Insulating attic converted to cathedral ceiling


Old 10-26-04, 05:16 PM
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Arrow Insulating attic converted to cathedral ceiling

We have knocked out the ceiling in our two story house (leaving the ceiling joists exposed), removed the old grungy batt insulation, and plan to insulate between the rafters and put in some roof windows (skylights that open). The roof has a steep (12:12) pitch. We hope to transform our cavelike bedroom into a more open, light, cheery space. We will have the skylights installed professionally, but first need to put new insulation and drywall in. We have talked to several roofers and have learned that we probably need to allow an air space between the insulation and the roof to prevent moisture and heat damage to the roof. Rigid closed cell insulation appears to be one option, but I get the impression this requires a ridge vent. Venting at the eaves on one side of the house might be problematic because of a first-story addition. We read in This Old HOuse magazine that sprayed on foam might be an option and the article seemed to imply this option would require no venting. We have had difficulty getting clear guidance from the roofing contractors we have spoken to--they seem more experienced with adding insulation on top of the roof. Our roof is sound, so we don't want to go this route. Any advice is greatly appreciated! -Jill and John
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Old 10-26-04, 09:19 PM
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Location: USA
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The purpose of attic ventilation is to bypass the low vapor permability of roofing shingles. All heat has moisture in it and since insulation, regardless of the type of insulatiion, does not stop heat flow, it only slows it down, there is a constant flow of heat that contains moisture flowing through the insulation in an attic. So either you misread the "This Old House" article or it is wrong.

Spray foam or Foam SIP (Structural Insulated Panels) require cold roof installations. This would mean ripping off your existing roof. Which is probably the only way you can get the look you want for this room and have the roof vented properly.
Old 10-27-04, 12:29 PM
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Did what you want to do!

I have a ranch on cape cod with a closed attic. After putting structural support between the roof rafters, I removed the cieling joists. I then used the pink foam panels from Home Depot that sits in the rafters and creates a space between the roof deck and insulation. Then I filled the rafters with insulation, vapor barrier (6 Mil Plastic sheet), strapping, and wallboard/plaster.

However, I put in a ridge vent and replaced the Sophets vents with the vented kind, and drilled holes where needed to keep air running through the air gap created by the foam panels.

One thing I did not do, but wish I had, was to drill holes between the rafters below the two skyligths I put in. these currently have no cross flow. I hope this does not create a problem down the road, but I need a new roof soon (not as a result of this project) and may add some then.

I have had this setup for three years now, so if there are any problems, they have not shown yet.

I did lots of research on this, and think i got it right!

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