I have R19 fiberglass batts in my attic. There are large soft insulated HVAC ducts covering the batts in some places. I would like to put in new R30 on top of the R19. What is the proper way to install it near the existing HVAC ducts? Should it go over them, under them, or up to them?

Thank youenter image description here

2 Answers 2


Those ducts are flex ducts they do have insulation in them but it is limited.

If you have enough length in the flex duct I recommend adding insulation under the duct because the flex duct is not hard to collapse.

The ducting is lightweight and although it will slightly compress the additional insulation the new insulation will work better if it can be laid continuously under the duct. Where the duct is at the roof if you don’t have enough slack to put underneath stop right at the duct for best results.

  • Thanks Ed! What you said makes a lot of sense Commented Jan 11, 2021 at 1:03

If there is enough slack and the ducts can be raised install the new insulation under the ducts. What you may want to consider is supporting the ducts from the rafters in order to have them not compress the new R-30 (if there is limited space).

Compressed insulation looses it's R value when compressed. Supporting the ducts is a simple matter of running some flexible steel duct strapping under it and securing the ends to a rafter. Do this every 4-5 feet.

If raising the ducts isn't possible than install the R-30 right up to and as tight against to the ducts. You'll loose the extra R-30 because of the ducts blocking that area, but not enough to make a huge waste of energy.

The other solution is to fill the attic with blown-in insulation. No need to move the ducts just cover it with the blown insulation. Most hoe centers will rent the machine and sell the bails of insulation. Very DIY.

  • Thanks Ojait! That's a great explanation. Commented Jan 11, 2021 at 1:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.