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My condo does not have a basement. I have a subfloor with three dummy registers. I don't know why they are there. My heating ducts are in the upper part of the side walls. In the winter the cold radiates two feet above the floor making it uncomfortable to keep my feet off the couch.

I wanted to insulate the space under the subfloor through the dummy registers or from the garage inside wall.

Is this something I should look into? Would it work?

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what do you mean by "dummy registers"? – mike Sep 29 '13 at 17:46
I can't imagine why anyone would install "dummy registers". These could be for return air, blocking them will at best cause your system to work improperly and at worst burn down the building. Do NOT block these until you are absolutely sure they serve no purpose. – bcworkz Sep 29 '13 at 20:56
Is the floor above another condo or above the garage? – littleturtle Oct 1 '13 at 14:09

You can easily unscrew off cover of "dummy register" and see if there is a duct on them. No matter what they are a doubt they are big enough to fit through.
Most of the time an access hole is in the floor of a closet or utility room. If you dont have one I would create one there. Look hard though you might find an opening somewhere. If there is ducts down there probably also plumbing. The plumber had to get down there after the house was built to install them. There is probably a foundation wall and box/floor joist in the way from the garage or anywhere else outside unless an openning was created when house was built.

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