Insulating HVAC ducts
GBA Editor| Posted inMechanicalson
I have a feeling this is a dumb question — but I don’t get it:
Nearly every house in heat-pump-land has ductwork in the attic … insulated to R-6 or R-8. Sure, it should be in conditioned space, but for millions of structures it isn’t.
Why aren’t there products that offer thicker insulation for attic ducts? Or conversations about DIY ways to improve R-value?
What happens if someone adds a couple of layers? Or wraps ducts in wall batts?
I’m sure there’s a perfectly obvious reason based in physics to explain for why such a simple idea as this is really bad. I can’t remember, or find, an explanation hereabouts.
Thanks for your patience in advance.
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Replies
Ducts in attic w/ R8 insulation are popular because it’s price competitive (aka cheap) to build and install, in most cases. Fact is, HVAC equipment should never be designed and installed in a ventilated attic, and a properly designed, installed and commissioned system, inside the conditioned space, costs a lot less than a system that is not; or at least that is my experience for the last 15-20 years.
It really all starts with education and the home design, but you must coordinate several trades, as PPI (Plan, Program & Implement) becomes critical. Too many designers, builders and trades take the easy, cheap and lazy way out. You get what you pay for…and since builders and HVAC contractors do not pay for the energy bills of their clients, they have no incentives to do better. Sad but true.
Thanks Armando -- but I'm wondering why there isn't a way to improve the mistake/design in the millions of homes that have to live with the error. If I wrap extra layers, does that help the problem or make it worse? why do you think no one advertises a "better" way?
The apparent absence of a solution strikes me as weird. It seems kinda simple to improve.
Joe - with existing duct systems, I would be concerned with airsealing and duct condensation. Maybe some experienced GBA member can tell you a good way to fix ducts and HVAC systems in an attic. I would imagine Carl Seville has done many of those fixes in GA.
Joe,
Check Allison Bailes blog:http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/54332/release-the-kraken-the-ductopus-is-bad-for-air-conditioning? - How can you possibly insulate and fix that!!!
LMAO! Fill the whole attic with foam maybe.
Joe, Armando hit on the two main problems with more insulation on attic ducts: too expensive for builders to bother with it and condensation if you add more insulation over the existing wrapped duct insulation. If you do add more insulation, you would need to make absolutely sure the outer insulation has a good Class I vapor retarder (i.e., a vapor barrier) that's sealed. By putting insulation outside the wrap on the first layer of duct insulation, that first wrap will be at a lower temperature. If attic air leaks in, it could condense on the 1st wrap, which is (or should be) a Class I vapor retarder. For that reason, I'd recommend NOT adding insulation unless you also remove the first insulation jacket. Since they don't have Class I vapor retarders, that's also why you wouldn't wrap ducts in wall batts.
Glad you guys liked the Ductopus!
Thanks for answering, Allison. Much appreciated.
So if the ducts need to be sealed it could be an excuse to take off the old stuff.
It would be nice if someone made a product that would handle it right ... it would take enough tape to do a mummy, as well as a heavy dose of labor, to get any product I know sealed at the seams.
thanks. It seems so simple.
There's also the little matter of the air handler which usually comes as a package with the attic ducts. Not so easy to slap a fiber wrap around that unit even if it were a good idea. When faced with this kind of problem we've had some success with tenting the mechanical unit to bring it into conditioned space, then adding enough cellulose to completely bury the ductwork in the remainder of the attic. For whatever reason we've not seen a condensation problem with this approach.
Tenting the unit, James? You mean building a sealed insulated enclosure around the handler? And I should assume the ducting you buried was insulated with the standard foil-faced R-8?
The attic access is usually adjacent to the air handler, too ... it could be a way to kill two birds w/one stone, huh?
That's promising, though as you say it seems like condensation should be an issue around the ducting.
Thanks for the idea.
Yes and yes. Michael Chandler has written about the insulated enclosure approach on GBA.
Yes, it's a good side benefit.
Allison brought up the condensation concern not I. We have not experienced those problems here in NC, don't know if this is a local climate concern or the difference between f/glass and cellulose and/or the thickness of the added insulation. YMMV.
James, the condensation issue is spotty. It doesn't always happen, but anytime you bury ducts in attic insulation in a humid climate, itcould引起冷凝。我不会冒这个险my ceiling. Also, that problem has occurred in North Carolina, which is why John Tooley and Advanced Energy don't recommend doing it. Here's what Tooley wrote in a comment toan article I wrotein May of this year:
I would like to see my Dallas Ductopus in a fight with Allison's Ductopus. I took this pictures yesterday from a $2 million house under construction in Dallas... Nice job, eh? Did you notice the nice radiant barrier?
Owwwww! You're making my brain hurt, Armando! I thought GBA filtered out that kind of filth. ;~)