I have a 20'x40' greenhouse in North Carolina. I have a double layer of clear poly greenhouse plastic that I keep inflated with a blower fan, which I installed with the understanding that its primary purpose was keeping things warm inside during the winter. However, is there any reason to keep the blower fan on during the warmer months? Thanks!
While the insulation factor is important (it can be as important in the summer as in winter) there is also the issue of movement of the plastic against the structure; this can cause wear on the warm, soft plastic where it comes in contact with hoops and crossbars. When the plastic is deflated it hangs more loosely on the framework, allowing for more movement of the plastic in windy conditions.
Watch carefully at crossover points where a nut or bolt head stands proud and sticks into the plastic. Continuous inflation holds the plastic more firmly against the structure so there is less movement. Well constructed houses use carriage bolts with rounded heads to minimize such damage. Consider too that seams in metal tube construction can be rough with jagged edges and need to point to the inside of the house when present.
For such a small greenhouse you may have no need for positive ventilation from one end to the other, but when air moves vigorously both inside and outside the structure there is even more opportunity for abrasion. And any abrasion becomes more pronounced as the plastic hardens under the effect of UV light.