We are thinking about putting a small green house against the south side of our house (mobile home). I am thinking steel hoops and plastic cover http://www.northerngreenhouse.com/products/ OUr neighbours use this plastic and it works great.
My concern is, will this greenhouse cause humidity issues in the house? There is no window or door directly from the house to the green house, but I have been thinking of adding a door.
I love my attached greenhouse in Ladakh, but we have a much sunnier winter than northern BC, and our greenhouse does go below freezing for January nights. It's just simple single plastic, no particular insulation, so you'll probably do better. Our thermal mass is the attached adobe house, and it works great. I love going out into the greenhouse with my morning coffee and filling my eyes with greenery and flowers in the middle of winter, doting on seedlings, nibbling on herbs and salad greens, and basking in the sun while I can hear my students ice skating outside.
But our outdoor minimum temperatures are more like zone 5 I think, whereas you say you're colder at Zone 3, and I know that Northern BC can get a lot of cloudy weather in winter. So if I were making a greenhouse in Northern BC, I'd focus on maximizing insulation, having moderate thermal mass, and installing some form of affordable back up heat for those coldest nights.
I don't have a humidity problem with the greenhouse attached to my house, but that might be for three reasons that might not apply to you: In my greenhouse, the ratio of watered bed to dry floor is pretty low; the adobe and earthen plaster of the building has an amazing ability to moderate humidity; and this region is a very arid desert. So in a wooden house in damp Northern BC with planting area maximized, I think you might have a humidity issue. We do have more humidity in a different greenhouse at our school, where the attached rooms are very small and the growing area is bigger: the doors do swell a little, and the greenhouse rains on your head on cold mornings. We remove our greenhouses for the summer, because they start causing overheating in April even with the end doors open, and would make the houses unliveable by June.
But oh my god, I love having an attached greenhouse all winter!!! Lettuce, arugula, spinach, chard, kale, edible chrysanthemum greens, claytonia, mustard greens, mache, carrots, dill, fennel, oregano, chives, dragonshead (lovely local lemony herb), mint... plus showy perennial mums in Nov-Dec, iris and other bulbs in March, calendula all winter, bachelors buttons in April... All while outside scenery is absolutely leafless from October to early April: Ooooh!
Works at a residential alternative high school in the Himalayas SECMOL.org . "Back home" is Cape Cod, E Coast USA.