I had the same issue in a house we bought and we caulked all gaps at the wall and where the cabinet sides meet the cabinet floor. Our cabinet had no back. If your cabinet has a wood back, maybe you can caulk all the corners and seams.

We also had a large gap on the back side wall and added a piece of wood and caulked it too. It helped a lot. Still a little warm, but nearly as bad.

Make sure to keep the vents open to the room.

Base on what you have described, my guess is that there is no duct between the grille on toe kick and the supply vent. Contractors will do this when installing a cabinet. They will use the empty void under the cabinet as the supply duct. The other possibility is the duct became disconnected when the cabinet was put into its place.

Are you able to remove the grille and look inside with a flashlight. You should be able to verify my theory. If that is the case the right way to fix this is to either remove the cabinet or cut an opening in the bottom of the cabinet floor and install a flexible duct between the grille and the supply. You can then place a sheet of plywood on the bottom of the cabinet to cover the hole.

Adding insulation will slow down the heat transfer but eventually the cabinet will become warm and cause problems.

Hi, Darochet,

If there is no duct connecting the supply ductwork and the grill installed at the toe kick, my biggest concerns would be the potential for mold/mildew developing on the wood in air conditioning mode and the limited access for cleaning. If you have temperature-sensitive materials stored in the cabinet, you can cut rigid insulation and install it on top of the cabinet bottom to reduce the temperature inside the cabinet. I recommend foil-faced polyisocyanurate for that application.

There is a product designed specifically for toe kick applications which we have been using for some time. https://toekickductor.com/ It works great.

That sounds like a code violation to me.

If the air coming through the vent is really hot enough to melt plastic bottles, then you have a serious problem that needs to be corrected immediately. The air should be no hotter than about 90° F, nowhere near hot enough to melt plastic.

"That sounds like a code violation to me." Although I'd argue it's not a best practice, I don't find anything in the 2018 IRC which precludes the use of cabinet toe kicks as plenums for heating and air conditioning. The IRC specifies a maximum flame spread index of 200 for duct materials. The flame spreads of wood species used for cabinet construction are well below that. The ignition temperature of wood is also much greater than the 125F to 130F +/- temperature of supply air from a gas-fired furnace. The melting point of body lotions is designed to be at or less than body temperature and the temperature of the cabinet bottom could easily be above that if the OP has a gas-fired furnace. My concern is the potential for mold/mildew to feed on wood and any organic materials that might accumulate in the space when the system is in air conditioning mode.

If toothpaste is getting hot enough to melt, I’d worry. I’m a Nervous Nancy.

Is there no fire danger?