Question about adding lean-to greenhouse to existing wall

kudzu9November 23, 2011

I posted the following issue in the Greenhouse Forum, but didn't get much in the way of informed replies, so I'm re-posting here. Any thoughts would be appreciated.

I live in the Pacific NW and the winters are mild: most of the season it only goes below freezing for a few days. I have a friend who wants to construct an attached, lean-to greenhouse against one wall of his house. It would consist of a cedar frame and salvaged, double-pane windows and a sliding glass door, all with aluminum frames. The footprint would probably be about 8' X 10' or 8' X 12'. The backside of the greenhouse would be a south facing wall. This wall is a garage wall clad in horizontal wood siding; it is insulated, there is a tarpaper moisture barrier, and there is finished sheetrock on the inside wall. The garage is unheated, but there is enough heat from some exposed ductwork that the garage never goes below freezing.

Here is the issue. He wants to attach the structure directly to the existing exterior wall, but I suggested to him that he should research whether he might end up with some kind of moisture problem within the wall. Can anybody advise on whether this is something to worry about? And, if so, what is the best way to deal with this? The greenhouse will not have much, if any, supplemental heat; however, it will have moisture, and the temperature inside it could range from below to well above the ambient temperature inside the garage, based on time of day and how much solar gain there was on a particular day.

Finally, it would be nice to have a solution that looks decent when finished, so something like just slapping up a piece of Tyvek on the wall is a less-than-ideal final solution.

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I am not a builder, but I would think a good hardware store or lumber yard could advise on whether any sealing treatment on the exterior wall would be helpful or necessary. Of course, exterior walls are meant to take weather, so maybe nothing needs to be done.

Preventing mold from growing. Maybe all that is needed is regular cleaning to prevent the moisture from leading to mold.

    Bookmark   November 30, 2011 at 5:32PM
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Fori is not pleased

Good question! I am in the SF area so much less damp. I think it depends a lot on what is planned for the greenhouse. This is stuck on the stuccoed (and windowed) (and wooden soffited!) back of my "new" house's garage and has been for over 50 years. The house is holding up as well as anywhere else.

I think this greenhouse was intended to start seeds in the winter--not to grow orchids or anything steamy--but I really can only guess.

We should try growing something in there this winter to see if it gets humid, but I'm a little nervous about that vintage heater!

    Bookmark   November 30, 2011 at 9:08PM
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I agree that properly constructed outside walls are meant to take moisture, but they are not impermeable. They are constructed in a way that regulates moisture and prevents it from migrating in and condensing inside the wall. By adding another structure to the outside, one is possibly changing how effectively the moisture transfer mechanisms occur. My concern is that damage could happen inside the existing wall from moisture, without anything being visible on the outside of that wall.

Thanks for the info. I'm familiar enough with SF weather, to know that it's not entirely different from my winters, just milder and less damp. My friend will not maintain it as a hothouse, but simply a place to protect sensitive plants and to start seeds in the spring. In any case, your response is at least encouraging. I'm curious: 1) Is that back wall, which is part of the house, insulated? 2) Is the inside of that wall heated living space, or unheated space?

    Bookmark   December 1, 2011 at 4:59AM
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Fori is not pleased

The wall there is partly garage which is unheated and (probably) uninsulated and partly (where the windows are) a chunk of garage that was carved out during construction to serve as a potting room with an open doorway. It's a little weird. Anyway, it's not quite the same as your friend is proposing.

It has ceiling vents (which are stuck in the open position) for ventilation and looks like it was a pretty nice kit when installed, although it's just single-paned glass. I'm not sure I would have chosen to install it like that, though. It's slightly higher than the roof and presented a challenge for the roofer.

Once we try growing things in there and using water in it we'll have a better idea of the humidity. I'm hoping it'll be more like a sunroom or enclosed porch than a swampy hothouse.

I'm looking forward to cleaning it up after the earthquake. No wimpy acrylic ceiling glass here! :P (Don't let your friend put glass over his head--you guys are due too!)

    Bookmark   December 1, 2011 at 12:26PM
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