Cathedral ceiling – R-38 plus 1 or 2″ of taped polyiso?
Howdy,
I am finishing the cathedral ceiling insulation and I am trying to decide between 1 and 2″ of polyiso sheets. It’s deep truss design and standing seam steel roof. There is about 12″ of void space above installed unfaced R-38, ridge vent, soffit vents.
Just for a reference, walls have cedar shingle siding, 1×4 strapping, 2 layers of taped polyiso, typar, 1/2″ OSB, 3+” of 2lbs spray foam and 3/4″ T&G pine as wall finish. Probably close to R-40 (about R-25 at framing)
Either way I will likely have to install strapping before putting the T&G pine as my ceiling finish, since 2.5″ finish nails will have a bit short grab on single layer option (they are shot a bit on an angle, 3/4″ T&G, 1″ ISO then about 1/2″ grab in the truss, which just isn’t enough).
I am in zip code 53951 and wonder if I should spend the extra $$ on 2nd layer of foam on my ceiling.
There would is some air space between top of the polyiso, where it faces fiberglass, as well as below of polyiso where it faces the T&G (thanks to strapping)
If I were to install just 1 layer, I would do it reflective side down, tape, 2x or 1x strapping.
If 2 layers, 1st would be shiny side out, tape, 2nd layer shiny side down, tape, 1x or 2x strapping.
Either way I would leave a 1/2″ gap on the perimeter of the polyiso (where it meets the floors, walls and peak) and foam it for airtight seal.
1 layer would add up to about R-47 and 2 layers would be about R-54.
Beside 1 vs 2 layers, would you advise 1x vs 2x strapping? Difference would be mainly air gap and finish nail hold strength (kinda doubtful)
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Replies
Nick,
Your zip code reference was rather obscure, like a clue for a scavenger hunt. But evidently you are in climate zone 6 in Wisconsin.
A layer of rigid foam under your trusses is fine. I think you are overthinking whether to go with 1 inch or 2 inches. Either way will work.
A less expensive way to improve your attic insulation would have been to choose cellulose insulation instead of R-38 batts. The batts are difficult to install well and allow convective currents. You could have added as much R-value as you wanted, cheaply, by installing deep cellulose instead of the batts.
1-inch ceiling strapping is fine; you don't need 2-inch strapping.
It's too bad that you installed a classic foam sandwich on your walls (covering the OSB on both sides with vapor-impermeable foam). Open-cell spray foam would have been a better choice for your walls than closed-cell spray foam. When you install rigid foam on the exterior side of your wall sheathing, you want your walls to be able to dry to the interior of your house.
Yes, SE Wisconsin, zone 6. Should have been more concrete.
Wouldn't I need R-49 in my roof, so 2 more layers of polyiso? Staggering joints is appealing feature too.
Too late for cellulose. Install would be hard too, with roof already on. Batts are already in. 1 vs 2 layers of polyiso would result in different orientation of first layer (shiny side up vs down) - that's why I wanted to resolve it first.
And on the wall assembly - I had some mildew problem (not sure you remember the discussion) and CC spray foam was the best possible solution.
Nick,
To get R-49, you'll need 2 inches of polyiso. Most brands of foil-faced polyiso have foil on both sides. If foil faces an air space, you'll get an extra R-2 from the air space.
Brand I am getting locally has shiny side and gray side (which are 2 sides of same material, actually) Obviously I want the shiny side towards the air gap. So first layer shiny side up, where there is 1 to 2" uneven space under the batts. And 2nd layer shiny side down, where it will have 3/4" gap towards the T&G made by strapping.
Is R-2 achieved just by the air gap or by reflective property of polyiso?
Nick,
Foil is a radiant barrier. It has no R-value.
An air space has an R-value. The R-value of the air space varies depending on the emissivity of the surfaces that bound the space. If these surfaces have a low emissivity, the R-value of the air space rises.
The R-value depends on whether the air space is horizontal or vertical, but in rough, general terms, you can figure R-1 for a 3/4-inch air space without foil, and R-2 if one surface has foil.
I understand that foil has no R-value on it's own. I just wondered if insulated panel with reflective foil has better insulation properties than without it. I know that when taped it created a solid air and vapor barrier. Is there anything else?
You described an air gap between 2 insulating materials.
How much different is it from an air film on?
Nick,
The emissivity of the exterior or interior surface of any building assembly affects the R-value of the entire assembly, even if the low-e surface isn't facing a sealed air space. That's why you can put a hard-coat low-e coating on the interior surface of a double-pane window and still improve the window's R-value. It also explains why radiant barrier roof sheathing works when the foil faces the attic.
So the foil doesn't have to face a sealed space. As dust accumulates on the foil, the R-value of the building assembly will go down.
The exterior facing of the polyisocyanurate has several purposes; one purpose is to slow the evaporation of the blowing agent that gives it a high R-value. Over several decades, the blowing agent will slowly be replaced by air, and the R-value of the polyiso will drop, eventually hitting R-5.5 per inch or so.
Thanks for some good info.
Would you recommend taping the first layer (grey side) using red tyvek tape? 2nd visible layer would get proper aluminum tape (On the exterior wall I actually ended with 2 layers shiny side out, so both got aluminum tape). Both tapes are very sticky.
Nick,
I don't know what your gray facing is made from, so I can't recommend a tape. Experiment and see what works.
Grey facing is the same material as the shiny facing. It's grey on one side and shiny on the other. I noticed that when I had a damaged sheet in my hands.
Grey side seems to be little more dent and rip proof.