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Vented vs. non-vented attic

I live in zone 2A. I am building a raised home for myself. It has a galvalume roof laid on top 2x4 strapping which is over 3/8-inch sheathing with a radiant barrier underneath.

It is an equal span gable, 8/12, with living upstairs. A ridge vent sits on top of the roof.

I was planning to insulate the roof with foam inside the knee walls up until the ceiling levels out at 9 vertical feet. I figured on the 2" airspace.

I want to use open cell foam with damp spray cellulose. Is this a good system for Louisiana north of Lake Pontchartrain?

Asked by dwight foreman
Posted Jul 28, 2011 10:48 AM ET
Edited Jul 28, 2011 12:14 PM ET

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3 Answers

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1.

Dwight,
I don't understand your proposed construction details.

1. Do you want to insulate a sloping roof, a vertical kneewall, or a flat ceiling? Or all three?

2. Where is the 2-inch air space?

3. Where is the open-cell foam? How thick will it be?

4. Where is the damp spray cellulose? How thick will it be?

Answered by Martin Holladay, GBA Advisor
Posted Jul 28, 2011 12:17 PM ET

2.

Thanks for responding Martin.My main question is for zone 2A should I keep my ridgevent system and insulate it appropriatly . In the upstairs my rafter assembly slopes at 8/12 for 8 feet then levels off using 2x6 collar rafters. Yes I want to insulate a sloping 2x10 sloping roof. Forget about the kneewalls. The open cell would be utilized in between the rafter bays blown onto rigid foam allowing an airspace between the rigid foam and roof sheathing.

Answered by dwight foreman
Posted Jul 28, 2011 8:40 PM ET

3.

Dwight,
Your description is still confusing, but I'm getting closer. Is this right?

Your sloped roof assembly will consist of an air space near the roof sheathing; then a layer of rigid foam; then a layer of open-cell spray foam; and then a layer of damp-spray cellulose. Is that right? If so, it sounds like a complicated sandwich.

If you are insulating a sloping roof, and you are including an air space between the top of your insulation and the bottom of your roof sheathing, you still need soffit vents and a ridge vent.

Answered by Martin Holladay, GBA Advisor
Posted Jul 29, 2011 4:18 AM ET
Edited Jul 29, 2011 6:55 AM ET.

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