Insulating attic-side ductwork

[matt_i]

All

Need some advice on how to insulate an 8" x 18" rectangular metal duct x 40 foot run, that lies above the bottom chord, on the cold side of the ceiling.

I have a couple of ideas with pros and cons. Assume that I do my best work in taping and/or mastic all joints first and so its on the good side of the "airtight" realm of possibilities as I don't want to waste my money heating the attic! The underside is going to rest against loose-fill cellulose so I really only need to insulate 3 sides. Also compounding (adding interest to?) the problem is that its in a small triangular area in the tail of the truss.

1. Spray foam the duct using DIY kit. I love the idea of just spraying the foam and boom-done. I don't love the possibility of overspray nor dealing with the chemicals. A ton of ladder or scaffold work to get into the tight side of the truss down next to the wall. Seems pretty expensive, I looked at the Dow Froth Pack kits, I have roughly 40ft x (8 + 8 + 18)/12 ft x 2" thick = 226 ft-ft-in (same as board feet) which puts me just outside of a single 200 bd-ft kit. Likely have to ship or special order. R6.6 per inch, R13 final.

2. XPS the duct, basically slide pieces of 2" rigid foam beside it and on top of it and use the water-based foam-appropriate construction adhesive plus some sort of tape on the corners. R5 per inch, R10 final. Available locally.

3. Roxul the duct 3 sides with 3-1/2". Not sure how I would secure or seal the edges. Alternatively, the sides and top wouldn't be sealed, unless I could use a blanket of Tyvek or the like, but the plastic-based material doesn't thrill me. I do have leftover Tyvek material and tape from the project. Seems like at worst operating condition the membrane is not going to see a lot of heat. R15 final. Available locally.

Hoping for your thoughts. Thanks!

 

[mrpizza]

We wrap new duct with reflectix insulation, you could do 3 sides and secure it with aluminum tape.

 

[Jeepster04]

Whats up with people thinking its a good idea to leave duct work uninsulated in the attic? My return is setup just like you described. I added some blow in insulation when I first bought the house 6 years ago and meant to go back and do something with the ductwork but never have. Laziness I suppose. I did pile the blow in up along each side of the duct work but its not full height.

My plan was to just buy R 13 fiberglass insulation and wrap around the ductwork then wrap the cheap aluminum tape tight around the whole shebang to keep it tight. Its scary how warm the air feels going back into the air handler in the summer. :/

 

[larry4406]

In new construction we try to never run ductwork in attics. That said sometimes there is no other way.

When this occurs we build a 2x6 wall parallel to the duct on top of the bottom
chord. We put R21 kraft faced batt in the stud bays with the kraft paper towards the duct. We then cover the exposed face with thermoply. For the lid we use deep framing so we can put in R30 kraft faced and again seal with thermoply. Our Energy Star rater is content with this approach. We follow up with blown in insulation of the attic to R43.

We try to keep about a 1" gap between the kraft paper and the duct.

 

[DC73]

2. XPS the duct, basically slide pieces of 2" rigid foam beside it and on top of it and use the water-based foam-appropriate construction adhesive plus some sort of tape on the corners. R5 per inch, R10 final. Available locally.

#2 is a good approach but I'd do all 4 sides if you can get to the bottom easily enough.

But, consider polyisocyanurate foam. You can get it at Home Depot. It's about R6.4 per inch and is foil backed. No need for adhesive if you do all 4 sides. Just use a good quality foil tape (also available at Home Depot in the plumbing section). The only drawback is that polyiso is not as easily found in 2" thicknesses so you would have to double up the 1" panels. Note that HD's website incorrectly lists the R-value at R6 for the 1" but they do correctly list the R-value for 1/2" to be R3.2.

We wrap new duct with reflectix insulation, you could do 3 sides and secure it with aluminum tape.

Don't use this bubble wrap stuff. It's basically only a radiant barrier and has little true insulating value. The fine print usually claims some "effective" R-value that has no basis in fact. Building scientists generally consider this stuff snake oil for routine insulation but it does have a valid use as a radiant barrier.

DC

 

[mrpizza]

When its fully wrapped with a spacer between it and the sheetmetal it works very well.

Also works very well in thousands of pole buildings to keep the metal from condensing, must be good at insulating something?

 

[NUTTSGT]

matt, when I installed a new wood furnace in 2016, I was able to add some ductwork and plumb heat to the other side of the garage. I, however unlike you, used 8" metal round duct.

My plan was to use flexible duct as insulation after I found out the Reflectix wasn't rated for the higher duct temps that I might experience.

I started off by pulling the flexible duct out of the center so I could slide it over the metal duct. Since I was working in the limited space of the attic, it didn't work. I ended up cutting the foil cover, wrapping the fiberglass batt over the duct work, holding it inplace with a wrap of duct tape. Once that was done, I wrapped the outer foil cover back over the fiberglassed wrapped metal duct. That was duct taped in place and then I added a layer of Reflectix.

The Reflectix comes in a roll 48" wide so I cut pieces about 26" long (8" duct x pi) to try to make it easy to workl with in the attic. Once it was wrapped, I used foil tapeat the edges and along the seam.

