Replies

1. 
   davidmeiland | Mar 16, 2012 12:45am | #1

   Seems like it depends on whether or not the foam is a vapor barrier, or is going to have one applied to it. What brand of foam and what thickness is proposed?

   With closed cell foam I don't think the engineer would be asking the question.

2. GBA Editor
   Martin Holladay | Mar 16, 2012 04:02am | #2

   John,
   The insulation contractor is right and the city engineer is wrong.

   David Meiland is also wrong. According to the code, the question of whether the insulation is vapor-permeable or vapor-impermeable is irrelevant. The only relevant question is whether the insulation is air-impermeable. Open-cell spray foam is air-impermeable, and can therefore be used in this application.

   More information on these code questions can be found here: How to Build an Insulated Cathedral Ceiling.

3. 
   LqmAj3mDu8 | Mar 16, 2012 09:38am | #3

   Martin,

   Thank you for the quick response. After reading the article it appears I can do it the way the inspector wants it done as long as the roof deck is completely sealed. However, it makes no sense and could potentially create future issues. Any advice on convincing the city engineer/inspector to go away from his old notion of how this needs to be done?

4. GBA Editor
   Martin Holladay | Mar 16, 2012 09:43am | #4

   John,
   Unfortunately, it's not uncommon for local inspectors to insist on installation methods that have no justification in the code.

   You generally have two choices:

   1. Do it the inspector's way, because it's not worth fighting City Hall.

   2. Politely ask the inspector to show you the code section that mandates the method insisted on by the inspector.

5. 
   davidmeiland | Mar 16, 2012 11:55am | #5

   Martin, it does look to me like there is a vapor "retarder" question, in zones 5 and up. I don't know what zone the OP is in. It's R806.4 item 4. If he's in zone 4 or lower, the code apparently doesn't care about vapor control, but what is the best practice?

6. GBA Editor
   Martin Holladay | Mar 16, 2012 12:11pm | #6

   David,
   All of Virginia is in Zone 4, and section R806.4 of the 2009 IRC doesn't have any vapor-permeance requirements for insulation installed in unvented rafter bays for climate zone 4.

   As you correctly point out, in more northern climates, that code section does address vapor permeance. The applicable section states, "In climate zones 5, 6, 7 and 8, any air-impermeable insulation [installed to create an unvented attic assembly] shall be a vapor retarder, or shall have a vapor retarder coating or covering in direct contact with the underside of the insulation."

   If you are using open-cell spray foam in a cold climate, the best way to meet this requirement is to spray the underside of the cured foam with vapor-retarder paint.

7. 
   LqmAj3mDu8 | Mar 17, 2012 10:50am | #7

   I even talked to the manufacturer and they say the same thing as David which is in line with what the contractor states. The type of installation the city wants me to do (baffles between sprayfoam and roof sheathing) raises the question:
   - Can this create a case in which dewpoint is reached and I am getting the underside of my roof sheathing wet which may cause warping as well as greater fluctuations in expansion and shrinkage? This could happen in both summer and winter.
   - Also if my baffles fail over time (due to extremes in heat and cold) and I reach dewpoint, then I would have water leaking into the insulation.

   Are these very likely or not very likely scenarios?

8. 
   wjrobinson | Mar 17, 2012 01:17pm | #8

   A see a dog, chasing its tail here....

   Ready, shoot, aim is next

9. 
   davidmeiland | Mar 17, 2012 09:27pm | #9

   John, as a building contractor for quite a few years, I have an instinctive dislike for the idea of foam sprayed on roof sheathing. At some point, some sheathing will need replacing and when that happens, the insulation is going to get damaged. It will need to be repaired, probably from above, so a foam sub will have to be brought in on an otherwise simple roof repair. The idea of spraying foam against baffles makes some sense from this perspective, but I've really never heard of anyone doing it. People always spray the roof deck.

   If you were to spray against baffles, I think you'd need an approval from the manufacturer that the foam does not need solid backing, and that it will perform in the way you want to use it. Maybe the manufacturer has published details for this, you could ask them. Of course, you would lose some insulation thickness.

   I don't see any increased risk of condensation with baffles, or at least it's safe to say that the vast majority of roofs are vented, in your neighborhood and just about everywhere else. With ventilation, your sheathing will be very cold in the winter, but with an adequate air barrier there should be no warm, humid interior air leaking out to it.

10. 
    LqmAj3mDu8 | Mar 18, 2012 12:20am | #10

    What does this mean
    "A see a dog, chasing its tail here....Ready, shoot, aim is next"

    Please elaborate.

11. GBA Editor
    Martin Holladay | Mar 18, 2012 04:23am | #11

    John,
    I agree with David that there is no danger in having a ventilation channel between the top of your spray foam insulation and the roof sheathing. He's also right that this approach will facilitate future sheathing repairs.

    This approach only makes sense if you have a very stiff site-made baffle to spray against -- something like plywood or rigid foam.

12. 
    wjrobinson | Mar 18, 2012 02:27pm | #12

    John, post 9 and 11 should give you your answer.

    All I was inferring w my post is that the thread was becoming a circular cipher project for me anyway.

    Hopefully, the circle has been broken and the dog thoroughly tired has laid down to rest.