Hi all:
I’m having siding replaced and we have removed the plywood ceiling of a small outside porch area. This porch area overhangs the foundation such that part of the second floor bedoom is above it. You can see the ceiling of this porch in the photo – taken from inside the house fhrough the sliding glass door.
There is a 6″ round HVAC duct pipe that is fed from a rectangular trunk in an adjoining outside wall (not shown). This 6″ pipe goes to a register that supplies forced hot air to the bedroom above.
The fiberglass/plastic duct sleeve is all compressed and in poor conditions. I have looked at new ones but they all have halogen flame retardants such as decabromophenyloxide. The EPA is looking at phasing out such compounds due to concerns they are carcinogens. Originally it was thought they were not volatile, but some researchers seem to feel they can be volatile. Since indoor air quality has been a big concern in this project, I’m not sure I want any of these newer duct sleeves on this HVAC pipe.
What are the forum’s thoughts on removing the duct sleeve and simply using formaldehyde free insulation batts on top and bottom of the length of the pipe run? Afterall, this is all the trunk like that feeds it has around it that is in the outside wall I mentioned. My only concern in not having a metallized or plastic lined duct sleeve is the possibility for condensation. I’m not sure if these duct sleeves are used to prevent condensation, or just because they are a convenient way to wrap the pipe. If the latter, it would seem I could simply remove the sleeve and use my batt idea.
The only situation for condensation that I can imagine (I’m in Colorado), would be for a hot humid day when the AC is on. It might be possible for hot humid air to get into the porch ceiling area and condense around a cold HVAC pipe – IF I don’t use a duct sleeve with the plastic or metal vapor barrier.
FInally, regarding insulating the joists (since I removed the old insulation which was in poor shape), should I use faced or unfaced batts? The upstairs bedrrom has a OSB subfloor and carpeting – and we may want hardwood floor in the future. I understand when hardwood floor is installed, they often put down roofing felt paper, so if I use faced insulation (with the Kraft in contact with the OSB), would this create a double vapor barrier?
If the board thinks I can remove the duct sleeve, should the batt that I insert above the pipe be faced or unfaced? Their is only maybe an inch or two clearance between the top of of the pipe and the underside OSB subfloor.
Thank you.
Replies
I'm not sure
what I'm seeing in the photo, and what your objection is. It sounds like you're saying that's a metal duct with insulation wrapped around it, and that you don't like the plastic that's part of the wrap. I'm sure you know this, but there are millions of miles of flex duct installed in houses... inner plastic duct with fiberglass insulation around it, surrounded by another layer of plastic. No doubt we are finding that plastic is not quite as inert as we had thought, and hoped, but I digress.
You can simply make sure that the entire duct run to the register is metal, and that all the joints are well sealed with duct mastic (or silicone), and then insulate the duct however you wish. I have used fiberglass batts to do it, and I've used duct insulation as well. Given that you're concerned about condensation, the main thing to focus on is air sealing the cavity that the duct runs in, so that humid air can't easily get in. Drop the duct, install a layer of rigid board above it, re-install the duct and box the sides and bottom with more rigid board, sealed at the joints with canned foam. Or, use duct insulation with a plastic outer wrap and make sure all the joints are sealed well. I like the rigid board more because you can get some decent R-value that way. You have a duct that's basically outdoors, never a great idea but sometimes unavoidable. Can you get 2" thick polyiso on all four sides of it?
Thank you David:
I'm very much into indoor air quality so adding the rigid boards might raise other concerns in my mind.
Do you think that infiltration of warm humid air during the summer would cause condensation on the galvanized HVAC pipe if I remove the plastic lined duct sleeve and just pack fiberglass around it? It is very dry here in Denver and although it can get hot for a handful of days in the summer, it is rarely very humid. I'm inclined to use no plastic lined duct sleeve and just pack unfaced fiberglass batts around the pipe but don't want to do that if I'm going to cause condensation issues.
Also, what are your thoughts about my other question as to whether or not the batts in these bays should be faced or unfaced? If faced, I think obviously I would want the Kraft oriented up against the OSB underside of the subfloor above - but then I got to wondering if I had hardwood floor installed upstairs in the future, I think they use felt paper and didn't want to create a double vapor barrier.
Fiberglass
could certainly be used around the duct. If there is already felt under the flooring, then I would consider using faced insulation and orienting the facing to the bottom. What you want to do is prevent exterior air from migrating into the joist space and hitting the duct. The facing can help accomplish this, but I would consider your air sealing details carefully. Really, the entire ceiling should be detailed as an air barrier, to keep exterior air from getting into the floor assembly.
I don't see a double vapor barrier issue here. If you don't use impermeable materials like poly sheeting you should be fine.
I'm not familiar with Denver's climate. A/C is not common here, so condensation on ducts is not an issue I see very much.