Question about insulating ducts in attic
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| Posted in Energy, Heating & Insulation on
Why does the duct insulation on my cold air return ducts in my attic have an outer foil layer?
I live outside of Chicago, and for half the year, the air that’s running through those ducts is much warmer than the air in the attic. Isn’t that a problem for moisture?
Replies
The foil, which much more importantly is also a vapor barrier, is mostly for air conditioning. In Chicago design conditions, the dewpoint of outside air is around 74 degrees. The air inside the supply duct during cooling is (hopefully) in the 55 to 65 degree range. The return duct will have air at the conditions inside your house, anywhere from 70 to 80, depending on what is comfortable for you. The warm moist outside air will condense moisture on any surface cooler than its dewpoint. The returns ducts generally don't get that much condensation. This is why the foil/vapor barrier is important, especially on the supply ducts. If the ducts were wrapped in insulation with no vapor barrier, the insulation would quickley become soaked with water and useless.
All ducts in an unconditioned space should be insulated. The foil faced, 1-1/2" fibergalss "duct wrap" is the most common, least expensive bulk insulation available from the HVAC suppliers. The supply ducts are insulated as well, just differently. The black or silver plastic flexible ducts in the attic (most likely) are the supplies, and they have the insulation "built-in".
Edited 10/4/2006 9:31 am by Tim
But don't I have the opposite problem in the winter? I would think that with warm air in the returns during the heating season, assuming that the ducts aren't 100% air-tight, I'm going to get wet fiberglass at that point.
The cold air returns in the attic are strange. The adapters that go from the wall to attic ducting aren't insulated at all. There is a backbone that runs along the ceiling, and that has no insulation or vapor barrier. But the ducts that run between the two have foil-faced insulation that is just tied in place with string, and the foil isn't anything close to air-tight. I guess before I insulate the attic, I need to rip all of this out and do it right.
In the winter, the warm air inside the house, is typically very dry compared to the outside air in the summer with the dewpoint being in the 30's and not the seveties. The concern is real and you may have condensation and ice forming in and on the insulation. But air leaking out of the ducts (supplies only) and through the insulation would be dispersed into the very dry air in the (vented?) attic. Return ducts leak in, not out so the warm air stays in and the cold air leaks in and mixes with it. Supply ducts leak out.
Just to be sure that we're on the same page, when I refer to a "return", I mean ducts/grilles that are "returning" air from the space back to the air handler/furnace. A "supply" is a duct/grill that is supplying air (heated or cooled) to the space.
From what you said, I take it that you have return grilles located in the walls, with wall stacks (ducts that fit inside of stud walls) that pass through the top plates into the attic, connected to branch ducts that are collected into a central trunk line, (the backbone). For the purposes of insulating, it doesn't matter because every bit of exposed duct should be insulated if at all possible.
Unless it is particulary degraded, I would not remove the existing insulation. You can seal it with the silver foil tape (not the basic gray duct tape). If there is enough wrap in place where you can pull the two edges to overlap enough, and fold them over and staple the joint, and then tape it, that would be ideal. Just a good tape joint is plenty, though. You can buy the foil-faced duct wrap at some if not all of the home mprovement stores ans well as the tape. For insulation the thinner, more flexible (and subsequently cheaper) foil tape is best. The thicker stuff is more suitable to connecting dryer vent peices and sealing duct work.
Yes, I'm talking about returns in the same way you are.
If that's the point of the vapor barrier, is to enough to blow in enough insulation to cover the return ducts with several inches of blown in insulation when I insulation the rest of the attic?
I would recommend that uninsulated portions be insulated first. As far as the parts that are insulated and not well sealed, they should be fine.
I'll do that. Thanks for the advice.