Replies

1. GBA Editor
   Martin Holladay | Dec 15, 2017 06:12am | #1

   Building Newb,
   Buried ducts can be at risk of condensation unless you follow the rules for buried ducts established in the 2018 IECC and 2018 IRC. (For more information on these code requirements, see this GBA article: Buried Ducts Allowed in 2018 Energy Code.)

   In Zone 5, insulated ducts must be rated at minimum of R-8 to be safely buried. So the answer to your question is no, you don't want to bury R-4.2 flex ducts.

2. 
   BuildingNewb | Dec 15, 2017 04:13pm | #2

   Would you mind explaining to me why there would be a condensation risk?

   I would figure adding more insulation would serve to further prevent condensation.

   Is it because the insulation gets very cold in the winter and the insulated ductwork isn't enough to separate the hot air vs cold insulation temperature? Thanks.

3. GBA Editor
   Martin Holladay | Dec 15, 2017 04:32pm | #3

   Building Newb,
   The worrisome condensation occurs on the exterior of the polyethylene jacket (the jacket on the insulated flex duct) in the summertime. Thin duct insulation under the jacket increases the risk (by lowering the temperature of the polyethylene jacket). Deep cellulose also increase the risk (by isolating the polyethylene jacket from hot attic temperatures, thereby keeping the jacket cool).

   Blown-in fiberglass and blown-in cellulose are not air barriers, so hot humid attic air can access the polyethylene jacket. Hence the condensation risk.

4. 
   BuildingNewb | Dec 15, 2017 04:34pm | #4

   Gotcha, but if I upgrade to R8, I should be safe?

5. GBA Editor
   Martin Holladay | Dec 15, 2017 04:43pm | #5

   Building Newb,
   Yes, that's what I wrote in Comment #1.