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Old 02-19-2015, 04:45 PM   #1
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Help with Insulating Ductwork in Garage


Hi folks!

I'm undertaking a project to redo the insulation in our garage and need some advice on insulating the heat vents. I want to remove the drywall ceiling, expose the joists and ductwork, and install new batt insulation.

I'm in New York. The garage gets in the 30f or colder range in the dead of winter. I don't get hot air in the rooms above the garage, just cold to start, then eventually it turns luke warm. Also, the floors ARE FREEZING even with a well-padded carpet.

The steel ductwork coming from the furnace and into the garage has a 2" layer of insulation over it. The round, steel ductwork that branches off the main duct have NO insulation. I want to lower the ceiling of the garage and install R30 between the joists, but how do I insulate around the branch ductwork? They're pressed up against the subfloor above and there's wooden cross bracing between the joists that obstructs my ability to wrap it.

If I use fiberglass insulation over the branch ductwork, do I use faced or unfaced insulation? If I use faced insulation, does the faced side go against the ductwork or away from it? Or do I use unfaced and just do my best to cut and tuck it around the bracing and wrap it around the vents?

Thanks for taking the time to address my questions,

-Paul
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Old 02-19-2015, 06:03 PM   #2
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Use the silver faced insulation. Silver side out.
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Old 02-19-2015, 06:25 PM   #3
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How about Roxul insulation (stone wool batts)? Any opinions on this over standard fiberglass? Is this okay to use around the ductwork?
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Old 02-19-2015, 07:22 PM   #4
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I used roxul in the walls of the two rooms I've taken down to the studs so far. Its slightly higher performance than fg (r-value per inch thick) but I like it most because it's not itchy and because you can cut and pressure fit to irregular spaces really easily. Fg loses most of its r value when compressed and squeezed into those same places.

I like it a lot and will continue to use it but it's not miracle insulation. If you really want to keep the floor warmer you have to provide continuous insulation at the underside of the joists. Rigid board insulation. Foam needs 1/2 drywall min for thermal protection. Roxul makes a board too but it was special order at big box last time I looked.
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Old 02-19-2015, 07:32 PM   #5
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I planned to use 5/8" fire-rated sheetrock. Also, I planned on building a bulkhead to surround the entire main duct coming from the furnace. This would be insulated within to R30 (if I can achieve that) and drywalled. I'm even toying with taking out the old 40-year-old branch ducts and replacing them with new insulated ducts.

I'm toying with installing 2x2s on the bottom of the joists to drop the ceiling a bit (it's a finished garage) to accommodate as much insulation as possible. It appears the R30 Roxul may provide me with enough insulation to get the job done without having to create extra work and cost.

I'm scoping all this out now, which is why I wanted to get opinions from all of you. Special ordering material isn't a problem. I don't plan to start work until it warms up a bit.

Any suggestions, please offer them!
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Old 02-19-2015, 07:44 PM   #6
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The continuous insulation offers a lot and if you're not in a rush, have space and have money it's what I'd recommend. It decouples the floor above from the floor below. And then put whatever cavity insulation fits in your existing joists. You'll be fine and I wouldn't bother with at all about R30 minimum if it meant making the joists deeper. Most people use foam but I would order the roxul. Search YouTube for the ASTM burning tests for foam insulation. Its scares the hell out of me.
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Old 02-19-2015, 09:00 PM   #7
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if it were me = roxul all around the ducts, as best you can = which should be pretty good. then a layer of 1" XPS caulked to the joists. then drywall.
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Old 02-20-2015, 10:50 AM   #8
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Excellent idea of adding XPS foamboard before installing the sheetrock. Thanks for that suggestion.

Do I need to install a vapor barrier? I wonder if I should call our town building inspector. Since I'm not really changing structure, I'm wondering if I can skip pulling a permit for this project. Anyone know?
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Old 02-20-2015, 03:15 PM   #9
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If it were my house, I would use a combination of insulating materials.

On the bottom of the floor above, I would put in rigid foam glued to the plywood between the joists. I would caulk the rigid foam where it meets the joists.
Then I would put rigid foam on either side of the vertical joist faces, butting it up to the foam above and trimming it flush with the bottom of the joists.

Then I would fill in this upside down "U" shaped rigid foam cavity with what every insulation you want, fiberglass or roxul.

Then I would put rigid foam on the underside of the joists, like how your drywall was originally installed. I would tape or caulk all joints in the foam

And then finally I would put new 5/8s fire rated drywall up. Tape and mud all the joints, then paint it all a color my wife wants.

Over the top? Well a bit, but that is what I would do....



You may have to move down or extend any outlets (garage door opener), if that's the case, you might have to pull a permit.
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Old 02-21-2015, 09:14 AM   #10
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Quote:
Originally Posted by bobinphx View Post
If it were my house, I would use a combination of insulating materials.

On the bottom of the floor above, I would put in rigid foam glued to the plywood between the joists. I would caulk the rigid foam where it meets the joists.
Then I would put rigid foam on either side of the vertical joist faces, butting it up to the foam above and trimming it flush with the bottom of the joists.

Then I would fill in this upside down "U" shaped rigid foam cavity with what every insulation you want, fiberglass or roxul.

Then I would put rigid foam on the underside of the joists, like how your drywall was originally installed. I would tape or caulk all joints in the foam

And then finally I would put new 5/8s fire rated drywall up. Tape and mud all the joints, then paint it all a color my wife wants.

Over the top? Well a bit, but that is what I would do....


that would be VERY labor intensive. with little to be gained.
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Old 02-21-2015, 10:38 AM   #11
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Yeah, I agree... I would think creating a thermal break from the material in contact with the unconditioned space would be more beneficial. Using XPS foamboard on the underside of the joists would limit thermal transfer to/from the joists and the sheetrock below them.
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