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Old 01-03-2019, 04:12 PM   #1
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Insulate slightly flat ceiling?


Hey DIYers!
I am almost done building my gym! still working on the roof install as the rain rolls in this weekend. As I think ahead to the insulation part of the build I wanted to get some opinion. I have a slightly flat roof - modern style structure with about a 10-15 degree angle.

Do I need create some type of airflow between the plywood and insulation? This should prevent condensation.

Drill some holes? What is the best option?
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Old 01-03-2019, 04:21 PM   #2
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Re: Insulate slightly flat ceiling?


You should vents at both end of all joist bays. 1 1/2" next to roof sheeting
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Old 01-03-2019, 04:27 PM   #3
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Re: Insulate slightly flat ceiling?


Hard to know what needs to be or should have been done with that little info and no pictures.
Cathedral ceilings?
Is there soffit and ridge venting?
What size are the rafters?
https://insulationinstitute.org/im-a...uch-do-i-need/
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Old 01-04-2019, 03:48 PM   #4
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Re: Insulate slightly flat ceiling?


Just took come pictures of what I am talking about. No vent of any kind currently and the roof is about 10-15 degree - flat roof.

Rafters are 2x6

I am thinking to just drill a few 1/2 inch holes for venting in the front and back of the building in each bay. Will insulate with 2" foam boards in ceiling, would this idea work for condensation stand point?
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Old 01-04-2019, 04:14 PM   #5
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Re: Insulate slightly flat ceiling?


Maybe 5 or 6 holes 1 1/2" diameter. As close to the sheeting as you can.
You would like an open cavity 1 1/2 inches between insulation and roof sheeting.
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Old 01-04-2019, 04:31 PM   #6
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Re: Insulate slightly flat ceiling?


If you have a permit and are following codes you will need more insulation than fits into a 2x6 cavity. Otherwise use the overhang on each side to add ventilation and drill holes through the blocking on top of each wall. Then install r-15 high density fiberglass insulation with Kraft facing so you can staple it up Cover with drywall and finish.

It is just an accessory building so not sure if permits and codes apply. But you are in CA and they have their own rules.

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