Do radiator covers reduce efficiency ?

Hello Eddy

A radiator is a device with which heat is dissipated to the surroundings.
Therefore any cover will reduce the natural airflow around a radiator, and thereby reduce the efficiency.
The question should not be if, but by how much (in %).
As long as most of the air can freely flow around the radiator I would not worry too much.

:)

A lot depends on the design but there is little lose with most covers. Think of it this way, where would the heat go? If the cover restricts the flow of warm air you loose some convection but the cover gets warm and the heat returns as conduction. Kind of half a dozen of one and 6 of the other.

Of course they do. Radiators are found next to exterior walls. The cover traps heat there, resulting in more heat escaping through that wall to the outside - more than if the heat could have been quickly transferred to the rest of the room.

I'd bet if you could take photos from the outside with an infrared camera, you'd see more heat loss resulting when a cover is in place.

Related to that: I'm wondering what kind of heat shield I can place between the radiators and the wall that they're next to.

I've read that putting a shield of sheet metal behind a radiator cover will reflect heat away from the back wall and back into the room you're trying to heat. I was reading about building radiator covers on the This Old House Web site and they recommend putting sheet metal behind the cover for this purpose. I've seen it on other sites as well.

When I say behind the cover, I mean on the inside of the cover in the back.