Adding additional insulation to rectangular sheet metal trunk line
rawelch30
| Posted in General Questions on
Hi!
My HVAC and ducts are in my unconditioned attic. The sheet metal ducts on my trunkline are probably 25 years old and I noticed there is only a very thin layer of insulation inside (I’m guessing around R2). I found some “Duct Wrap” at Home Depot that is R6.
Will this help reduce the current supply temp variations throughout the house? (is it worth it?)
Also, will I have any condensation issues on the ducts with this? I live in the hot/humid southeast.
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Replies
How long is the duct run, and is it located within conditioned or unconditioned space? If it’s a short run located in conditioned space, that’s the situation where extra insulation will have minimum benefit. If the run is long and in unconditioned space (like an uninsulated attic), that’s the situation where you see the maximum amount of benefit.
Duct profiles with a high ratio of surface area to contained cross sectional area will also see more benefit than ducts with a lower ratio. That basically means skinny rectangular ducts will see more benefit than square or circular ducts.
BTW, the usual commercial method to insulate large ducts is to attach rigid fiberglass panels using nail-like fasteners that are tack welded to the duct with an arc welder. Seams are then usually taped for a neat appearance. This type of insulating work isn’t normally done in residential settings, but sometimes it can make sense.
Bill
Thanks, Bill. It's in an unconditioned space (hot attic in the SE). I'm going to take the supply temps in various areas of the house this week and I'll report back with the differences at the longer runs. I did notice that I have some of the older gray ducts (not shiny) that have started to separate on the outside (ducts in tact though). The R-Value is only 4 and I know you can get double now. Why do these old ones split? Getting baked in the attic??
Ryan,
Whether you end up insulating or not, air-sealing, or re-air-sealing the ducts is time well spent.
Ryan
It depends on the type of variations you are seeing. If the temp dissapates quickly as you move down the trunk then insulation might help. If you are seeing the variations randomly it is most likely a sizing or leakage issue. Either way Malcom is correct - focus on air sealing which should give you more CFMs which should allow you to balance things out a little bit. In my experience wrapping ducts in a retrofit situation especially in an attic can be difficult. We have had better luck with the buble type wraps like you have show in conjunction with a good spray adhesive as opposed to the foil backed fiberglass wraps that are out there. Better yet - have 2-3” of open cell sprayed on your ducts. Air sealed and insulated done in one step. You can also have them hit the boots where they penetrate the ceiling which is typically a difficult area to seal and insulated. Our labor and material for an attic job is typically comparable to a small spray job that can be done in one mobilization
In CZ1-3, the code now allows to install R13 min. insulation wrapping the ducts in ventilated attics to avoid condensation issues. We install one layer of R8+ one layer of R6, giving us total R14 in CZ3. All joints still need to be sealed with mastic.
Thanks Armando - I read about that recently and heard you could bury them at that R13 value. I had a contractor last year that just wanted to blow right over the old R4.5 flex ducts in my attic and I was concerned about condensation.
How do you accomplish that R13 with the flex? Do you use R-8 flex and then wrap again with R6 wrap? Will the vapor barrier in between condensate?
I use 2015/2018 codes. Read this code compliance from the Building America Solution Center: Ducts Buried in Attic Insulation and Encapsulated. Scroll down. You have some options.
https://basc.pnnl.gov/resource-guides/ducts-buried-attic-insulation-and-encapsulated#quicktabs-guides=6
Sorry, I didn't answered your question. Yes, and we install metal ducts only, but first seal the ducts with mastic, then install the R8, SEAL IT, and then wrap it with R6 SEALED! Our HVAC guys have an awesome duct tape and then they cover it with mastic.
Im sure metal ducts make a huge difference! Wish I had them!
>"Yes, but first seal the ducts with mastic, then install the R8, SEAL IT, and then wrap it with R6 SEALED! Our HVAC guys have an awesome duct tape and then they cover it with mastic."
YES YES YES !!!
Air sealing the ducts is going to do more for the comfort issues than any amount of additional insulation the duct, and it's absolutely the FIRST step! Putting a higher R on the ducts will improve the efficiency (which is well worth it), but usually isn't going to make or break the comfort issues. Air seal EVERY seam and joint on the system with duct mastic, even if it looks tight.
The initial question after all was:
>"Will this help reduce the current supply temp variations throughout the house? "
Answer: Maybe, not usually, but still "worth it."
Supply temp variations are about flow more than insulation. Often that flow is being limited on the return path side. An Energy Star duct system is balanced well enough that room to room pressure differences are never greater than 3 Pascals, or 0.012" water column. That has to be a all air handler speeds, room doors open/closed. Are any of the "too warm" rooms doored-off without dedicated return registers or jump ducts to keep the pressure differences low?
Hi Dana
The rooms that do get "doored off" at night are the small bedrooms that are often too cold during the summer. The warmer rooms are at the end of the trunk line. I did find out that the flex duck to them is the old gray R4 flex line, so I'm putting R8 flex on them and see if that helps. The temps on a warm day are usually 5 degrees warmer at the supply ducts at the end of the trunk line.
I probably need more return vents. I have two at each ends of the hallway. The smaller doored off rooms actually get too cold in the summer!
Try to straighten out the flex duct as much as possible, or better yet, replace it with rigid metal duct. Air flow is more about the size and type (rigid or flex) of duct than about insulation.
The old duct that cracked is probably some kind of plastic material. Older plastics would get brittle and sometimes shrink with age, which would cause them to crack and separate.
Bill