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Posted by the_tool_man on September 2, 2008, 4:27 pm
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.
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Posted by ransley on September 2, 2008, 5:16 pm
> Hi all:
>
> I'm finally finishing that bonus room project, and am investigating
> insulation options for the cathedral ceiling. =A0The roof structure is
> standard 2x6 rafters on 16" centers with OSB sheathing and shingles
> above. =A0I need to achieve R30, and would like to do so without loosing
> much headroom. =A0From what I have learned, this will almost certainly
> require a foam insulation of some kind. =A0I have investigated three
> options:
>
> 1: A local installer of Icynene foam (trade name for a polyurethane
> foam). =A0He has a minimum $2000 charge, and wants $2500 to do the whole
> room (ceiling, walls, floor, stairwell). =A0He 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. =A0Tiger Foam
> sells a 600 bdft kit for $630, but I would need just over 1300 bdft of
> foam just for the rafters (1 bdft =3D 1 sq ft x 1 in thick). =A0Again,
> almost $2000. =A0This 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. =A0Enough 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. =A0This would require a lot of cutting and fitting of 2
> layers of foam between the rafters and a third below. =A0The walls,
> floor and stairwell would be insulated conventionally at a cost of
> about $400 in materials. =A0Thermax 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. =A0Right 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). =A0I am wondering what else is out there. =A0I find i=
t
> hard to believe that with all the green building movement going on,
> there aren't more options for acheiving high R-per-inch. =A0What other
> options are out there?
>
> Thanks in advance,
> John.
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.
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Posted by John Grabowski on September 2, 2008, 8:33 pm
> 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?
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.
http://alsnetbiz.com/homeimprovement/insulation2.html
http://alsnetbiz.com/homeimprovement/insulation3.html
http://alsnetbiz.com/homeimprovement/drywall.html
http://www.insulation-r-values.com/
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Posted by the_tool_man on September 2, 2008, 10:39 pm
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.
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Posted by PerryOne on September 3, 2008, 2:01 pm
> 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.
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.
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