Will a thin foil insulation serve as an effective vapor barrier to prevent condensation in my crawl space before wrapping it with fiberglass for insulation?
Asked by Marius Harestad 
8 months ago
Last Updated: April 19, 2024
I’ve got some condensation in my crawl space coming from the air ducts. Do you think that thin foil insulation would do the trick as a vapor barrier to stop the condensation? And then I can add some fiberglass insulation on top of that for extra warmth.
Where is your location? Do you experience condensation when heating or cooling? Are the crawl space vents open? Is the condensation coming from inside or outside the ducts?
I’m not quite following what you’re showing here. To prevent condensation, you need R-value to increase the surface temperature and eliminate the cause of condensation, which is the drop in air temperature that leads to moisture in the air settling on the surface. A layer of foil alone won’t suffice. The foil simply blocks moisture from passing through. Insulation with a vapor barrier works by reducing moisture movement from the moisture side (usually inside a house) to the exterior surfaces (cold side) of the house. The same principles apply to duct insulation.
This is exactly what I mean. Right now, I just need something to stop condensation and I’m not concerned about R-value at this moment.
This really helped clarify things for me. I was focused on preventing moisture from reaching the ducts and condensing, but forgot that the moist air can condense on the foil if the duct isn’t insulated.
The insulated ones with condensation could pose a problem. There might be others with condensation that isn’t visible at the moment. This could potentially become an issue down the line. Moisture can accumulate inside the insulation without you realizing it, especially with the seam positioned higher up and away from the bottom.
The majority of the insulated ducts are in good condition. However, there are a few with condensation. The uninsulated ones are definitely dripping.
The vapor barrier is not very effective with the seam wide open like that. Have you experienced any moisture-related issues with the insulated ducts?
Hey there , thanks a lot! About 90% of my ducts have insulation made of fiberglass, with a cloth layer on top. I believe the cloth acts as a vapor barrier and the fiberglass is directly against the metal ducting.
I had some rodents that chewed through the insulation and used it to build their nests. I’ve been busy working on the crawl space for the past year.
I waterproofed the crawl space, laid down the vapor barrier, insulated the rim joists with foam board and rockwool, got rid of the rodents, and sprayed for bugs and mold.
My final task down there is to address this duct work issue.
Hey , it’s possible that gas heat tends to be dry.
Regarding your initial question, if the current duct work is metal, I recommend using R9 duct wrap. It’s a single step process for adding insulation and a vapor barrier.
If the existing duct is uninsulated flex duct, my suggestion is to replace it with R9 Insulated duct.
Working with flex duct can be a DIY project as long as you’re comfortable maneuvering under the house.
There is a way to line the inside of your ducts with foam, but that’s a job for a professional contractor since the equipment and materials are specialized.
Right now, the humidity in the house is at 53%.
Hey , it seems to be propane. Maybe during winter, it doesn’t happen. Just a guess.
I agree with you , it is strange to have condensation during the heating season. It’s possible that along with leaks in the duct, the humidity in the house is high and warm moist air leaks into the cold crawl space, causing condensation. Similar to a dryer duct leak.
Hmm. Dealing with condensation can be quite tricky for me. It seems strange to have condensation when the heating is on … because when warm, moist air hits the duct, the relative humidity drops, so condensation shouldn’t happen. It’s the opposite in cooling mode … where moist warm air hitting a cool duct causes a sharp increase in relative humidity, leading to condensation (similar to a glass of ice water).
Not sure what you mean by thin foil insulation. Tin foil is not typically used as insulation. It could act as a radiant barrier … but that wouldn’t be helpful in this case.
Typically, you would install a vapor ‘barrier’ on the moist side of the insulation. If your crawl space is relatively damp, you should apply duct insulation with the vapor barrier on the exterior. Before that, make sure your ducts are well sealed (which is a good practice in any case). Avoid using grey duct tape for sealing as it’s not effective. Seal all joints and seams properly before installing insulation. Ensure that the vapor retarder on the insulation is also sealed to prevent air from the crawl space reaching the duct surfaces.
I believe that’s the correct approach. Let’s wait to hear other opinions as well
From Oklahoma. Having a vented crawl space with a vapor barrier on the dirt floor. Condensation is a problem in both summer and winter, occurring outside the ducts and dripping onto the vapor barrier. It seems like condensation is the main source of moisture in the crawl space
Consider using spray foam for the ducts.
I decided to go with the foil material. I wrapped it around the ducts where insulation was lacking and managed to do a pretty good job.