Insulation in vaulted ceiling

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Old 06-19-03, 06:22 AM
NoVa
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Insulation in vaulted ceiling

I'm enclosing my side porch to make a TV room, and I will be removing the joists to vault the cieling. There is NO peak on the roof of the porch-- it just slants up at about 25 degrees until it butts into the side of the house. I understand that I need to allow for airflow between the roof and the insulation, so I will be using high-density (8-1/4") R-30 in the 2x10 rafters. I also understand that I'll need to install a soffit vent at the low end of the roof, but what kind of vent do I install at the top end of the roof where it butts into the side of the house? The county inspector mentioned some kind of vent that runs the full length of the roof, kind of like flashing, which allows hot air to flow out the top of the roof but no water can get in. Is anyone familiar with this kind of vent? I'd really appreciate any input. Thanks!
 
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Old 06-19-03, 08:27 PM
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The 2 types of vents the run the width of a roof are ridge and drip edge vents. Neither one applies to your situation. The drip edge vent can apply to your soffit or eave area but not to the higher portion of your roof that butts up against the wall of the house.

Vaulted ceiling are not cathedral ceilings. The difference is not all of the roof is used for the ceiling in a vaulted ceiling. The advantage to a vaulted ceiling is that it has the same affect as a cathedral ceiling and at the same time it allows you to put vents on the walls supporting the roof to obtain adequate free venting.

For example, once you remove the ceiling joists, measure 2 feet down from the highest point on your roof rafter on the wall. Install a ceiling joist at that point. This forms a right triangle that includes the wall of the house, the roof of this room and the new ceiling joist you just installed. Anything below this joist the roof is part of the ceiling and anything above it the roof is not part of the ceiling. In this right triangle you can install vents on the walls supporting the roof.
 
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Old 06-20-03, 11:01 AM
NoVa
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Vaulted cieling

Thanks. It sounds like vaulted (as opposed to cathedral) is the way to go. Is 2 ft a pretty standard distance to drop down for the new cieling?
 
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Old 06-20-03, 01:01 PM
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Two feet is about average but it depends upon the look you wish to achieve with the ceiling and the size vent you want on the walls that support the roof. The vent has to fit inbetween the rafters, wall of the house and the ceiling joist. I recommend measure twice, cut once. After the ceiling joists are down go out and get a vent that you think will fit in this space. Tack up the vent from the inside to see if it will clear the wood members. Also remember the vent has to clear any insulation you put inbetween the ceiling joists. You may have to drop the ceilng joist a little bit more or you might be able to raise it if you want. Just make sure once the vent is installed, there isn't anything that will obstruct air flow through it.
 
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Old 06-20-03, 07:07 PM
JBar1
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Yes, & I wish I had known about it before I had my roof done!
It's called a Flash FilterVent put out by Air Vent Inc. (certainteed).
You can see it on their web site certainteed.com under specialty vents. Good Luck!!
 
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Old 06-24-03, 05:26 AM
NoVa
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Size of attic vent

Thanks. I decided to go ahead and drop a cieling with vents on the two walls supporting the sides of the roof. Is there some kind of formula for deciding how big my vents need to be? I don't want to drop the cieling any lower than I need to. The roof will cover an area that only measures about 8' x 12'. Will a 22" round gable vent on each end be enough?
 
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Old 06-24-03, 08:16 PM
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Since you are probably insulating with a vapor barrier, the rule for free venting is 1 : 150. The calculation for this is as follows :

12 feet times 8 feet = 96 square feet.

96 sq. ft divided by 150 sq. ft. = 0.64 sq. ft. of free venting required for this area.

This is the minimum amount required and probably the 2 gable vents you intend to install exceeds 0.64 sq. ft. On most vents they usually give the net vent area. But in my opinion you really don't need to be that exact. The vents you intend to install are just fine.
 
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