Replies

1. GBA Editor
   Martin Holladay | Oct 25, 2018 05:26am | #1

   Gordon,
   If you want to install cellulose between your rafters, codes require either (a) a ventilated air space between the top of the cellulose layer and the underside of the roof sheathing, or (b) the installation of an adequately thick layer of rigid foam above the roof sheathing.

   Option (a) will only work if your roof has no valleys, hips, or dormers. For more information on option (a), see "Site-Built Ventilation Baffles for Roofs."

   For more information on option (b), see "Combining Exterior Rigid Foam With Fluffy Insulation."

2. 
   gmgranteh | Oct 29, 2018 04:21pm | #2

   Thanks Martin.

   I see R806.5 in the code for use of the rigid foam for condensation control. Is there a part of the code that mentions the ventilated air space?

   Thanks.

   gg

1. Expert Member
   Dana Dorsett | Oct 29, 2018 04:47pm | #3

   R802.2 and more likely, R806.3 are probably the parts you're looking for.

   https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction?site_type=public

3. 
   Jon_R | Oct 29, 2018 05:33pm | #4

   Joe Lstiburek wrote:

   "The valley issue can be handled by dropping the valley rafters. See figures from my Cold Climate Builders guide. Hips are handled the same way. This also extends to framing for dormers and skylights."

   Also review SmartVents by DCI and scissor trusses.