If it's really cold outside and it's 72 where your plants are and that's as cold as it gets, why worry about heat? You are 100% fine in the 72 and up range for seedlings. Honestly, I know you put a lot of thought and effort into this, but now that your ambient temperature is revealed, I for one think you'll be just fine without any type of heat mat or anything else. Just two cents.Thanks Skottelgoed. My lights also emit some heat. I downloaded a room temperature gauge phone app. I'm sure it's not going to be 100 % accurate but it's got to be within a few degrees I should imagine. I am getting a reading of 22 C (72 F) inside and it's quite a cold day I think. We got some rain today. Did you?