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 2
x6 I believe.
My ductwork is 6 inch flex. I am replacing some of it with 6 inch sheet met
al 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 swe
ats. I noticed the flex feeding my two bathrooms is sweating as well and th
e 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 n
ot 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 fibergla
ss 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 in
stall duct). Once the matic is dry, installing frost king self adhesive foa
m insulation around the duct. (The reason for this is less about insulation
and more about helping to insure everything is sealed appropriately.) On t
op of that sliding regular insulated flex duct over the self stick insulati
on and duct.
My question is, would it help to insulate the floor joist "bay" as well? Th
e duct runs in this bay from the large 16 inch supply pipe that runs down t
he center of the house to the bathroom floor register. I was thinking it wo
uld 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 su
b floor. I could just force some regular r-19 with the vapor barrier attach
ed to the subfloor, but I have read the paper on this insulation does not l
My second though was to put some xps rigid insulation of the appropriate th
ickness on the sub floor, then some xps attached to both floor joists, then
stuffing regular fiberglass insulation in the spaces between the xps on th
e joists and the duct.
I also thought of using the reflectix bubble insulation attached to the sub
floor then down the sides of the joists, then using regular fiberglass stuf
fed 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, fo
r a neat appearance, and to protect the duct from further damage. Do you se
e a problem with this?
Sorry for all the questions, I appreciate your help!