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Knee wall attic insulation and ventulation questio


Mark P
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1 ½ story house, upstairs is finished except for small knee wall attic that is insulated on all walls and roof. Is it correct to say the vapor barrier on the wall between the finished space and the attic is on the wrong side, but the vapor barriers on the roof and exterior walls is correct? There is no ventilation in the area, but with the roof insulated is it needed? Or should roof insulation be removed and a vent added?

Thanks as always

Mark

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Ideally, there should be a single vapor barrier, DIRECTLY against the conditioned areas with ventilation provided in the unconditioned attic eaves. Additionally, there should be an air space above insulation installed on the underside of the roof sheathing to allow air to flow between the soffit vents and ridge vent.

The vapor barrier as shown will trap moisture in the eave attic space.

I would recommend removing all of the insulation and starting over!

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  • 1 month later...

Hey Mark, how's business. I'm sure that you won't remember me but you rode along with my AHIT class in Jan. '06.

It looks to me like that is inside an unconditioned attic area and that the insulation was installed on the bottom of the rafters. If this is the case, insulation is not needed. There should be channel vents between the insulation and the roof decking to provide for ventilation of the underside of the roof decking. This should extend from below the bottom edge of the insulation to the ridge between every rafter/truss. Built up heat will greatly reduce roof life.

Vapor barrier (and insulation for that matter) is not needed if the installation is in an unconditioned area.

Also, keep in mind that the vapor barrier is a fire hazard and it should state on the craft face that it should NOT be left exposed.

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Built up heat will greatly reduce roof life.

So SIPS roofs don't last long? They're well insulated and unvented.

There should be channel vents between the insulation and the roof decking to provide for ventilation of the underside of the roof decking. This should extend from below the bottom edge of the insulation to the ridge between every rafter/truss.

That's only effective if there eave and ridge vents.

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Built up heat will greatly reduce roof life.
I can't get my hands on it right now, but I'm about 99% sure that there are legit studies that take an opposing view. That is, there are folks who say that attic heat doesn't have much effect on roof shingles.

Next time I'm killing time, I'll search my files, and see if I can find the black & white.

WJ

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