Replies

1. GBA Editor
   Martin Holladay | Oct 20, 2011 11:02am | #1

   Jeremy,
   If you are going to the expense of installing spray foam on the underside of the roof sheathing, why not make the attic part of your home's conditioned space? Then it will be warm and dry, and could even be used for living space if you want.

   Take the money you want to spend on the cellulose insulation, and use that money to beef up the R-value of the insulation that follows the sloping roof.

   Incidentally, you forgot to tell us whether anyone intends to insulate the gable walls.

2. 
   gkeNVPcxD9 | Oct 20, 2011 11:32am | #2

   The developer did not indicate whether or not they would be insulated the gable end walls. I don't think they do. I'll wait to to post my thoughts to see what others say. Thanks all!

3. GBA Editor
   Martin Holladay | Oct 20, 2011 12:12pm | #3

   Jeremy,
   Insulating the underside of the roof sheathing without insulating the gable walls is a total waste of time and money.

4. GBA Editor
   Martin Holladay | Oct 20, 2011 01:25pm | #4

   Armando,
   5.5 inches of open-cell spray-foam insulation is not air-permeable.

5. Expert Member
   ARMANDO COBO | Oct 20, 2011 06:16pm | #5

   Jeremy,
   1. You need to read 2009 IRC 806.4.5.2. If you are installing air-permeable insulation directly under the roof decking, then you must install rigid foam on top of the roof decking.
   2. You should have enough roof deck insulation between the over/under to get the R38 required in CZ5, creating a non-vented attic. Always remember that a roof assembly is no different than a wall assembly on an angle (per Dr. Joe).
   3. Either you install insulation on the over/under roof decking or install a vented attic with R38 cellulose above the ceiling; both are an unnecessary cost. Also, if your foam insulation is creating a conditioned attic, then the cellulose is doing nothing for you.

6. 
   user-659915 | Oct 20, 2011 11:07pm | #6

   If the vapor seal at the ceiling is not perfect, will enough heat leak into the unvented attic through the R38 to prevent condensation? Beyond wasting money does this approach not additionally create a vulnerable attic space which is both cold and unvented? Bear in mind the roof deck insulation will prevent daytime heat gain as well as night-time heat loss.

7. 
   wjrobinson | Oct 21, 2011 12:49am | #7

   Condensation on the foam? Less likely then if no foam.

   Cellulose also will somewhat help regulate extremes of humidity and moisture.

   IMO....

8. 
   user-659915 | Oct 21, 2011 09:32am | #8

   It's not condensation on the foam I'd be concerned about AJ, it'd be on the exposed roof structure. Yes, the cellulose would act as a moisture reservoir but in this case I'm not sure that'd be a good thing. I may be worrying unnecessarily, but Jeremy should get someone to run the numbers for his locale before implementing this idea.

9. 
   gkeNVPcxD9 | Oct 21, 2011 11:02am | #9

   I don't think they planned on air sealing, or as James put it, "vapor seal", the attic ceiling.

   @James: I'm not sure what you mean by "exposed roof structure". Also what numbers should I run?

   I don't see how adding R21 ocSPF at the underside of the roof helps with the R value of the total assembly. Here are my thoughts:

   Your thermal barrier and your air barrier should align. In this scheme they do not. Your thermal barrier is the attic ceiling, but your air barrier is the roof. Since we are insulating the attic ceiling, what will the temperature of the attic space be? If we don't air seal the attic ceiling, moisture laden air can enter the attic space. Is there a possibility for condensation on the face of the R21 ocSPF? It's also important to not that the home will have exhaust only ventilation so the home will be negatively pressurized, so "semi-conditioned" air from the attic space will be drawn into the living space.

   I'm just not seeing how this design is effective.

10. 
    user-659915 | Oct 21, 2011 08:37pm | #10

    By 'exposed roof structure' I was referring to those parts of trusses or rafters withing the attic space not covered by the foam. Cold framing + moist air = trouble, no?