From what I have found, 72 F is ideal for lettuce germination. I am new to farming, so I do not have years of experience behind me. I built a germination chamber in my basement, mostly for growing in the summer. Here in Canada, it gets quite hot in the summer and if your soil/medium is above 80F, your seeds, apparently, will not germinate. Also, I have read that if you can have them under non-heat producing light, the germination rate is much better. (I have also read that red light is best, but this seems a little excessive.)