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Discussion Starter #1
I’m having issue finding the right size insulation batt at my local Home Depot and Lowe’s.

The measurement for my ceiling beams are 22”W x 9”D x L

found R30, which is 7.25”D

Wood stud R-30 Rock Wool Batt Insulation with Sound Barrier (23-in W x 47-in L) https://www.lowes.com/pd/Wood-stud-R-30-Rock-Wool-Batt-Insulation-with-Sound-Barrier-(23-in-W-x-47-in-L)/3693532

And this

R-30 Kraft Faced Fiberglass Insulation Batt 24 in. x 48 in.

This is 10”D

https://www.homedepot.com/p/303653997


Is my ceiling not standard size or something?



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retired framer
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Are you just doing a repair or the whole ceiling?
I have done 5 1/2 and 3 1/2 to make up full depth then if you ever want to add you can go the other way over top with out gaps.
If you use paper face the paper goes down against the ceiling.
The depth of the joist is not about standard, it is about the span between bearing walls.
 

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Discussion Starter #3
Are you just doing a repair or the whole ceiling?
I have done 5 1/2 and 3 1/2 to make up full depth then if you ever want to add you can go the other way over top with out gaps.
If you use paper face the paper goes down against the ceiling.
The depth of the joist is not about standard, it is about the span between bearing walls.


I’m just doing a repair. The area is about the size of a drywall 4x8.

My old insulation have paper face go down against the ceiling. It’s fiberglass, which I want to avoid now.

So you used two batts in the past to make it 9”?


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retired framer
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I’m just doing a repair. The area is about the size of a drywall 4x8.

My old insulation have paper face go down against the ceiling. It’s fiberglass, which I want to avoid now.

So you used two batts in the past to make it 9”?


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Yes I have done that before, then I covered the other way with another 3 1/2"
 

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A modern day 2x10 will be 9.25" unless they have changed again. But, you have what you have and the best approach is to fill the cavity full and be neat. I also like the Roxul and you can use Neal's approach of 5.5" plus 3.5" and it should be full. All batt insulation comes compressed so will fluff over time.

I don't see your location but wouldn't worry about it, VBs are less important tham sealing air leakage.

Bud
 

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Discussion Starter #6
A modern day 2x10 will be 9.25" unless they have changed again. But, you have what you have and the best approach is to fill the cavity full and be neat. I also like the Roxul and you can use Neal's approach of 5.5" plus 3.5" and it should be full. All batt insulation comes compressed so will fluff over time.

I don't see your location but wouldn't worry about it, VBs are less important tham sealing air leakage.

Bud


If I were to use 24”W x 10”D do you think if I compress it in and it cause any issue?


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If I were to use 24”W x 10”D do you think if I compress it in and it cause any issue?
The "fluffiness" of insulation (airspace around the glass fibers) is part and parcel of the insulating value of batt insulation. If needed, batts should only be compressed just enough to make it hold between a joist space. But we're not talking nuclear physics so sticking 10" (already compressed from the packaging) should be fine in 9 1/4". Cut the 24" to fit in 22".

There seems to be info missing to cause concern - is it under second floor? under roof? under attic?
 

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Discussion Starter #8
The "fluffiness" of insulation (airspace around the glass fibers) is part and parcel of the insulating value of batt insulation. If needed, batts should only be compressed just enough to make it hold between a joist space. But we're not talking nuclear physics so sticking 10" (already compressed from the packaging) should be fine in 9 1/4". Cut the 24" to fit in 22".



There seems to be info missing to cause concern - is it under second floor? under roof? under attic?


This is under my roof. I have a flat roof and do not have an attic, so right above it is plywood then roof.
I just realized there is no 10” on the wool insulation. I guess I have to stack up the R15 and R23 together to get me the 9”


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3 on the tree has brought up a good point, apparently there is no attic space above this ceiling. Guidelines for insulating just below the actual roof specifically avoid any air permeable insulation directly in contact with that roof. There needs to be an air space above the insulation that is vented to the outside.

Maybe I'm off track so corrections are welcome.

Bud
 

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Discussion Starter #10
3 on the tree has brought up a good point, apparently there is no attic space above this ceiling. Guidelines for insulating just below the actual roof specifically avoid any air permeable insulation directly in contact with that roof. There needs to be an air space above the insulation that is vented to the outside.

Maybe I'm off track so corrections are welcome.

Bud


So if that’s the case am I suppose to leave some space open like use 7.25”D instead of the full 9”?


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So if that’s the case am I suppose to leave some space open like use 7.25”D instead of the full 9”?


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That's a question I should have asked earlier. Yes there should be venting under the deck above.
Do you know what you had before and do you see a gap with the rest of the space.
 

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In addition to batts you need a source of airflow into and out of every rafter bay. That usually occurs where the rafters overhang a wall and form a soffit. If the roof is flat you need vents on both ends and hope that wind provides the air movement.

Tell us more about how this roof is constructed and pictures if possible. Also some history.

Bud
 
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