Replies

1. GBA Editor
   Martin Holladay | Jan 28, 2011 08:49am | #1

   Steven,
   I'm not sure I understand your question correctly, but I'll take a stab at interpreting it. I think you are asking whether it makes sense to have ventilation chutes under the roof sheathing, fiberglass batts between the rafters, and rigid foam above the sheathing.

   If that's your question, the answer is no. If you install ventilation chutes between fiberglass batts and the roof sheathing, it makes no sense to install rigid foam on top of the roof sheathing.

   If I guessed wrong about your question, please rephrase your question.

2. 
   knobhead | Jan 28, 2011 12:01pm | #2

   Martin:
   Really sorry this is not coming out right. My initial plan was build envelope utilizing rigid only on advantech sheathed roof, but leery using 6+ inches rigid(fastening). Apparent need to compromise with NYS code and of course independent township enforcement. What insulation system can I use on roof to get me cathedral or modified cathedral, and for that matter, the rest of house with flat ceilings, using EITHER 2X lumber or trusses?? The plan is r-26 rigid walls (2x4 walls to keep window box out to minimum). Shooting for Huber zipped walls, and hoping to get minimum air changes, and using fan only (Braun) ventilation. Is there a simple roof insul. system that will agree with town/state and my energy anticipations?

3. GBA Editor
   Martin Holladay | Jan 28, 2011 12:13pm | #3

   Steven,
   That's a very broad question, and of course there are a huge number of answers. For an insulated sloped (cathedral) ceiling, you can use any of the following:
   1. Thick foam above the roof sheathing.
   2. SIPs
   3. Nailbase on top of roof sheathing.
   4. A combination of foam above the roof sheathing and cavity insulation in a hot (unvented) roof.
   5. Spray foam insulation between the rafters.
   6. Dense-packed cellulose under a ventilation chute.

   ...and that's just a sampling. There are lots of ways to build!

   For an insulated flat ceiling, you can use:
   1. Thick cellulose
   2. Thick blown-in fiberglass
   3. Spray-foam insulation

   ...and many other types of insulation.

   Can you narrow your question?

4. 
   knobhead | Jan 30, 2011 08:02pm | #4

   Boy! Let me try to better explain.....Whether i use rafters with a collar tie,OR trusses with a bottom plate that is lower that collar tie, I still have an air space somewhere with un-conditioned space that architect says needs to be vented to code. I'm thinking that whatever insulation system used,IF it is applied ABOVE air space, such as rigid on sheathing, with possible 1-2" flash under sheathing, then I would be fine, code wise, to not require venting,which I DO NOT WANT!!I realize there is some heat loss from below modified cathedral, thru drywall, to un-cond. space, but with a hopeful 1-2 Ach/hr home, I should be good. Does this game plan meet with building code specs in your opinion?

5. 
   user-869687 | Jan 30, 2011 10:33pm | #5

   Steven,

   I think your architect is correct that if there is an unvented assembly with rigid insulation above the sheathing, then there should not be any air space between the underside of sheathing and any additional cavity insulation. If you don't think it's possible to install cavity insulation without this undesired air space, then your alternatives would be either (1) rigid insulation only, thicker with long fasteners as you say, or (2) cavity insulation only, with venting as required by code.

   With the latter you would want to pay special attention to air sealing the drywall at the ceiling, to meet your goals for air tightness. Avoid can lights and seal all penetrations carefully before installing the insulation.

6. GBA Editor
   Martin Holladay | Jan 31, 2011 11:39am | #6

   Steven,
   Your writing is unclear, and I have no idea what you are talking about. Let's start at the beginning.

   1. Question one: are you talking about a cold, unconditioned attic or an insulated sloping roof?

   2. Question two: do you prefer a ventilation gap or do you prefer an unventilated ("hot") roof?

7. 
   steve bumpus | Jan 31, 2011 01:47pm | #7

   (1) insulated sloping roof
   (2) prefer unventilated (hot) roof

8. GBA Editor
   Martin Holladay | Jan 31, 2011 01:55pm | #8

   Steve,
   Insulated hot roofs are allowed in most building codes. In the 2009 IRC, for example, the requirements can be found in section R806.4.

9. 
   steve bumpus | Jan 31, 2011 03:33pm | #9

   Thank you Martin!!!