Insulating cathedral ceiling

[the_tool_man]

Hi all:

I'm finally finishing that bonus room project, and am investigating insulation options for the cathedral ceiling. The roof structure is standard 2x6 rafters on 16" centers with OSB sheathing and shingles above. I need to achieve R30, and would like to do so without loosing much headroom. From what I have learned, this will almost certainly require a foam insulation of some kind. I have investigated three options:

1: A local installer of Icynene foam (trade name for a polyurethane foam). He has a minimum $2000 charge, and wants $2500 to do the whole room (ceiling, walls, floor, stairwell). He claims an installed R- value of 20, but claims that because it is foamed in place, the air permiability is virtually nonexistant, so the "effective R value" is much higher.

2: Spray foam kits can be ordered, but are very expensive. Tiger Foam sells a 600 bdft kit for $630, but I would need just over 1300 bdft of foam just for the rafters (1 bdft = 1 sq ft x 1 in thick). Again, almost $2000. This does not include walls, floor, or stairwell, and I would have to do the work.

3: Use Dow Thermax precast polyisocyanruate foam, available locally in 2" thickness. Enough to do my ceiling would be just shy of $1000, and would require a 3/4" layer of foam on the underside of the rafters to achieve R30. This would require a lot of cutting and fitting of 2 layers of foam between the rafters and a third below. The walls, floor and stairwell would be insulated conventionally at a cost of about $400 in materials. Thermax is available in 4" thickness, but not locally, and I would have to order a truckload to get any, so 2 layers of 2" will have to do if I go this route.

Option 2 is definately out. Right now, I am leaning toward option 3, as it is the least expensive (yes, I am ignoring the value of my time to do the install). I am wondering what else is out there. I find it hard to believe that with all the green building movement going on, there aren't more options for acheiving high R-per-inch. What other options are out there?

Thanks in advance, John.

[ransley]

1 The guy says R 20 for 5.5" thats under R 4" not very good, there are foams sold at R7 spray on for R 38, you have to shop around. 2 Polyisocyanurate board is R 7.2" so 5.5" is R39.6, it is sold in 1/2" sheets. If you look you will find an installer doing R7.2" spray foam. But R 30 is nowhere near code in my area of Zone5 so you must live south where heating isnt so expensive. Zone 5 optimal is around R 60, my Minimum code is R35, remember codes are minimums.

[John Grabowski]

I'm thinking that a double layer of drywall with a radiant barrier in between will help a lot, but I am no expert. I agree with what someone else said about having an air layer directly under the roof. Hopefully you have ridge venting.

[the_tool_man]

In an effort to be somewhat brief, I apparently left out a few important details. I neglected to mention it, but I plan to install air baffles directly to the underside of the roof sheathing. They are about 1" thick, leaving 4.5" of space for insulation. By "just finishing", I meant that the room is currently unfinished, as in, no subfloor, no insulation, etc. I added two new gables so that windows could be installed. There are numerous other changes being made, but they are not particularly relevant to the question at hand.

[PerryOne]

This is something that you need to get right first time. If you spray polyurethane, you will make sure that every possible hole is filled - it is the holes that are left unfilled that loose heat and cost money. One that is done then fix almost vapor proof plastic sheet under making sure there is no possible access for water vapor to get into the roof space, then cover that with two inch polystyrene closely butted sheets, to stop the transmission of heat through the fabric of the roof by conduction, followed by drywalling. This will give you a sound dry warm roof. Keeping all that in mind, heat always moves to cold, so your expensive heat will then look for the next weak point to escape, this means doing the rest of the room to the same standard. Best of luck.