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Small Cabin Forum / General Forum / Hot Roof
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paulz
Member
# Posted: 26 Jul 2020 11:10
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So nice and cool at my cabin in the woods. Unfortunately I have to come back to the house in the city every few days for various reasons, mail, laundry, shopping etc.. Short 20 minute drive.

The bedroom, 300sf. bakes under the summer sun, hot nights unless I run the window AC box. Tar and gravel roof over 2" foam board over open beam, that's it, no attic. No shade trees.

I read a bit about roof cooling, the two easiest things are reflective coatings and mist cooling. Might try a sprinkler up there tonight. What about a reflective tarp, like silver maybe? Just trying to live with this place until I can move to the cabin full time, I hope.

sparky30_06
Member
# Posted: 27 Jul 2020 07:17
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sprinkler during the day will help, the water evaporating will cool it down some. If you can stretch a reflective tarp above the roof with a slight pitch to it, the tarp will take the solar load of the sun and the slope of it will allow natural convection to occur allowing cooler air to be sucked in from the bottom and the hot air out the top. I would put it about 4" above the roof.

https://youtu.be/jorrPb6e_i4

paulz
Member
# Posted: 27 Jul 2020 11:02
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Sounds good sparky, and I'm stuck here tonight, it's going to hit 90F so I'll give it a try and report back. I have a big silver tarp..

paulz
Member
# Posted: 27 Jul 2020 11:36
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Can't find the silver tarp. But I've been reading online, seems to be some debate over which color would be best. One says white absorbs the most heat, another says black keeps covered objects coolest.

What's the deal?

sparky30_06
Member
# Posted: 27 Jul 2020 14:33
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i'd say as long as there is a gap to create convection it doesn't matter. you just need something to take that solar load.

90F, i can wait to get back down to that!!! lol

sparky30_06
Member
# Posted: 27 Jul 2020 14:34
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remember you need that airspace

Brettny
Member
# Posted: 27 Jul 2020 14:35
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The problem is that you only have 2in of foam insulation. That's like r13-19 at best.

NorthRick
Member
# Posted: 27 Jul 2020 14:38
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Quoting: paulz
One says white absorbs the most heat, another says black keeps covered objects coolest.


That's funny. Get two pieces of sheet metal, paint one white and the other black and lay them in the sun for a while. Put your hand on them and see which one is hotter.

paulz
Member
# Posted: 27 Jul 2020 14:43
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Thanks guys. Hey when I was searching colors I read that black is a better color to wear in summer! News to me. They say white reflects your body heat back at you, which is worse than the sun. I have always worn a white T shirt but I'm going to give it a try.

https://io9.gizmodo.com/the-physics-that-explain-why-you-should-wear-black-this-59039 56#:~:text=When%20all%20that%20body%20heat,in%20from%20the%20sun%2C%20sure.

sparky30_06
Member
# Posted: 29 Jul 2020 08:39
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Paul, did you put a tarp up?? My electric bill have been bad this year and my living room has been stupid hot. I have some large floor to ceiling windows that face west and after trimming some trees they get full sun all afternoon. For a quick fix I just stuck some foil backed foam insulation in the windows to keep the sun out and it has dropped my electric bill by 16% and my ac doesn't run as much. Guess I'll be replacing those windows soon and adding either ceramic tint or solar shades to them

NorthRick
Member
# Posted: 29 Jul 2020 13:18
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Quoting: paulz
Thanks guys. Hey when I was searching colors I read that black is a better color to wear in summer! News to me. They say white reflects your body heat back at you, which is worse than the sun. I have always worn a white T shirt but I'm going to give it a try.

https://io9.gizmodo.com/the-physics-that-explain-why-you-should-wear-black-this-59039 56#:~:text=When%20all%20that%20body%20heat,in%20from%20the%20sun%2C%20sure.


Read the first response under that article. He does a pretty good job of explaining why the premise of the article (black clothes keep you cooler) is wrong. The article is also talking about the human body and not a house.

paulz
Member
# Posted: 29 Jul 2020 17:17
Reply 


Sparky, haven't put the tarp up yet. I knew I had a silver tarp at the cabin (a failed drive in movie screen, it was terrible), so I'm bringing that back tomorrow.

Rick, that's a very scientific explanation. I have been conducting my own slightly less scientific approach: been wearing a black t shirt last couple days. But the heat wave has subsided. I haven't felt any worse at least.

DaveBell
Member
# Posted: 31 Jul 2020 08:37
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Quoting: paulz
Tar and gravel roof over 2" foam board over open beam


That is R-13. Minimum for a wall. Roof should be around R-40. I have 12 inches in my attic.

Quoting: paulz
it's going to hit 90F

Band-aids will not help with ambient air temperature.

Take the roof off and build it back correctly.

paulz
Member
# Posted: 31 Jul 2020 21:39
Reply 


Sounds like a good project for the next owner. I'm moving to the cabin.

So today I brought home a silver tarp and put it over the bedroom roof. Didn't get home until afternoon though and threw it up there quickly, but the breeze has been doing a good job of keeping it off the roof surface. Now, 6:30pm, I measured the ceiling temp inside with the digital temp gun, 84 above the tarped area, 87-89 untarped area, with the sun still hitting.

I need to do a better tarp job and give it a full day, will report back.

sparky30_06
Member
# Posted: 1 Aug 2020 07:53
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how much space did you put between the tarp and roof???

paulz
Member
# Posted: 1 Aug 2020 09:19 - Edited by: paulz
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It was like a tent, draped over the chimney at one end, 3 ft or so, and a crate at the other. Like I said I was beat yesterday from 3 days at the cabin (working on the roof of coincidentally). I'll do a better job this morning, what would you suggest? I have some 20' sticks I could make a frame...

sparky30_06
Member
# Posted: 3 Aug 2020 07:10
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The ones i have done in my blinds have been only a few inches. If it was me I'd do 4" to 6" but if it's easier to do a few feet try that. The idea is to make shade, let the tarp take the solar load and convection to move the heated air away

paulz
Member
# Posted: 3 Aug 2020 09:47
Reply 


Thanks Sparky. I left it the way it was until last night and measured a few more times, definitely 3-5 degrees F cooler ceiling temp in the area under the tarp. It was an old tarp and the wind finally ripped it in half so I'll have get another one if I want to pursue.

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