Thanks for finding us & your detailed comment! I’d probably start with the fact that many new greenhouse owners overestimate what it can do for heat retention. I’d tell you, having done both the UV plastic insulation and foam barrier (recently as of this year), that the UV plastic makes way more of a difference in heat retention. Neither are a miracle against cold outdoor temperatures, though, and the tendency for things to equalize with outdoor temperatures is strong. However, the insulation does ensure that whatever external heat you do provide is reasonably effective and not is not just paying to heat the outdoors.

Our gusset decision was mostly based around comparative cost, ease of creation/installation and effectiveness against the snow loads we needed to protect against. (Usually exceeding 7-12 feet of annual snowfall.) The plywood gussets are fine, and likely would even work well in our circumstance. For me, though, it’s a matter of trusting steel more than I trust wood. Thus why we bolstered several aspects of the design with steel and various fasteners. I just don’t want to wake up one day and realize my building was crushed by snow! (Which is not an uncommon thing where we live!)

Hope you enjoy your new greenhouse! Ours is still working great, many years after building it. It’s a vital piece of our kit and the investment has easily paid for itself! Thanks for the comment and checking us out!

Excellent! I’ll try to break down the concept of what’s at play here as I think it’ll affect your situation. You may still need to deal with that air gap in the outer layer.

The second layer of plastic is not what provides the insulation. It’s the air gap in between that is insulating the inside. The temperature change of the actual materials happens very quickly, but it takes time to heat up (or cool) the air gap in between. If the outer seal is not “relatively” air tight (it doesn’t have to be perfect, but not leaking like a sieve) then the insulation barrier will not work quite as effectively.

As for our door, we didn’t insulate it. More importantly, I want to put in some wood to act as a door stop that I can apply weather strip to that will create a better air seal for the exterior of our door. That’s currently our weakest link. We also want to put 2″ foam insulation in between the studs at the bottom of our greenhouse. We might do these things this year if we have time. Then, it might make sense for us to put plastic on our door. You lose the overwhelming majority of heat at the very top of the greenhouse, not further down, so that’s the most important area to insulate.

Also, one last thought. When we eventually resheathe our greenhouse, I am going to put plastic on the outer layer as well and then sheathe over it. (So, basically two layers of plastic.) This will make an even better airtight seal and will make it work even more effectively.

Hope that helps!