Different Take on Attic Ductwork Insulation – Seeking Feedback
Hello GBA Experts,
This is my first post, and it’s with bit of urgency.
Long story short.. We had a massive tree fall on our house in one of the recent storms (everyone is ok!). The roof and 2nd floor ceiling needed to be replaced giving me an open attic to play with. I took the opportunity to seal some horrible air leaks on top plates and a few other locations, add ceiling fans, etc.
My house has no AC, we rely on noisy 5,000 btu window units in the bedrooms and struggle with even cooling on the second floor. first floor has a mini split I installed that works well. I do not want mini splits in the bedrooms.
I’ll start by saying that I know ductwork in the attic is not ideal and is the last resort. For my case, it’s either ductwork in the attic or window units for the foreseeable future. I have an available closet (conditioned) on the second floor to put my air handler and the return would be centrally located at the top of the stairs. The supply goes up into the attic and feeds 4 bedrooms and two bathrooms. The ductwork is rigid and currently 1-2 inches above 2×4 trusses on the attic floor. two 6in ducts run next to each other for about 15 feet until one takes a turn for a different room. Attic is unconditioned with a soffit and ridge vent. R49 blown fiberglass will be added once ductwork is complete. I am aware of the condensation concerns burying ductwork in a mix-humid climate (4A Near Philadelphia) and I did read the 2018 reports in SC/NJ regarding the successful insulation and burying of supply ducts and believe I can hit those requirements in a few ways. I searched many articles and there is one method (method 2-3 below) I have not seen talked about and would love professional feedback.
Method #1: Seal the ductwork with mastic and encapsulate with 1.5-2in of closed cell foam. Duct work would rest on the rafters/attic floor and would have minimum of R15 on all sides. Total R value around the metal duct would be about R24.
Method #2: Ductwork in this case would be spaced off the rafters using 1/2 foam board so there isn’t direct contact. Seal the ductwork with mastic, add R6 insulation sleeves (might not have time to order the R8). Using 1in rigid foam board, build a three sided box around the entirety of the ductwork (4th side being the attic floor, sealing the foam around the rafters and drywall with spray foam. The ductwork in the box, which would have an r6 jacket, could also be surrounded by loose fill insulation or batts.
Method #3: Same as method #2 but no additional fill in the box, 1/2in foam for the box, and the exterior fully encapsulated with closed cell foam.
The idea for methods 2-3 is to try and created a “conditioned space” for the ductwork and blocking it from contact with raw attic air. I figured it was similar to ductwork in an exterior wall.
I hope I gave enough information upfront! Condensation being the ultimate concern, what are expert opinions on these options?
Thank you for your time!!
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Replies
"The ductwork is rigid and currently 1-2 inches above 2×4 trusses on the attic floor. "
I want to be clear on what you mean here. So there is a truss, made out of 2x4's? Since you say "attic floor" I assume that it's a style that has a clear area on the inside? And there is a space of 12" or so between the attic floor and the ceiling of the room below, which is where the insulation is going to go? And the duct is 1-2" above that floor?
If that's the case, ideally the duct would go through the trusses, the thickness of a 2x4 above the ceiling. It would be buried in the blown insulation.
To Bill's point below, if you're stuck running them above the floor you can still run them in loose insulation. Just build a "coffin" around the ducts and pour loose insulation into it.
Thank you for the reply!
Sorry if that was a little bit confusing. Yes they are howe trusses constructed using 2x4's, 24" on center. Attached directly to the bottom side of the 2x4s are the 2nd floor ceilings. The ducts would ride right along the top side of the bottom span of the truss or be spaced using a small piece of foam board so its not in direct contact with the truss.
The stack up would be 2nd floor drywall - 2x4 truss - air/foam gap - ductwork - blown in insulation (R49 or maybe less depending on how much I can fit over the ductwork before contacting the baffles on the roof deck.
Whether the ductwork runs parallel or perpendicular to the trusses, it will remain about 3.5-5.5 in above the 2nd floor drywall. Aiming for R15 under the ductwork, not including the insulation around the duct itself.