Dirty, itchy, nasty work but it does work.

 

[86turbodsl]

No question, spray foam all 4 sides. I have a long set of hoses for froth paks you can borrow if you want. I believe you're local... When i sprayed my attic ductwork in my house, all my problems stopped. Heat bill went way down too. ANY HVAC leakage in attic is a BIG problem.

 

[matt_i]

Thanks for the offer, I did consider that but not super happy about the overspray which I felt like would stick to everything and need lots of masking.

I ended up going with the polyiso R-6 insulation for 1" thick, all 4 sides, foil taped. Going to attempt to go to 2" thick on 3 sides after that.

Its a bit of a puzzle as I am putting the rectangular duct thru a "lower triangle" in the truss system. The rectangle duct has to be snapped together in-place rather than on-the-floor. Also the pieces of insulation have to be roughly in-place before the duct is snapped together unless I want a bunch of extra tape joints. Then there's the problem of bending the tabs over on the plenum takeoffs, either I have to access the rectangle open end of the duct and reach in, or else put the takeoffs in on the bottom half of the duct before snapping in the top. Its all moving along but its slow. Hardcast 1402 tape on the important seams and a Nashua aluminum tape on the long seams.

 

[jeeper46]

A timely, interesting thread for me. I have to run a return air through the attic, in the "lower triangle",too. About 20 feet, with four takeoffs. I was wondering how I was going to insulate it, and I think I will use your method.

 

[Milton Shaw]

If you are using the ducts for a/c in the summer you need to have the duct insulated and sealed to keep it from sweating and dripping through the insulation and ruining the ceiling. Wrap the duct in standard duct insulation with foil on the outside and tape all joints.

 

[86turbodsl]

I cannot emphasize enough how important sealing and insulation in attic ducting is. If you have returns, do them too. If you have a leak on return, you take humid hot air back to your handler, and if you have a leak on supply, you lose cooled/heated air to attic when it's needed in the house. It's a lose/lose. Ask me how i know...

 

[tvtaurus]

I cannot emphasize enough how important sealing and insulation in attic ducting is. If you have returns, do them too. If you have a leak on return, you take humid hot air back to your handler, and if you have a leak on supply, you lose cooled/heated air to attic when it's needed in the house. It's a lose/lose. Ask me how i know...

Yea, I was in the attic of the new house one day and bound one of the air returns just disconnected. Had to reattach and seal it all back up.

 

[toyotadriver]

Best thing to use to attach foam sheets to things is spray foam in a can. Works much better than any foam rated adhesive in a tube.

 

[matt_i]

I cannot emphasize enough how important sealing and insulation in attic ducting is. If you have returns, do them too. If you have a leak on return, you take humid hot air back to your handler, and if you have a leak on supply, you lose cooled/heated air to attic when it's needed in the house. It's a lose/lose. Ask me how i know...

Exactly. I don't have A/C currently, but its like heating the sky. A proposition I am currently not in a financial position to excel at

One thing that helped me tremendously, having to assemble the various bits and pieces inside the "triangle" was to layout a centerline, then layout an edge, then snap a chalkline the length of the building across the top face of the bottom chord of all trusses. This gave me a way to align the duct as the various takeoffs are coming in from both sides, and its impossible to slide it both of those directions at once without a wicked twist.

 

[dreasoner]

I don't know if this is the most correct method, but it works. I have wrapped existing attic ductwork with 6" unfaced fiberglass insulation on three sides and tuck the insulation under the bottom of the ductwork. The attic was then insulated with blown in type of insulation.

 

[TractorJeff]

When we moved into our current house which has a furnace in the Attic, the cold air returns and ductwork were all uninsulated! Not knowing what to do, I took Kraft 3.5" insulation and wrapped the returns and ducts with it. I have A/C and haven't experienced any dripping issues after 20 plus years. Insulated the plenums up to the furnace also as none of it was insulated either!
Did I mention that the electrostatic air filter had obviously quit a long time prior. Try cleaning that **** out of the "A" coil! I replaced the fan motor and now use 6 inch pleated filters with seemingly good success?

 

[martyre]

matt, when I installed a new wood furnace in 2016, I was able to add some ductwork and plumb heat to the other side of the garage. I, however unlike you, used 8" metal round duct.

My plan was to use flexible duct as insulation after I found out the Reflectix wasn't rated for the higher duct temps that I might experience.

I started off by pulling the flexible duct out of the center so I could slide it over the metal duct. Since I was working in the limited space of the attic, it didn't work. I ended up cutting the foil cover, wrapping the fiberglass batt over the duct work, holding it inplace with a wrap of duct tape. Once that was done, I wrapped the outer foil cover back over the fiberglassed wrapped metal duct. That was duct taped in place and then I added a layer of Reflectix.

The Reflectix comes in a roll 48" wide so I cut pieces about 26" long (8" duct x pi) to try to make it easy to workl with in the attic. Once it was wrapped, I used foil tapeat the edges and along the seam.

Dirty, itchy, nasty work but it does work.

thanks for sharing it's very useful for me