Insulating Garage Ceiling - Joists Unevenly Spaced

[pbywallace]

Hi all - we've got a very old detached garage and plan to insulate the ceiling and then cover with plywood.

I'm planning to buy batt insulation in 23inch widths/6.25 thick but I measured my joists today and most are 24 OC but in some instances they are 23 or 22.
Also, the joists are only 5.5 inch tall; so would I still be able to use 6.25inch thick insulation? Does that just cram under the plywood?

thank you!

 

[The Cobbler]

you would cut the batts to fit the smaller sizes .
compressing insulation = loss in r value.
either drop down a size in insulation, or strap the joists 1"

 

[pbywallace]

thanks!

So - since R19 is 6.25 inches, and R13 is 3.5 inches, should I just go with R13 given that my clearance is only 5.5 inches? I won't be heating/cooling this garage but I'm hoping to insulate the ceiling so that it doesn't get crazy hot in the DC summer.

 

[The Cobbler]

double check your measurements, r-20 usually fits a 2x6 which is 5-3/8

 

[nadogail]

Can you blow in cellulose fiber on top of the existing ceiling?

 

[Leaflessshadetree]

Just put the insulation in. You aren't compressing much. It will reduce the R value a little but not as much as dropping to the thinner insulation.
You can trim the width if necessary. I compress it and use a sharp knife, heavy scissors also work well. For 23"OC (even 22") spacing it probably isn't worth the effort.

 

[MushCreek]

I just insulated part of my barn, and the stud bays were all over the place. It was actually rare to find one with the correct spacing, and it was only built a few years ago. The framers were real hacks. I just trimmed the insulation to fit. It was a PITA, and took a long time. For a ceiling, I'd have cellulose blown in. The best would be foam, but $$$. I am NOT looking forward to doing my shop, which is the other half of the same building.

R-19 batts are intended for 5-1/2" deep walls.

 

[BearsFan315]

Hi all - we've got a very old detached garage and plan to insulate the ceiling and then cover with plywood.

I'm planning to buy batt insulation in 23inch widths/6.25 thick but I measured my joists today and most are 24 OC but in some instances they are 23 or 22.
Also, the joists are only 5.5 inch tall; so would I still be able to use 6.25inch thick insulation? Does that just cram under the plywood?

thank you!

welcome to my world... every joist was different, had to custom cut most of them. was tedious but worked out. laid them out on the driveway over large pieces of cardboard and cut with a razor knife. also used the strap to assist in holding it up and in. been working great !!

Garage Roof/ Rafter Insulation... To Vent or Not to Vent ?

same instance under my house when i did that, also had to cut the ties !! oh the joy.

 

[LB-1911]

thanks!

So - since R19 is 6.25 inches, and R13 is 3.5 inches, should I just go with R13 given that my clearance is only 5.5 inches? I won't be heating/cooling this garage but I'm hoping to insulate the ceiling so that it doesn't get crazy hot in the DC summer.

The chart supplied by Owens Corning should prove informative.

Building Insulation Compressed R-value Chart

 

[pbywallace]

Thanks for the advice guys.

I hadn’t thought about blown insulation - would that make sense even though I still haven’t closed up the ceiling with plywood?

FYI I’m only working with about 300 sq foot of ceiling.
Would blown be more cost-effective than R19 batts?

 

[Bert_]

Are you putting up the plywood for the ceiling or does it already have a ceiling and the plywood is going to be an attic floor?

As long as you already have a ceiling, blown in is the way to go.

 

[pbywallace]

We are putting up plywood for the ceiling. Right now it is just exposed joists with no attic.

 

[Kevin54]

6" insulation is fine as it can stick above the joist. As far as compressing it 1" sideways to fit, I wouldn't worry about cutting it as the R factor loss won't even be measurable. If you are using papered batts, just stable them to the joist, then put your plywood over that. Or if you are using unfaced batts, you can put your ceiling in sections at a time, then drop the batts in from above. Once you get to the last section of ceiling you can run duck tape across the joist to keep the batts in place from falling.

 

[itb45]

The thickness is fine, sounds like you have standard 2x6's which would be R19.

If you are that worried about squishing them an inch on the width and some are wider, rather than cutting 1" off on some, and trying to add a 1" piece on the others, cut them in the middle but 0.5" off from center. Put two small pieces on the skinny joists, and two wide pieces on the wider ones.

If these are going up against the roof, you really shouldn't have them contact the roof, there should be an air gap, for moisture. If they aren't going against the roof, and there is space above(attic), I would go R30 rather than R19, they can stick above the joists.

 

[rayra]

R-19 is commonly used in 2x6 voids. Don't get hung up on the literal dimensioning or expect any exactitude in fiberglass insulation.

 

[CraigStu]

I agree, shove it in there. It's just not worth worrying about. Heck you are lucky none of the spaces between joists are over sized. This is one of those deals where the phrase 'don't let perfection be the enemy of good to go' or something like that.

 

[PWC Repair]

Just put the insulation in. You aren't compressing much. It will reduce the R value a little but not as much as dropping to the thinner insulation.
You can trim the width if necessary. I compress it and use a sharp knife, heavy scissors also work well. For 23"OC (even 22") spacing it probably isn't worth the effort.

I agree, just put it in. Been there done that in my 1958 model house

 

[K'ledgeBldr]

Put a ceiling in- blow the attic.

Since you're not heating and cooling- 6" would be plenty.
Or, you could do a cost comparison of using radiant barrier ply for the ceiling and skip the insulation.