p>To build a passively heated greenhouse in a cold climate that can operate for 3 seasons you would need to find a way to store the heat. Insulation, vapour barriers and air barriers are important, but in order to retain heat from the day you would need to include some sort of <a href="/guides/2200/the-pros-and-cons-of-thermal-mass-in-buildings/" target="_blank">thermal mass</a>.  I would say your first step is to check out our page on building greenhouses –</p> <p><strong><a href="/guides/3336/how-to-build-a-greenhouse-in-a-cold-climate/" target="_blank">How to build a greenhouse in a cold climate</a></strong></p> <p>Finding a way to store the heat will not only allow it to stay warm longer, that can also be a way to prevent it from overheating as well. Since a greenhouse will obviously have a lot of windows or clear plastic where there is little to no R value, insulating some portions of wall will help, but only so much. </p> <p>Since you say you started already it's a bit late for us to help too much unfortunately, but...we recently installed a <a href="/guides/3368/solar-air-heated-radiant-floor-kit-diy-problem-soils-expansive-clay-leed-passive-house-zne/" target="_blank">slab on grade with a solar air heated floor</a>,  it’s not fully ‘passive’ as it requires a <a href="/guides/1075/all-about-solar-air-heaters-diy-or-purchased/  " target="_blank">solar air heating panel</a>, but that only requires a fan which can be run that off a very small PV panel.</p> <p>As for:</p> <p><strong>Vapour barrier</strong>: Check with your municipal building inspector, for a higher humidity environement they may have requirements beyond the standard 6-mil poly barrier. </p> <p><strong>Best insulation </strong>- maybe Rockwool, it is unharmed by moisture. Same for the rigid insulation, EPS and XPS will stop moisture, Rockwool will let it pass right through, read more here -</p> <p><strong><a href="/guides/2254/polyisocyanurate-foam-is-a-hot-new-building-product-learn-where-and-where-not-to-use-it/" target="_blank">Choosing the right rigid insulation panels </a></strong></p> <p> </p