Duct condensation, floor insulation, and other questions
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 2×6 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!
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Replies
Encapsulating the crawl space would be a far simpler way to solve your problems.
Don. See this article for more information on building an unvented crawl space: https://www.greenbuildingadvisor.com/blogs/dept/musings/building-unvented-crawl-space
Don,
If your ducts sweat during the summer, then clearly, you shouldn't be inviting hot, humid air into your crawl space. Close the vents -- but do it properly. See Building an Unvented Crawl Space.
I cant encapsulate it right now and the way it is layed out would not be easy to do. Knowing that, how would you go about insulating the space?
Don,
The problem you describe is a classic problem, and the solution we suggested is the proper solution.
Insulation is a good idea -- but the insulation belongs on the crawl space walls, not between the floor joists. You also need to make sure that you have a good vapor barrier on the dirt floor of your crawl space, and you need to make sure that the vents are sealed.
Your attempts to address this problem with fiberglass batts and Reflectix bubble wrap are doomed to failure. Once again, I urge you to read my article (Building an Unvented Crawl Space) and to follow the advice given in the article.
I have a vapor barrier on the floor. The problem is the amount of effort in my situation to totally seal the crawlspace. It is difficult because my garage has a storage space that goes into the crawlspace.
I find it difficult to believe that it is impossible to stop condensation on ductwork in a regular vented crawlspace.
I understand most people's opinion is encapsulation is the best thing to do, but I have read conflicting reports that enclosing the crawlspace can increase radon levels. I am not aware of a radon problem in my house, but I live in an area that generally has very high radon levels, possible due to the underground make up of this region. (Mammoth cave).
If I cant get an answer on a way, other than encapsulation, to condensation proof my duct, can someone tell me if using the reflectix around the sub floor and joist bay that the duct runs in, would eliminate the potential for moisture damage around the duct?
"I find it difficult to believe that it is impossible to stop condensation on ductwork in a regular vented crawlspace."
Impossible, no (not usually, but sometimes damned-near). The physics are pretty straightforware- either the duct surface has to be above the dew point of the ductspace air, or the dew point of the air has to be reduced to something lower than the duct temperature. In a humid climate the temperature of a vented crawlspace in an air conditioned house can run at or very close to the outdoor dew point, and it can take quite a bit of air-impermeable duct insulation to prevent condensation under summertime humid-day conditions. In practice it's often easier to seal off the crawlspace from the outdoor air so that the dew point of the crawlspace air tracks that of the fully conditioned space.
Using Reflectix to air seal the joist bays with the ducts might work if it's air tight and there is some air leakage in the subfloor to the conditioned space, but it wouldn't do any better than air-tight 6 mil polyethylene would. Making it sufficiently air tight might be pretty difficult to pull off too. The experiment isn't likely to be worth the trouble.
That brings up another question. My only reason for recommending the reflectix was the fire rating. Is it acceptable to use 6 mil plastic as a substitute?
Also, if the insulation does get wet but dries out, is the insulation still good? (fiberglass)
Don,
You are aware of some of the problems inherent in your approach. The biggest problem is hidden condensation that lurks for years, until mold or rot become so bad that they become obvious.
It's impossible for us to evaluate the quality of your workmanship, or whether there is any possibility that humid air might contact a cold portion of your ductwork, even after you attempt to create barriers to that air. Suffice it to say that excluding humid air from contacting your cold ducts is extremely tricky. It is far safer to keep the humid air out of your crawl space in the first place by closing the vents.
I'm definitely NOT recommending 6 mil polyethylene (or Reflectix) as a solution here. The likelihood of making it sufficiently lair tight even initially (let alone for the long haul) is a dubious prospect at best.
When fiberglass or rock wool gets wet then dries it's insulating function is restored. It can take a very long time to dry out though- long enough to start growing mold.
In a zone 4A (all of Kentucky) climate a vented crawlspace always adds more moisture to the house than it purges. Laying down a ground moisture & radon barrier (EPDM membrane or 10 mil polyethylene, sealing it off from outdoor air then insulating the walls to ~R10-R12 is also cost rational from an energy use point of view in that climate.
The physics of the phase change of water are straightforward, and the laws of physics are self-enforcing- you can't really cheat even if you wanted to. If there is a way to section off part of the crawlspace, building a good air-barrier around all of the ducts and air handler you could get there without fully encapsulating the crawlspace, but would still have to figure out how to deal with the make-up air for your furnace. I'd be surprised if that were easier than bringing the entire crawlspace inside the pressure & thermal envelope of the house.
Dr. Peter Sparks: "Physics is not democratic. It just is."
In other words, we don't get to vote.