.
If rebuilding the roof, perhaps the Hunter Cool Vent panels (which I found at a good price, with 1" polyiso board, to which perhaps I could add another inch or two of polyiso) would be a good solution?
Thanks for any feedback you might have!
I think the simplest would be to frame up a new ceiling bellow the rafters. The bottom of the ceiling joists can start at the top plate on the outside wall and can be either flat or mostly follow the slope of the roof. This way you don't loose any height but gain a bit of space. What you want to end up with is a gap under the rafters near the top to allow for cross ventilation air flow.
Insulate the roof with thick batts, these will get squished a bit near the bottom but you'll get full R38 elsewhere.
Venting can be either a roof/wall vent or simpler gable vents on the side. Soffit intake vents don't hurt but for a such a small space, I would not loose sleep over not having it.
It looks like there are two skylights as well. These are moisture issues in almost any old house, make sure to detail them properly. You want an air tight and well insulated box (plywood or lumber) from the ceiling all the way up to the skylight. The box is then insulated around the outside with rigid insulation so there is no chance of it falling down over time.
Make sure this box is well sealed to bottom of the skylight, air leaks here can cause a lot of condensation in the winter time. What I have done is use peel and stick membrane to cover the inside of the box and tie it to the drywall groove of the skylight with a flex flashing tape. Bottom is taped to the ceiling vapor barrier. Drywall is also caulked at the skylight as backup.
Akos,
Sorry for being so slow at replying. Thank you SO much for all the suggestions!
I should clarify that the skylights have been removed and the holes boarded up. The photo I had enclosed was taken after the skylights had been removed, but before the job had been completed.
I confess that I still don't understand how the addition of vents would vent that roof, as the "channels" run parallel to the house, not perpendicular to it. So, to achieve the goal of venting it from soffit to a roof/wall vent, one would have to rearrange those boards, hence remove the shingles and the sheathing. Maybe I am missing something, and don't fully understand how that could work. Likewise for the gable vents on the side: from where to where would the air flow?
I like your idea of framing another ceiling, so as to be able to have thicker insulation. Yet, assuming that the roof can be vented, of I find cheap iso boards, wouldn't that become an option again, with no need to thicken the ceiling?
Or, if I were to give up on venting the roof, shall I just go ahead and hire a contractor who would install closed-cell spray foam? I might actually post a separate question about that; yet, in brief: The township's inspector wants me have the 5 1/2 cavity with closed-cell spray foam, to reach R-38 for the ceiling. I actually would rather prefer filling the 5 1/2" cavity with OPEN-cell spray foam, trimmed at the rafters, and add a one-inch polyiso foam board. Yet, the inspector insists that that would not give me R-38 (disregarding the fact that, between thermal bridging and the empty space and exposed wood that inevitably the closed-cell foam will leave, R-38 will not be reached that way either).
Matt,
What you want to do is essentially build a mini attic near the top of the low slope section. Since the ceiling joist are now bellow the rafters you have a continuous open space that connects all the rafters so you can use gable vents. You won't get as much airflow as with a proper soffit to ridge vent but if your ceiling is reasonably air tight bellow it should work.
A while back I pulled off a similarly constructed roof that was about 40 years old (zone 5, so cold). This had a single sad mushroom vent near the top and whatever air could flow through the old T&G soffits as the intake. The old plywood roof was in such a good shape that I re-used it as rain screen strapping for the walls.
Unless you are in zone 3 or bellow, unvented roofs need closed cell SPF. Open cell spf even with the layer of rigid under the rafters is not the same thing, that roof without venting might fail.
Akos,
Sorry if I go back to this older suggestion of yours: of framing a new ceiling below the rafters, to add insulation to my sloped roof. . ., but I am still not quite sure I understand how the ventilation would happen. What I am thinking is that, if I were to take this approach, I could apply some baffles to the old T&G sheathing, between the rafters; create a vented soffit at the lowest side of the room, and a SmartVent at the highest side. That would create ventilation under the sheathing and above the insulation, following the channels the rafters create. Is that compatible with what you were thinking?
One worry I have about taking this approach is that of ending up with what I have on the top floor of my house (where the insulation is now R49, and the floor of the attic has been meticulously sealed): it still gets pretty hot in the summer. That is, a "traditional" attic for that four-season room I fear would not really give me the year-around comfort we need for that room, and which instead I think the external insulation, above the roof deck, would give me.
In summary, according to your suggestion, the venting would be between the sheathing and the insulation, right? And you don't think it would be risky in terms of year-round comfort?
Thanks, Akos!
I'll have to add a longer and more detailed answer tomorrow, perhaps with some pictures attached. In brief, I do not know whether that (shed-style) roof is currently vented. I don't believe it is; also, it does not seem -- to me -- to be that easy to vent. Hopefully, things will be clearer tomorrow, when I can better explain the situation with that roof, and add a picture or two. Perhaps, the whole matter would be worthy of a separate question...
In any event, yes, 1) I was thinking of a cut and cobble install, though 2) with also a 1" polyiso board attached to the beams, and the ceiling (drywall or shiplap) screwed/nailed to the beams, through that last layer of insulation -- so as to reduce the thermal bridging. (The cut and cobble install is made necessary by the need of reaching R38 for that roof, which no fluffy insulation would give me with only 5 1/2" available. Polyiso boards filling the 5 1/2" space would get me close enough, and the city inspector said he would approve it; yet, I am planning to add that additional board across the rafters anyway).
One quick question on that thermal bridging: a 1/2" board -- instead of 1" -- would not be enough, right? (I could use all the room I can get, for lowering the ceiling as little as necessary.)
In any event, the real problem, which your question raises, might be that that roof, alas, is not vented, and maybe is not easy to vent. So, I might have to look at some more aggressive solution. (On that, more tomorrow!)
Again, thank you so much!
Akos, Thanks! All things considered, I'll go for close-cell spray foam for the ceiling, and most likely will add a 1" layer of polyiso nailed to the beams. The ceiling will be finished with beadboard (likely shiplap boards), and I guess those could be nailed to the ceiling beams directly (that is, without furring strips), using longer nails.
Now I am all taken by the wall and floor insulation plans! LOL I might post separate questions for those. Hopefully any ensuing discussion might be interesting/helpful to other people as well.