Kevin Wang wrote:House exterior walls are not waterproof. If you have super high humidity air on the outside, it will make its way into your house's walls and feed mold, rot wood, etc. cement is not waterproof. stucco is not waterproof. wood is not waterproof. tar paper may be pretty waterproof, but usually when the side of a house is layered with tar paper, kraft paper, or house wrap, it's layered, much like shingles. it's designed so that gravity will pull water droplets (i.e. from rain) away and out from the building. If you build a greenhouse, humidity is air, not water droplets, and will happily go upwards into your house.
I would not recommend an attached greenhouse, because of the humidity.
An attached/enclosed porch is different, because there is no moisture.
i see your point, siding is generally oriented in mind with water travelling towards earth and there is lots of air gap between each shingle or sideboard. with long enough exposure, high humidity will seep upwards through the gaps in the siding.
perhaps it would be a good idea to extensively caulk any air gaps and put a strong layer of paint on top of the wall which will get the attached greenhouse .
