Replies

1. GBA Editor
   Martin Holladay|Apr 28, 2015 10:13am|#1

   Advice to GBA readers: Always finalize your insulation plan before choosing your framing members. If you have to figure these details out after you've got your trusses on site, then you are thinking about insulation too late in the game.

2. GBA Editor
   Martin Holladay|Apr 28, 2015 10:17am|#2

   Aaron,
   The best way to insulate these trusses is with a blown-in insulation material (for example, dense-packed cellulose or blown-in fiberglass) or with spray polyurethane foam.

   If you decide to use cellulose or blown-in fiberglass, you'll either have to include a ventilation channel between to top of the insulation layer and the underside of the roof sheathing, or you will have to install an adequate thickness of rigid foam insulation above the roof sheathing.

   For more information on these details, seeHow to Build an Insulated Cathedral Ceiling.

3. Expert Member
   Dana Dorsett|Apr 28, 2015 11:59am|#3

   Damp-sprayed cellulose or damp sprayed JM Spider (fiberglass) will fill in just fine, and will not settle, since it is glued in place by the water activated adhesives.

   If you don't have exterior insulation above the roof deck you'll have to use some fraction (determined by climate zone) of the R as closed cell spray foam on the underside of the roof deck to meet IRC 2012 Chapter 8 code:

   http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

   The prescriptive values for US climate zones 3 & higher are predicated on R49 total R. If you go higher-R than that, you need to proportionally increase the foam fraction for dew point control, since it's the ratio that determines the mean temperature at the foam/fiber or roof deck boundary.

   But you can relatively safely cheat those numbers, by applying 2" of closed cell foam against the roof deck and using a smart vapor-retarder detailed as an air-barrier on the interior side of the insulation. At 2" cc foam is a Class-II vapor retarder of sufficiently low permeance to avoid excessive moisture adsorption in the roof deck over a winter, but sufficiently vapor open that it can still dry at reasonable seasonal rates. Smart vapor retarders are also class-II vapor retarders when the relative humidity of the proximate air is fairly low (as it would be in winter), but become far more vapor open when the RH of the air is high (when the roof deck & fiber insulation is releasing any accumulated moisture.) This limits the amount of moisture accumulation in the insulation over the winter, and allows it to dry very quickly. It doesn't meet the letter of code, but it works.

   See:http://www.buildingscience.com/documents/bareports/ba-1001-moisture-safe-unvented-wood-roof-systems

   This document only analyzes the moisture at the roof deck, and does not include the smart vapor retarder enhancement, which only improves the situation for the roof deck, but would dramatically reduce the moisture levels in the fiber layer. In climate zones 5 & lower cellulose would safely buffer and redistribute that moisture, but in climate zone 6 & higher with only 2" ccSPF there is a potential for frost accumulation at the foam/fiber boundary if there are air-leaks to the interior or the vapor permeance to the interior side is too high. Smart vapor retarders would prevent that eventuality (again, if air-tight.)

4. 
   nvman|Apr 28, 2015 09:02pm|#4

   Thank you to all.
   I had a plan but thought I should broaden my scope and get info from the experts.
   I forgot to mention that I am including a site built ventilation channel consisting of a 1x2 spacer and quarter inch plywood sealed all around.
   After allowing for the ventilation channel, I will still have at least 14 inches for insulation.

   I am in North Vancouver, a marine climate zone.
   I don't really want to use spray foam but will make some enquiries into damp sprayed cellulose and JM Spider. They would be really labour saving.

   Thanks, again.