Greenhouse growing temps

[dmacch]

I grow catts and phals in my GH. I have the day temps set at 75 deg and nights at 64 deg. My question has to do with day temps and cloudy days. Should I just have a minimum temp set on the thermostat only and let the sun warm the GH or do I leave the setup the way it is so the phals arent chilled on a cloudy day. The problem with letting the sun do the work of heating the GH is that a couple of hours after the sun starts to heat the GH it gets blocked, for about twenty min by a nieghbors tree, then hits the Gh again. So my main concern is keeping temps constant. Any thoughts on this subject. Thanks