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Ducts in unconditioned attics

K

kyhowey

Silver Member
Joined
May 26, 2011
Messages
100
Location
Elizabethtown, Ky
To bury or not to bury? Do you allow the HVAC installers to lay the ducts on the joists and then have them buried in the insulation? Or do you have them elevate the ducts above the insulation so you can get full R-value of the insulation? I've read plenty of opinions on both sides and I'm trying to establish a consistent interpretation for our area.

The state HVAC inspectors have concluded that they will allow either. But they also say the final decision is up to the building inspector. The only drawback for burying the ducts I can find is condensation in high-humid climate zones. Kentucky is not in the high-humid climate zone, so condensation is not a huge problem. I do worry that the insulators will not add enough insulation over the ducts to keep it R-38 causing those areas to violate the building envelope.

My main concern is large supply/return trunks that run the length of the house and how you would insulate those areas. Thoughts?
 
The main problem would be the reduction in r value where the ducts lay. If that problem is taken care of you should be good to go.
 
IMO it would not make much of an impact.

R-value.jpg


Francis
 
When you allow the ducts to lay within the rafters you are displacing the required R-49 (in my area) with 2 layers of R-8 duct insulation. Some will blow in extra insulation on top of the ducts but if you have some serious wind in your area then a washing of the blow in insulation occurs and you end up with littel or no insulation on top of the ducts.

I don't know if there realy is a good solution for this one
 
# # #

IMO, blown in insulation will settle significantly over time.....Plus,

with any vibrations on the ducts, ...again, over time, the "blown in"

type will settle significantly. [ i.e. - off of the ducts that were

initially buried in insulation ].

If your concern is to maintain a minimum R-value on all ducts,

then it would seem that the ducts would need to be wrapped and

sealed onto the ducts themselves, or install ducts with the

[ thermal ] insulation on the inside of the ducting, and not rely on

adding more "blown in" insulation as a long term R-value cover.

& & &
 
mtlogcabin said:
When you allow the ducts to lay within the rafters you are displacing the required R-49 (in my area) with 2 layers of R-8 duct insulation. Some will blow in extra insulation on top of the ducts but if you have some serious wind in your area then a washing of the blow in insulation occurs and you end up with littel or no insulation on top of the ducts. I don't know if there realy is a good solution for this one
Click to expand...
The really good solution is to run ducts in the conditioned space.
 
brudgers said:
The really good solution is to run ducts in the conditioned space.
Click to expand...
Correct you are

I should have remembered that from the miles of ductwork I installed in South Fl in dropped halls and soffits

If your concern is to maintain a minimum R-value on all ducts,then it would seem that the ducts would need to be wrapped and

sealed onto the ducts themselves, or install ducts with the

[ thermal ] insulation on the inside of the ducting, and not rely on

adding more "blown in" insulation as a long term R-value cover.
Click to expand...
My main concern is providing the correct attic insulation coverage When the ducts displace that insulation
 
Sorry, I didn't make it real clear. I am assuming the ducts are sealed and insulated per code. I am only concerned with maintaining the R-38 in the attic.
 
& & &

It would be my recommendation to remove the ducts from the

equation, by either suspending them above the ceiling insulation,

or installing them in the conditioned spaces, and then, ...install

the required thermal insulation as normal.

I'm guessing that installing them in the conditioned spaces

would be an atypical install [ i.e. - cost more money ].

# # #
 
In our area, the majority of homes that have ducts in the attics are homes/multi-family on a concrete slab. It's not prevalent, but we get several per year.
 
It would seem to me that you'd want to get the full R-value in the ceiling by not burying the ducts in the cavity. Insulating the exposed ducts to meet your local requirements seems a lot more energy efficient.
 
Perhaps we insulated up at the deck and this is an unconditioned space on the conditioned side of the envelope.

It seems obtuse to have the ductwork breach through the conditioned space and back again due to rvalue concerns (which doesn't require you to not factor in the structure).

Perhaps this all needs to go back to the drawing board and we re assess what the real physics of the world is telling us to do versus what we hav legislated to do in the codes.
 
How is this any different than installing skylights? I think you're overthinking this.
 
I misunderstood the question; to "lay the ducts on the" 2x8, 2x10 or 2x12 ceiling "joist" or are they trusses 2x4's, open web or TJI's? I did not interpreted it "in-between the joist".

The code simply requires that the R-value be met and not compressed or reduced except at the eaves.

Regardless if not in humid climate; if a vapor retarder is used it should to be below the duct in contact with the ceiling.



Francis
 
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