insulation of open joist bay in bathroom
pulper_11
| Posted in Energy, Heating & Insulation on
I want to insulate the open joist bay in my bathroom that I’m renovating, before putting subfloor down in this area.
there’s a lot of uneven spaces in this area due to blocking (some that i’ve added and some previous).
i read that using fiberglass insulation wrapped in a plastic bag can work. I did that with one area here. i’ve attached a pic or two, including a pic where this is discussed on another site.
My questions are:
1. is there an issue with flammability with this tactic? nothing was mentioned in the sites discussing this.
2. since the plastic bag completely wraps the insulation, creating a vapor barrier, is there any issue with moisture forming on one side of it?
Thank you.
Replies
The insulation in a bag creates a pillow that is stuffed into a cavity. It is an attempt to air seal as well as insulate. The problem is fiberglass insulation relies on the trapped air for its insulation. Stuffing the pillow into irregular shapes will squeeze the air out, reducing the insulation value. It only works well if the opening is a farily smooth regular shape, with minimal compression. I use that technique for insulating attic access hatches, as it can be easily removed / replaced as required with no mess. ( Roxul and heavy duty garbage bags work well)
Mineral wool insulation ( Roxul) is stiff enough that it can be cut to fit around the blocking and is self supporting. No need to bag. Drywall knives help when sliding the insulation into narrow gaps.
I might get a bag of cellulose (kind they blow in) and use a thin stick to help push it in. and then put a vapor barrier across the first bay, and up the wall. (you can slit the existing vapor barrier to avoid the double barrier issue)
Thanks to both of you for your replies!
I decided to go out and get some mineral wool insulation, as suggested.
here's a couple pics of what i've done so far. can't do the right portion of the wall yet since I haven't added blocking there.
let me know if you see anything wrong.
thanks!
Tiger Foam. Done and done.