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Old 09-13-2010, 09:34 AM   #1
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insulation under a metal roof


we have built a cabin but it will not be used in the winter, was thinking about just installing 6 mil plastic sheeting on the loft roof rafters and skiping the cost of insulation, was thinking the plastic would work fine ??

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Old 09-13-2010, 12:53 PM   #2
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insulation under a metal roof


No insulation, no vapor barrier! You're pretty certain to have a major condensation problem that way.

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Old 09-13-2010, 02:00 PM   #3
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insulation under a metal roof


Actually the plastic would be a vapor barrier
But it won't hold much heat
There are plenty of times in the Spring/Fall when its cold enough that insulation would help
Are you insulating the walls ?
There won't be any heat source at all in the cabin ?
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Old 09-13-2010, 02:03 PM   #4
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insulation under a metal roof


If no insulation I would just leave the rafters open and paint eveything.
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Old 09-13-2010, 02:51 PM   #5
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insulation under a metal roof


how do you keep the sweating of the metal roofing from the fiberglass insulation?
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Old 09-13-2010, 08:12 PM   #6
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insulation under a metal roof


There needs to be a layer of roofing paper or felt under the tin.
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Old 09-13-2010, 08:16 PM   #7
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insulation under a metal roof


Insulation works in both cold and hot seasons.
Without Insulation, you need to be pretty well shaded by trees in the summer to avoid over-heating the cabin.
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Old 09-14-2010, 06:04 AM   #8
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insulation under a metal roof


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Insulation works in both cold and hot seasons.
Without Insulation, you need to be pretty well shaded by trees in the summer to avoid over-heating the cabin.

so I'm thinking I should staple plastic in between each rafter before installing the insulation? (to keep the rain effect off the fiberglass insulation)
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Old 09-14-2010, 06:29 AM   #9
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insulation under a metal roof


I wouldn't use plastic. Try something that breathes and won't become a vapour barrier.
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Old 09-14-2010, 06:35 AM   #10
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insulation under a metal roof


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I wouldn't use plastic. Try something that breathes and won't become a vapour barrier.
like what? and how do we keep the moisture from the insulation or will the moisture stop when the insulation is installed?
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Old 09-14-2010, 09:25 AM   #11
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insulation under a metal roof


are you insulating or not?

Are you talking a vaulted ceiling over a sleeping loft?

Id do sheeting, paper then the metal at a minimum for somewere you are going to try to sleep in during a rain storm.
I'd leave the rafters open and use a shiny or white metal roof to reflect the heat.

If you want to add insulation later, it will be much easier with open rafters.
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Old 09-14-2010, 11:14 AM   #12
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insulation under a metal roof


Understand the problem: http://www.buildingscience.com/docum...f%20insulation

Research other solutions: http://www.buildingscience.com/searc...f%20insulation

I’ve never done metal roofs, so I won’t answer. Hope this helps...


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Old 09-14-2010, 11:36 AM   #13
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insulation under a metal roof


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are you insulating or not?

Are you talking a vaulted ceiling over a sleeping loft?

Id do sheeting, paper then the metal at a minimum for somewere you are going to try to sleep in during a rain storm.
I'd leave the rafters open and use a shiny or white metal roof to reflect the heat.

If you want to add insulation later, it will be much easier with open rafters.
the roof is already on we are going up this weekend to do the insulation and finish the lost ceiling? and if we just install the insulation with out plastic before it won't the moisture ruin the fiberglass?
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Old 09-14-2010, 12:03 PM   #14
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insulation under a metal roof


there needs to be airflow beween the roof and the insulation. that airflow should maintain near outside temps on the roof side of the insulation to eliminate the condensation.

if you want to just fill it all up with insulation, I think the spay in foam is about the only way to do it without creating a mold problem.
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Old 09-14-2010, 12:24 PM   #15
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insulation under a metal roof


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there needs to be airflow beween the roof and the insulation. that airflow should maintain near outside temps on the roof side of the insulation to eliminate the condensation.

if you want to just fill it all up with insulation, I think the spay in foam is about the only way to do it without creating a mold problem.
so should we just forget the plastic and take our chances with the insulation?
your thinking is the air gap left with prevent moisture?

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