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johnk44 11-23-2011 01:03 PM

How would you insulate this???
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Long time DIY website visitor, first time poster....

I'd like some advice on how to add insulation over the duct work in the attached photo.

The space is above my garage and is unconditioned. The main HVAC header in the picture is insulated with Owens Corning Soft-R with an R5.6 and the flex duct is basic fiberglass insulation with R4.2.

I am concerned about condensation issues if I were to simply add blown-in or fiberglass batt insulation directly on top of the duct work. I could build a box around the header and then add insulation on top of it but that will be a royal pain given it's location and orientation. An insulation contractor recommended spraying closed cell insulation over the header and flex duct but they want $2.65 per sq-ft for insulation and ignition barrier. That will run me about $400.

BTW, I live in eastern Tennesee and have already added XPS rigid foam insulation over the fiberglass batts on the knee wall to the right.

Thanks in advance,
John K

Windows on Wash 11-24-2011 10:04 AM

First and foremost, make sure all the ductwork connections are tight and sealed.

HVAC installers will hide a bunch of garbage behind the exterior layer. Duct leaking outside the envelope is a real energy waster.

After you have checked all the connections, the easiest thing to do would be to wrap the duct lines with more fiberglass and if you wanted to go an extra step, you could add an additional radiant barrier over that.

Foam is a great idea but its primary benefit in this application is to seal it up against leaks. Do that by hand and if you want to apply more insulation, do that.

FYI, make sure you cut a strip out of the plywood at the kneewall and seal up the joist bays.

johnk44 11-26-2011 07:09 AM

Thanks Windows on Wash,
I will make sure to check all connections.

My concern is with condensation under the additional loose-fill or fiberglass batt insulation. I'd think that using batts would be the easiest but how much R value should I add to feel safe that I'm not getting any condensation? And if using fiberglass batts, should I use faced or unfaced? If it's faced, should the paper side be 'down' against the existing foil facing on the duct or 'up' so that it's exposed?

Again, thanks a bunch for your help.

Windows on Wash 11-26-2011 12:17 PM

Do not put a faced batt across it.

If you want to, build a false wall around the trunk line and blow cellulose overtop of it.

Wrap the spider lines with fiberglass (high density) and wrap that with tyvek to keep the wind wash off it.

johnk44 11-28-2011 01:26 PM

I'd like to avoid building a box around the main trunk line since the location of the rafters will make that a major pain.

So here is what I'm thinking...
1) Confirm tight seals at junctions of trunk and two spider legs.
2) Wrap trunk with 23" wide R13 unfaced fiberglass batts to fit between the rafters. Cut each batt to length (38" long) to wrap the circumference of the main trunk line which is a 12" diameter.
3) Wrap spider legs (6" diameter) with the same unfaced batts in step 2 but run it length-wise since it will cover the 19" circumference.
4) Cover all insulation added in steps 2 and 3 with radiant barrier and tape the seams with good quality foil tape.

Thoughts? Concerns? Other recommendations?

Windows on Wash 11-28-2011 02:25 PM

If you want to put a radiant foil on it, make sure the ducts are super tight and do not leak. Otherwise you will get condensation and inability to dry out with the vapor impermeable radiant barrier.

Moisture + fiberglass = mold/mildew. Not so much of an issue in the FG surrounding the ducts but you don't want it spreading to the ducts.

johnk44 11-28-2011 02:36 PM

Good point. The radiant barrier that I have is perforated. It also has a class A/class 1 fire rating (ASTM E84-10).

johnk44 12-05-2011 06:37 PM

Just checking to see if anyone has concerns with me using perforated radiant barrier (also class A/class 1 fire rated) to cover the HVAC ducts after wrapping them with R13 fiberglass??


johnk44 12-11-2011 02:48 PM


Windows on Wash 12-12-2011 07:49 AM

If the ducts are tight, you should be fine.

johnk44 12-12-2011 08:11 AM

I pulled back the insulation at the junctions on both of the spider runs and sealed the unions with mastic, repositioned the insulation, added a zip tie over the insulation, and re-taped all seams.

I feel pretty good about their tightness. Thanks WoW.


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