I hope that helps clear things up.
OK, that sounds pretty good. If at the end of the day you want the insulation deeper over the ducts you could box around them to hold the insulation.
The box/coffin was something I thought sounded nice in concept, but i wasn't sure if insulating the pipe then boxing it in with foam and packing in more insulation around it would cause any issues. I guess the box would technically be a second vapor barrier, but I'm not sure if that's good or not. Although the bottom of the box is just the 2nd floor ceiling, so not a complete vapor barrier. I feel like enclosed in a box the duct would be just as, if not more, insulated than ductwork on an exterior wall?
Since this is attic work, and attic work is miserable (if it's NOT miserable, you're probably not doing it right :-D), I'd put some thought towards minimizing your installation labor here. I would consider the following:
1- Seal all duct joints with mastic as you mentioned, and do it right with a "nickle thick" layer. It shouldn't look like a layer of paint when you're done, there should be some noticeable thickness to it.
2- Use insulation sleeves over the ducts, or something like Reflectix wrapped around the duct. You don't need much insulation on the duct themselves if you bury them in something else, you're mostly just looking for some vapor control.
3- Use some cut blocks of polyiso or XPS to support the ducts off of the trusses if you want to do that. I'd cut semicircular saddles out of the insulation material to hold the ducts. This is relatively quick and easy, and the saddle-type design will help keep the ducts in place. I'd avoid using EPS here since it's much more crumbly than the other two materials.
4- Bury the ducts in loose fill insulation. I'd strongly consider using cellulose here instead of fiberglass. Cellulose will likely be cheaper, is more R per inch (sometimes a plus if the attic is tight on headroom), and cellulose will help a little if you have any air leaks in the attic floor -- although I'd still put some effort into air sealing the attic floor prior to insulating.
In this way, most of the insulating is done by the loose fill insulation, and you can just build that layer up thicker over the ductwork so that you don't have thin spots in your insulation layer. This is relatively easy to do, and not very expensive. If you have no alternative to running ducts in the attic, this is the method I would use.
Bill, who has been working in attics all day today and so is rather focused on the misery of such work at the moment...
Thank you for your response, Bill!
I decided to rent a vacuum and remove the old insulation myself...I didn't get more than 10 minutes into it and deeply regretted my decision. Insurance covered the rental cost, but it didn't cover the misery! I wanted to get eyes on the structure to see if I could find any hidden damage before the contractor got started. If misery is an indicator of right vs wrong, I believe I'm on the right track!
Responses to your comments:
1- Yes, no sense in rationing product, it's going on thick!
2- I considered sleeves as well as gutting a flex duct so I could get a longer run of insulation. Not sure if that's possible. Only reason I considered the spray foam is because the insulation guys were coming out anyways and the sleeves came out to about $400 plus my time (misery). I'd rather pay more and have someone else do it, in this case at least.
3- Understood, I like this idea.
4- I should be able to get at least ~R23 around the ducts, not including whatever insulation I wrap the ducts in. I didn't like cellulose because of how it aged in my attic. I had a mix of cellulose and fiberglass and the fiberglass looked almost new, while the cellulose was extremely dusty. With that said, R value is paramount and I will reconsider my choice based on your feedback.
Do you think there is any benefit to building the box/coffin around the ducts and packing the box/coffin, as DC said, with loose fill? The box/coffin would be capped and sealed on all edges. I'm all for more work if it minimizing potential failures!
I don't see a problem building a box around the ducts as DC mentioned. I don't think you'd have a double vapor barrier problem there, either, BUT I would build it a bit differently. I would build a top with 1/2" or 3/4" plywood, and the sides with 1/4" waferboard (my favorite cheap panel product). I would frame the top to be able to have someone walk on top of it. At some point in the future, someone is bound to step on the duct box, and if the duct box can support that someone, you avoid a major failure. I now build boxes over bathroom fans using 3/4" or 1/2" plywood and 2x2s/2x4s for this reason, originally it was either Akos or Malcolm who suggested boxing the fans in a more substantial manner.
Bill
Adding some early photos of the ducts/plenum roughed in.