Insulating a vaulted ceiling

Old 10-05-02, 10:39 AM
Sharon K
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Question Insulating a vaulted ceiling

Hello! For the past year we have been remodeling a 100 year old home and now it is time to insulate. We have vaulted the ceiling in most of the upstairs of our home, making the insulation process a little more difficult. We are unsure how to insulate the ceiling due to such a limited space (roof rafters are 2x8's) and to allow enough room for air ventilation. In addition, we want to make sure that it is done properly in order to prevent moisture build-up. Any suggestions on the proper materials to use and the correct way to do it, would be wonderful. Thanks!
Old 10-05-02, 11:26 AM
Mike Swearingen's Avatar
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I would use 6" insulation bats with vapor barrier stapled to the edges of the rafters facing the heated area. That will leave 1.5" of air space between the insulation and the sheathing.
Old 10-07-02, 11:12 AM
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The use of FG in vaulted ceilings is nothing short of a disaster.

Over the years I have developed a insulating system that is more effective than anything I have seen so far.

You have four problems with vaulted ceilings.

1 You have a larger ratio of surface area, include gable ends, than you do with a horizontal flat ceiling. This increases the cooling load by a large factor. In fact one of the most common complaints about vaulted ceilings on this fourm is the inability for the a/c to cool and the coldness of the room in winter.

2 Even with the most efficient insulatiopn between the rafters you have heat energy being directly conducted to the drywall from the roof sheathing. MUCH, much, much heat. So you have to have an insulation system that will control that heat path too.

3 If you are in a cold climate and one of the ceiling surfaces faces pretty much north, then you have condensation problem. The room moisture travels thru the rafter, pass the insulation and condensates on the roof sheathing. Bad, bad news.

4 Fiberglass is only about 10% efficient plus it is about 98% air spaces. Since "down" heat gain is about 95% radiant that means heat energy, traveling at the speed of light, goes right thru

Install a type 3 (two layer) radiant barrier (RB) insulation between the rafters towards the bottom surfaces of the rafters. Install a non perforated, single layer RB to the bottom of the rafters like a vapor barrier, which it is, and 7/8" steel furring strips acropss the rafters and then the drywall. Since the rafter surfaces are covered with a low emissivity ( .05%) foil the amount of BTUs being radiated to the drywall is about 4-5 BTUs/hr/sf.

For ridge vents see

If the walls are already insulated see If not you can install RBs there too.

Let me know if you want more info.

Thank you for considering my opinion.

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