I am planning on building a solar collector for heat for a greenhouse. My question is what is the best covering, or most efficient, cover for the collector. I have read several places where they talk about glass polyethylene, poly, polycarbonate, etc. Can not find good info. on comparisons of these various coverings. IR transmission, insulation, etc. This is for the collector only, not the greenhouse.

Your lack of specifics indicates more research needed. I suggest starting at builditsolar.com. Glazing choice depends on the type of collector. I can tell you that double glazed polycarbonate (greenhouse type) will not hold up for flat panel type hot water or air collectors; the temperature gradient between inner layer and outer will make the panels bow and warp, prematurely failing. I switched to single layer 1/16 polycarbonate which works fine, but doesn't perform as well in very cold weather, and you should design your collector to accommodate the expansion and contraction from heating/cooling.

If money is no object than double glazed tempered glass (high transmission) would be hard to beat for flat panel collectors.

Hi Bruce Thanks for the comeback. At this time , this part is very preliminary. Just today started working on moving dirt to get ready for foundation. Want to get the main building up, so just thinking along the solar thoughts at this time, and gathering idea's and info. I hadn't run across builditsolar before. Looks like a lot of good info. Thanks