I’m trying to decide which to use in a loft ceiling.
The rest of the house will be cellulose, but this area
only has 2×8 rafters for a cathedral ceiling.
Then it occurred to me that they will only put about
3.5″ foam.
So that’s 3.5″ x R7 per inch= R24.5
7.25″x R3.5 per inch= R25.3
Am I figuring this right?
And are the air sealing differences so great that the foam
is still preferred?
Replies
Why will they only put in 3.5 inches? I have 12 inches of foam on my ceiling.. Now granted I live in the arctic regions of Minnesota. We're not quite on the edge of the world from here But I've been assured that you can see it from here..
To answer your question partially, yes foam is a superior seal to any other form of insulation. and foam doesn't have the same issues of compaction and deterioration from moisture that other insulations do..
However I've seen foam sprayed to a whole variety of depths. and I'm firmly of the opinion that if sum is good, more must be better, and too much is just starting to get fun!
Your R7 is for closed cell foam, which is way more expensive than open cell, which is only about R3.5. But the seal is absolutely the best with foam. And you don't need as much foam...the R factor isn't as important, because foam seals everything so well. Just yesterday I had a 2x6 cathedral ceiling sprayed with open cell....only 5 1/2 inches thick (shaved off after being sprayed). The R factor is only about 18 or 19. Closed would have been about 35 or more, but would have cost about $1500 more. Last night we didn't even turn the furnace on, and yet we noticed a remarkable difference in our bedroom's temperature. There was a great article in Fine Homebuilding a few months ago, all about foam. Read it. There is a law about 'diminishing returns' on foam. A lot isn't much better than a smaller thickness. Foam is the future!
"There is a law about 'diminishing returns' on foam. A lot isn't much better than a smaller thickness. "The foam will indeed be better under the roof than a FG batt, but the conductive heat loss will vary with 1/R (=U). Double the R, half the heat loss. Half of how big a heat loss is always the question. Henley is in NY, not Kansas City. Doing much better in the roof than 25 (less bridging effect of the rafters) certainly is worth it in his climate. I don't think R25 for a roof even meets code there.
No 25 isn't code here at all.
My confusion is the 3.5" of closed cell that is often
done. Why is this an excepted practice? I understand the air sealing advantages of foam, but at some
depth dense packed cellulose must effectively seal it also. so, at what depth does the scale of rewards vs cost tip?
I tend to think a 2x8 isn't deep enough, yet it's hard
to quantify (at least for me).
I don't know your location, so I've no idea of what your heat gain or loss shall be but, I know that the newly adopted building code calls for a heck of a lot more than you are talking about, it varies depending upon your region I believe. And if you want to get a 30% tax credit to reduce you gross $ outlay, You must meet or exceed the new building code whether your municipality has adopted it or not.
When I say that it's required, I mean that the new federal energy conservation guidelines have specified that the IRS only recognize those installations that meets or exceed the minimun recommended levels. And in my opinion, the IRS has a lot more leverage than your local bi. Drat! How do I eliminate this underlining?
I would go for the full depth in closed-cell foam. That would give you about r-45.
Edited 10/15/2009 3:43 pm ET by CLupis
His location is just outside of Cooperstown NY!
I'm in Central NY, as in it gets cold here.
The tax credit is a good point.
For this one area, the difference in foam cost would just
about eat those savings for the entire house though.
Hey henley, why not put 2" of board foam up and blow the are solid with cellulose?
I am extreemly leery of site made foam. I have some still in my house from the 70's.
Terrible job!
Head room is in high demand for this space.
I don't think I'll drop it anymore no matter what. Yeah, I agree about foam installs. When it's absolutely
necessary foam is a god send, other then that I'd rather
go another direction. It's hard to get a real world handle on the performance
differences between cellulose and foam. They both seal the air and I've never lived in a truly
well insulated house anyway! So, in this case the R-values would be about the same
(substandard supposedly) but the foam is fine according
to the inspectors?
Hard to believe cellulose isn't sealed at 7", while it's
equally implausible that foam doesn't need R38 to perform
well also.
Thought I'd follow this thing up with my conclusion. Talked to another foam guy (the third one) and
asking about the three inches commonly used, He said
that he doesn't do it that way and often looses bids because of it.
He has asked the codes officer about it and was told it
depends on the inspector, but he would not OK it personally. Sounds like another case of Upstate NY half #### building practices. So, I'm going to spray 3" closed cell foam and dense pack
the rest of the bay with cellulose.