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For an unvented cathedral ceiling is a roofing deck sandwich of 1/2" plywood and 2" of both sides foil faced polyiso, OK ?

I am planning the building of a new year round ENERGY STAR VERSION 3 lakefront home on Andover Lake, CT. The home will have an unvented cathedral ceiling with the following insulation and sheathing configuration. I refuse to use OSB or SIPS in the construction of my home due to their inferior water resistance and durability, (NOTE: California does not approve SIPS in residential construction in many areas.) The insulation/sheathing configuration are the following::

The 2 x 10 rafters 16"OC will be filled with 3" closed cell spray polyurethane foam against the underside of the roof sheathing. The remainder of the rafter cavity will be filled with R19 FG batts. The roof sheathing of this unvented cathedral ceiling/roof will be a sandwich configuration consisting of 1/2 " plywood, 2" of both sides foil faced polyiso, 1/2 ' plywood, synthetic underlayment, and 30 year architectural asphalt shingles. Polyiso was choosen because of its greater stability at higher temps compared to XPS. Do you see any problems with this insulation/sheathing configuration? Will the foil face on both sides of the 2" polyiso trap moisture in the lower plywood sheathing layer between the foil face and the impermeable closed cell spray foam below causing rot of this plywood layer?

I will also be using vinyl siding with 3/4 " foil faced both sides polyiso over 1/2" plywood sheathing with 2" closed cell spray foam and R13 FG batts betwen the 2 x 6 wall joists. Do you see any problems with this configuration? The home wiil be heated by a ground source heat pump,closed vertical loop well with air ductwork and an ERV energy recovery ventilation system.

I am particulary interested in your recommendations for the specifications on the plywood sheathing, i.e. ply, waterproof glue, CDX,, thickness, etc for both the walls and roof. I am also concerned with the use of duplicate foil faced polyiso, such as Dow Tuff R and the possible trapping of rotting moisture in the configurations above? Thank You.


Posted May 31, 2012 12:42 PM ET
Edited May 31, 2012 1:03 PM ET


12 Answers

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Q. "Do you see any problems with this insulation/sheathing configuration?"

A. Yes. First of all, manufacturers of synthetic roofing underlayment do not allow their underlayments to be used on unvented roof assemblies. They are only suitable for vented roof assemblies, so I suggest you use ordinary asphalt felt.

Secondly, I advise anyone interested in using foam insulation (whether rigid foam or spray foam) to put the foam on one side of the roof sheathing or wall sheathing, but not both. If you put foam on both sides of the roof sheathing, the sheathing can never dry out if it ever gets wet.

If you want to use the flash-and-batt system, go ahead. But if you do, skip the rigid foam on top of the roof sheathing.

If you want to install rigid foam on top of your roof sheathing, go ahead. But if you do, don't install any closed-cell spray foam under the roof sheathing.

For more information, see How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay
Posted May 31, 2012 2:18 PM ET


Dear Martin,

The reason I put the rigid foam board insulation on the exterior of the roof sheathing is for thermal bridging considerations. If I put foil face polyiso below the rafters, I will trap moisture in the rafter cavity between the impermeable blown in spray foam and the poly iso. What type of rigid foam board would you recommend below the rafters to limit thermal bridging? Thank You.


Posted May 31, 2012 2:48 PM ET


Your plan to put rigid foam above the roof sheathing is fine -- as long as you don't put any spray foam under the roof sheathing. If you read the article I linked to, you'll find that there is a minimum thickness for rigid foam installed above roof sheathing; the minimum R-value varies with the climate.

Answered by Martin Holladay
Posted May 31, 2012 3:26 PM ET


Dear Martin,

What I failed to tell you is that I have a legal height limitation on my home in order to preserve the lake view of my neighbors living uphill across the street. Therefore in order to obtain the maximum R value for my roof I must use the rafter cavity with some means of limiting thermal bridging. My house plans already have my ceiling heights at a minimum/maximum and the overall house height at a legal maximum so that I can not stack rigid foam board insulation on top of the roof. My apologies for not informing you of these limitations in the beginning.


Posted May 31, 2012 3:43 PM ET


Then I advise you to stick with 100% closed-cell spray polyurethane foam or flash-and-batt.

Answered by Martin Holladay
Posted May 31, 2012 4:43 PM ET


Hi Martin,
I've been pondering a similar unvented roof arrangement as Joseph. My current roof deck has closed-cell spray foam between the rafters. I need to replace the roof shingles so was considering taking the opportunity to increase the overall roof R-value by adding 2" of XPS or polyiso on top of the roof deck, re-sheathing with 1/2" ply and new asphalt shingles.

I understand that this would be creating a vapor impermeable layer on either side of the original roof deck. However, would ice & water membrane near the eaves, synthetic underlayment elsewhere and asphalt shingles not equally prevent drying to the exterior? In other words, do asphalt shingles not, by themselves, prevent drying to the exterior?

Thanks Martin.


