Condensation on interior of duct
kpklein
| Posted in General Questions on
I’m planning a house in Climate Zone 3 (hot-dry) with a traditional vented attic. I know it’s best to get the ductwork out of the attic in this case, but if I do just run them up there (and accept the energy penalty), can mold grow inside of the ducts? A recent Build Show (Risenger) video said that was possible. I can see how condensation could happen on the outside of the attic ducts, but the inside too? Please advise.
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Replies
Since you are in zone 3, you can do an unvented roof with the insulation in the rafters as long as you have a diffusion ridge vent:
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
This would move the insulation above the ducts so the ducts are in conditioned space. This solves any condensaion issues plus all the other problems with ducting in unconditioned space.
Thank you! I had not seen the Lstiburek document you provided the link for. I like the idea of the typical insulation on the attic floor with vents near the peak (but only open for vapor, not fully open). This is not an option I had seen before. The problem with other methods, like insulation on the underside of the attic roof, is the reluctance of local builders to do things they are not familiar with.
Akos is right. Be aware however, the moving the building envelope up to the roof adds surface area to the house and that increases overall heat gain. Whether the tradeoff of better duct insulation makes up for it depends on a lot of factors. For most people, gaining a conditioned attic is enough of an extra benefit to go with insulating the rafters.
As far as your original question, you won't get any condensation inside ducts in a hot climate. The ducts are colder than the attic, so condensation happens on the outside of the ducts. This is one reason why air sealing the insulation on ducts is so important. In heating climates, it is possible for condensation to form inside the ducts between heating cycles, but it is relatively rare, as the heated air is also usually very dry. The worst cases of condensation and mold inside ducts that I've seen are when the ducts are used only for A/C in summer, with something else for heating in winter. It is possible for warm/humid house air to enter the dormant ducts and condense throughout the winter. That can get ugly.
Thank you Peter! That's what I thought concerning the condensation on there inside of ducts while running AC. I had not, however, considered the problem of hot/humid air getting inside the (dormant) ducts in winter and causing problems. Mine would not be dormant, and would be used for heat in winter. The problem here in SoCal of a full unvented/conditioned attic is that the builders have never done this type of attic, and are uncomfortable with trying to do trusses (or anything else) and insulation that is not typical. Also the roof pitch is low (Spanish style) at 4:12 or so, which limits the functionality of a conditioned space. And of course, another problem is cost. So I had pretty much given up on trying to do a conditioned attic that would be within the thermal envelope of the house. I am, however, planning on mechanical rooms which will keep the HVAC equipment within the house envelope (only ducts above). Another idea was to put the ducts in soffits, but I have move away from that, and am uncomfortable with burying them and making them inaccessible, plus the other complications that arise from that approach.
When you buy an existing home, you are stuck with all the choices made by its builder.
Often the builder see value in eliminating the utility room. He is selling living space and will never pay the 25% higher heating bills with the equipment in a vented attic. If you pay for spray foam upgrade you get a lower R value spread across a larger area your bills will only be 15% more.
If this is a new custom build given some planning you can make much better choices.
If this is more of a production build, they will not give you many options and any changes will come with big price tags.
Consider plenum trusses.
https://www.greenbuildingadvisor.com/article/use-plenum-trusses-to-keep-ducts-out-of-your-attic
Buried ductwork
https://www.energyvanguard.com/blog/buried-ducts-allowed-2018-building-code/
Concealed ducted miny splits
What You Should Know About Concealed Duct Mini-Split Systems (pioneerminisplit.com)
Walta