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Air Sealing Roof Retrofit

We are in the beginning phase of repairing & renovating a 100 year old wood frame church in Somerville, MA.

We are considering our options to substantially improve the energy performance including upgrading the exterior envelope and overhauling and/or replacing mechanical systems. This will be a long process, maybe over 10 years. However, we need to replace the roofing immediately. We are going to remove the existing asphalt shingle roofing down to the sheathing and install new asphalt shingle roofing. The roofer has proposed Grace Tri-flex synthetic underlayment.

The majority of the roof is an unvented pitched roof cathedral ceiling (which makes sense it being a church!!) with a clear story on either side of the main axis. This roof is currently uninsulated. The roof structure is 1x plank sheathing on dimensional lumber (roughly 2x10). There has been some consideration of installing insulation (that would double as the air barrier) over the roof deck before new roofing but we are not ready to proceed with this and we need to replace roof NOW so defaulting to insulating the roof from the inside sometime over the next year or two. Anticipating that a closed cell spray foam that uses a blowing agent with a low GWP will be available by the middle of 2013... by which time we will likely be ready to remove old decaying plaster and wood lathe ceiling from interior, spray several inches of closed cell foam on underside of roof deck and fill cavities below with batt or cellulose.

Is there any point in exploring the potential to use the synthetic roof underlayment as an air barrier? If so how? tape? What about the permeance of the underlayment in this scenario? Does any of this make sense?

Asked by Nathaniel May
Posted Dec 17, 2012 12:01 PM ET


13 Answers

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First, most synthetic roof underlayments can't be used on unvented roof assemblies. The manufacturers require this type of roof underlayment to be installed over a ventilated attic or a ventilated cathedral ceiling. If you think you might be installing spray foam from the underside, then you should install asphalt felt underlayment, not Grace Tri-Flex.

Second, you are missing out on a golden opportunity that only comes around ever 25 years or so. Now is the time to install a layer of rigid foam on top of your roof. Ask the roofer to get a price on a layer of nailbase (rigid foam plus OSB) -- the thicker, the better.

Remember, your proposed spray foam won't address thermal bridging through the rafters.

Answered by Martin Holladay
Posted Dec 17, 2012 2:02 PM ET


Martin - thanks. I appreciate you emphasizing the opportunity! The nailbase makes sense but I have to check the added weight. We've had the existing structure reviewed by an engineer but haven't proposed any changes so I'm going to pursue that.

Question on the underlayment - wouldn't a vapor permeable synthetic be suitable on an unvented assembly?

Answered by Nathaniel May
Posted Dec 17, 2012 2:29 PM ET


Yes, you could use a vapor-permeable synthetic underlayment like Cosella-Dörken Delta-Foxx -- but it will cost you a lot more than asphalt felt. You definitely don't want to use Grace Tri-Flex, which has a vapor permeance of only 0.04 perm.

Answered by Martin Holladay
Posted Dec 17, 2012 2:34 PM ET
Edited Dec 17, 2012 2:35 PM ET.


Nathaniel. Have you considered strapping foam boards and using steel roofing over the strapping? Steel is light, strong and cheap.

I'm with Martin. Don't miss out on this chance if you can.. But if you have to get it done now, I get it.

I just used trifex under steel over a vented roof. You can't use it in your unvented roof due to the low perm.

Steve in Waltham

Answered by stephen edge
Posted Dec 17, 2012 3:09 PM ET


Martin - what are your thoughts on an unvented cathedral ceiling assembly in which we install nailbase over the existing 1x plank sheathing and then asphalt shingles directly on top of that? Then down the road we insulate the cavities between the rafters from the inside. I've just started to research different nailbase options including thermal values, air barriers, and installation methods. Lots to consider...

Answered by Nathaniel May
Posted Dec 17, 2012 3:13 PM ET


Stephen - there was some internal musing about steel early on no real investigation assuming it was cost prohibitive. Can you point me to what you used?

Answered by Nathaniel May
Posted Dec 17, 2012 3:18 PM ET


It sounds like you are willing to consider my recommendation. The reason I recommended it is that I think it is a good idea.

