Is Vinyl Backed Roll Insulation Worthwhile or Trouble?

[CA_Tallguy]

Hey guys, I'm struggling to decide if I want to install wide rolls of that vinyl backed insulation for metal buildings on my ROOF (not on the walls). I'm in East Texas so we have some heat and humidity. Here's the stuff I'm talking about...

http://metalmarts.com/insulation.php

Again, this is for the ROOF and I will also have 4 inches of EPS rigid foam insulation on top of it, plus venting above that before installing an R-panel metal roof. The R-panel is "self venting" according to specs due to the shape but I'm also gong to have furring strips on top of the rigid foam to provide additional vented airspace.

I'm concerned about the vinyl backing on the roll insulation that will face the inside of my shop. It seems like it is pretty impermeable. The rigid foam is fairly impermeable as well, especially 4 inches of it.

There was some disclaimer on one site that I found selling that vinyl backed stuff saying that it can't be used against wood sheathing, and I'm assuming that's due to the impermeability of the vinyl facing as the wood could rot.

Am I looking at trouble by trying to use that stuff? Or will the 4 inches of rigid foam above it help ensure that I don't hit the dew point in the fiberglass or on the vinyl? I was thinking it could augment the rigid foam sheathing and do like the idea of having that vinyl facing towards the inside of the shop ceiling as a "finish" layer. But I can't figure out if it will cause a problem. And if so, what else I should consider.

My building is primarily metal but the contractor wants to install furring towards the inside of the shop so that he can have a wood to wood connection from the furring above the rigid foam down to the inside of the shop. That would be alongside the metal purlings.

I'd greatly appreciate any advice you can offer!

 

[toyoguy81]

I used the same thing for my pole barn, did it myself with my fiance', alot of hardwork but was well worth it, R19 6inch thick vinyl faced. i used 1x3 across the span of trussed on bottom side of top chord and used roofing nails with plastic washers to attach to bottom side of purlins, it was a budget project but it turned out good and definite difference already, it was 39F this morning and 58F inside shop when I opened up to start working on a truck, here are some pics.

 

[Fyrme]

Yes it is. I installed the 3" thick vinyl back on my shop roof. You wouldn't think it would make that much of a difference with such a low R value, but it is the reflective surface of the vinyl that does all the work. I think the fiberglass is more for noise and separation. While I was installing the roof panels, I could only work up there until about 2p, (in July) because the roof surface was well over 120˚ and probably hotter. It was so hot that it burnt my hands through my leather gloves if I left it in one place for too long. However I could take an ungloved hand and put it on the vinyl underneath and it was only slightly warm to the touch. Can't get much more proof than that. I'm sold

 

[nine4gmc]

My old shop had 3" vinyl backed and the difference was about 10-15 degrees between the bare metal roof and the vinyl.

 

[CA_Tallguy]

Thanks guys. So what about venting that stuff? If the vinyl backing is basically a vapor/moisture barrier then I'm freaked out about any condensation not being able to dry out if it is pressed tight and sealed into squares where the metal panels hit the purlins and stuff.

Reading some specs, I see companies are saying plain vinyl backing is 1.0 perms, so that would breathe a bit.... which could be good or bad, I don't know. Then most other types of backings are saying 0.02 perms, so that *IS* a moisture/vapor barrier.

I have no idea if I want high or low perms in the backing, and furthermore, if I want to try to allow some venting at the edges so air can circulate behind the fiberglass part.