Adapting Josh Salinger’s Roof Assembly
GregH_207
| Posted in Green Building Techniques on
I like the look of the roof design in Josh Salinger’s article “a new take on insulating a roof” in issue 299 of Fine Homebuilding. I’m wondering whether the basic approach would translate to a my zone (northern Maine, 7a) by using a parallel chord truss sized to allow for ~2 feet of cellulose to get to R-80. Would it need changes such as increased the strapping or using sheathing to support the additional weight of the cellulose, or perhaps different barriers/membranes because of our climate? Are there examples of similar roof structures available online?
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Replies
Greg,
The assembly is very similar to one that 475 High Performance Building Supply had on their website. I have some reservations about attaching the membrane tightly enough to keep the ventilation channel clear, and it's a bit hairy for the framers to work on rafters or trusses covered in membrane with no strapping or sheathing, but evidently it's doable as it's been done.
Strapping the underside of the trusses at 16"oc will help support the cellulose, and I'm not sure it matters what membrane you use on the top side as long as it is permeable to allow drying.
Two tricks: Check the stretch factor of whatever membrane you use. It will bulge a bit but shouldn't touch. Also, you can use two layers of 2x for extra venting depth. A good practice when the roof is a lesser pitch as well.
Greg, Josh's roof design is a good one. As Malcolm notes, it's similar to a standard 475 detail. I first learned about it from Chris Corson at Ecocor here in Maine; they use it a lot and call it a Sarking Membrane roof. A couple of weeks ago I visited a job designed by Robert Swinburne, who commented above, under construction by Mindel and Morse Builders, with a similar system. Those who have not done it worry more than necessary about working over the membrane; it's really not that different from normal practice, all things considered. Certainly no weirder than many of the other things we do on high-performance homes.
Since you're building in Maine, you'll have strapping on the ceilings. I recommend using 2x material, or doubling up the strapping--not for weight but to allow the use of low-profile LED fixtures and other wiring, all on the interior of the vapor control membrane. If you won't have lighting or wiring in the ceiling, regular 1x3 strapping is fine. Strapping should always be no more than 16" o.c.. Nothing additional is needed for R-80. I've done up to R-140 with cellulose in a similar assembly with no problems. Dense-packed cellulose weighs 3.5 to 4.0 lbs/ft³ which is much less than your snow load (https://www.maine.gov/decd/sites/maine.gov.decd/files/inline-files/Maine_GSL_Listing_by_Town_2015.pdf), which will be your main structural concern.
For membranes, you can use Pro Clima products from foursevenfive.com, or Siga products from sigatapes.com. Rothoblaas products are now available in the US as well but they're a little harder to find.
A couple of small changes can simplify your roof build.
If you are going with trusses, having a small mini attic at the ridige would let you go with loose fill insulation. You still build the roof in a way to contain the insulation in the rafters and have a clear vent channel.
The idea is to overfill the mini attic with insulation, this way any settling in the rafter space would be taken up by this extra bit of insulation. This also avoids the extra cost and weight of dense packing and you might be able to get away with no strapping for the ceiling.
If you can find space for a structural ridge beam, another option is to go with deep I-joists for rafters as you can use the top flange to build the vent space. For a quick and cheap vent channel, cut a roll of house wrap to a width a bit less than the space between the I-joist webs and staple it to the bottom of the flange.
This avoids the extra labour of building the vent channel out of 2x above the trusses plus the frames won't have to try and walk on trusses covered in house wrap.