Replies

1. GBA Editor
   Martin Holladay | Aug 13, 2012 08:10pm | #1

   Daniel,
   The best place to install the rigid foam insulation is on top of the roof sheathing. Is there any reason you can't do that?

2. 
   user-735982 | Aug 13, 2012 08:15pm | #2

   Martin,

   We are pushing a roof height limit allowed by the zoning code. We might end up lowering the ceiling height if we have to (below 8 feet). If the clients don't want to do this, what are our options per the original question?

   Thanks,

   Daniel

3. 
   gusfhb | Aug 13, 2012 08:19pm | #3

   Why not use iso on top of sheathing? screws are available for over 6 inches and it is much cheaper than cutting between rafters. It is the way the roof membrane was designed to be installed.

   edit: responses did not show prior to my posting.

   I would still count my inches to try an put foam on top

4. 
   user-735982 | Aug 13, 2012 08:21pm | #4

   Hi Keith,

   I understand but we are limited by a roof diaphragm/interior ceiling height issue that makes the "typical" rigid over roof sheathing not as attractive, at least initially.

5. Expert Member
   ARMANDO COBO | Aug 13, 2012 09:04pm | #5

   Daniel, on your previous question I asked you for more details to help you... "It would help if you describe your roof assembly for truss, TJI or dimension lumber, and height. Any ducts, plumbing or electrical? Where are you located, or what’s your climate zone?"

6. 
   user-735982 | Aug 13, 2012 11:32pm | #6

   Armando,

   TJI's (12 depth approx), climate zone 3, no wiring, no ducts, San Francisco (non coastal).

   Thanks,

   Daniel

7. GBA Editor
   Martin Holladay | Aug 14, 2012 05:55am | #7

   Daniel,
   Q. "Might we substitute rigid foam board on the underside of the roof sheathing (cut to fit in our framing bays) and then fill the rest of the cavity with blown in dry cellulose (or wet)?"

   A. Possibly, depending on how your local code official interprets the code. In many areas of the country, such an unvented ceiling assembly would be governed by section R806.4 of the 2009 IRC. This section requires that the rigid insulation be installed above, not below, the roof sheathing; however, your inspector might allow you to install it underneath.

   The code section I cited requires at least R-5 of rigid foam before you can install cellulose underneath (in Climate Zone 3). If you go this route, remember that each rectangle of foam needs to be carefully air-sealed at its perimeter.

   For more information, see How to Build an Insulated Cathedral Ceiling.

8. Expert Member
   ARMANDO COBO | Aug 14, 2012 10:21am | #8

   Daniel,
   California uses the 2010 California Residential Code not the 2009 IRC. In Section R806.4 (page 489), which is very similar to the 2009 IRC on Section R806.4; it does specify clearly the three different ways to build a non-vented attic assembly, as Martin said, but it does have a few modifications for California that can impact your roof assembly.
   Your idea to install rigid foam under the roof decking (cut to fit and calk-sealed) and then cellulose (R806.4.5.3), it is allowed; however, that is a lot of work and your air-sealing and moisture management detailing must be impeccable. If you choose to go with cavity fill cellulose, you should know that cellulose may settle and render the rigid foam useless, either above (R4) or bellow the sheathing. Good installation and density is the key. If your client had a bad reaction to spray foam insulation in the past, you need to use no-VOC calks to seal the rigid foam on the inside.
   If I was you, I would go to the building department and explain that your client has health issues with spray foam, and by installing 1” rigid foam on top of the roof decking, you can cavity-fill with cellulose as a healthier alternative than to bring foams on the inside. I did the same on a Huntington Beach project not long a go, where the client wanted 4" of rigid foam above the decking, 2" above zoning height, and it was allowed on a zoning variance. It's worth a try.

9. 
   user-735982 | Aug 14, 2012 01:03pm | #9

   Thank you Armando and Martin. We'll try the 2" rigid (were in Climate Zone 3 - relatively benign) and fill the framing cavity with cellulose. I presume that wet cellulose application would be the best as long as adequate drying time is given. Sidebar question: Armando, you mentioned that the rigid board would be rendered useless if there was a little settling of the cellulose - the assembly would still have the same insulation capacity, would it not? Are you thinking that there would be convective loops like in walls?

   Lastly, can you steer me towards a rigid foam board spec that would be appropriate for the roof (polyiso I presume?).

   Thanks Again!

   Daniel

10. 
    user-735982 | Aug 14, 2012 01:05pm | #10

    Clarification - we'll go with the ABOVE sheathing rigid board option if we can.

11. 
    user-735982 | Aug 14, 2012 01:49pm | #11

    Martin,

    I read your link re: flat roof assemblies. To be extraordinarily clear, I presume that we can use dense pack cellulose insulation IF and only if we use the appropriate amount of rigid foam above the sheathing (or spray foam below). I was thrown for a loop when I read this:

    "Can I use dense-packed cellulose insulation in an unvented roof assembly?

    In a word, no — the code explicitly forbids this method."

    I trust that you a referring to SOLELY using dense pack insulation with NO rigid or spray foam which has led to ugly dew point formation within these assemblies.

    Thanks,

    Daniel

12. GBA Editor
    Martin Holladay | Aug 14, 2012 02:00pm | #12

    Daniel,
    Thanks for pointing out a sentence that could have been more clear. I will re-write it.

    You are correct: you can use cellulose insulation in an unvented roof assembly as long as the assembly includes enough rigid foam above the sheathing or enough closed-cell spray foam below the sheathing.

13. Expert Member
    ARMANDO COBO | Aug 14, 2012 04:28pm | #13

    Answers to #9:
    1. If the cellulose settles, yes, it can create convective loops like in walls. Whether you use dense-packed or wet spray, quality installation is a most, as both can settle.
    2. Polyiso is a good choice; and you only need 1” where you live. The rigid foam above the sheathing also help prevent thermal bridging.
    I think you said in your other post that you plan to use can lights; I would reconsider it and replace with track or surface mounted lighting.