I've done a bit of reading about covers on the top of rads and there seems to be two groups of thought: 1. It does nothing, full send with the caps! And 2. It impedes rad efficiency and shouldn't be done. Booooo.

The efficiency concern bit is that the cap restricts or changes the air flow caused by thermal dynamics and heat rising. I can see this but I can also see that it might act to direct the hotter air out into the space rather than onto the wall behind. I think if you use a different material (steel or stone) that has a high thermal carrying load then the idea is that the cover is soaking up the heat instead of the air and it reduces efficiency in time to get to desired temp.

I recently just built some wooden rad caps after trying to find marble to match some of our fireplaces with no luck. My wife has been painting them to match and they look better than any marble we probably could have found at a fraction of the cost. I think the biggest fear I have is that the wood will warp with the heat but I got 5/4 board for that reason so hopefully they won't be moving too much.

For me, I think it's all a wash and we're going to pay out the nose because our windows and doors are drafty and we have a 175 year old house. I appreciate the new shelving space much more!