Insulating Scissor trusses without raised heel
Greg Bailey| Posted inEnergy Efficiency and Durabilityon
我有一个12”之外的我的房子was built with scissor trussed without the raised heal. Near the exterior walls I only have 3 1/2 or less insulation. I live in Central New York and get over 200″ of snow almost every winter. this is causing ice dams and leaks. I plan on stripping the current shingles. Then spraying DOW Froth Pak 200 Foam Insulation through small hole to fill the whole cavity. Then cover the sheathing with 2″ of Polyisocyanurate Insulated Sheathing. Screw on 3/4″ furring strips to the trusses for venting Re sheath the roof with plywood and shingle. does this sound like a good way to get rid of the ice dams?
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Replies
Greg,
You're on the right track. The issue is complicated by a few factors; for one thing, I assume that you have an air space above your existing insulation for most of the length of the scissors trusses. That makes it harder to add insulation above the roof sheathing and still obtain the high R-value you want.
If there are existing soffit vents and ridge vents, you have to carefully seal those so there is no air flow between the older insulation at the bottom of the scissors truss and the new insulation on your roof.
Moreover, in your climate, you really want more than 2 inches of rigid foam above your roof sheathing. For more information, seeHow to Build an Insulated Cathedral Ceiling.
Thank you for the quick response to my situation. I did just read the article and still have some concerns. If I were to spray DOW Froth Pak 200 Foam Insulation through small holes to fill in above my current fiberglass batt would the expanding pressure of the foam cause my ceiling sheet rock to pop? also if I were to apply say 4" of Polyisocyanurate Insulated Sheathing over the existing sheathing how do I vent it at the soffits? I have been unable to find details to accomdate the the wider fascia. Does someoce make a special soffit channel for this type of installation?
Greg,
Q. "If I were to spray DOW Froth Pak 200 Foam Insulation through small holes to fill in above my current fiberglass batt would the expanding pressure of the foam cause my ceiling sheet rock to pop?"
A. Probably not, because the foam should be able to expand sideways. But use common sense, and spray a little bit at a time.
Q. "If I were to apply say 4 inches of Polyisocyanurate Insulated Sheathing over the existing sheathing how do I vent it at the soffits?"
A. There are various ways to redesign your soffit and fascia to disguise your thicker roof. Get out a pencil and paper and play with the alternatives. Perhaps a GBA reader will post their favorite solution.