# preventing condensation on metal roof



## Sparticle (Nov 1, 2004)

Hi folks. We're living in an RV this summer and building a roof over it to help us keep from overheating. Wanting to insulate quickly with the thin blue board stuff. Wondering where to actually put it. Directly underneath the metal and on top of the 1x4's? Or underneath the 1x4's that sit on the 2x6's? Thanks for the help.


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## MO_cows (Aug 14, 2010)

Sparticle said:


> Hi folks. We're living in an RV this summer and building a roof over it to help us keep from overheating. Wanting to insulate quickly with the thin blue board stuff. Wondering where to actually put it. Directly underneath the metal and on top of the 1x4's? Or underneath the 1x4's that sit on the 2x6's? Thanks for the help.


I would put it right under the metal. If you leave a gap between the steel and the insulation, into that gap is where the condensation will drip. So your 1 X 4 "runners" would be getting wet every time conditions are right for this condensation. Also, putting the insulation directly under the metal will reduce the noise every time it rains.


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## texican (Oct 4, 2003)

I don't know if insulation would actually do you any good or not. I'd consider doing a 'test' before you go and actually commit to the whole project. Set a piece of the metal roof out at the same angle as it'd be over the rv, and test the temp right below it.... at the same distance it'd be over the rv.

I'd think the shade would be more important. The higher the roof is over the rv, the less 'absorbed heat from the tin' you'd feel on the rv roof.

High metal barn roofs beat the dickens out of low roofs... I thought I was going to die last summer, hauling hay, and moving it through a low roofed (8') goat loafing shed area. Was a good 30 degrees hotter than ambient air temp.... the heat absorbed the heat, and radiated it down. My big barn (18' tall roof) doesn't have that problem... the radiated heat stays within a foot or so of the metal roof, and down a few feet below, it's actually slightly cooler than ambient.

Trees are your friend, if you have any!


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## tiogacounty (Oct 27, 2005)

texican said:


> I don't know if insulation would actually do you any good or not. I'd consider doing a 'test' before you go and actually commit to the whole project. Set a piece of the metal roof out at the same angle as it'd be over the rv, and test the temp right below it.... at the same distance it'd be over the rv.
> 
> I'd think the shade would be more important. The higher the roof is over the rv, the less 'absorbed heat from the tin' you'd feel on the rv roof.
> 
> ...


Believe it or not, a layer of 1" blue styrofoam, or any brand of extruded polystyrene, will have a dramatic effect on temperature when installed directly under the metal roof. About a dozen years ago, I worked on a giant project in the Phillipines. We basically built a new village of 260+ low cost homes. The architect called for a layer of smooth tin on the rafter (for insect control and light reflection) then an inch of blue styro, topped of with the corrugated galvanized metal roof. The locals thought it was simply crazy. The architect had run some tests on two roofs, side by side, with and without foam. The temps. under the foam roof were much cooler, 30-50* lower in the 100* summer sun. 

Texican is correct, a high roof, and shade are both highly desirable. Obviously a white or silver roof is also a big plus. Don't forget about heat gain on the walls also. A white tarp. like a painter's drop cloth, strung tightly like a wall, a few inches off the south side of the trailer would really help also. Good luck. BTW, it's 46* here this morning :happy0035:


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## MELOC (Sep 26, 2005)

i think a similar thread discussing this suggested that one should use a spray foam insulation directly on the metal. doing so leaves no gap in which condensation can accumulate.


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## Sparticle (Nov 1, 2004)

All good advice. Thanks one and all!


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## jwal10 (Jun 5, 2010)

A lot cheaper to leave a good space for air movement between the 2 roofs. No need for insulation that way. Good ridge venting keeps the heat from building up in between the 2. Built as a pole barn so when I moved the "trailer" it became equipment storage....James


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## Ray (Dec 5, 2002)

Had a new steel roof put on a while back. The co. that put it on over shingles, said it had to have the 1x4 boards to put the steel on or the condensation could not evaporate, and would cause the roof the rot. I did some research and found the same answer. I guess its up to you whether you go with that or not, ray


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## SteveO (Apr 14, 2009)

YOu might look at a leanto roof (no peak) you get a continous air flow as heat rises.


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## Guest (Jun 10, 2011)

I owned a home that had a shingle roof that needed replaced . I decided to install a painted steel roof over the shingles . After consulting with various builders & roofers I had about a 50/50 mix of being told to put strips on first & being told it wasn't necessary . I contacted the manufacturer of the steel roofing & was told to cover the shingles with felt & install the steel roof directly over that . That's what I did . I owned the home for several years after that & had no known problems . I sold it last year .


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