I know I don't need a poly vapor barrier when I have polyurethane spray foam in my attic, but is there any benefit to having poly? Then the spray foam would be stuck to the poly not the drywall, so I guess it would be easier to repair drywall. But what do y-all think?

What kind of foam?

Where area do you live in?

Urethane spray foam in Minnesota.

The density/perm rating of the foam counts, as does the construction.


Is this open-cell or closed? (Brand name/vendor of the foam is...?)

On the attic floor or applied to the roof deck?

If attic floor, is the attic vented?

If roof deck, is there a vent channel above the foam, or a secondary vented nailbase for the roofing?

On top of the drywall ceiling, vented attic, closed cell. Don't have other details yet.

With a vented attic the poly buys you nothing from a moisture transfer point of view if you're going more than a couple of inches with any 2-lb polyurethane. With mid density stuff it might buy you a tiny amount, but only if the vent area is ridiculously small.

If it's just a release function to ease gypsum repair you're after almost anything would work (housewrap, kraft paper, etc.), but poly is fine.

FWIW: To go high-R with closed cell foam is a bit pricey- 2" of 2lb goods would be sufficiently vapor retardent & air-sealing to allow you to do the rest with cellulose or high-density fiberglass (but not low density fg, which has performance issues in attic apps.) Even if you're in the R38 code-minimum parts of MN rather than the R49 zone from a $/R point of view going to R60-R75 with foam/fiber stackup would be cheaper & more cost effective (and more thermally effective) than R38 of closed-cell foam only. The extra mechanical rigidity of 2" of closed cell foam would also mitigate sag issues on the gypsum that can occur with high-R cellulose with 24" o.c. joists or trusses. Adding ~15" of cellulose above 2" of ccSPF would total ~R65, (settled-R, higher initially), and if it buried the joists/truss chords it would provide a terrific break of the thermal short-circuit of the timber. You could get the higher R at somewhat reduced weight with JM-Spider or Certainteed Optima blown-in-blanket to 1.8lbs density, but at higher cost than an open-blow of stabilized cellulose (but still under the cost of the other 4.5" of ccSPF that it would take to hit only R38). It's literally like cutting the heat loss to the attic in half, for the same money or less.

Sorry to thread-jack, but do you get the same or functionally similar 'mechanical rigidity' by using open cell foam? I'm mostly interested in the air sealing qualities of the foam and the open cell variety is substantially cheaper. I'm planning on stacking ~15" of cellulose on top of it.

Posted By galnar on 02 Feb 2011 12:39 PM
Sorry to thread-jack, but do you get the same or functionally similar 'mechanical rigidity' by using open cell foam? I'm mostly interested in the air sealing qualities of the foam and the open cell variety is substantially cheaper. I'm planning on stacking ~15" of cellulose on top of it.

Open cell foam has a very LOW compression strength rating, is quite flexible, and provides almost no mechanical strength to the assembly.  But it would be able to support 15" of cellulose.    It is also quite vapor permeable in comparison to closed cell foam, and you'd have to adjust the vapor retardency by other means, and you'd have to use heavier/stronger gypsum to support the cellulose over a 24" span.

A 3" shot of half-pound open cell would cost about the same as 1" of 2lb closed cell,  but would have ~70% more R value (~ R10-11 as opposed to R6-6.5).  But that small difference in R could be made up very cheaply with another 1.5-2" of cellulose.  OTOH, if you need to use a vapor retarder and heavier gypsum anyway, an all-cellulose insulation stackup + air-seal detailing with foam/caulking would cost even less.

Bummer. The local expert (there's only one and he is not an installer) is completely opposed to closed cell foam anywhere in the attic except directly on/around the rim joist. His opinion is that all roofs will leak eventually, and if you're not diligent about checking for water up there it will pool and eventually sag and destroy the sheet rock on the ceiling. I'm not confident in my builder's ability to effectively seal the space using foam and caulk so I planned on spraying the whole thing. I didn't even think about needing a vapor retarder if going with the open cell option.

If your roof leaks, and you have the most common type of homeowners policy, that covers "risks of physical loss, unless excluded" They will pay for the damage. If you are worried about it use magnesium oxide board which is unaffected by water. That is what I am using.

I was asking about poly, because I thought it may help for the temp heat. I fastened XPS to the underside of the roof trusses to hold in the heat in.

I think next time I would like to permanently attach rigid insulation, maybe InSoFast, to the trusses. NUDURA also makes a product for that application, it is EPS.

The most important benefit of polyurethane insulation foam spraying is the flexibility it offers in terms of insulation. While the majority of insulation products must be installed before walls are completed, spray foam can be installed anytime.

Polyurethane foam is a particularly good insulator because of its high R-value. R-value is a measure of how well a material resists thermal penetration, and higher R-values mean higher levels of insulation.