Do Radiator Covers Save Energy or Waste It?

Over at the Old House Web, Amy Hayden wrote about Five Benefits to Using Radiator Covers. This has started a bit of a debate: Are rad covers useful or do they waste energy? Amy writes that "Radiators are a good source of heat, but they also take up valuable square footage...By purchasing a radiator cover, you can reclaim the flat surface on top to display books, picture frames or hardy plants."

But there is a problem with rad covers.

Convection

They may be called radiators, but they should probably be called convectors, since the bulk of the heat that we get from a traditional rad is moved by convection. In convection, air warmed between the fins of the radiator rises to the ceiling and is pushed around the room in a circle.

Some warmth is transferred by direct radiation, but not all that much and not to the right place, namely throughout the room.

Reflective Backing

Amy writes:

Radiator covers with the proper backing can distribute heat more efficiently than an uncovered radiator. Instead of the heat going directly to the ceiling, the back allows it to be pushed into the living space.

It is true that radiators should have a proper reflective backing; I use foil-faced bubble wrap insulation; it reflects a little of the radiated heat that would have been absorbed by the wall back into the room and the radiator. But more heat would be lost by blocking the convection upward with a cover, particularly if it is holding books or plants; you want the heat to go to the ceiling, that is how the radiator convects heat.

Radiator Types and Covers

Some radiators, like copper finned modern rads, come with integral covers, often with dampers to adjust the convection; they, like steam rads, need covers because they are too hot to touch. But for a traditional cast iron rad that one finds in old houses, connected to a hydronic system a cover isn't needed for safety.

Radiators are designed to expose the maximum surface area to the air flowing by them so that it can rise up; that is why the fins are perpendicular to the wall instead of parallel, which would maximize radiation. Anything that blocks airflow reduces their efficiency.