Greenhouse project planning

We talked about doing something like that, but it’s mostly untreated pine that already shows patches of mold, so probably couldn’t withstand the humidity and heat of a greenhouse. Also, one point of contention in greenhouse selection has been aesthetics, and apart from the mold issues that would probably be the ugliest option… But if all other options end up being too costly, that’s definitely still on the table!

Years ago I worked on a farm that had many untreated wood frame greenhouses, mostly framed up on 3-4 ft centers of rough cut lumber.
I would say that most framing lumber lasted 10 + years.
These were poly covered , a double layer on roof with a inflation fan so it would puff up and shed water.
Every ~ 4 years we would put new greenhouse plastic on .
At that time we would inspect and replace rotted wood.
Usually just a board here and there needed replacing.
The main frame lasted decades .

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There are a ton of answers, but most of them did not consider the basics of the use to which this greenhouse will be put. You want to grow Avocados and tender annuals in winter. The site MUST have maximum solar exposure. This eliminates all but 2 of the listed options. The other consideration is type construction which seems to currently be leaning to a wood frame. The rest of the advice I would give is to make it larger than 150 square feet. If you can, go for 250 to 300. I’ve seen first hand the limitations of a small greenhouse.

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Thanks for the reply! Turns out we’re going with the garage demolition option (not originally listed), and assuming we reuse its foundation that’s 16’ x 20’, so yep over 300 sq ft


I included the garage as one of the two viable options for sun exposure. :slight_smile:


another vote for stripping the existing structure to its framing and sheeting over that… you’ll be surprised at how well the wood holds up. the mold in these situations is usually only on the surface, wood needs to be in contact with dirt or sitting in a puddle and exposed to bugs to degrade quickly. plywood sheeting will get spongy from humidity but dimensional lumber usually doesn’t. you can paint all the wood with an oil based primer and it should be good for a couple decades. total project cost could be little more than some paint, plastic, wiggle wire channels, irrigation/fan/heater and controls. and if you don’t like it you can always fully demo it later


After getting quotes for the demo and custom greenhouse I’m starting to think the only thing we could afford would be something like that anyhow. I wonder how much custom polycarbonate panels would be rather than using sheeting, and whether they could be firmly affixed to the existing framing… I think my wife has a firm no-sheeting opinion because of how it looks.

Thank you for all the feedback everyone!


look at farmtek for budgetary pricing, you may want to go local in the end but farmtek is usually pretty competitive


Perfect! Thanks again

I will add that this garage is about 70 years old, so there’s a decent chance some of the wood is not in the greatest shape, but other than a few areas with mold near the foundation I haven’t seen any signs of insect damage or serious rot.

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Considering asthetics and using the existing structure. You could strip it down to the framing and then paint it. That way it would improve the look and also make the wood a bit more durable to the elements inside the greenhouse.

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Agree with the others who suggested repurposing the garage. You don’t need to remove all the siding- leave the siding below about 3’ and leave the North side alone. That will make it look more like a purpose-built structure. Then all you need is to put twin wall poly onto the existing framing, add vents etc and you are done for a fraction of the cost of building a new greenhouse.

Heck, you might be able to get away with leaving three (or at least two) of the sides intact and just remove the roofing materials and the siding on the south side. With that flat roof you will still have a ton of solar gain.


Does anyone have thoughts on the best way to ensure sufficient ventilation if we’re reusing the structure and going with the farmtek 8mm double-walled polycarbonate?

Would it be enough to put a couple exhaust fans at the front and rear of the roof gables, and a few passive intakes around the base, or should we include roof flaps? Seems like that’s more complicated but I don’t want to do all this work and have it get too hot every summer.

No doubt a ridge ( roof ) vent would be best.
As that is where the hot air is.
But they are expensive to buy.
Could make one ?
Just a long panel with a hinge that lifts up at the ridge line .
That or large vents on the gable ends with large roll up side wall vents should do it.
I have a 30 x96 high tunnel.
With 6ft roll up sides on both sides, and several 4ft vents on the gable ends as high as I could put them.
This creates a natural convection , the heat goes out the vents at the top, drawing in cool air along the side walls.
It stays amazingly cool on hot days. No fan involved.
I like the roll up sides. Lots of ventalation


There are as many ways to ventilate a greenhouse as there are ways to skin a cat. IMO, a flap at the end of the greenhouse near the top is the best solution for a small greenhouse. Put it at the opposite end from the door so air will circulate.

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Compared to the overall cost of this project, adding a couple of these wouldn’t be prohibitively expensive, and might be cheaper than getting exhaust fans, especially including the electricity costs over time even if the fans only run on hot summer days:

I’m starting to feel like this is really going to happen. Thanks for all the help everyone!

For a small greenhouse, only one automatic opener would be required. However, you might want to build in a smaller vent that can be manually opened as a backup.

I figured a 320 sq ft greenhouse was big enough that two would be a good idea (one on either side of the center of the roof?), but one sure is cheaper than two! :grin:

I’d definitely put hand-openable vents at the top of each gable, where I was thinking of putting the exhaust fans before.

Also smaller ground level intake vents on each end, plus the windows + door that can be opened. Hopefully that should do it.


It is not possible to have too much ventilation. In my greenhouse 1/4 of the roof opens automatically and I have a huge automatic vent fan and a screen door and a shadecloth and it still gets way too hot in the summer. You don’t really need a greenhouse in the summer, so don’t sweat it if it gets too hot. Most hobby greenhouses in the temperate zone are useful fall through spring. In the summer it’s a sauna.

Don’t think you don’t need ventilation the other seasons. Yesterday in Maryland my greenhouse hit 115 degrees with the roof open and vent fans running.


Having lived in Maryland, I’m hoping that the (much) milder Seattle summers won’t be quite as bad, since our summer lows are too cold for citrus or any true tropicals to flourish outside and I’d hate to have to move them in and out every day, but for stuff like avocados yeah the plan is to spend summer on the patio.

I just came in from my greenhouse. It’s 114 in there with the door and roof open and all my fans running. Feels really good with snow all around! One thing to consider is make sure you can humidify effectively. Need a lot of moisture in the air at high temps and that’s not easy. I always drop down to like 10-15% humidity sometimes for hours because my small home humidifier just can’t keep up (and open doors and vents doesn’t help).

But yeah, your experience will be totally different. It’s hard to give good advice really because every situation is totally different. You should be able to easily insulate the North wall which is super helpful at maintaining temps. But yeah ventilation is super important and there is no possible way to add too much. Water is crucial, having running water makes your life so much easier. And having a good electrical line coming in is key too. I run three space heaters all night long in the winter and LED panels sometimes so I need every watt. You can’t do it with an electrical cord.