I need to insulate a 3rd floor open ceiling (attic above) with some R30 insulation batts (not loose fill). Problem is that my joists are about 21-22” on center leaving a 17-18” cavity. Is my best or only option to buy 24 inch insulation rolls and rip it to width? Also what would be the best way to rip it?

I also have to insulate a portion of the roof rafters between the ceiling and the knee wall - so my challenge there is conforming rafter vents to the odd size cavity- any advice there?

 

Actually not worried at all about the cost, insulation is cheap. I am only worried about time and a good job. Pretty difficult to spray foam into an open cavity?? I guess I could use it on the roof rafters but that is a small job for all the startup overhead and besides I hate spray insulation and have a lot of mechanicals in that area. My issue is just dealing with the odd size - thanks anyway for your reply

 

Hi mlupa, unfaced batts either ripped to length or cut to 19" lengths and fitted across each bay. As long as you get a good fit, either works. If your spacing is fairly uniform, I have taken a hand saw and trimmed a couple of inches off of a roll before I unroll it. Since you have a knee wall, here is a helpful link:
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As for the roof vents, you should be able to find one you can trim to fit, although as long as you have one per cavity, the ones for 16"oc should be fine. Or make something to fit out of rigid foam, EPS or XPS. Just avoid a foil faced or plastic faced so you don't add a vapor barrier where it shouldn't be.

Hope that helps,
Bud

 

Spray foam should never be used between rafters, in the event of a leak, it can involve a lot of work trying to find the leak.

Spray foam between the joists is best, as it is a better insulation than most things and it is easy to do, with a air tight fit.
However, spray foam is very expensive.

The best bet is polystyrene sheets, these can be cut with a sharp knife, and cut to a push tight fit.
The downside, polystyrene requires careful measuring and fitting but, it does a very good job, and will maintain the same level of insulation for the life of the building. Plus, in the attic you can load things on to it, without lowering the insulation level.
With its sealed cell construction, it is wind proof and waterproof.

It will stay in place between the rafters as well.

 

You say not to use spray foam because it's waterproof and will make it tough to find a leak -if one occurs- yet you advise using polystyrene and fitting it tight so as to make a waterproof fit? If the goal is a tight windproof fit (waterproof should not be a big deal as when a leak occurs most people roof the entire house anyway) then you CANNOT beat spray foam plus it adds a better R value per inch than fiberglass. If i was building a new home and had the exposed skeleton to insulate I would 100% use foam.

If cost is an issue and you can buy the 4' x 8' polystyrene sheets cheaper then by all means save the money as long as your time isn't an issue because trying to cut multiple 2" layers of that sheeting and fitting it tight will take FOREVER

 

I am doing the exact same project. I purchased a hot knife. It cuts the EPS very fast, almost like drawing a line with a pen.
I removed fiberglass batts cut "sideways", tiny little 14" -> 17" inch widths stapled up; what an absolute waste of someone's time and money. All the fiberglass did was filter the air before it went out the roof vent.