Roof insulation with I-beam rafters
I have built a shed roof using iLevel “2 x 10″ rafters. These have an I-beam profile. They are 16″ on center. I want to insulate the cathedral ceiling to R30 using fiberglass insulation. The width between the narrow part of the I-beams is 15 1/2”. That’s a problem as insulation is cut for 14 1/2″ width. Also, I want a 1″ air space above the insulation so I need 8 1/2″ thick insulation. I see that Owens-Corning has that (R-30C). Another problem is the blocking that separates the soffit spaces from the rafter bays is made from the same iLevel I-beam material. There are 2 gaps, one on either side of the blocks but I need some holes for the air flow in that top 1″ of space. I guess I’ll drill some holes in that 2″ x 2″ that is at the top of the blocking. Is that what you guys have done?
Replies
It sounds like you are just now getting around to doing some of the planning that was left off at time of the drawing board work.
Drill holes and use cellulose or chopped FG blown into the spaces.
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Another option is full width batts (full 16" should be available). Drill holes in the blocking? Probably, don't think they are structural, really; if so, you'd have to allow the insulation to squish a bit near the soffit and drill lower. (i.e. install a baffle near the eave to allow the air flow in lower and then up and over the insulation).
Edited 7/28/2009 8:01 am ET by Clewless1
I believe that if you check most I-Joist manufacturer product literature on informing you how to intall, what you can and can't do, you'll find you are not to cut or notch into the top flange of the I-joist.
To provide a 1" airspace above your insulation, you can use a baffle board. This is a preformed styrofoam u-shaped product which will hold the insulation back.
The air in the airspace above the insulation needs to have somewhere to escape. The roof sheathing on your shed roof needs to be cut back 1" at the top and a continuous venting strip needs to be used to let the hot air out of your shed roof assembly. Cor-A-Vent is a product that I typically spec out for these types of situations.
Don't forget you need to provide soffit vents to allow cool air to enter into your assembly to push the heated air above your baffle board towars the continuous vent strip above. Everything works naturally.