Replies

1. 
   Dana1 | Feb 14, 2015 11:28am | #1

   Closed cell foam is FAR more expensive than fiber insulation + "smart" vapor retarders such as Certainteed MemBrain or Intello Plus. On an unvented roof assembly you DON'T want a true vapor barrier, since there is zero drying capacity for the roof deck in either direction, making it a moisture trap. At the IRC 2012 code-min R49 for US climate zones 6 & 7 (most of NH is zone 6), it would take 8" of closed cell foam (at about $8 per square foot!), which would have a vapor permeance of only 0.12-0.15 perms, which is too low. The threshold for Class-I vapor retardency is 0.10 perms, which is a true vapor barrier. Typical 6 mil polyethylene vapor retardency is about 0.05 perms.

   A safer/cheaper assembly would be a 1-2" of closed cell foam on the underside of the roof deck, with the remainder of the R-value as cellulose. Without a smart vapor retarder this will still work in NH, but would not meet code. (Code would require 50% of the total R be impermeable foam against the roof deck in a zone 6 climate- that's 4" of closed cell + R25 fiber) But with a smart vapor retarder on the interior the code is silent- the low vapor permeance of the vapor retarder when the proximate air is dry (under 35% RH) makes it a class-II vapor retarder during the winter, but in the spring when the roof deck & cellulose warms up an releases it's accumulated moisture the RH goes higher, making the vapor retarder more vapor open than standard latex paint. This variable characteristic gives the assembly the drying rate of having no vapor retarder, but the moisture accumulation rate of a Class-II vapor retarder.

   It's possible and desirable to swap out the HVAC unit for something that is direct-vented/sealed combustion. It's not desirable to include a humidifier. The only reason you need a humidifier in winter is high air infiltration rates, probably driven by the air handler in the HVAC unit in the attic. Any duct leakage or duct imbalance creates pressure differences between rooms, and between the indoors & outdoors, which causes outdoor air to enter in some leaks, and conditioned space air to exit via other leaks. When you air seal at the roof deck it brings the whole shebang inside the pressure boundary of the house, which reduces the air-handler driven infiltration considerably.

   In a NH climate it's better for the house to limit the interior humidity to no more than 35% RH @ 70F, which is still in the "healthy & comfortable" range for humans. If you go above that it increases the amount of wintertime moisture accumulation in the sheathing & studs. If your house is made reasonably air tight you won't have much of a problem with humidity dropping below 30% RH, and it can usually be kept under 35% RH by being religious about using bath & kicthen exhaust ventilation. Combining ventilation with heating or cooling functions is a generally bad idea, since it tends to over-ventilate during periods of high heat load,which is when the outdoor air is driest, but may not always ventilate sufficiently when loads are neutral.

2. GBA Editor
   Martin Holladay | Feb 15, 2015 07:09am | #2

   A.G.,
   Dana gave you good advice. Yes, you can make your attic into an unvented conditioned attic; there are lots of options for insulating the sloped roof assembly. Yes, this work will require you to swap your existing atmospherically vented appliances for new sealed-combustion appliances. No, you won't have to change your ductwork (although it always makes sense to seal duct seams).

   And you should definitely disconnect that humidifier.

   For more information, see Creating a Conditioned Attic.