HVAC Ducts In Unconditioned Space
So I’m building this house (I’m the owner, mind you). In Omaha, NE. Not exactly what you’d call a warm subtropical climate, right?
We’re getting bids right now for HVAC. Talking to an HVAC contractor, he sez that they’ll run the ductwork up to unconditioned attic space, then branch off to my bedrooms, baths, etc. He says that doing so isn’t a problem because the ducts will be under all that insulation.
I’m a bit puzzled by this…even with the insulation, it seems a bit counterintutitive to me. Is this standard practice? Is it what you’d call “best practice?” Or is it simply the easy way to do things?
Jason
Replies
Given that your floor joists are likely running in one direction, I don't think all of the ductwork could be under insulation if he is referring to the insulation in the joist cavities. Is part of the bid to wrap the main trunk in insulation as well.
I'm not a professional so I wont speak about best practice but here in CT, I can say it's common practice.
I would recommend against ducts in an unconditioned space. Most contractors that propose such and justify doing so as this one has, will not attempt to seal the ducts as they should be in an unconditioned space, will not insulate them properly and just being "under all that insulation" is NOT good enough. This is cutting corners, and you can be relatively certain of having others cut (by this contractor) as well. Important little details like performing a proper load calculation, including adequate returns, sizing the duct work properly, excessive use of flex duct, etc.
Do you have any other HVAC contractors bidding the job? Do you have other means to run the ducts? What is the construction of your house? Single story, slab on grade? Two story over a full basement?
Is it standard practice? Yes, unfortunately, but a poor one. There are no real "standards" for good installation in a residence. IF it seems like a bad idea, it usually is.
My opinion is that this contractor has eliminated himself from further consideration. Find another one.
Some important questions you may wish to ask prospective contractors are:
What are the calculated heating and cooling loads on my home? Under what design conditions?
Do you propose to use flex duct? How much and where? How do you seal the duct?
Did you include returns in every bedroom? How many total returns in the system?
Do you put volume dampers at every branch?
BTW, the design conditions for Omaha, NE is -10 degF for heating and 95/75 for cooling. You have an average of 6000-6500 heating degrees days per year.
I would look for a new HVAC contractor.
Here in San Jose, CA, ducting is in either the attic or the crawl space - and it's ALWAYS insulated. Not particularly well, maybe, but it's insulated.
I shudder when I think of the condensation you'll have when the A/C is running and pushing cool air thru uninsulated ducting in a humid attic - lol.