Hi, my wife and I live in western New York in a house we bought last year. Our house is well insulated and weather-tight except for one area. This area was once attic space that the previous owner converted to a loft overlooking the kitchen. It’s ceiling is framed with 2x6’s and the finished ceiling (drywall) is sloped/vaulted. There is currently R-19 fiberglass jammed into the 5.5” rafter cavity with a cheap Styrofoam type baffle that is most likely crushed in several spots. This area is the only one that continually melts snow in the winter resulting in ice dams. Heat during the summer also builds up in the loft due to the roof heat coming into the house.
I would like to take the dry wall down and replace the fiberglass with rigid board. This is the only way I can get close to R-38 without lowering the ceiling too much (peak of the ceiling is approx 4’ above the loft floor). I have looked into spray expanding foam, but even that doesn’t give me the R-value without adding more rigid board. The only thing I am concerned with is how much the rafters expand due to summer humidity. If I re-insulate during the summer tightly squeezing the insulation boards between the rafters, will the foam boards become loose during the winter when the rafters dry out and possibly shrink? Also, do I need to get the insulation value up to R-38, or is R-30 enough to keep the snow on the roof?
Most foam boards have an R value of 5 to 6 per inch of thickness, In a 2x6 rafter space, all you can get is about R-20 or R-24 and still allow an air space above the insulation. I wouldn't worry about the boards fitting the space in either winter or summer. The expansion amount is microscopic.
So it's hardly more than what you have now; however, positive ventilation could help against ice dams.
Architect (NY) and Home Designer (PA)
Posts: 2859 | Location: Tobyhanna, PA | Registered: 24 October 2005
Thanks for the response. The way I figure it, forming a 1" airspace with the top insul. board, I would have 4.5" within the cavity and another 1" covering the entire ceiling, will give me an R value of ~33-36 (DOW tuff-r = 6.5/inch). I think I am figuring this correctly, and will probably go this route. Thanks again. Any other comments are welcomed!!
Most foam board insulation has a maximum R value of 4.5 per inch.
This mean that for the 4.5 usuable inches of insulation space you have in a 2x6 rafter (less the 1 inch required air space above) you are likley only to get a maximum insulation value of R20.25 uaing any type of rigid foam boards.
New York State energy code requires at minimum an R-49 for your location.
Foam will provide less than half of recommended insulation when stuffed into your rafter cavities.
That said, you need to look to other methods to insulate.
One method might be to increase the depth of the rafter by sistering 2x12s to the existing 2x6s.
This way you could increase the depth of insulation between the rafters.
Alternately, you could insulate the roof deck under the shingles using several inches of foam in addition to foam board or other types of insulation placed between the rafter below.
But as described, your plan doesn't come close to meeting minimum energy requirements for your location, and using foam compared to other insulation materials just makes your plan one expensively bad mistake.
