I'm planning a lean-to passive solar greenhouse approx 9x16. I'm tearing down an existing greenhouse that is falling apart and reusing the 6mm twin-wall polycarbonate panels. The panels are 15 years old. I'm planning on doubling up the panels ie one on outside of framing one on inside. Glazing is only going on south facing wall. The doubling is for strength (as after 15 years they have gotten a bit brittle) and for insulating purposes.

I'd like to keep the greenhouse above freezing for the winter. Record low in winter is -27 with average low during the coldest month of 13. There will also be thermal mass added via water barrels to achieve heat retention.

My question on doubling up the polycarbonate is are there any potential problems with this? As an alternative I could use bubble wrap on the inside and make it removable. Might my greenhouse be too hot in the summer? I'm in a dry climate but might I create condensation problems?