Insulating HVAC ducts in unconditioned attic.
I have 40 feet of square metal ducting in an attic. It is subject to extremes of temp and humidity. I am located in climate zone 5A, southern NH. Currently it is insulated with a pitiful 1/2″ of fiberglass wrap and vinyl.
The ducts sit 6″ above the ceiling joists. Total distance from ceiling drywall to bottom of ducts is approximately 12″. The duct is held off the ceiling joists by 2×6 spacers. and metal ties.
There is plastic vapor barrier on the attic side of the ceiling drywall.
FG batts combined with blown in FG insulation fills the space from the ceiling up to the underside of the ducts.
I intend to strip the existing insulation and seal the ducts (which was not done during installation).
I then want to make a 3 sided structure of 2″ foil faced polyiso insulation board over the ducting, anchoring it to the ceiling, which would be the 4th side. I will seal the joints between the polyiso boards with gap and crack foam. This would make a box that brings the ducting into the conditioned space of the house and isolates it from the unconditioned attic. In effect, an upside down soffit.
I intend to leave the present attic insulation in place under the ducting when I enclose it.
The 2″ polyiso will be R-13+. The recommended R value for attic ducts in my climate zone is R-13.
My primary question is, can I use 1″ polyiso board to fill in the space between the ceiling joists, beneath the ducting? The 1″ is only R-6 but it will be backed up by the insulation that is under the ducts now, that I will enclose. I’m not sure what the effective R value of that arrangement would be.
My main concern is to avoid condensation on the surface of the Polyiso board when running the ac on a 72 degree dew point day.
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To avoid condensation you need a sealed vapor barrier on the exterior of the insulation. You can see in your photos that the existing insulation came preinstalled with this. Moisture will want to condense on any cold surface that it can reach, like your metal duct when it is in cooling mode, so however you insulate it you want to be sure to have a vapor barrier out beyond the point in the insulation where the dew point will occur (typically the exterior of hte insulation). If you use polyiso, you probably want to use foil faced polyiso (with the foil on the exterior acting as the vapor barrier) and tape any seams/joints with aluminum duct tape.
Are you sure you're getting much leakage from the duct?
Yes I wrote that I will use the foil faced polyiso. Without taking the current insulation off I'm not sure of the extent of leakage. However, the fittings for the takeoffs from the main trunk to the the branch flex ducts were not sealed at all and there were a lot of gaps between the bent over tabs that attached them to the main duct.
As I said, the main question I had was whether the 1" polyiso was good enough R value to block off between the joists, beneath the main trunk. I may have to make a drawing to make clear what I'm asking.
The r-value for the blown in insulation is around r-2.5 per inch and the polyiso is 6.
Thank you for the answer.
I think you will find this article interesting.
https://www.energyvanguard.com/blog/buried-ducts-allowed-2018-building-code/
The big concern if moisture in the cooling season. My guess is in VT the cooling season is about 2 weeks or so. LOL Really your cooling season is almost too short to be a problem.
The R2 fiberglass never became a moisture problem I think the higher the R value the less likely you are to have a problem. I would not spend time and money building a box around the duct.
I would air seal the duct before I buried it. Clean the metal first with something like 409 to remove all dust and dirt then seal every joint with foil tape. If you can not clean it then cover the joints in mastic, it can handle some dirt and dust. Then turn on the blower and hold a burning incense stick near every joint if the smoke gets disturbed you missed a leak fix it. Then bury the duct in cheap the same fluffy insulation you have already.
Walta
Hi, thanks for the response.
The high heat and humidity index in southern NH lasts a lot longer than 2 weeks.
The reason that I suspect that the R2 insulation never had a moisture problem is that it was not enough insulation to allow the surface of the vapor barrier to get down to the dew point.
With an ambient attic temp of 100 F, the surface of the vapor barrier will be 100 F before the ac starts. Even if the air in the duct is 55 F when the ac runs, for argument's sake, the volume of the ambient air is so much greater than the cooled air in the duct, that the surface of the vapor barrier will rarely, if ever get down to the dew point. If it does, it will do so only briefly, and once the ac shuts off the temp of the surface of the vapor barrier will quickly rise above the dew point again. Thus the opportunity for condensation of vapor will be very limited. It would be a greater problem if the temp and dew point were close together.
I'm afraid that if I applied your suggestion, and buried the duct without a vapor barrier on the outside of the insulation, that the insulation would keep the duct surface cool for a long time but allow water vapor to penetrate the loose insulation to condense on the metal duct.
Perversely, increasing the insulation without a vapor barrier would cause a problem where one does not exist now.
I would consider the following:
- Remove the existing duct wrap insulation, and seal all joints in the ductwork with mastic. Use a mesh tape for larger gaps in the ductwork (if they exist).
- Install 2” duct board to the rectangular ductwork, and take the seams with a high quality metal duct tape.
- Wrap the run-outs with 2” foil-faced duct wrap.
Your best bet is to find a HVAC supply house that can get the good stuff material-wise.
This sounds like a solid plan of attack.
From a design perspective, I treat southern NH as zone 6. In fact, several municipalities in this area of the state require zone 6 standards despite the mapping.
Your goals should be well beyond minimum standard.
Thank you all for the answers. Local supply houses only stock R8 FSK duct wrap. They won't order 1 or 2 rolls of R13 for a residential customer (or commercial one either I suspect). This won't change until the code for climate zone 5A demands it.
The price quoted for R13 FSK wrap on line is astronomical.
This leaves me with spray foaming the ducting or the 2" polyiso boards. I will use the spray in areas where it is difficult to put the boards.
I will concentrate on the main trunk because it is the most exposed to the attic air, has the most joints and the most surface area. Cutting the pieces to fit, lugging them up through a 2x2 ceiling hatch on a step ladder is giving me nightmares and I haven't even started yet, and the temps are rising.