HVAC in the attic
Hello,
I have part of my ductwork in my attic. My furnace and most of the duct is already in the conditioned space and it seems to be very complicated to bring the attic ducts back in the envelope.
My attic ducts are insulated to what I believe is R-6 and the attic is currently insulated to R35 with blown in fiberglass.
I am in zone 6 by the way.
After reading a few articles on duct insulation, in my zone, would it be better to add a layer of closed cell spray foam on top of my existing insulation and then to bury the ducts in more blown in insulation? Or can I just add more blown in insulation right away without fear of condensation?
As an aside question, would you go with cellulose or fiberglass blown in insulation?
Thank you for your answers,
Sofiane
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Unless the same ducts are used for air conditioning there is no condensation risk from simply burying them.
Cellulose is more air-retardent than fiberglass, but heavier per R. If going for more than R60 in cellulose it's good to verify the static load capacity if it's a trussed roof or if the attic floor joists are smaller than 2x10s. It's fine to blow cellulose on top of blown fiberglass, and the air retardency even improves the performance of low-density old-school open blown fiberglass products.
Thanks Dana.
I realize I forgot to say that I also have a central air conditioning unit. We mostly use the air conditioning unit sparingly two months of the year and almost daily in July and August. Would this create a problem?
As for the weight of the blown in cellulose, I live in an area where we get quite a lot of snow accumulation on roofs to the point we had to remove snow from the roof this year as we had a lot of ice accumulation from freezing rain and snow. In my scenario, I would assume going the fiberglass route would make more sense as long as I make sure to attic is still adequately air sealed. Does that sound right?
I knew nothing about house construction and design prior to buying a first home and starting to read from this website and a few other. I greatly appreciate your contributions.
Yes, air conditioning with the same ducts presents a potential condensation problem.
The snow load bears down on the rafters and walls, not the attic floor. They're not really related issues.
Ahh oops...for the attic floor part...Well, I have much to learn.
What would be my options, I remember reading an article here where it was said that ducts could be sprayed with closed spray foam and then buried, but I couldn't find any information on ducts that are already insulated. Can closed spray foam be sprayed on top of the existing insulation? Or does the existing insulation need to be taken out? Is there a better alternative?
Thank you once again!
Sofiane,
Here are links to two relevant articles:
"Burying Ducts in Attic Insulation"
"Solutions to the Attic Duct Problem"
What type of ducts are you talking about? Do you have insulated flex duct? Or do you have galvanized ductwork wrapped in duct insulation?
Hello Martin,
I actually have read both and the “No Sweat,” as well as I couple of others. I understand the general principle, it's the specifics of my particular case that still elude me.
To be precise : I have 15 year old but relatively high efficiency furnace (92%) in my basement as well as an air conditioning unit. The vast majority of the ducts are in the basement and made of galvanized steel. I have a galvanized steel box in the attic and galvanized steel circular duct runs that provide heat/air conditioning to the rooms on the second floor. Obviously, there is a duct running in the conditioned space between the first and second floor all the way to the attic. The galvanized ductwork is wrapped in duct insulation (fiberglass + reflective foil) to what should be R6.
I do have a small run of insulated flex run as well in the attic insulated to R6 as well.
What I want to do is get to the equivalent of Buried ducts inside conditioned space as written in :
"https://www.greenbuildingadvisor.com/article/buried-ducts-allowed-in-2018-energy-code."
I was planning improve the air seal of the attic and increase its insulation to at least R60 already. Now I don't know whether I need to add closed cell spray foam on top of the existing duct insulation, add further insulation to the ducts or simply add the attic insulation and leave the ducts as they are to get to a "duct in the conditioned space equivalent"
I hope that makes it more clear.
Again, thank you so much for your help.
Sofiane,
If you want to go the "buried ducts" route, you have to follow the rules. That means that you need to wrap the ducts with a minimum of R-8 duct insulation (that is, insulation that includes an exterior polyethylene jacket) before you bury them.
The details are provided in my article, "Burying Ducts in Attic Insulation."
Can I add a layer of insulation on top of the foil or do I need to remove it first?
Sofiane,
Your first step is to upgrade the duct insulation from R-6 to R-8. You can either (a) remove the existing R-6 duct insulation and install new R-8 duct insulation, or (b) wrap the insulated ducts with new duct insulation -- that is, insulation that includes a polyethylene or foil jacket -- that adds at least R-2 to the existing duct insulation.
Once you've done that, you can bury the insulated ducts with blown-in fiberglass or blown-in cellulose.
Great discussion.
Re: moving air handler to conditioned space, what if there's no space for air handler and furnace on the 2nd level and the vented attic is the best place for it (2 zone heat). With space limitations Is it common to put the second zone equipment in the basement (with a long inefficient run) or build-out an insulated machine room in the vented attic?
>"With space limitations Is it common to put the second zone equipment in the basement (with a long inefficient run) or build-out an insulated machine room in the vented attic?"
Without a field survey of existing homes it's hard to say how common it is, but installing the mechanicals for supporting the loads of second story spaces in a basement with long duct runs isn't rare, and isn't as big an efficiency hit as installing in an unconditioned vented attic. It's easier and cheaper to install the equipment in the attic- no duct chases to build, cutting the register holes in ceiling gypsum rather than flooring, etc.
Building out an insulated mechanical room in the unconditioned attic IS rare, and probably more expensive than alternative solutions.
I’ve been struggling to get any HVAC contractor on board for the retrofit so far. I’m very new to anything construction related. I’m actually doing my first project - air sealing and improving the insulation of my rim joist with cut & cobble - that I don’t find particularly easy, but doable as I had a number of penetrations in one side and I am dealing with a finished basement.
All this premise to ask whether it within the realm of a DIY newbie to upgrade the insulation around the ducts that are in the attic?