TPO and vapor barriers

[randynavarro]

Hey guys. Personal dilemma trying to figure the best scenario for re-roofing my house.

My 60's rambler roof structure is made of 4x8 beams at 6' o.c. The roof deck is only 2x6 T&G boards across the beams. That's it. Inside, all you see are the open beams and T&G decking. Just 1 1/2" of wood between us and the sky! No insulation. Slope is 2:12.

I will be installing 2.5" of poly-iso board on top and re-roofing with TPO.

Question: would you recommend a vapor barrier on top of the T&G before the insulation? No vapor barrier? I can't find any information from the manufacturer (Carlisle); at least on their website.

Our marine climate warrants vapor barriers on the inside but of course there's no where to put in on the open vault.

What do you think?

[Chad Fabry]

The poly iso is a vapor barrier. Carefully detail the seams in the insulation to avoid having warm interior air contacting the TPO or you will have issues when the condensate seeps back through the decking.

I'd leave at least a 1/4 inch gap between the panels then fill the gap with spray foam.

[kurt]

That sounds about right.

What's the lowest average temperature in winter out there?

[randynavarro]

Thanks, Chad.

Wondering if laying a sheet of 6 mil or even buying extra thin-mil TPO (it's surprisingly affordable) directly on the deck would be a better option.

I bought the foam already-used from Insulation Depot. It's in very good condition; however, there are the occasional dings and nicks out of the corners and edges. Really big holes that could take a lot of foam.

Mainly just inquiring if vapor barrier is critical. I don't want to rely on the poly-iso only.

Cooling degree days average 35-45 deg. Not extreme.

[kurt]

A decent layer of foam, sealed, and it would be fine imho.

[Tom Raymond]

Skip the spray foam and install at least 3 layers of EPS in alternating directions with the seams staggered. It will solve the condensation issue and prevent ghosting patterns in the dew on the roof. Plus it's lots less messy.

[kurt]

Even better.

Why not XPS? Or, am I mixing up my acronyms again...(?)...

[randynavarro]

Skip the spray foam and install at least 3 layers of EPS in alternating directions with the seams staggered. It will solve the condensation issue and prevent ghosting patterns in the dew on the roof. Plus it's lots less messy.

That's good but I've already got the 2.5" board. One layer. . . that's all I get.

Nothing's ever as ideal in the real world as on paper, mostly because of $$. That helps me a lot when discussing options for repairs with clients.

[Marc]

EPS - Expanded Polystyrene

XPS - Extruded Polystyrene. Has greater insulation value than EPS and is stiffer.

Marc

[Tom Raymond]

Ah yes. My turn to get busted.

Extruded. Always extruded. Expanded is for coffee cups.

[Liz Morgan]

Hi Randy,

I live in Portland, OR and have as similar roofing scenario as you. Now that you're nearly 9 years out, what did you land on and how did it go??

 

Thanks!

Liz