Answered by Ewan Smith
Posted Jul 6, 2012 7:45 PM ET


This is my opinion as a roofing contractor. I agree with Martins post about using the foam on 1 side or other, not both. I have personally seen wringing wet closed cell foam. (The foam guys are PO'd now)

I have seen soaking wet closed-cell Iso board tearing off flat roofs. I have also tore off closed cell SPF roofs where the foam chunks were so heavily saturated with water I could barely lift them. Call it bad foam, or whatever. There is also an issue of closed cell foam in boat hulls that are found soaking wet I have read about.

What about the option of using above the deck ventilation. A 2 inch air space above the plywood. This would help with thermal bridging and help your shingles last longer. I know it don't mean much but the 30 year shingles you are referring to probably have a lifetime warranty now.

What caught my eye on this thread was Martin's post "...manufacturers of synthetic roofing underlayment do not allow their underlayments to be used on unvented roof assemblies." I appreciate Martin pointing this out as I have used them before in this situation. Titanium does make an allowance for it on their website.

"Q. Can Titanium UDL be used with foam insulation?

A. Blown foam insulation by its very nature can make a roof attic not well ventilated. Titanium installation instructions state to use "above a properly ventilated areas, as it is considered a vapor barrier". Titanium UDL underlayment can be used with a blown foam system if the attic area is part of a closed loop, meaning it is part of the HVAC system. Therefore the attic air temperature and moisture is being controlled by the HVAC mechanicals."

I am checking and getting a clarification from an other mfg as well. You can understand what the synthetic mfgs are trying to do here.

Back to original question. I am not so against OSB as you are. I use to be. Products like Advantech and seeing CDX cupping so bad on roofs have changed my mind. As far as thickness, if the plywood requires a H-Clip then it is too thin. Go up a size then use the H-Clip if you still want. (Just my opinion) You will appreciate the difference if you ever get up there and walk on your roof.

Answered by Ronnie Allen
Posted Jul 7, 2012 4:57 PM ET


Hi Ronnie,
Thanks for your reply... appreciated.

Your suggestion to incorporate ventilation above the existing roof deck and below the new 1/2" ply was something I had considered in conjunction with the added 2" XPS or polyiso. I had thought about using 2x4 sleepers on-edge with 2" thick insulation between the sleepers, providing a 1-1/2" ventilation channel below the new 1/2" ply sheathing. I felt it was getting a little fussy for the sake of an additional R10 or R13, but I'll maybe take another look at this option.

I had also given some thought to using EPS foam insulation instead of XPS or polyiso to provide greater vapor permeability for outward drying. However, again, I didn't think the additional R8 justified the expense, and I'm loathe to add much more than 2"... I want to minimize the amount of eave extending/rebuilding required.

Interestingly, I was actually looking at the sales sheet for Titanium UDL earlier this afternoon and noticed both the perm rating (0.06) and the blurb about "must be installed above properly ventilated spaces". This lead to several frustrating hours of reading GBA blogs, BSC documents and FHB articles, all with seemingly conflicting views on underlayment permeability in the context of unvented roof decks and asphalt shingles.

Thanks again.


Answered by Ewan Smith
Posted Jul 7, 2012 10:48 PM ET


Joe, Have you considered changing the layout of you floor framing to reduce the floors thickness? This could shorten the structure allowing you to get all of your foam outside the first layer of sheathing.

Answered by John McCormack
Posted Jul 8, 2012 8:27 AM ET


Ewan, some of your roof ideas are not something I would build. I would leave the exterior alone since you have interior foam. Exterior foam is best done taped in two layers continuously and then vent space above. And with no or permeable insulation inside.

You will never have a payback either.

Answered by aj builder, Upstate NY Zone 6a
Posted Jul 9, 2012 1:44 PM ET
Edited Jul 10, 2012 12:25 AM ET.


AJ Builder,
Thanks for your candid response... appreciated. I've been slowly coming to much the same conclusion myself... I don't think the foam "sandwich" is worth the risk (however minimal) for the payback.

The current roof deck has approximately R30 (3-1/2" of closed-cell + 3/4" XPS on the underside of the rafters), which isn't where I would like it to be for our climate (Ottawa, ON), but not bad for a 1940s era 1-1/2 storey home. The roof deck performs nicely in the winter (no ice damming or appreciable snow melt), but with no central AC, the upstairs gets pretty steamy in the summer. My plan for adding insulation to the roof was primarily for the summer season, but I don't think it's worth it.

I think I'll use a vapor permeable roof underlayment (felt or synthetic), re-shingle and be done with it.

Thanks again.


Answered by Ewan
Posted Jul 9, 2012 11:58 PM ET


I got a reply from Alpha ProTech on their synthetic felt use over unvented roofs.

Their reply was "Our Website states that you meet the requirements of the Applicable code. Your good to go."

Thanks again Martin for pointing this out. I am a fan of synthetics and pretty much have solely used them for at least 8 years.

Answered by Ronnie Allen
Posted Jul 11, 2012 2:05 PM ET

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