# Insulating narrow joist bays



## Bud9051 (Nov 11, 2015)

How deep are your joists? Ideally you should fill the joist cavities to the top and then add additional perpendicular across that. This way you cover the joists whis is important.

As for fitting your batts into irregular width spaces I have used a couple of methods.
1. When cavities are wider I have added narrow strips then the full width bat.
2. In other cases I have cut the batts to the desired length for a press fit and then one by one tucked them neatly into the cavity side by side.

As for installing a batt that is an inch or so wider a little compression doesn't hurt.

To slice off a thin piece use a long piece of metal, maybe a piece of angle iron to press down and slide your razor along the edge.

Bud


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## Nealtw (Jun 22, 2017)

I would buy rolls of 23" and cut my own 1/2" longer than the space is wide and scrunch them together . So when installed each piece would be 22 1/2" long.


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## gyzhor (Apr 2, 2021)

Bud9051 said:


> How deep are your joists? Ideally you should fill the joist cavities to the top and then add additional perpendicular across that. This way you cover the joists whis is important.
> 
> As for fitting your batts into irregular width spaces I have used a couple of methods.
> 1. When cavities are wider I have added narrow strips then the full width bat.
> ...


My joists are just 2x4s, so any reasonable insulation is already going to go way over the top. Would it still be necessary to cross-layer them? 

So compressing and extra inch should be fine, but 2" not so much? Are we talking buckling or curling at the sides of the backing, then? I was hoping for a simple solution that didn't involve cutting, but that kind of looks like the best option at this point, unless I want to pay out the nose for that 15" stuff I mentioned (which now everyone seems to be sold out of!)..


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## gyzhor (Apr 2, 2021)

Nealtw said:


> I would buy rolls of 23" and cut my own 1/2" longer than the space is wide and scrunch them together . So when installed each piece would be 22 1/2" long.


Yeah, cutting down 16" to fit looks like the best option. Doing that is just going to ad so much more time to the project.
Guess I just need to get off my lazy ass then, right?


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## Nealtw (Jun 22, 2017)

gyzhor said:


> Yeah, cutting down 16" to fit looks like the best option. Doing that is just going to ad so much more time to the project.
> Guess I just need to get off my lazy ass then, right?


Normally they make 15" to fit the 14 1/2 between joists that are 16" on center or 23" for joists 24" on center. 
I would buy the depeth of 2x4s, fill the joists and then do another layer over the top.


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## Bud9051 (Nov 11, 2015)

You will see when you try to crunch in a piece 2'' too wide it will want to crinkle and not be so neat. 

Fill the 2x4 joist space with r-13 or 15 no facing. Then over the top cross wise with your thicker layer. 

If you know you will be going up there occasionally create a walking path and cover it with that thicker layer. Hang markers from the rafters to locate all electrical boxes.

Also, while up there air seal all penetrations from below.

Bud


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## mike44 (Feb 3, 2021)

gyzhor said:


> I'm insulating a older house. The joists in the attic are both close together and unevenly spaced (between 14 and 14.5" apart, or 16 and 16.5" center line).
> 
> I'd like to install r-38 or greater, but I'm having a hard time finding anything that fits. Knauf makes a 15" r-38, but it costs nearly twice something like Owens Corning.
> 
> ...


If the framing was perfect, which it never is, the space between joists would be 14-1/2". 16 or 16-1/2" centers on your joists are still 14-1/2" between joists. You are getting concerned over nothing. Just install the batts, any bays that are way to small then cut them to size. As an example , 12" ID bay . Cut the batt 13" Pull paper on cut side down a bit and push the 1" of insulation up out of the way. Staple the uncut side, then you have 1" of paper on the cut side to staple.
Another easier way is to rip the batts to size and use spring wire cut longer than the joist ID. Push the wire up to hold the batts. I use a small bolt cutter to cut these wires. A Dremel or similar tool will cut the hardened wire. 
Don't use side cutters or any type of pliers. The spring steel is too hard and will chip the cutter. 
mike


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## gyzhor (Apr 2, 2021)

Lots of great advice here. Thanks all. Looks like just filling the joist bays to height and layering on top of that is the best way to go.

~gyz


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## Domo (Nov 9, 2018)

I always used a machete to cut insulation. The blade is long enough to go all the way through, sharp enough to cut evenly and easily held at a 45 to the insulation while remaining vertical for a good cut. Easiest way to cut I even found.

Trim your own to fit properly rather than cram insulation in, which defeats the purpose of it being fluffy (that's a technical term...).


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## Randy Bush (Dec 9, 2020)

Personally I find using blowin a lot easier and more effective then bats. And with places like HD doing rent free on the machine is even better.


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