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Exterior construction/insulation for a new home

I am about to start building a 3000 sqft, 1 1/2 story home in zone 3A W/H (central Alabama).
The house will have a standing seam metal roof and a drilled pier foundation with open crawl space. Air handlers and ductwork will be above the first floor ceiling, on the back side of the second floor knee wall.

My current construction/insulation plans are:
- Roof: Metal roof / Ice & water shield membrane/ 7/16 OSB/ Closed cell SPF sprayed 5" thick between 2 x 6 rafters
- Exterior Walls: Cement board siding / membrane / 1/2" OSB / Closed cell SPF sprayed 3 1/2" thick between 2 x 4 studs on 16" centers / 1/2" gypsum board
- Bottom floor: Hardwood floor / 3/4" T&G Advantech OSB / Closed cell spray foam sprayed 4" thick between 2 x 12 joists

The builders I have talked with have concerns that the enclosure will be too air tight from the CC SPF. I intend to address this with a ventilation duct to outside air tied into the HVAC system. They are also concerned about trapped moisture causing the OSB and other wood to rot.

Would you recommend any changes to my exterior construction/insulation plans?

Asked by Thierry Swinson
Posted Fri, 07/18/2014 - 15:59

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3 Answers

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1.
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Thierry,
It's very difficult to install closed-cell spray foam at a thickness of 3.5 inches between 2x4 studs. Since this type of foam is so difficult to trim, you are more likely to end up with about 3 inches of insulation.

Yes, you will need a ventilation system. Here is a link to an article that explains your options: Designing a Good Ventilation System.

I don't know what type of "membrane" you plan to install on the exterior side of your wall sheathing. If you mean a vapor-permeable housewrap or asphalt felt, you will be fine.

Your roof assembly is a type of assembly that makes it impossible for your roof sheathing to dry in either direction. Opinions differ as to how risky this approach is. If you want your roof sheathing to be able to dry out in at least one direction, you should create a ventilation channel between the top of your insulation and the underside of your roof sheathing. You can do this with a product like AccuVent or with site-built insulation baffles that are strong enough to resist the expansion pressure of the spray foam.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 07/18/2014 - 16:49
Edited Fri, 07/18/2014 - 16:51.

2.
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I was planning on closed cell spray foam in the walls (instead of open cell ) to get the higher R value. I understand your point about the difficulty in trimming the closed cell foam. Even if I only get 3 inches with the closed cell foam, the R value will still be around 18, whereas with open cell (which trims easier) R value is only about 13 for 3 1/2" thickness.

If I used a product like the AccuVent between the roof sheathing and the CC SPF, would I need to provide a path for venting at the the top end near the roof ridge? Looks like air could come in from the bottom end at the eaves but would have no place to go unless the top end was also vented to outside.

Answered by Thierry Swinson
Posted Fri, 07/18/2014 - 17:48

3.
Helpful? 0

Thierry,
The detail at the ridge is called a ridge vent. Any roofer you talk to will know what a ridge vent is.

If you've never seen what a ridge vent looks like, just Google "ridge vent."

Answered by Martin Holladay, GBA Advisor
Posted Fri, 07/18/2014 - 19:40

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