If the "house" is insulated just close up the vents with 1" foamboard and turn on a 60 watt lightbulb. That should be enough to keep it from freezing. That's all we did when I lived in So. Cal when we had temps at night below freezing and we had to drape material over the orange and grapefruit trees. A 60 watt lightbulb (incandescent of course) was all that was needed to keep them from getting frost bit, and we had some nights in the lower 20's. The one night we hit 17, even with the Remay Cloth and the light some got hit lightly.
