Replies

1. 
   Mongo | Dec 17, 2009 08:35pm | #1

   Closed cell runs about R6.5 per inch. You'll see values thrown around from 6 to 7.

   A lot of people can't afford full thickness foam, so they'll spray a couple of inches and fill the remainder of the bay with FG batts or cellulose.

1. 
   MHolladay | Dec 18, 2009 12:00pm | #3

   Simon,
   Don't be tempted to
   Simon,

   Don't be tempted to skimp on R-value. Obviously, code requirements represent a minimum. The only reason that spray-foam contractors try to talk people into skimping on R-value is because they're afraid that high prices will scare away customers.

2. 
   fletchnj | Dec 18, 2009 08:47am | #2

   I'm in the same situation
   I'm in the same situation you're in, but I'm in NJ. I'm insulating a bedroom with a cathedral ceiling and that's partially over a garage. Here the recommendation was for 2" in the walls and 2 1/2" in the ceiling. I've been trying to find some "authoritative" source that justifies this approach, but so far I haven't had any luck. I have found a few sites (all foam installers) that talk about an "Efficiency Rating" of 92% that supposedly makes 2" of closed cell equal to R40, but I haven't found anything from the DOE or BSC or any other third party source.

   I've used closed cell foam in the past and I'm familiar with all the benefits of a better seal, etc, but I'm unsure how those benefits make what should be R13-14 equal to R40. Most third party sources (DOE, BSC, etc) talk about closed cell foam having an R-value of 6.5-7 and talk about it in the conventional manner, i.e. to get R40 you need roughly 6".

   I'm still looking for someone who has done the 2-3" approach and is satisfied with it. At this point I'm leaning toward going with a thicker install (5 1/2" in my case).

   fletch

1. 
   DickRussell | Dec 19, 2009 11:21am | #4

   When 2" of closed cell is equivalent to "R40," the reference ceiling is one that is only claimed to be R40 but is so poorly done that its actual performance is only R13.

   As Martin Holladay pointed out, some foam contractors claim that a few inches of closed cell foam is all that is needed because the stuff is so expensive relative to other materials that a quote for a proper level of insulation would scare off the customer.

   As to the "92% efficiency rating," that too is a gimmick to convince the customer that a lower total R foam installation is "enough," implying that substantially increasing the thickness won't save enough more heat to make it worth the cost.

   Any statement that a certain amount of insulation has already saved X% of "the heat" must be based on some reference level of heat loss (percent of what?). One can easily assume a starting point consisting of a thin metal skin between indoors and outside, then show that an inch of foam saves 95% of "the heat" and that more than R5 in the wall or ceiling is not justified.

   In your case, you have limited space for insulation, so that closed cell foam may be a good choice, although expensive. You are correct in going for the maximum R value you can get into that space. I hope the roof above is sound, as an unvented roof assembly insulated with a low-perm material such as CC foam has a much lower drying potential.

1. 
   aworkinprogress | Dec 21, 2009 11:33am | #5

   Mr Russell, makes a key point that Must not be underlooked. The existing condition of the roof structure. For with that degree of minimal perm rating can become a disaster in the making. (think Eifs) What about a different approach to uncouple the sheathing for future repair/replacement? Something to consider

1. 
   fletchnj | Dec 21, 2009 02:10pm | #6

   The roof was just replaced 2 months ago and any problem areas in the roof sheathing were replaced. At this point, I'm going to trust the advise here (and my original instincts) to go with a thicker installation of closed cell foam for the underside of the roof.

   fletchnj