Insulating @ 24" on center....

[Beetle-Kill]

Trying to figure this out. This is for a semi-gable/hip roof addition. I know how that sounds, but it's as close as I could come to describe it. It was a '76 addition to existing. Plus, trusses are non-dimensional 2x4's, 24" OC. Batt insulation will be R-13, so how to seam up the spacing? How would you fill the spaces? Thanks, JB. (Btw, I don't want to sister new 2x's to close the tolerances.)

 

[JonP]

Home centers or building supply centers will generally sell insulation in varying widths including 24".

 

[SolarAndWood]

I've used foam board to fill in the weird bays I knew I wasn't going to get the batts to go clean. I've also gone 2" foam board across the face and sheet rocked over it without too much trouble.

 

[dave11]

If you can't get batts pre-cut to perfectly fill those joist bays, I'd go with 2 inch foam board as well. Cutting batts the long way is a real pain, and will inevitably leave gaps. Any open space around the batts will greatly lower their effectiveness, as will any compression or "puckering."

Alternative would be put the drywall up first, then fill the spaces with blown-in insulation.

 

[velvetfoot]

I've cut a bunch of 2" foam the long way on a table saw, and it can grab if you're not careful.
You could cut it a little narrow and use great stuff foam for the gap.

 

[gpcollen1]

Either locate some 24" insulation or cut some pieces down the middle to make the difference. That is what i did. Cutting a bunch of insulation sounds like a pain but not once you set up and are doing it. I set a sheet of plywood on some horses and cut on top of that. You cut with the vapor barrier up [if there is one] and hold the insulation down with a 2x4, compressing it so it is easy to cut. Make sure your blades are sharp. Seriously, not a big deal...I was doing it under my house, a converted deck that is 16x20.

 

[midwestcoast]

If you can't get batts pre-cut to perfectly fill those joist bays, I'd go with 2 inch foam board as well. Cutting batts the long way is a real pain, and will inevitably leave gaps. Any open space around the batts will greatly lower their effectiveness, as will any compression or "puckering."

Alternative would be put the drywall up first, then fill the spaces with blown-in insulation.

+1 to all. Perfect install of batts is essential to performance.
Blown in would be cheaper than foam board though, you'd have to be carefull to fill bays dense enough to eliminate settling. If looking for higher R-Value you could put up some foam board, blow in behind it, foam the holes closed then drywall over it.

 

[Beetle-Kill]

Thanks for all of the ideas. I appreciate the input. I'm bracing the existing trusses, when done I'll incorporate these ideas. Thanks all. JB

 

[pyper]

The walls in my house have random width stud bays. I put 5/8 inch R-matte board, and then fiberglass. Since the bays were all different sizes, and different at the top and the bottom (not square) I got normal size and cut it all to fit, putting up one piece whole, and then slicing the second top to bottom I made measurements in three spots for each.. Anyplace where there was a gap I cut some more to fill the gap.

 

[richg]

Spray foam. The only way to go in vaulted/cathedral ceilings.

 

[joel95ex]

I am pretty sure johns manville or owens makes a 24" batt. Make sure you have correct ventilation below the roof deck to the soffits.... the builder of my house blocked the soffits with batts so i had to install the dura-vents/baffles... incorrect ventilation is very common

 

[kofkorn]

I've cut a bunch of 2" foam the long way on a table saw, and it can grab if you're not careful.

X2 - Be very careful, I had a brother-in-law that cut about halfway through 2 fingers and a thumb when 2" foam insulation caught on a table saw. Keep the blade height as low as possible and use whatever you can to keep your hands out of the way. Worse case, let it go, it's much easier to replace a whole sheet of 2" foam as compared to one finger.

 

[Beetle-Kill]

Thanks all,I appreciate the input. I went a different direction, reinforced the trusses, but left it a "cold roof". Added R-13 foil backed yellow board to the enclosed area, and am rebuilding a hollow-core door to be R-26 insulated. (that may not work out so well, but I'm gonna try it). Dbl. stack R-13 foil board makes for a very thick access door. thanks again, JB

 

[gpcollen1]

Gotcha - I think. I am renovating a room with a cathedral ceiling and ventilation that is piss poor. So I put 2" XPS up against the rafters and left the entire bays for air flow. I built a ceiling below it so I could get some R38 in there. Add the 38 to the XPS and I am above 50. One of my friends was puzzled as to why I did this. I guess he is used to seeing just stuffing insulation behind the plastic venting and living with some R19 or so. Forget about that. That roof should never ice up once I am finished.