Then coat the outside of the joint after it has been fastened. With the fitting in its final position, coat seams with mastic. The seams in the boot itself also should be coated with mastic. The supply register creates back pressure that forces air through this gap. To seal around the boot, remove the register and seal the gap between the boot and subfloor.
Some mastics are available in tubes for use in caulking guns.
If you're using mastic in a bucket, protect any carpet with a drop cloth and wrap tape around the carpet edge. In new construction, register boots are available that have flanges to overlap the floor or underlayment. A bead of sealant under the flange and a dab to seal each corner are all that's required to complete the seal. Sealing boot penetrations. Toe kick registers Toe kick registers are a common kitchen and bathroom supply that dumps warm air under a cabinet.