I have a 1910 bungalow that has a 8/12 pitch attic that was converted to a MBR. It gets awfully hot up there in summer. There aren’t any nents in the soffit or roof. I would estimate they used 2×10 rafters. I am due for new shingles and that got me wondering…
Can I strip off the old shingles and TP, and rip some 2x4s in half and screw them directly on top of the existing rafters and then sheath the ‘new’ higher roof? I could then have a vent cavity while maintaining the insulation depth. I would want to use a Cobra vent at the top and cut in continuous vents at the soffit. I would even use the new plywood with the metallic facing for heat reflection.
It all seems plausible, but I am not a roofer. What am I not seeing? (that little voice is talking again)
Replies
Yes you can. It is a time honored technique called a cold roof
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Seems like an awful lot of work for little benifit. And your attic still wouldn't be vented. Plus you would have to figure out what to do at the eave line.
Why not tear off the roof and insulate while things are open? We did that on my Brother's house, and it made a tremendous difference.
Being as you're in Wisconsin, the insulation would do you a lot of good in the winter too.
If you're going to all that trouble, look into adding a layer of rigid insulation and re-sheathing over that.
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Skip the 2x4s all together? How do I vent then?
Can you explain further?
Skip the 2x4s all together?
Yes.
How do I vent then?
You don't. You eliminate the need for venting.
http://www.hpanels.com/pages/pdfs/Lit_Prod_Color/H-Shield-NB.pdf
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What are your thoughts on the effectiveness of using that foam vs. spraying the underside of the sheathing? (aside from the issue of extending the trim to meet the new roofline)And if he's in Wisconsin, isn't that too far north for a hot roof? I was under the impression that an unvented assembly would be ok where I am, but in the land of snowy roofs you'd want some air movement to ward off ice damming, no?'Man who say it cannot be done should not interrupt man doing it' ~ Chinese proverb
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What are your thoughts on the effectiveness of using that foam vs. spraying the underside of the sheathing? (aside from the issue of extending the trim to meet the new roofline)
I don't really like adding insulation on top of the sheathing. But, in this case, the ceiling is already finished so this would be an option.
And if he's in Wisconsin, isn't that too far north for a hot roof?
He's got one already. Ice damming doesn't seem to be his issue. He's got a summer cooling problem. I'd gues he has little or no soffit.
I was under the impression that an unvented assembly would be ok where I am, but in the land of snowy roofs you'd want some air movement to ward off ice damming, no?
Not necessarily. The object is to keep the snow from melting and refreezing at the eaves. Mucho insulation reduces the heat loss that melts the snow. Venting directs the lost heat out the vents instead of thru the roof.
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If it's a summer cooling issue, wasn't there a foamboard product that also had a reflective barrier on the topside?I'm guessing that the r-value on spraying the underside is greater than you'd get with the foam sheeting. And it wouldn't throw everything out of proportion.'Man who say it cannot be done should not interrupt man doing it' ~ Chinese proverb
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If it's a summer cooling issue, wasn't there a foamboard product that also had a reflective barrier on the topside?
I believe there is - I have no experience with it.
I'm guessing that the r-value on spraying the underside is greater than you'd get with the foam sheeting. And it wouldn't throw everything out of proportion.
An inch of poly iso has the same approx R value whether it's sprayed or applied in sheets. The sheets would eliminate or reduce thermal bridging thru the rafters. But there's that rake end problem. The eave is easy enough to deal with in most cases. All that being said, all we've ever installed the poly iso sheets under is EPDM or TPO.
On another note, I haven't heard from ABC yet. I'll call them tomorrow.http://www.quittintime.com/ View Image