Insualte Return Duct

T Joesephson

I'm trying to lower my heating bill and was wondering if I should insulate the return duct for my 2nd floor furnace which sits in the attic. The return duct is about 40 feet long and has a diameter of about of 14 inches. The unit is uses propane as a fuel source and heats about 1,000 square feet.
-T

tim smith

Absolutely, we insulate all our duct work. R-8 foil face duct

wrap is what we use.

Brad White_191

Questions for you

Is the attic space where the duct is, within the thermal envelope or is the attic essentially above/outside the heated space?

My first order of business regardless would not be insulation but sealing. Do check that. I do not mean "duct tape", but the brush-on acrylic compound made for the purpose. I like "Hardcast" products, so give them a shout and they can tell you a local source of supply.

The reason I am bullish on duct sealing is two-fold:

One is that you otherwise cannot control the quality of the air you are drawing in. Attic are? What is up there? Fibers? Bats? Mrs. Bates? (Mrs. Bates?)

The second is ducted efficiency- you are paying to move air from A to B, not from A to B with a lot of extra passengers. Control your air delivery.

As for insulation, yes, I would if you have any doubts about the ambient temperature, but insulate only after thorough sealing.

In MA here at least, the energy code says at least R-5 if within a conditioned space and the temperature difference is 15 degrees or less. R-5 is typically what 1.5" thick foil-faced duct wrap will give you.

Otherwise at least R-8. R-8 is typically representative of 2", but personally, I would use two layers of 1.5" with staggered seams.

The other thing you have to ask yourself is about humidity. Can the ambient ever get hot and humid while the return duct might be below the dewpoint? Vapor barriers must be sealed.