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Old 09-08-2010, 10:08 PM   #1
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4" Trusses & ice dams; enough space for good insulation?


I live in a 1 1/2 story Cape Cod style house in Minnesota. After spending a few winters in this house, it became clear that there are roofing/insulation/venting problems since ice dams form when there is a heavy snow and I don't use a roof rake. The snow is melting on the roof above the area where there is no crawl space. My house is the only house in the dead of winter that has no snow on the roof because it has melted. If you took a cross section of the top 7 feet of the roof you would have the 4" trusses, whatever insulation is between them, and the sheetrock on one side, the asphalt shingles on the other. When there is snow, it melts right away and then travels down the roof to where its colder and then it freezes up again.

Now I believe most of these problems can be fixed with better insulation and ventilation in the crawl space, I don't think this is the case. A handyman friend of mine suggested that unless I wanted to put in 6"trusses, a fix would be to tear the existing asphalt shingles off, line up firring strips vertically on the roof every 16", add insulation (spray or roll?) between the firring strips, lay down new plywood on top of the firring strips and then shingle. Is this a good idea?

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Old 09-09-2010, 06:17 AM   #2
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4" Trusses & ice dams; enough space for good insulation?


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Originally Posted by ben5048 View Post
I live in a 1 1/2 story Cape Cod style house in Minnesota. After spending a few winters in this house, it became clear that there are roofing/insulation/venting problems since ice dams form when there is a heavy snow and I don't use a roof rake. The snow is melting on the roof above the area where there is no crawl space. My house is the only house in the dead of winter that has no snow on the roof because it has melted. If you took a cross section of the top 7 feet of the roof you would have the 4" trusses, whatever insulation is between them, and the sheetrock on one side, the asphalt shingles on the other. When there is snow, it melts right away and then travels down the roof to where its colder and then it freezes up again.

Now I believe most of these problems can be fixed with better insulation and ventilation in the crawl space, I don't think this is the case. A handyman friend of mine suggested that unless I wanted to put in 6"trusses, a fix would be to tear the existing asphalt shingles off, line up firring strips vertically on the roof every 16", add insulation (spray or roll?) between the firring strips, lay down new plywood on top of the firring strips and then shingle. Is this a good idea?
The best insulation option for this is rigid insulation. There are products available with sheathing and venting integrated. By furring as you propose, heat will short circuit through the furring strips. You'll have a bunch of melted stripes on your roof.

http://www.hpanels.com/2009/pages/pd...g_CoolVent.pdf

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Old 09-10-2010, 08:43 PM   #3
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4" Trusses & ice dams; enough space for good insulation?


great info. Thanks!
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Old 09-10-2010, 10:11 PM   #4
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4" Trusses & ice dams; enough space for good insulation?


Before worrying about that, have you already taken care of any stack effect, or uninsulated heat sources (exhaust pipes, furnace ducts, hot water lines). A long Saturday with several cans of blowfoam and caulking may cure 80% of the problem and pay for itself in a MN winter month on heat savings alone. Its the first thing I do when I move.

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Old 09-11-2010, 10:59 PM   #5
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4" Trusses & ice dams; enough space for good insulation?


great suggestion. While I don't think that's the main cause, since the melting is occurring where there is no air in contact with the pipes/stacks you're referring to, its something that I will improve on when i replace the roof. Thanks again!
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Old 09-12-2010, 07:07 AM   #6
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4" Trusses & ice dams; enough space for good insulation?


I think I screwed up when I read your OP. We're talking about melting on the other side of sloped finished ceiling, right? Sealing against stacking won't fix that but it will definitely reduce the amount of water that backs up behind ice dams, and will save you money. You don't need to wait to reroof to do it either, just go plow around - carefully- in the existing insulation (or perhaps not at all if its asbestos).

If you are a long way from reroofing, but are doing any inside remodeling, you could also add insulation on the inside just where the ceiling is sloped. You'd lose a little head room, especially around the perimeter where the sloped part will move down the wall 2-3 inches.

Meanwhile..... how did you neighbors fix the problem? I assume they have similar homes, probably built by the same crew. Do you have open gable vents above the flat part of the ceiling?

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Old 09-15-2010, 09:12 PM   #7
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4" Trusses & ice dams; enough space for good insulation?


While I do have gable vents at the top, i've been told that for the space they are covering, it's probably not providing sufficient ventilation. As far as the neighbors, my house was one of the first in the neighborhood in '49 and it doesn't appear that the others used similar construction methods. At one point in time, there was an addition, extending one end of the house. Because of the way this was built, it really didn't leave sufficient space for proper ventilation in the section that's most concerning. I really can't afford to reduce any headroom on the inside. Thanks
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Old 09-15-2010, 09:24 PM   #8
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4" Trusses & ice dams; enough space for good insulation?


How much insulation is in there? Attics should be at least R40-R60 for best results.

I recently moved around some insulation in my attic and did not get around to putting it back (was WAY too hot in there that day) but I plan to fix it up when I'm done my project in there. Having just 1 bat (R20) is just not enough. I feel it get cooler faster in the house and it's only a small area that I moved the insulation around. If I left it that way for winter pretty sure I'd freeze!

And I can relate with head room issue. My roof is like 10-15 degree slope and is cottage style. Near impossible getting within 5 feet of the corner of the house from up there!
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Old 09-22-2010, 11:28 PM   #9
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4" Trusses & ice dams; enough space for good insulation?


my guess is that its about a r 30 in the crawl space.

I had a contractor come out today to give me an estimate to fix the heat transfer problem along the section of the roof where there is only the 4 inch trusses separating the ceiling from the inside and the shingles. His suggestion was to tear away the ceiling from the inside, install closed cell spray insulation. The best i can tell is that this amount of spray insulation will give an R value anywhere from 24-32. Various internet sites suggest a minimum R value for cathedral ceilings of R-38 for homes in minneapolis.

Is it a mistake to settle for a solution like this? I'm sure it'll be better than what I have now, but not sure if it will fix this part of my problem.
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Old 09-23-2010, 12:38 AM   #10
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4" Trusses & ice dams; enough space for good insulation?


Page 6: http://www.ecohomemagazine.com/energ...ms.aspx?page=5

SPF and thermal drift, lose up to 30% of claimed R-value: http://www.kwikbuildpanels.com/pages/article_facts.html

You need to thermal break the rafters from the living space, as mentioned already by Seeyou: http://www.coloradoenergy.org/procor...f/r-values.htm

http://danperkinsroof.com/1108_JCL_Perkins_A.pdf

Gary

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