Replies

1. 
   Riversong | Aug 23, 2010 12:50pm | #1

   If the roof assembly is insulated, then the attic is already within the thermal envelope, but not likely within the air barrier. It sounds as if you're trying to bring the air barrier up to the roof assembly and improve the thermal "hat".

   You don't indicate where you are (what climate zone), nor how deep the rafters are or how much fiberglass is in place. You should be bringing the roof assembly up to IECC standards.

   A vented roof is almost always more durable than an unvented roof, particularly if you reduce the inward drying potential. 2" of foam board, unless it's EPS, is semi-impermeable at best (2" XPS would have a perm of 0.55).

   If you're going to re-install the fiberglass batts, then you need something better than Propa-Vents. The vent channel should fill the cavities from rafter to rafter and allow 1½" air channel. Wooden nailing strips and some kind of sheetstock works well. That also creates a secondary drainage plane in the event of a roof leak.

2. GBA Editor
   Martin Holladay | Aug 23, 2010 01:06pm | #2

   Chris,
   You should never use fiberglass batts to insulate an unvented sloped ceiling.

   Although taped rigid foam under the rafters would improve the situation -- by reducing the chance that warm humid air would migrate through the batts and contact the cold roof sheathing -- the detail is still risky, especially if there are any wiring penetrations.

   I think you have no choice but to remove the fiberglass batts. You can either put them in a Dumpster or set them aside for reuse. If you want to re-install them, you'll have to first establish ventilation chutes, following Robert's advice.

   if you put the fiberglass batts in the Dumpster, there are several ways to insulate the rafter bays, including closed-cell spray polyurethane foam without ventilation chutes, or dense-packed cellulose with ventilation chutes.

   If you go with the ventilation chutes followed by cellulose or batts, I would recommend rigid foam board under the rafters.

3. 
   jnarchitects | Aug 23, 2010 03:08pm | #3

   Thanks for the comments...

   1. Climate Zone 5, currently only R-19 batts.
   2. Yes, the attic is currently within the thermal envelope, but not the air barrier.
   3. Silly question, but does the IECC have a supplementary document that goes into specifics, i.e. R-7.5min. is recommended for exterior rigid in Zone 5? Is that determined based on the Equivalent u-factor tables or do they spell out specific wall, floor, roof assemblies?

   4. The client wasn't too excited about the additional cost of tossing the batts and foaming the rafters with closed cell. But I suppose factoring in cutting a ridge vent, soffit vents, removal of batts, install of ventilation channel, reinstall of batts, install of 4" of foam to get to r-39 might not be much of a savings....

   Thanks

4. 
   Riversong | Aug 23, 2010 04:59pm | #4

   Chris,

   I don't think the IECC specifies exterior insulation R-values, but the IRC has a provision to reduce the vapor retarder from class II to class III (1-10 perm) when the conditions of table N1102.5.1 are met, either with vented cladding or insulated sheathing sufficient to eliminate most condensation potential.
   (http://www.buildingscience.com/documents/guides-and-manuals/irc-faqs/irc-faq-insulating-sheathing-vapor-retarder-requirements)

   The National Building Code of Canada has a chart of interior/exterior insulation ratios to prevent condensation with interior RH maintained below 50%:
   http://www.nrc-cnrc.gc.ca/eng/ibp/irc/ctus/ctus-n41.html

5. 
   Riversong | Aug 23, 2010 07:54pm | #5

   Correction: the NBC in/out insulation ratios are for indoor RH ≤36%, with low vapor-permeance exterior sheathings (wood sheathings with gaps are excempt) that are also low air permeance (less than 0.1 L/(s•m2) @ 75 Pa)

   You can view the same table converted to Fahrenheit HDD65 at:
   http://www.flickr.com/photos/riversong-housewright/4921912080/