My coop is slightly smaller than yours and I just had a heat lamp going in there. My flock will be about 28 this winter so one should do it. My coop is also insulated. I only have one door (Chicken door out the back) that isn't a weather tight door and I seal that up with a block of foam wrapped it in a garbage bag and it works like a charm. It is toasty enought that no frost bitten combs the last two years but I have to but in a heated water base.
