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jludman
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# Posted: 28 Feb 2012 11:12
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A couple years ago I inherited 3 seasonal (summer only) cabins in central NH, USA. The 20x20 one was nicknamed "The hot box" by the handyman, and is frequently 20-30 degrees above the outdoor temperature. The roof is shingled, flat, slightly sloping down to the back. So, it is just shingles, thin wood (held up by joists 24" apart), then indoors.
The walls are just cheap fake wood paneling. Some airgaps to the outside (probably keeps it from being even worse).
Is there a good low cost option for keeping this cabin cool? It's my understanding that the roof situation is the bulk of the problem.
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TomChum
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# Posted: 28 Feb 2012 11:36 - Edited by: TomChum
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I doubt that insulation inside will help much. After the heat soaks thru the roof, it's already inside the building. You need to prevent the heat from entering the building enclosure. Vent the roof somehow that the heat stays outside.
Maybe you can panel the rafters, and liberally vent the joist-space for airflow. If the roof is just about flat there won't be much airflow though. Maybe build a peaked and well-vented roof above, that puts the existing roof in the shade.
The best advice I have at the moment is to add some pictures to your thread.
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MtnDon
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# Posted: 28 Feb 2012 12:51
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Quoting: jludman ....good low cost option for keeping this cabin cool?
define low cost as a ballpark $$ figure...
Insulating the ceiling/roof without insulating the walls will not help a lot.
Is the goal to try and keep the interior cool on a typical summers day?
What size are the rafters?
Is the roof basically at ceiling height; 8 foot or so??
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jludman
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# Posted: 28 Feb 2012 14:22
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Maybe "economic" is a better term than "low cost". Last Fall we installed a 15k PTAC unit for heat and A/C, previously there was no A/C. If I was relatively confident that the cost could be recouped in 4 years by reducing the cooling load, I could go as high as $1,000. More, if it made the cabin nicer. Looking up at the actual roof is a little bit too "rustic" :P.
The joists are 2x6s I believe. The cabin is about 10' in the front, and maybe 7' in the back.
The main goal is to keep the place passively cooler during the summer. I use the place a couple weeks for personal use in the Spring and Fall, but it is generally rented during the hottest times.
I am pretty sure that the roof is the main culprit. It must be direct solar gain to get the place up a lot higher than the outside temperature. Most of the sunlight hits the black shingle roof.
The best solution I came up with was to attach some reflective barrier directly to the joists, then I guess add lots of venting. I did find 1 reflective barrier that had white facing on the other, but it was only available online. I think I'd have to cover up something like Reflectix or it would look really ridiculous.
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MtnDon
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# Posted: 28 Feb 2012 16:52 - Edited by: MtnDon
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2x6? 20 foot span, one side to the other, no posts in the middle?
Seriously undersized rafters!
A 20 foot clear span, flat roof (meaning no gable end... rafters just go from one wall to the other) should be using 2x12 in most commonly used species, #2 grade and that is using only 20 psf snow load. I don't know what your snow loads are; much of NH is based on local figures as local conditions can vary widely in the distance of a few miles. My bet is the snow load average is more than 20 psf though.
I would not spend any money on insulation until the roof is corrected. It should really be rebuilt into a gable style.
That slope, from 10 feet down to 7 feet translates into a pitch of less than 2/12 and it is way too flat for any place with any snow; also too flat for best shingle performance.. Shingles do best from 4/12 and up (12/12 is a 45 degree angle)
You could use those 2x6 for ceiling joists and buy new stock for rafters. You could even leave the ceiling sloped like that and build an uneven gable roof; one side steeper or shorter thasn the other. Depending on the snow load 2x6 rafters for a 6/12 or steeper pitch might work, although 60 psf would need 2x8. Then you could put a real good amount of insulation in the attic; enough to make a difference.
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jludman
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# Posted: 28 Feb 2012 17:44
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My measurements were estimates. It's possible the joists are 2x8, and there is an interior wall to separate the bedroom, so presumably that provides some structural support. Maybe it is closer to 12' at the front of the cabin. The cabin has managed to stay standing since it was built in the '40s somehow, though its sister was demod 5 years ago.
I can't really afford to put a preventive upgrade roof on it, but it's good to know that some March I may come up to a collapsed cabin. I will avoid upgrading something I need to amortize over too many years, until it gets a new roof (maybe the next time it starts leaking).
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Martian
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# Posted: 1 Mar 2012 07:15
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Is $1000 more than you can afford to spend? That is the approximate amount it would cost you, in materials, to put on a new metal roof. If you laid 2X4's spaced at 24" across the rafters, filled the gap between with 1 1/2" stryrofoam, and cover that with reflective barrier, and then put light colored metal roofing on, you would greatly reduce the solar gain.
Tom
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