Down here we use open cell above grade and closed cell below. The building department has finally relented and no "chutes" or other nonsense "ventillation" is required now. We spray the bottom of the roofing deck.

i used cellulose on my last house. never again. its a year old and the dust is still bad. we have tried everything to stop it but every few days the furnature is coated.

Building Science Corp.'s document "Unvented Roof assemblies for All climates" says ccf is suitable for all climates.

johnstaci said: Close Cell Polyurethane Foam:

-Does not allow moisture to flow (potential moisture issues)

CCF is semi-permeable and, according to BSC, "acts as a throttle to control outward vapor diffusion during cold weather and inward vapor diffusion during warm weather." It is also "excellent as a secondary rain water barrier."

There's no reason to vent the space you're insulating.

The oc foam doesn't provide the R49 recommended for your climate by the Department of Energy. As well, oc foam's much greater permeability means it provides no secondary rain water barrier.

Building Science shows cellulose coupled with sheets of 1" XPS to the interior to increase R value.

"Down here we use open cell above grade and closed cell below."

That is what we were told also. Worthy, I am curious about your opinion on that. Specifically, we were told that we did not want closed well in our attic roof because if we got a roof leak we would not know it and would end up with a rotted roof deck. Whereas with open cell the water would leak through, damaging the drywall but letting us know we had a roof leak. We have 12" rafters, so more space than the OP has to work with.

(OP, sorry to hijack your thread, but I think the answer might be relevent to you as well)

Yes, I have heard the same thing about closed cell. Another potential issue is the humidity problem in the summer if the house has a nearly impermeable cap on it. I think I am more concerned about a humid/stuffy house than having lower R value.

I have heard the wet cellulose (with adhesive) is a good way to get the insulation to stick into tight spaces, yet allow the house to breath, and at a reasonable price. It just lacks the R value, but I am going to have to give in on something..

Am interested in more info on wet blown cellulose if anyone is familiar w/it.

John

Down here we use open cell above grade and closed cell below

Building Science articles do mention both cc and oc being used in attics, but they explicitly recommend cc. See "Unvented Roofs for All Climates" and "Understanding Attic Ventilation" (See Link below.)

As I mentioned above, cc is excellent in attics as it acts as a secondary liquid water barrier. At the same time, it is permeable, allowing vapour to move both ways depending on the season. As for "hiding" leaks, I have seen commentary from foam appliers who handle all kinds of foam say that in case of leaks cc focuses the water, so it's easier to spot the source. Because oc foam is more permeable and holds water, the R value is degraded, which means the sheathing is more easily reaching dewpoint, which is not desirable.

The same logic applies below grade. The one time I used oc foam, it was applied in cold weather and shrunk back considerably around the rim joists; this is recognized in the literature but brushed off as a minor quibble. When I've used cc foam in basements, rim joists, garage ceilings and attics (but not under the decking) I had none of those problems.

Clint Eastwood's Dirty Harry had a famous comment about opinions--that I won't use here. That's why I try to reference mine to Building Science, whose principals are among the leading building scientists around; and who are also the source of much info at the US Energy Department --Energy Efficiency and Renewable Energy--Building Technologies Department.

Here is a link that might be useful: Vented and Unvented Roof Recommendations