Building a home with cathedral ceilings. Getting to required R value and roof shape requires spray foam. What shingle issues am I in for?

[OK EE]

So I'm working with a builder and he mentioned using spray foam to insulate the roof deck. We're in climate zone 3A so R38 is required and getting that in a 2x12 or 11.5" TJI cavity with insulation is almost impossible, not to mention the roof shape has intersecting lines that would make venting it difficult if not impossible. The roof was also engineered for 2x10 rafters or 9.5" TJI so there would be added cost upsizing the rafters. Spray foam in the 2x10 cavity would easily meet the R value requirements, but it sent up immediate red flags due to my brothers lousy luck on his house using this setup and the warnings most AI search results/shingle manufacturers websites give.

I know this technology has advanced significantly in the last few years and that installation is everything. Following manufacturers guidance to a tee is required.

That said, anyone have experience with this? This is hurricane country so we're looking at GAF Windproven systems.

Or am I just better off swallowing the pill and going to a standing seam roof system. I know it's significantly more expensive but in this area (coastal ENC) maybe the cost/benefit analysis says spend money here.

 

[Fermion]

I might not exactly be following what you are trying to describe but the upstairs of our home we built and designed ourselves has cathedral like rafters and ceiling in part of it. What we did to get the required R-value was first put in those rigid thin foam vent sheets (forget what the name is called) which vented the underside of the roof plywood up to the peak vents. Next we put in regular R-38 insulation batting (we had 2x12 rafters IIRC, would need to consult the designer wife because I can't remember). Finally, to bring it up to R49, we installed 2" polyisocyanurate rigid sheeting OVER the rafters and aforementioned insulation. I think the 2" has a R-value of R13, so the total was R38+R13 = R51. We attached drywall directly to the foil sides of this polyiso using 3" drywall screws (the rigid foam won't hold a screw but is quite strong as a spacer.

Edit: Actually I think we used 2x10 after all. It was 2" for the vent, 5 inches for R21 batting, then we inset 2" of rigid polyiso sheet and then put another 2" on top of the rafters like I mentioned. So it was like R21 + R13 + R13. This is a pic I found of a dormer in the ceiling showing the outside 2" polyiso:

 

[pb4uski]

When we rebuilt we went with a "hot" roof with closed cell spray foam insulation applied to the underside of the roof deck. The foam filled the cavity (2x6's IIRC) and slightly covers each roof joist. With a hot roof there is no need for venting, in fact it shouldn't be vented. We haven't had any problems but we rebuilt only 16 years ago.

If I was doing it again, I would put 3/4" thick straping horizontally across the top of the roof joists before putting down the roof deck so when the closed cell foam is applied it oozes between the roof joists and theuse a bottom of the roof deck and mitigates thermal bridging everywhere other than where the roof joists and strapping intersect. We did this on the exterior walls but didn't think of doing it on the roof. When we have a frost now, I can see the shadow of the roof joists due to thermal bridging from outside the house.

The other thing that I would do differently is to put adhesive membrane (Ice & Water Shield or similiar) over the entire roof before installing the asphalt shingles. As I recall, the contractor just did it around the perimeter of the roof and the valleys.

ETA: Upon further review, I guess I would only Ice & Water Shied the eaves, valleys and other penetrations and a vapor-permeable synthetic underlayment elsewhere.

 

[Fermion]

The other thing that I would do is to put adhesive membrane over the entire roof before installing the asphalt shingles. As I recall, the contractor just did it around the perimeter of the roof and the valleys.

Ice barrier? Usually you put down something like Titanium 50 before the shingles but on the bottom 3 feet around the perimeter of the roof you put down a adhesive ice barrier (it is like a rolled up giant asphalt sheet with adhesive backing). We did put it on the entire mansard roof of the tower.

 

[skipro33]

My roof is vaulted as well, using 12" rafters and R30 10" x 24" pink insulation with no foam. I bought a infrared camera that shows hot spots. The rafters and the main ridge beam are hotter than where insulation is, but not by much. The attic has never gotten to 100 degrees even with summer highs of 108 outside. Our 2300 sq ft house with 23' ceilings is easily kept cool with a 2 ton A/C unit
There is no venting. The roof sheathing and interior 1x4 pine ceiling nailed directly to the rafters. This is a great fire Safety feature since no attic space over the main living area. The bedrooms have 9' ceilings with 12' tall attic open space.

 

[Sunset]

How about going with a metal roof ?

 

[hopefullyoneday]

All I know is that spray foam is awesome. It has a higher upfront cost than traditional, but you get that back within a year or 2 from heating and cooling savings. I went with the envelope style foam insulation. I have 2x6 exterior walls sprayed full of open cell spray foam and 2x10 roof rafters sprayed full under my metal roof. My house is in Texas and 2300 sq ft and I never been over 1000kw in a month. My average electric bill is under 100 per month. My attic which is where my hvac system is within 10 degrees of whatever my house interior is. It gets above 100 here in TX a lot and my attic never gets above 80.

 

[old medic]

Or am I just better off swallowing the pill and going to a standing seam roof system. I know it's significantly more expensive but in this area (coastal ENC) maybe the cost/benefit analysis says spend money here.

BTD Buddy, BTD.... WNC and we went with spray foam and metal roof. I'm 63 and if I live to 100 I won't need to worry about this roof.

 

[Onda]

Rockwool should get you to the required R-value in that assembly, and in many ways I’d consider it superior to spray foam: non-combustible, vapor-open, water-resistant, dimensionally stable, and much more forgiving if there is ever a roof leak. It also avoids the off-gassing risks associated with spray foam.

If a roof with spray foam under the deck leaks, the problem is that the leak can be hidden for a very very long time.

Standing seam is also worth pricing seriously. It is not just long-lasting; with the right color/coating and assembly, it can be a very energy-efficient roof system, and in coastal/hurricane areas the durability argument becomes stronger.

 

[ransil]

I did standing seam on my roof a few months ago, price was not as bad as I thought it was going to be.

 

[COcheesehead]

We have a complex angle cathedral roof with open space up to 25’. The builder used spray foam and as far as I can tell, it’s the best insulated home we’ve ever had. Our roof is metal.
The house is quiet, has a “soft” feel to it.

I documented much of the build and took a picture of the product used.