R-60 with scissor trusses?
user-742626
| Posted in Energy Efficiency and Durability on
I’m finishing an attic, framed with scissor trusses that do not have an energy heel. (Roof slope is 12:12, interior is 8:12). I’m trying to achieve R-60 but I’m wondering how to treat the diminishing roof cavity near the exterior walls. Does anyone have a suggestion for this detail? I am also wondering whether to spray in cellulose above the ceiling or install batts at the rafter level. Is there an economy to using batts (22″!) vs. spraying foam? Project is on Lopez Island, Washington state.( I’ve read the posts from as far back as July, so I know I’m opening a can of worms with this question!)
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Replies
Lydia,
How many inches do you have to work with at the eaves? To achieve R-60, you need:
-- 9 1/4 inches of closed-cell spray foam, or
-- 16 1/4 inches of open-cell spray foam or cellulose.
When you write that you need 22 inches of fiberglass batts to achieve R-60, you are assuming that the batts provide only R-2.7 per inch. That's a little low -- you could probably go with something less than 22 inches. But you'll start an argument, because many people will point out that fiberglass batts don't really perform as well as their labels imply.
Martin -
The space between the top and bottom chord of the trusses diminishes to approx. 9" at the top plate, but an inch less than this if we accommodate venting. (Washington State doesn't officially allow unvented roofs until July). Based on your reply, I'm considering spraying 9-1/4" of foam against cardboard venting baffles, tapering off to 8" at the eave - accepting something less than R-60 there- and continuing the foam up the roof until there is sufficient room to accommodate 16-1/4" of cellulose. I'm assuming that the cellulose is less expensive than the foam.
Lydia,
Your plan will work. Be sure that your eave detail allows the top plates of your perimeter walls to be totally covered with insulation.
Thank you!
Lydia,
Be sure to extend the wall sheathing (wind wash barrier) to the underside of the venting baffles to hold the spray insulation in place.
Lydia,'
The 9ΒΌ" of closed-cell foam is going to be very costly and you may need to bring in two different insulators for the two materials. Additionally, the expansion of the foam may compress the cardboard vent baffles, partially closing off the vent channel.
8" of cellulose will yield R-25-R-30, depending on density, and you can compensate by blowing more cellulose in the upper parts of the truss roof so that you get an average of R-60. This will also put more insulation in the higher part of the cathedral ceiling where there will be more warm air stratification and more potential heat loss.
This will be a less expensive and equally efficient approach, and it won't risk compressing the baffles..
Lydia.... Robert Riversong is giving you good advice... great insulaion.... easy install and the lowest price... along with the lowest chance of future moisture failure.
aj
Lydia, I think your approach is reasonable. Doug's point about extending the sheathing up to the baffles is important. Note that Corbond, and I assume other closed cell insulation, are normally sprayed in 2-1/2" to 3" layers, so your installer will need to make several passes.
Reading this thread, I was reminded of a home I took pictures of years ago when I first became interested in foam insulation. The photos I took relate to your questions, so I created a link to these photos so that you and others can see a typical installation.
http://gallery.me.com/dvanderhoop#100033