Well, I'm no expert, but I checked with my DH who used to be a contractor, and he said the shop lights wouldn't get hot enough to cause problems, especially since the top is open and the foil would be reflecting the heat away from the foam. It might be different if you were using hot bulbs like metal halide or high pressure sodium for example.
Nice set-up, by the way. You might put something white, maybe a vinyl table cloth on top of the chest to get even more light reflected(just my 2 cents).

Oh that's a good idea! Thanks for the input. I was thinking it should be reasonably flame retardant since you're supposed to put it in your walls, after all. But I figure it never hurts to check. Thanks much!

Here is a link that might be useful: my blog

You should check the reflectivity rating of that material. Clean flat white paint on wood will give you 85% light reflectivity. Sometimes people are surprised that many metallic "reflective" surfaces are actually worse (obviously some are better than the paint but it can be hard to tell).

If you are unsure I'd just go with flat white paint - 85% isn't bad and it's cheap. If you want to spend more you should look into some of these materials if you want a durable solution: http://www.anomet.com/reflective_products.html

I believe sheets of the 98% reflective aluminum can be shipped for something like $50 or so - the problem is that if you work them to shape them you can seriously damage their reflectivity if you aren't careful.

And there's always mylar if you want a less permanent solution.

I don't see heat as a problem, but I would be concerned about lack of air circulation which contributes to damping off disease of new seedlings. Al

Good point about damping off, I guess a fan would be needed in order to prevent that. Its a good idea though, I love anything built with duck tape. :)

I have pieces of 1" foil backed foam cut ready to do the same thing. I plan on leaving the front off of mine unless I need a lot more heat than I expect. I also have a thermometer and a thermostat set up to track heat readings.

My set-up will be in the shop so I wont be there to watch it all the time, but the thermometer will transmit the reading to the receiver beside my recliner, (I hope).

Larry

Thanks for the input, everyone. So far, I haven't had any problems with damping off (knock on wood). But maybe I'll open my foam doors during the day when it's warmer, and close them up at night when they could use the extra protection. The picture is a little deceiving in that it makes the top look more "closed" than it is. But I'd hate to lose 14 tomato seedlings that are doing beautifully at the moment.

Larry, my set-up is out in my garden shed, unheated, which is why I wanted to add a little insulation. This seems to be working good.

And yes, it was refreshing building something with duct tape! LOL My first idea had been to make a box out of some scrap wood I've got, then cover it with foil. When I saw that the 4X8 sheet of foam insulation was only $8 I decided I could "splurge". It was worth it!

If you add a small 4" personal fan on each end that helps with dampening off and also stimulates the plants. Just my two cents worth. I think this is a great idea