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Condensation on HVAC Ducts

I'm currently working on half a dozen projects in a 70 year old house.

Today, I was up in the attic space finishing off the installation of a bathroom light/vent combo, & decided to check on an issue I ran into when trying to lag a beam across the new opening for my bedroom closet. I used a 1/4" bit to pre-drill holes for the 3/8" lag bolts, & had done so previously with very good results. However, when tightening the lag bolts for this beam, they would go in about 95% of the way, but would not tighten up.

Having finished the bathroom light/vent early, I decided to take a look to see if I could spot a problem in the attic that might cause the lags not to go all the way in. While digging out the blown-in insulation over the closet space, I noticed that some of the insulation was wet, not just damp, wet as in I could squeeze moisture out of it. Since it hasn't rained in 3 days & the roof is less than a year old, & after running my hand along the bottom of the duct, I'm 99% positive that the moisture is comming from the ductwork. The duct in this location was covered to the top of the duct with the blown-in insulation which appears to be either cotton or wool fiber. I also noticed that the floor joists & attic floor were damp from the condensation. Short of running the air conditioner less, is there anything I can do to reduce or eliminate the condensation?

As for the lags, I don't see anything that would prevent them from being fully tightened, so I may just remove them & use 3/8" Grade 5 Hex head bolts & lock washers to secure the beam with.

Re: Condensation on HVAC Ducts

By your description it sounds like the duct work it self is not wrapped with appropriate duct insulation rather just in the raw and was simply covered over with blown insulation used for the attic.
Well that would create the issue of condensation you seem to have discovered. Since the attic is hot and any leakage of the ducting for the cold A/C air and just the cold surface of the exposed ducting will create condensation.

I recommend to seal all the joints of all the duct work with metal duct tape ( not the cloth based duck or sometimes called duct tape ). You can use the cloth based duck tape for anything else but not for duct work ... heck you might even use it for securing the beam and forget about the lags.:D ( just kidding )

Then once you have all the joints taped and sealed wrap all the ducting with appropriate insulation and use the dust tape to seal that as well. Then cover all this with the blown insulation. The heat from the attic will boil out any moisture in the blown insulation and it will likely be fine.

Re: Condensation on HVAC Ducts

We have had customers with the same problem. The easiest way to solve this issue is to have the duct work insulated with 2lb closed cell spray foam insulation. This type of insulation will seal any cracks that their may be, and it will insulate better than any other product available on the market today. 2lb spray foam has an average R-Value of 7 per inch. We recommend a min. of 2" coating on all sides.

This will solve your problem and you will see a difference in the temperature of the air that exits your ducts, both with the AC and heat being that the duct work is in your attic space. If you look into the insulated ducts that are available, you will find they are insulated with an average R-value of 2-4. 2" of the closed cell foam will give you a consistant R-Value of 14.

With the cost of energy now a days, its worth the little bit extra, and it will pay off in the long run.

Re: Condensation on HVAC Ducts

yep ... that'll do it perfectly.;)

Re: Condensation on HVAC Ducts

I thought it might be something like that.

I've been looking at the various spray foam's... & I'd love to go that route, only problem for me is the price. Even though it could save us a bundle on electricity costs, I simply cannot afford it upfront :( 90% of the house is without insulation as is, but maybe in the future... for now I'll see about getting some duct wrap.


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