Ok, shorter question - all else is details.    What is the downside of using medium density ccSPF on the roof deck for a low slope roof?     And, can I achieve a higher r-value if I first put down rigid foam and use the ccSPF to seal it instead of relying on tape?

Thanks




Sure could use some advice.   Am trying to figure out best alternatives for my budget on a retrofit of not only the roof but also some other assemblies for my house (other topics).  Sadly, a DER simply isn't feasible.  So, starting with the roof ....

Basics:
Zone 4A (Maryland)
6880 sf of roof (yikes), 5500 of it is cathedral ceiling (yikes again)
All roof slopes are 4:12 , all rafters with 2x10's 16" o.c.
Current cathedral ceiling has R-9 batts at the bottom half of the cavity for a natural air channel
We've removed the can lights.
Roof overhangs are a honkin' big 3' (linear), making it a full plywood sheet on the roof slope as overhang.
Roof is currently asphalt and needs replacing
Cost of 1" used rigid polyiso on our deck is ~$1340 plus trucking it in
Installation cost of rigid foam unknown, but very few know how to do it and it also includes that $36 a roll tape...
Am assuming installed cost of ccSPF at $1.00 per board foot

I need help with cost tradeoffs compared to risk of improper installation, and of course, having the roof open to the elements long enough to do everything. I've crunched enough numbers to know I can't afford ripping the deck off AND do adequate insulation.  With no existing air channels, I can't just pop in dense pack, and ripping down the drywall in the interior...well, it has other costs and living condition issues.

So, current deck stays intact.  This means insulation on top, adequate air sealing at roof/wall juncture, and dense pack in the cavity.    We plan to do a metal roof -- and before you scoff at the frivolity, our quote for a metal roof with 2x4" purlins is THE SAME as the quotes for asphalt. (Screwed in roof, Amish installers, well-recommended. I will do my homework on the screwed in roof vs. standing seam.)  I'm in favor because of reusable material, moss on parts of current roof, emissivity/reflectivity of paint, and -- most importantly for this discussion --- NO second layer of plywood/OSB required for fastening the roof. 

Here's what I came up with -- and I'm trying to be creative -- just not sure if it's crazy.

1. 3" used polyiso rigid board (or a combo of 1.5" polyiso and 1.5" EPS to minimize polyiso penalty in winter). 1x4" strapping, with 2x4 purlins attached to the strapping.   More rigid board on top is relatively cheap, but I have to pay for the dense pack too.  Risk of improper installation negating much of the work with poor air sealing at seams.   

2. 3" ccSPF on top of roof -- the regular 2 lb, not the roof-density.  With a metal roof, I don't have to worry about compressive strength or the UV coating (or the higher cost).  I won't have improper rigid foam taping, and I might be able to keep the existing shingles (ripping off existing shingles is a $3000 quote from more than one roofer).  Can I build up a frame around the roof and, on a low wind day, do this?  

I could lose a lot of energy before making up the cost differential between #2 and #1, so I'd like to explore a hybrid #3 below...?

3. A combination of rigid foam and ccSPF on top of roof.  Using the same thoughts as for #2, set the rigid foam against the deck stopping short of the roof edges, and pop on a layer of ccSPF on top, thicker around the roof perimeter. 

4. Other combinations?

Note, with the existing batts I'm assuming the dense pack will not be as crisp, so I'm assuming consistency at 8.5" pack in the rafters (3.5 R per inch = ~R29).
Also, our installed quote for completely replacing the decking with 5/8" plywood was ~20,000 for 215 squares.  It's driving some of this creative thinking above.  Does this seem high to you? 


Thanks so much.

 

To use ccSPF above the roof deck you need 3lb density foam, which is more expensive than 2lb foam.

The climate impact of even 1" of ccSPF is many orders of magnitude higher than even 10" of reclaimed foam. Open cell foam (ocSPF) at 3" air-seals far more reliably than 1" of ccSPF, and can be sprayed in lifts of up to 5.5" lifts between passes. It also uses water as the blowing agent, not a climate-damaging HFC.

Dense packed cellulose or fiberglass can be even more expensive per R-foot than thicker reclaimed foam above the roof deck, or ocSPF below the roof deck.

In a 4A climate as long as at least 30 % of the total R is above the roof deck you don't need an interior side vapor retarder. If you go with the 1.5" + 1.5" ~R18-ish rigid foam above the roof deck it means you can safely add up to R42 on the interior o.c. SPF. That means it would be fine to just fill the 9.5" of 2x10 depth with ocSPF (R33-R35), installed in 2 passes. You would be above code-min for center-cavity R, but it would outperform typical code min due to the R18 of thermal break over the rafters. At 9.5" the vapor permeance of half-pound density ocSPF is 3-5 perms, a "pretty good" Class-III vapor retarder, and about ideal for that stackup.