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I have a 63 year old house that a lot of people have apparently worked on in the past. Under the tub in one bathroom there is a very large hole (2' x 2', a little irregular) that is allowing cold air from the crawlspace to cool the tub and go up between the studs, cooling the room by as much as 20F in the winter. I was going to cut and glue/screw rigid foam board to stop the air from entering, but due to how many pipes are there I couldn't get even two or three smaller pieces of rigid foam board maneuvered into place.

What is the best way to stop the air from entering? I have some R30 unfaced fiberglass insulation I was considering just stuffing densely up in the hole, but I've read fiberglass insulation isn't really an air barrier. I'm unaware of any spray foam product that would work since the void is so large. The water heater is directly above, so there is a very hot pipe exiting the water heater in this void I have to deal with as well.
 

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Post a picture, we all know what your talking about, just have no way of seeing it from here to make a good suggestion.
 

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Without seeing it, I'd put batt insulation up in the hole to fill the voids. Then try to piece the rigid in the best you can.
you can also find insulation wrapped in vinyl- like is used in pole barns. This can be hung from the joists- under the p trap and water lines
 

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I second the Fiber Glass batts in plastic bags to get your major holes filled, then try and put some rigid on top if possible. But I'm in the same boat, looking up into the tub from the unfinished basement there are huge holes, and I've sealed a good deal of stud bays with fiberglass batts (no paper backing) in regular grocery bags. The plastic makes the air seal (sort of), but if you get enough fiberglass in bags into the space you could seal with polyethylene underneath to get a good air seal.

Basically stick as much fiberglass up in there in bags and then polyethylene seal at the bottom would by my advice. But it's hard to say without pictures.
 

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Oc, over a crawlspace, IMO a poly sheet would be a bad idea as would poly shopping bags because they are vapor barriers (maybe in Canada and only with full stud cavity fill and full-contact with the drywall). Because the OP has to add plywood for the fire-stop (hole in the decking), poly would be a mute point, don't you agree.... FB is good though.

Gary
 

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Maybe I was misunderstanding his original post, but I thought I took away from it that he couldn't even figure out how to attach multiple small pieces of rigid foam. From that I assumed he could not do even plywood, which is why I suggested bags filled with fiberglass. You are right about not doing polyethylene sheeting under fiberglass batts, my bad. It appears that if you want to do a vapor barrier it has to be on the hot side in this case.

http://inspectapedia.com/Energy/Basement_Vapor_Barrier_Location.htm
 

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The bags would work to insulate the area around the tub (as I said in Post 5 earlier), but the flooring requires a solid structure repair. With a 2x2 hole you allow fire/moisture/air through. I have screwed plywood under the bags after using low-exp. canned foam to keep the tub water warm longer. Foam board may also require a covering against fire other than fiberglass insulation or drywall (not in a crawl...). Check with local AHJ when using alternative measures other than prescriptive codes.

Gary
 
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