I used to have glass my new greenhouse has the plastic that looks like cardboard (three layers one corrugated in the middle) and I have to say that the plastic is way warmer than the glass even on cloudy days it is significantly warmer than outside. Tomatoes are powering away so its plastic for me but definately the triple layered stuff, its loks cheap and is cheap but it keeps in the heat and lets in the light
x sunloving