I finally have all of the bids, and I think I'll be able to afford an unvented attic. However, the foam costs vary drastically. I received a quote for the Icynene MD-R-200, which was drastically lower than the corbond. My concern is that it technically is not a vapor barrier, 1.3 perms at 3". Am I understanding that right? I live in Cold Climate (Milwaukee), and the roof is a 10/12 pitch, conventionally roofed. I was going to do 2-3" of foam, then finish with batt or dense pack cellulose. I'd like to use the Icynene, as it is nearly $4K cheaper, but I'm worried that isn't going to provide enough moisture resistance. Am I worrying about nothing? Also, if I did use a flash/batt with the Icynene in the walls, would I need a vapor barrier? Any help would be greatly appreciated. -Kyle

You're worrying about less than nothing:

If you look at the RR-1001 simulation results on P11 you'll see that 1" of ccSPF is sufficient for Chicago's climate with an asphalt shingle roof.  At 1" a "brand-x" polyurethane foam is ~2-2.5  perms (and only ~ R6), so methinks you'd be good to go with ~1.3 perms (& ~ R15) in Milwaukee's only slightly colder climate. At 2" thickness ccSPF is ~1-1.25 perms, only slightly less vapor permeable than nominal for 3" of 2lb Icynene- close enough to be considered equivalent.  Corbond is considerably less permeable than most SPF (0.6perms @ 1.5" thickness), but it's still the same order of magnitude.

With flash & batt walls you'll DEFINITELY need an interior side vapor retarder, but vapor-retardent latex and air-tight wallboard technique would be good enough.

But on both the walls & attic you'll do a lot better with wet-spray cellulose (even at standard density, not dense-pack) than any batt solution, and worth paying a premium over batts.  Spray cellulose (but not dry blown) by itself will also qualify as the ignition barrier for the Icynene at 3" or more.  With 3" of Icynene in the rafters the additional benefit of dense-packing the rest is small.

NO foam is a vapor barrier (<<0.1 perms) and should be considered a vapor retarder.  A 0.05perms poly sheeting at 0.006" passes substantially less water than a full cavity-fill of Corbond.  Vapor barriers are two edged swords- too much of a good thing:  They lock moisture out, but they also lock moisture in.  Since most of the year the OUTDOOR temperature in Milwaukee is above the dew-point of conditioned space air, it's good to design structures that can dry in both directions when temperatures allow, but pass moisture slowly enough that you don't get seasonal accumulations of moisture in the structural wood that are high enough to create rot conditions.  Closed cell foams are a useful tool, in that they do pass some water vapor, but slowly enough that it limits acute accumulation, but rapidly enough that the assembly dries.  Anytime it's more than 37-40F outdoors in your location, the assembly can dry toward the exterior, which is most of the year in Milwaukee, but not much of the winter, which is why it's useful to have an interior-side vapor retarder to limit  moisture flow as vapor during those winter months.  A 1-2 perms takes many weeks to push much water through, in EITHER direction.  A 0.1 perm it takes forever, and moisture can be trapped.

Dana,
I just noticed the bid was for 2" of Icynene, not 3". Should I have him rebid with 3"? Or does it not matter to much?

Posted By beckkl on 25 Jun 2010 10:33 AM
Dana,
I just noticed the bid was for 2" of Icynene, not 3". Should I have him rebid with 3"? Or does it not matter to much?

If the spec is for 1.3perms @ 3", it means it'll run ~2 perms @ 2", which is similar to an inch of generic closed cell SPF. 

If you're concerned you can buy some margin by going cellulose-only for the rest of the rafter bay fill to provide some hygric buffering, then putting up air-tight gypsum with 2-coats of standard latex on the interior, which will run ~2-3 perms.   Then you would have less ~ 1 perm total between the conditioned attic & the roof deck, while retaining plenty of inward-drying capacity to allow the moisture stored up in the cellulose over the winter to dry thoroughly over the spring/summer/fall.  Fiberglass or rock wool can't do this for you, and more moisture would end up in the rafters & roof deck.

Would it matter if the cellulose is dense packed? Oddly enough, the dense pack guy was a few hundred cheaper than the non dense-packed.

Posted By beckkl on 25 Jun 2010 12:28 PM
Would it matter if the cellulose is dense packed? Oddly enough, the dense pack guy was a few hundred cheaper than the non dense-packed.

Denser is better all the way around- if it's cheaper too, go for it!

At higher density the amount of air-transport of moisture due to convection currents within the cellulose layer to the foam becomes an order of magnitude lower, reducing the condensation volumes even further.  You also get more thermal-mass out of it, which gives it a performance edge over other insulation types at the same R-value (or cellulose at 2-2.5lbs density, but the same R-value.)  The thermal mass is a benefit primarily during the shoulder-seasons and to some extent during the summer, not the dead of winter, but it's real.