Impermeable Insulation Vs Roof Leak? - Insulation - DIY Chatroom Home Improvement Forum
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Old 02-09-2019, 12:04 PM   #1
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impermeable insulation vs roof leak?


Hello!

I'm redoing a bathroom ceiling in a dormer. It's sloped and the other side of this ceiling is the roof.

I'm redoing it because of a leak which collapsed the ceiling.

The rafters are not very deep, about 3.5".

I'm wondering if I should NOT insulate this ceiling with an impermeable insulation because then I won't be able to tell if there's a (rainwater) leak in the roof until I have really serious rot. What I mean is, if i use batt insulation, then a leak with make itself known to me, inside the bathroom (ie, below the ceiling). If I use a rigid foam board, won't rainwater leaking through the roof slide away into other areas of the house unbeknownst to me until major collapse happens? How do people deal with this potential problem?

In the extreme scenario, some people use a closed-cell spray foam, which is very impermeable, to seal the underside of their roof--but again, how do you know if you have a roof leak then?

For what it's worth, I'm in Vancouver BC which I understand to be Zone 4 climate.

Thanks!
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Old 02-09-2019, 12:58 PM   #2
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Re: impermeable insulation vs roof leak?


Welcome to the forum pmp. One of our contributors is from your area so he should have some specific advice.

In general you are dealing with both rain and condensation. IMO, leaking from above should be a very low probability. But, condensation without ventilation is a high probability. let's see what they recommend.

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Old 02-09-2019, 01:27 PM   #3
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Re: impermeable insulation vs roof leak?


What was the nature of the leak previously? If we worried about leaks being able to percolate through readily, we would never use closed cell foam.
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Old 02-09-2019, 02:26 PM   #4
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Re: impermeable insulation vs roof leak?


Thanks Bud.

And Windows on Wash, you asked:
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Originally Posted by Windows on Wash View Post
What was the nature of the leak previously? If we worried about leaks being able to percolate through readily, we would never use closed cell foam.
The leak previously was nothing special, just an old roof. I had a new roof put on (shingles with waterproof sheathing underneath, and they replaced any damaged roof deck wood).

So I don't expect a leak anytime soon, but my asphalt shingles are expected to degrade over time, so in the long run, how will I know when I have a leak problem?
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Old 02-09-2019, 03:22 PM   #5
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Re: impermeable insulation vs roof leak?


impermeable insulation performs best with roofs - you don't need ventilation channels/baffles and vents because moisture from inside can never get to the cold surface.

if you use batts and vapour barrier, you have to have ventilation and the vapour barrier can trap water from leaks, rot out the assembly without you knowing until the insulation is saturated and it starts dripping.

leaks are bad regardless of how you insulate.

impermeable is worse when it comes to detecting the leak.
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Old 02-09-2019, 03:28 PM   #6
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Re: impermeable insulation vs roof leak?


If it leaks again, you'll know it no matter what insulation is installed.
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Old 02-10-2019, 10:23 AM   #7
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Re: impermeable insulation vs roof leak?


Thanks user_12345a and Guap0_

Guap0_ you wrote:
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If it leaks again, you'll know it no matter what insulation is installed.
Could you elaborate? My worry is that the impermeable insulation will carry the water away from obvious areas, and into the inside of walls of the house. If it's impermeable--water and air tight--how will I ever see the leak?

Thanks!
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Old 02-10-2019, 11:29 AM   #8
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Re: impermeable insulation vs roof leak?


For bat insulation you would like 2x10 rafter and 1 1/2" continues venting from the soffit to the peak so I think foam is the way to go.


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Old 02-10-2019, 01:54 PM   #9
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Re: impermeable insulation vs roof leak?


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My worry is that the impermeable insulation will carry the water away from obvious areas, and into the inside of walls of the house. If it's impermeable--water and air tight--how will I ever see the leak?
It might be called impermeable but water has a funny way of going where it wants to go. It will appear somewhere.
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