hello-
just looking into helping a friend with his house. he’s trying to finish out a 22’x24′ attic space on almost no budget. has 2×4 roof rafters on a 12/12 pitch- the house is in the Chicago suburbs and is a 4-square from the 1880s, so the 2x4s are actually 4″ (and rock solid). he wants to try to get away with NOT furring out the 2x4s and doing something like rigid insulation or sprayfoam. furring out and down will eat up valuable headroom there, so is there any kind of insulation product that can go beyond the depth of the 2x4s, over the rafters, to both add extra depth and break the thermal bridge, and allow him to sidestep the furring out?
will this work? meet code? will it be ventilation-able, or need to be? any suggestions?
thanks.
Replies
he wants to try to get away with NOT furring out the 2x4s and doing something like rigid insulation or sprayfoam. furring out and down will eat up valuable headroom there,
Take a look at http://www.buildingscience.com . There is an article on roof design that may give you some insight into what you need to do.
My take on it would be to add enough to the budget to spray foam into the bays and then add a layer of 1/2" rigid foam board on the underside of the rafters. This becomes and unvented roof assembly and could cause problems in the future if it is not done correctly or only a part of the total roof area.
Since the house is old, both of you need to understand the dynamics of air movement, heat flow, and humidity in a leaky building envelope. The above site and the U.S. Department of Energy web site can help with that understanding.
Dave
Dave-
Thanks! and thanks for the link- i'll start my homework here. yeah, my sense is that things could go seriously awry here, dealing with moisture/ventilation issues, if i don't figure out how to do it just right. it's easier- with a budget- to just leave it up to the pros, but can't do it that way this time. then we have to meet the local energy code on top of it all...
around here, you can now do an unvented roof if it's classified as a flat roof, but i don't know about a 12/12 roof- will look into it.
-m
You may want to price dense-pak cellulose or wet-blown cellulose. It should be cheaper than foam and, like foam, won't need a vapor barrier. Make sure your roof is not leaking before you close in the underside of your roof!
As far as ventilation, you have a hip roof right? What kind of ventilation, if any, does it have now-a couple roof vents, or nothing? If there is some local code issue with venting roofs-are you sure this is a code issue for a renovation?-maybe talk to the BI and find out what would be satisfactory. Once you insulate the underside of the roof with foam or densely packed cells, the need to vent the roof should be eliminated.
Aaron
Ideal location for a Corbond install. If he wants, he can still run foil faced Thermax over the rafter faces and under the SR for added thermal break
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Mikeymo,
FWIW, I did exactly what both DaveRicheson and Piffin suggested on my house in Ottawa, Ontario (similar climate, I gather, to yours).Filled the 4" deep rafter bays with closed-cell polyurethane spray foam (Demilec HeatLok) and wrapped over the edges of the rafters to eliminate thermal bridging above the collar ties and behind the kneewalls.
On the sloped ceiling portion (i.e. where drywall will be installed), the foam was kept slightly recessed from the face of the rafters and I installed 3/4" XPS rigid insulation to eliminate thermal bridging and provide a vapor retarder over the exposed rafter edges. Did the same thing on both gable end walls.Worked like a charm this past winter... no ice damming where previously there was lots.Regards,
Ewan
Edited 4/6/2006 10:26 pm ET by Ewan
what did you pay for the install of the foam in Ottawa? (per sq/ft)
Drozer,
It wasn't cheap... $1.25/sq.ft/inch of thickness (pretty much MAsprayfoam's numbers but in Canadian dollars).Ewan
ewan
thanks for the reply.
Of course, as many have mentioned, the 2lb spray foam is the answer in terms of performance needed.
The big problem is the budget. Count on $1 per bd-ft, or $4 per sq ft at 4" thick. Sounds like that might be the deal breaker.
Stu
Consider this option. 1-1/2" foam board in 2x8' sheets, manufactured with channels for 1x3" firring strips. It will add high R/inch, prevent thermal bridging, if you use PL300 on the seams & housewrap tape the seams you stop airflow and get a vapor barrier in the process.
You can still use fiberglass R-13 or R-15 fiberglass behind it & it will have 1/2" air space for ventilation behind/above it.
Good insulation system for attics on a budget, even though it was designed for basements.
Edit to add: This goes over rafters or knee wall framing (perpendicular to framing)
Edited 4/8/2006 8:41 am ET by basswood