Greenhouse against south facing walk out basement
I would like a greenhouse. I have an unfinished, heated, south-facing walk out basement. The wall is concrete with exterior stone fascia. Would it be a terrible idea to build the greenhouse directly against the basement, converting a large basement window into a door to the greenhouse?
I figure the concrete in the basement wall can handle the moisture. I can open the basement door to warm the greenhouse. (Or heat it separately if needed.) And my HPWH can dehumidify the air. (If needed, I can run a dehumidifier, since the plants need the heat anyway.) And, I get the convenience of being able to start plants in the basement and move them out to the greenhouse as it warms up.
What building details should I consider if I do this?
Central MA, zone 5
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The biggest issue you'll have is moisture drive from the greenhouse towards the house. Greenhouses tend to run very high average humidity levels compared to what would be considered to be "normal" within a typical home. While the concrete won't care about being exposed to the high moisture levels, you want to keep that moisture from getting into your home's basement, so I'd consider a vapor barrier on the greenhouse side of the shared wall. You'll want to make sure that big window seals well too.
Aside from paying extra attention to dealing with the high moisture levels, and air sealing for the same reason, there is no reason you can't build a lean-to style greenhouse like this.
I agree with Bill. The types of plants you would typically grow in a Massachusetts greenhouse like humidity. The warm air in a greenhouse can hold a lot of moisture. If you mix that with cooler house air, you will get elevated moisture levels and the risk of mold and other microbial action. Greenhouses are rarely if ever airtight, so running a dehumidifier, which is expensive in a tight house, would likely be an exercise if futility.