Replies

1. GBA Editor
   Martin Holladay|Aug 16, 2012 06:49am|#1

   Randy,
   In a hot, humid climate like yours, burying ducts in a deep layer of fibrous (air-permeable) insulation is not recommended. Here's why: the deep fiberglass keeps the outer plastic jacket of the duct at a low temperature, allowing the moisture in the hot, humid attic to condense on the outside of the flex duct. Then the ducts drip, saturating the insulation (and your ceiling).

   The best way to address poorly insulated attic ducts is to replace older flex duct with newer flex duct with a higher R-value (at least R-8). If you need to bury the ducts in insulation, the only safe way to do it is to first install a 2-inch layer of closed-cell spray foam on the outside of the ducts. Then it's safe to bury them with as much fiberglass as you want.

2. 
   James Howison|Aug 16, 2012 12:20pm|#2

   I'm down in Austin and had considered this too (having the same flex duct as you). I spoke to a few locals and they aren't big fans of burying but the inspectors do allow it here, FWIW (without foam). I was worried that I'd go to a lot of effort but actually create a problem that's hard to locate and hard to fix, a downside risk I didn't want.

   当前的计划,推迟到够酷to work in the attic, is to "fur up" over my ducts, creating an air-tight tunnel through which they flow. This would either be built from taped rigid foam or taped drywall/plywood on 2x2 screwed into the joists and foam sealed to the ceiling, then fur out on top and sides of that to place rock wool batts (which are nicely friction fixable (I've used them) and will stay on the sides as well as the top, probably using the 2x8 size for R30.

   It's definitely more work and won't be possible everywhere, but I like the fact that I can get to the ducts in the future easily, make a repair and replace everything.

   Basically this would turn the furred up section into semi-conditioned space, if it was showing problematically high humidity I'd stick a small return/supply in there and make it conditioned space.

   Of course these are just plans and I haven't tried to implement this, but it seems a middle course between burying and sealing/insulating the roof deck which is impossible in my attic.

3. Expert Member
   Dana Dorsett|Aug 16, 2012 04:01pm|#3

   Insulating the ducts with 3-4" of open cell foam or 2" of closed cell (as Martin recommends) would work without the condensation risk, at which point you can bury it all in any fiber you like. With open cell you have very modest risk of accumulating a small amount moisture in the foam over the course of several weeks of killer-humid-heat, with a temporary reduction in effective-R value, but it gives the moisture back up pretty quickly over the following weeks without a permanent loss of thermal performance. Closed cell doesn't have that issue, but it's more than 2x as expensive.

   The ideal solution would be to put all of the ducts inside of conditioned space that includes the pressure boundary of the house, and put all of the insulation above. (Hard to retrofit in many houses, I know.) The practice of putting HVAC ducts & air handlers outside the pressure & thermal boundary of the house is a lousy one, but still allowed by code.

4. GBA Editor
   Martin Holladay|Aug 16, 2012 04:16pm|#4

   Thanks for the post, Dana.

   For anyone who missed it, here's a link to a relevant article:Keeping Ducts Indoors.