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Heating & Air Conditioning Basements

Insulating existing rectangular duct trunk supply

3 years ago

I am starting a basement project and want to insulate the existing rectangular duct that runs down the center of the basement (photo attached). Current plan is to build a bulkhead around the duct when I finish the basement. I would like to insulate this rectangular duct before I close it in. It is currently un-insulated, the top and bottom and one of the sides are mostly clear, but one side is against the main horizontal support beam for the house (On that side I might be able to squeeze some very thin foam board insulation in between the duct and the beam).

I have been searching the internet for ideas to insulate this rectangular duct. The link below looks to be the best option for what I am trying to accomplish. It involves installing 1in foil backed foam board

https://www.youtube.com/watch?v=s1EK8dMiZj8&feature=emb_logo

Any ideas or feedback is appreciated. Thanks

Featured Answer

Seabornman

3 years ago

I used duct wrap insulation, sealed with aluminum ductwork tape. It worked fairly well. Some spots you just can't get to, but it's not critical.

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Comments (7)

mike_home
3 years ago
When I finished my basement many years ago I did not cover the main duct with insulation. What I should have done was apply mastic seal to all the joints. The duct is running through conditioned space so there in not going to be much loss in heat loss or gain. There is no harm in adding insulation. I highly doubt you will see in any change in your heating and cooling costs.
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I use a mastic..paint on type in a bucket (hardcast..iron grip available at hvac supply) for all the joints and seams. When the unit is running you can feel the air leaking out so you will know where to seal..but seal everything. hopefully you'll not have to do this again! I also use a foil backed duct wrap and recommend the extra cost for R-8. The supply house will have a foil/paper adhesive tape with a nylon scrim that is the best for sealing duct wrap. I staple (again...special duct wrap staple gun) the duct wrap before taping. Ducts under homes come loose unless correctly installed and foil tapes fail faster than the duct wrap tape. Duct wrap has to fit properly and it is not good to wrap ducts a 'couple' of times as the foil is a vapor barrier and will condensate between the vapor barriers... putting you back under the house to re-do your diy. best of luck to you..and while you are under your house support any ducts that sag..another cost..duct strapping, but not much of a cost. I use a metal duct strap for hard duct and a 3" webbed duct strap for flex duct. best of luck.
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I like the mastic tape and use Hardcast brand, but surfaces have to be clean and dry for the tape to adhere. I've also used the material countrybomo posted..but when I used it the first mastic application would fill most of the woven areas once it dried the second application of mastic would make complete seal. So for me..the mastic tape works best..with clean surfaces. while you are shopping your hvac supply store ask for foil faced duct wrap..R-8 and the tape of similar material to seal the seams. The wrap will install after you have completed the sealing work.The larger gaps that you have mastic sealed will now be well insulated with R-8. The wrap has a foil faced paper with a nylon scrim and fg insulation backing. You'll want a sharp blade to cut it. cut your duct wrap in to sections. tape one end of wrap to surface wrap remainder around & under trunkline. You want the fg insulation in full contact with the metal trunk line. But not pulled so tightly as to compress the insulation and reduce R-value. A stapler for duct wrap will help to hold the sections and give them support while you cut & peel the backing off the tape. I don't have much faith in the foil bubble foil products, as install is hardly ever done properly with spacers and the performance is over rated. best of luck.
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Energy in equals energy out. Any space inside the exterior building instillation envelope does not necessarily need to be insulated unless required in local,state or national code. If the work was properly permitted and inspected the building official would have not allowed a final certificate of occupancy if not to that code. If the space in the basement contains any apparatus that contains water or is subject to freezing the basement temperature must be maintained above freezing. So to insulate the ducts then provide heat to the space would be redundant, unless the system is also used for cooling then it would be quite necessarily to individually insulate the ducts to avoid condensation from forming on the cold duct pipes. Joe M Corbett Mechanical Contractor Here is a link that might be useful: Corbett Cooling
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I thought the piece coming thru the floor was metal & that there was a transition to duct board at the point about 18" above the floor where what looks like a metal strap is nailed to the wall. I also thought that you already had the offset piece in your possession. But I guess not. I was wrong on both points. Why mess with the duct board? I would replace with all metal. Sturdier and easier to clean. Makes for an easier job using all one material. Would need to be insulated at least in the attic but that's not a big deal.
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Elmer J Fudd
3 years ago
I agree with mike_home insofar as the importance of sealing all accessible duct and plenum joints and connections with painted on mastic or, at a minimum, real mastic tape. Not standard garbage "duct tape". It works well and makes a big difference.

Do the same with return ducts as well as supply ducts.
Andrew
Original Author
3 years ago
Thanks for the feedback, agreed - I plan to seal (mastic and high quality/heavy foil tape) all seams as well.

Seabornman - Were you able to access all 4 sides of the duct? I may go the duct wrap insulation wrap as well, but I'm trying to figure out how to hold it in place on the bottom when the one side is against the beam.
Seabornman
3 years ago
It's been so long I don't remember. Buy the good aluminum tape. The cheaper stuff doesn't stick as well. Use a plastic spatula to rub it in. If you use your hand you'll get all cut up.
Bruce in Northern Virginia
3 years ago
What is the purpose of your insulating a duct running through conditioned space? Is it for noise reduction, or some other reason? I think sealing it well is more important than insulating it.

Bruce
Mini-Split Warehouse
3 years ago
Insulating HVAC Ductwork.

https://www.heating-air-conditioning.net/

When insulating HVAC ductwork, use a foil, faced fiberglass insulation with an R-6 or higher R-value. Use the type of metallic foil duct tape recommended by the insulation manufacturer to seal and hold the insulation in place.

Make sure the foil surface of the insulation is free of dust before applying the tape. Peel the paper backing off the tape as you apply it to prevent the tape from sticking to itself.

If your existing duct insulation is in fairly good condition, an additional layer of duct insulation can be applied over it. If the old insulation is in poor condition, remove it and replace it with new insulation.

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