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How to insulate ceiling and walls in zone 3 - new construction?

I am in the framing stage of a new house near Raleigh, NC (zone 3). Our strategy for an air barrier is to rely upon placing the air barrier on the outside at the sheathing using the Zip system on all walls and the roof with all seams taped. The house is situated on a concrete slab that is insulated with rigid foam (R-5). The sill plate is air sealed using Protecto-Wrap sill sealer gasket. Our plan (right now) is to insulate the walls using fiberglass batts (2x6 walls). I know that is not popular, but due to our region and the air barrier on the outside, we are willing to make this cost-based trade-off. The house has a utility room located in the center of the home (inside the conditioned space). All plumbing and HVAC components will be run only in the conditioned space of the home (never in an exterior wall or the attic - we are employing 20" floor trusses that make this easy). The house will employ separate, dedicated ducting for the heating and air duties from the ventilation system which will be an ERV.

My big question is the roof insulation. The roof is not "simple" enough to vent it properly (2 large gables, 2 large dormers, multiple valleys), consequently, I feel like I should be building a non-vented roof. The roof design calls for cathedral ceiling from the floor/rafter junction up to the 9 foot ceiling height. The remainder of the roofline will be attic (I have no intention of letting anyone go up there, no storage needs, etc.). The rafters are 2x10's.

My thought right now is to move the insulation barrier to the outer edge of the envelope throughout the roof (ie - the rafter bays) and not just in the areas of the cathedral ceiling. Our goal is R-30 for the roof structure. We are considering 3 possible methods: fill the rafter bays with 9" of open-cell spray foam, spray a 5" layer of closed-cell foam, or employ a flash and batt strategy (could be 1" of closed-cell with 8" of batts or 2" of open-cell with 8" of batts.

Cost is obviously a factor, but I want this to be a "safe" installation. Based upon the climate zone and my air barrier being the sheathing, what is the best route?

Thanks

Dave Harris

Asked by David Harris
Posted Sat, 12/28/2013 - 14:09

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5 Answers

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Dave,
Either of the three options will work. Even better would be a roof assembly that includes a continuous layer of rigid foam, to address thermal bridging through the rafters -- but that approach may not meet your budget.

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

Answered by Martin Holladay, GBA Advisor
Posted Sat, 12/28/2013 - 17:37

2.
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Martin

Thanks for the response. If I go with the flash and batt approach:

1. Does using open cell foam comply with the code? My understanding is the foam employed in a non-vented shall be impermeable?
2. Does using closed cell foam create a concern for trapping moisture in the sheathing if there is a roof leak? Would it hide a problem forming such that it would not be noticed until it was severe?
3. Does using closed cell foam create a concern for condensation forming in the batts? Or is this not a concern due to my region 3 location?
4. Does using open cell foam (assuming it is legal) provide for a tell-tale failure location if the roof leaks? In other words, does the open cell foam's slight permeability enable it to pass any roof leak through to the inside where it can be identified and repaired prior to a more catastrophic failure?
5. If forced to use closed cell foam due to the answer to question 1 above, do the answers to questions 2 and 3 drive me to a certain thickness for the foam?

Bottomline, which would be your preferred method and why?

Thanks

Dave

Answered by David Harris
Posted Sun, 12/29/2013 - 12:49

3.
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David,
Flash-and-batt only works with closed-cell foam. You can't use open-cell foam for a flash-and-batt job.

The jury is still out on whether roofs with foam applied to the underside of the roof sheathing will have a higher chance of sheathing rot due to unnoticed roof leaks. If you are concerned about this problem, you should live in a house with a traditional vented, unconditioned attic.

If you follow the usual guidelines for flash-and-batt jobs, you won't have to worry about condensation in your roof assembly. For more information, see Why Flash and Batt Makes Sense.

As I explained in my article ("How to Build an Insulated Cathedral Ceiling" -- link provided in my first answer), the minimum R-value of the spray foam layer for a flash-and-batt job in your climate zone is R-5 (about one inch of closed-cell foam).

Answered by Martin Holladay, GBA Advisor
Posted Sun, 12/29/2013 - 16:51
Edited Sun, 12/29/2013 - 16:53.

4.
Helpful? 0

Martin

Thank you again.

One last question and I will leave this alone:

I have not priced the "all open-cell" versus "flash and batt with closed cell" yet, but assuming they are close in price (or maybe regardless of price), which would you recommend? Better still, if it was your house, which would you use?

Thanks

Dave

Answered by David Harris
Posted Sun, 12/29/2013 - 17:51

5.
Helpful? 0

David,
If I had to choose between closed-cell spray foam and open-cell spray foam for installation on the underside of the roof sheathing of my house, I would choose closed-cell spray foam.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/30/2013 - 06:28

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