Magnetic Vent Covers 4 PCS - HVAC Premium Anisotropic 1.5mm Optimal Thickness - 5.5 X 12 inch - for Wall, Floor, Ceiling Vent Covers - Trap & Seal Air

Strong heavy magnets. Work well

The covers are sturdy, and with a good heft to them. I use them to cover ceiling vents, and so far, they are sticking very well to block summertime level air conditioning. Basically, I wanted to keep the cold air blowing upstairs instead of into the already cool basement. It is not as cold now in the basement, so looks like the covers are doing their job. As others have pointed out, you need to close the louvers on the vents first before using such covers. I did not try to use them on open vents so I will not know what will happen if I do (not curious). I will find out in winter if these covers can still stick with hot air ventilation.Update after winter: I can confirm that they stick securely to the ceiling vents all throughout winter, so they are good for year-round use.

These are a nice heavy weight, with a strong enough hold. The only problem I had is that they are perhaps 1/4 inch too small to completely cover a standard vent. I had to cover the edges with white tape to completely seal off the vent.

I am always cold, so I like to cover some of the vents. This brand is smaller and fits my vents a lot better, but they are also thicker, and heavier, and grip the vents better. I'm so glad I bought them.

Not long ago I had a tree fall through my home taking out the whole backside. I recently noticed that after the repairs some of the vent covers were broken and never replaced. I was going to replace them but came across these covers. They are thick heavy sturdy and covers the broken vents blocking the air like I wanted them to. A great find!

I was pleasantly surprised on how heavy these are. They work perfectly and stay over the vents we needed to block without budging. I was worried our dogs would try to scratch at them or take them (labs take everything!) but the magnetic pull is very strong ans we have had no issues.

Bought this and wanted to cover a window vent for my AC. It's a metal window vent. Thought this would be perfect and it would stick to the metal. It doesn't. Returned same day.

These are thick enough to be useful and strong enough to attach without sliding or falling off. I cut some in half and they still worked well to cover a heating vent.