Answered by Martin Holladay
Posted Dec 17, 2012 3:20 PM ET


Hunter Panels makes ventilated nailbase in a variety of R-values. You can order R-25 or R-28 panels if you want. Here are some links:



Answered by Martin Holladay
Posted Dec 17, 2012 3:27 PM ET


I used simple corrogated galvalume steel that you can get from home depot. You might want to go with painted for longevity. I found mine from a wholesale guy on craigslist in Laconia NH. He has a truck, cuts it to your length and can ship. But you'd have to pay a fuel surchage I'm guessing. Since you'll be buying a bunch, you can put it out to the Home Depot bid room for pricing.

Might be able to score some cheap foam board here. They are local I think. Worcester.

Here are some roofing suppliers.
http://everlastroofing.com/ (based of Maine)

If your roofer is not experienced with steel, he may try to charge you the same labor as asphalt. Move on if that is the case. Flashing can be tricky for the first timer and if the roof is not square you'll need some end caps etc.

Martin may have more advice and I'm sure has more experience with steel than I..

Answered by stephen edge
Posted Dec 17, 2012 3:33 PM ET


Martin - the configuration of the roof is complex including some parapets at gable ends, etc and so it makes sense to me to go with an unvented assembly. I recently used zipwall on an addition. Ideal in my mind would be zipwall panel with an inch of rigid foam and Zipwall has a product called Rsheathing that is just that but specs say not suitable for roof. Hunter has an unvented panel with polyiso foam - specs seem to suggest installing a vapor barrier on top of the existing sheathing before installing the panels. This makes no sense to me if the panels also have a perm < 1. Trying to find a nailbase type product that will primarily serve as the air barrier and thermal break and have some insulating value all in an unvented assembly.

Answered by Nathaniel May
Posted Dec 17, 2012 4:04 PM ET


In MA there are several vendors handling reclaimed roofing foam from commercial building re-roofing or demolition. Search your local beantown craigslist for "rigid insulation" and you'll get at least one or two, and there's also the Insulation Depot out in Framingham (insulationdepot.com) as Steven Edge points out. Pricing is typically 25-35% of virgin-stock goods, and is often cheaper per unit-R than virgin stock batts.

With a foam-over you can air-seal the roof deck from the exterior with lapped membrane goods such as Ice & Snow taking care to lap properly and seal the edges, then stack at least R20 (R24 is better if you want to go higher than code eventually) in rigid foam on the exterior, at which point you can bring it up to R50 with fiber on the interior.

If you use 2x furring on the outside of your foam through-screwed to the structural deck 24" o.c. with timber screws 24" o.c. (or use Hunter Panel's recommendations on fasteners & spacing) you can then mount an OSB nailer deck and and have a fully vented solution. At R50 and people sometimes skip the venting and through-screw the OSB to the original deck unvented. With an unvented approach it leaves at least a theoretical ice-damming risk in this warmer-wetter maritime climate, even at R50, but empirically that risk is pretty low, especially in buildings with simple roof lines and no skylights.

A deep energy retrofit in Worcester I was advising on this past year made extensive use of reclaimed roofing foam, on walls as well as roofs, saving many thousands out of the budget. The roof ended up with 6" of iso ~R35 on the exterior, which was enough to keep the roof deck plenty warm in winter even with at ~R55-60 whole-assembly average on the roof pitches. The nailer was unvented, composite shingle roofing. (They didn't take ALL of my advice, eh? ;-) )

Answered by Dana Dorsett
Posted Dec 17, 2012 4:13 PM ET


Dana - thanks for your feedback. I'm taking a look at reclaimed insulation route. I have some concern about ice dam risk but the roof is complex and I'm more concerned about proper execution of a vented system (expense is a big challenge). Cold rain and the holiday has probably bought us a couple of weeks of time to ask some more questions.

Answered by Nathaniel May
Posted Dec 18, 2012 8:22 AM ET


Empirically I've yet to see a house in this climate with an R50+ roof that has ice damming issues. Even at lower-R than that ice damming issues are much mitigated with as little as R10 above the roof deck to thermally break the stripes of melt lines at the rafters. The more complex the roof lines, the more opportunities there are for localized thermal bridging or other heat leaks, but unlike homes with 7 bump outs and 9 dormer, church building roof lines tend to be pretty simple. Skylights are another common heat contributing to ice damming, but aren't too common in 100 year old churches.

The higher the total R, the lower the risk, but even R25 foam above the roof deck is already ahead of the game compared to "typical" venting and batt insulation at 1980s code-min as far as ice dams are concerned.

Answered by Dana Dorsett
Posted Dec 18, 2012 4:11 PM ET

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