Hi mehitabel,
91F seems a little high to me. I would turn the mat off during the day with the lid nearly closed and then only operate the mat at night.

I usually keep my seedling trays at 75F day and night. I use my heat mats with a thermostat set at 75F. The thermostats have probes that I put under the domes with the plants.

good luck with the orchid babies

Hi, audmer. Thanks for the reply, it's much appreciated. Seems like good advice to me to not let it get quite so high. I'll stick to 80 for a max.

My mat does not have a thermostat you can control, but the temperature can be kept down by raising the dome a couple of inches.

Alternatively, maybe you could put the mat on a timer so that it runs only at night. That way you don't have to raise the dome so high and then you would keep the humidity up. What is the ambient temperature during the day where the seedlings are located?

Room temperature maybe 75, but would vary. I'm trying the thermometer upright right now, see if that makes a difference. I'm trying to measure the air temperature, anyway, not the temperature of the mat.

I'm going to experiment around with it today, see what works to keep it around 80-83F. That's what it was at leaf level under the downstairs lights this winter, and the phals did great down there.

But I appreciate your caution about the 90's being too high.