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Old 04-24-2019, 09:21 PM   #1
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insulating attic joists


I am converting an attic to a BR.
Its an older home where the ceiling joist spacing between joists is around 20" so 23" insulation would all need to be cut to fit.
Im thinking of laying 1" foam board on top of the ceiling joists for stability then rolling unfaced R30 on top of that. I could do loose fill but its only a 12x12 ceiling space probably not worth all the prep work for that.

My question is there any issue with the open space between the drywall and the foam board that would cause a vapor or mold issue. In other words the joist cavity would be uninsulated with the insulation above that space.
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Old 04-24-2019, 09:34 PM   #2
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Re: insulating attic joists


The term 'ceiling joist' is confusing here - do you mean the joists in the floor of the attic/ceiling of the room below or the roof rafters/truss system?
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Old 04-25-2019, 01:42 AM   #3
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Re: insulating attic joists


Quote:
Originally Posted by mtmprop View Post
I am converting an attic to a BR.
Its an older home where the ceiling joist spacing between joists is around 20" so 23" insulation would all need to be cut to fit.
Im thinking of laying 1" foam board on top of the ceiling joists for stability then rolling unfaced R30 on top of that. I could do loose fill but its only a 12x12 ceiling space probably not worth all the prep work for that.

My question is there any issue with the open space between the drywall and the foam board that would cause a vapor or mold issue. In other words the joist cavity would be uninsulated with the insulation above that space.
There is much to talk about when you are insulating an attic bedroom so we will need a bunch more info about where you are and the venting you have and the stage you are at.
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Old 04-26-2019, 09:25 AM   #4
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Re: insulating attic joists


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Originally Posted by mtmprop View Post
I am converting an attic to a BR.
I'll jump ahead and say this plan will NOT happen.

As to insulating in an attic... blow in some cellulose or rockwool.
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Old 04-26-2019, 11:57 AM   #5
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Re: insulating attic joists


I think I understand what you are wanting to do but please add more info so we can give you the right answers.
It sounds like you have an attic space where the roof is supported only with straight studs.
Are these roof supports what you are calling joists?
What is the dimension of these roof supports, 2x4, 2x6?
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Old 04-29-2019, 09:06 PM   #6
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Re: insulating attic joists


attic ceiling joists not floor joists.
these form the ceiling which ties the roof rafters together from outward stress.
Drywall is done, ceiling will need to be blow in now so my previous question is no longer needed to be answered.
thx for the responses.
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Old 04-29-2019, 09:15 PM   #7
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Re: insulating attic joists


I hate to be a wet blanket but did make provision for an air gap between top of insulation and and roof deck? In other words, what will keep air flowing above the insulation to prevent condensation and mold?
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