Insulate Floor Duct

I have a heating duct that empties up into the room at the perimeter of the house where it is cantilevered 18" over the 1st floor.
The duct ending is run between the two joists and is missing insulation. Can anyone think of a way to cut from inside the register opening to put insulation into the cavity? There isn't much room for a drill, even a sideways drill.
The only other option is to drill/cut from underneath outside. But that would involve removing the vinyl soffits, etc.
Any ideas would be appreciated.
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How do you know it is missing insulation if you can't get at it?
The metal is probably fastened to the floor wiht a few roofing nails if it's installed the way I have seen most. Getting it up and out will depend on the dimensions of the register verses the size ofthe flex that is attached to it. If you can manage to pull it out through the hole you coul stuff in some insulation and then put it back. Otehrwise you are ging to have ot remove something.
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Sanjay Bala wrote:

Is the ceiling finished in the 1st floor room under the cantilever section? 18" isn't very far to reach and stuff insulation if you can access the joist cavity from the 1st floor room.
If the register is fed from flexible duct you might be able to move it back and access the cavity from above, but flexible duct is also usually insulated already.
Otherwise, removing the soffit under that area is going to be your best bet.
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Can't you just drill some holes in the subfloor (close enough to the duct opening that the register will cover them back up) and go crazy with the spray foam?
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That sounds like the best thing to do, that foam goes everywhere, the only question is whether to use regular expanding foam, or one of the variants that do not expand as much.
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That sounds like the best thing to do, that foam goes everywhere, the only question is whether to use regular expanding foam, or one of the variants that do not expand as much.
I did not mention that I have hardwood floors. So I can't really drill down without having to patch the wood floors, which I'd prefer to avoid.
But that is definitely something I should consdier. I know I can buy face-grain oak plugs that should match the grain.
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Not endorsing the spray-foam idea without seeing it, but is this register flush to floor, or set into the baseboard? Either way, there is usually enough room under the removed register to drill a small hole or two, and have the register cover it. It doesn't have to be in the actual floor- it could be at a 45 degree angle just inside the opening. If the foam doesn't seal it, a dab of aluminum tape will. The hoses on those spray cans are pretty tiny. If you go that route, I'd use the low-expansion stuff, lest you collapse the duct, or blow out the soffit, or the ceiling of the room below. But do the math first- a joist bay is 14.5 inches wide x 7.5 (or 9.5 or 11.5) inches tall, by however long the overhang is. Swag a number for how much space the duct takes up, and make sure you get enough foam, and that the hose is long enough to reach deep in there.
Having said all that, going in through the soffit is the proper approach. If it is vinyl soffit, and there is no old wood soffit above it, they are usually pretty much snap-in-place panels, held at most with a self-tapping screw in a blind spot. Get on a stepladder, and push up on it- it should be self-evident how it goes together. Is the floor cold over the entire overhang? You may need insulation bats all the way along that run, not just where the duct is.
Stuff like this, I recommend doing it right and getting it over with. It won't be as bad as you think.
-- aem sends, on Google until 29th...
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