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Will furring strips be needed over sheathing if using steel roofing?

I'm Zone 5 - upstate NY, planning to build 1050 s.f. ranch home, using r-20 superior wall basement, r-33 double wall cellulose.
If i build roof using 2x10, and gusset 2x4 underneath, filling with cellulose, and if i use 1" foam board to baffle/vent roof 1-1/2" beneath sheathing, i could obtain over r-43. here are my thoughts/questions

*would it be acceptable to "create" gap of 1-2" between 2x10 and 2x4 using ply gussets to break thermal transfer between 2-piece rafter? if so, that gap could be spray foamed or kept cellulose along with rest of cavity, correct? either one better than the other? 2" gap would then become r-50+roof.

*using 1" foam board as baffle/vent, does it really matter if i use xps, eps, or foil polyiso?

* And original question.....the sheathing above vent....if i use steel roof, do i really need vertical or horizontal furring strips, or will design of steel be enough air movement?

thanks!
Steve B.

Asked by steven bumpus
Posted Mon, 05/21/2012 - 21:52

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4 Answers

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Steve,
Q. "Would it be acceptable to create a gap of 1-2 inches between 2x10 and 2x4 using plywood gussets to break thermal transfer between 2-piece rafter?"

A. Yes.

Q. "If so, that gap could be spray foamed or kept cellulose along with rest of cavity, correct?"

A. I would recommend blowing cellulose in the cavity, using the dense-pack method. There is no need for spray foam.

Q. "Using 1-inch foam board as baffle/vent, does it really matter if I use XPS, EPS, foil polyiso?"

A. I think that EPS isn't strong enough, so I wouldn't use EPS. Polyiso is more environmentally friendly than XPS.

Q. "If i use a steel roof, do I really need vertical or horizontal furring strips, or will design of steel be enough air movement?"

A. When steel roofing is installed over furring strips, the strips are installed parallel to the ridge to facilitate screwing. It's possible to install two layers of furring strips at 90 degrees in order to get soffit-to-ridge venting, but that is rarely done. It's also possible to screw the roofing directly to the plywood or OSB sheathing (as long as there is a layer of roofing felt, of course.)

My preferred installation detail: one layer of furring strips (either 1x4s or 2x4s) parallel to the ridge.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 05/22/2012 - 05:34

2.
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Martin:
Excellent! And Thank You for your many,well detailed FHB articles!
SJB

Answered by steven bumpus
Posted Tue, 05/22/2012 - 06:06

3.
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Have you considered insulating he attic floor with a raised heel rather than insulating the rafters? It's cheaper and safer. Can get to r60 in a ranch that way. No need for furring strips under the steel from my perspective. If you do them, perhaps you can run them diagonally so if you spring a leak in the steel, the water will have somewhere to go. Seems like a squirrel haven in the making though..

Answered by stephen edge
Posted Tue, 05/22/2012 - 08:12

4.
Helpful? 0

Your roofing supplier or installer should made available 1x2, 1x4 or 2x4 pressure treated notched battens; it allows moisture to run down as it provides ventilation. You need to install battens with stainless steel fasteners long enough to penetrate the framing members at least 1 1/2". Almost all steel roofing manufacturers provide installation instructions that you must follow or their warranty will void.

Answered by Armando Cobo
Posted Tue, 05/22/2012 - 10:39

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