Bubble wrap, expanded polystyrene sheets, corrugated cardboard, even a blanket or a folded bedsheet.

I have flats of cuttings that I'm rooting and also some flats of seeds on heat mats in my unheated greenhouse. The temp of the GH is in the 80's on a sunny day but dips down pretty low at night. I've placed the mats on a sheet of rigid styrofoam, the flats with clear dome lids on top and at night I go out and cover the flats with several layors of burlap (a blanket would do just as well)which I remove during the day. A few cloudy, cold days I haven't bothered to remove the burlap. I should add that my heat mats came with a thin plastic tray that they sit in as well, so they do not sit directly on the styrofoam.

Thanks for your ideas, the flats are supposed to be kept in the dark so I covered them with cardboard boxes and now the light on the thermostat has gone out. Apparently it didn't need much. Thanks, I will remember all of your ideas.
Karen_B

I have a problem with TOO MUCH heat from a mat. I have three altogether, two of them some years old. This morning I discovered that one of the older ones had gone wild and heated a flat planted with lobelia, candytuft and carnation seeds to 100 degrees Fahrenheit.

Of course, I've moved the flat to another mat. My question: have the seeds been cooked beyond germination? They've been on the bad tray for 48 hours.