I live in south central ky. My ac unit/furnace and ductwork is in a vented crawl space. The house was built around 1998. Subfloor is osb. Joists are 2x6 I believe.

My ductwork is 6 inch flex. I am replacing some of it with 6 inch sheet metal due to the fact some of my kids cats got under the crawl space and pawed at the duct, tearing some of it.

All of my ductwork sweats in the summer. It is all insulated, but still sweats. I noticed the flex feeding my two bathrooms is sweating as well and the sub floor is wet. I am pretty sure it is not that way in winter as I have crawled under their in winter to run an additional electric line and did not notice it. The duct to these bathrooms is not torn the best I can tell.

My idea is to replace this duct with 6 inch metal as well. Install fiberglass mesh tape and mastic around the joints as best I can. (It is hard to get the top with the duct installed and it is too long to pre assemble then install duct). Once the matic is dry, installing frost king self adhesive foam insulation around the duct. (The reason for this is less about insulation and more about helping to insure everything is sealed appropriately.) On top of that sliding regular insulated flex duct over the self stick insulation and duct.

My question is, would it help to insulate the floor joist "bay" as well? The duct runs in this bay from the large 16 inch supply pipe that runs down the center of the house to the bathroom floor register. I was thinking it would but a littler perplexed about how to go about this.

There is probably a couple of inches between the top of the duct and the sub floor. I could just force some regular r-19 with the vapor barrier attached to the subfloor, but I have read the paper on this insulation does not like moisture.

My second though was to put some xps rigid insulation of the appropriate thickness on the sub floor, then some xps attached to both floor joists, then stuffing regular fiberglass insulation in the spaces between the xps on the joists and the duct.

I also thought of using the reflectix bubble insulation attached to the subfloor then down the sides of the joists, then using regular fiberglass stuffed between the joists and duct. The purpose of the reflectix bubble is not to use it as insulation, but as a better vapor barrier than what comes on the paper faced insulation. It is also class A fire rated. (If that matters.. I live in the country and there are not a lot of local codes around here, I even have Amish neighbors!)

Lastly, no matter which route I go, I thought about putting a piece of XPS across the bottom of the joists in the bay to increase insulation value, for a neat appearance, and to protect the duct from further damage. Do you see a problem with this?

Sorry for all the questions, I appreciate your help!

On Monday, August 1, 2016 at 9:20:01 AM UTC-4, wrote:

Not sure what you mean by "floor joist bay". If you mean a bay that has
a flex duct or similar within it, I'd say closing it off would make things
worse. Any condensation that does occur will stay trapped and I don't
think you can prevent all condensation from happening. In any caase,
I don't think it's worth the trouble as long as the duct itself is
insulated.

If you mean a return duct, where they use the bay itself and just nail
sheetmetal over it to close it, then it might be worth it to put
insulation over the bottom of that. That metal would not be as cold
as the outgoing AC air, only at room temp, so condensation probably
isn't occurring, but it would save energy winter and summer.