Passive venting scenario - attached, recycled GH

17 years ago

I want to run this idea past you all, see if there are flaws, mainly re: ventilation. Highest summer temps here are a couple of weeks of 90F in July-August.

A 24x10 sq ft, south-facing, wood frame greenhouse attached to a workshop which has no electrical power;

-GH walls are recycled windows;

-the roof slopes from 10' to 6', and consists of three, 4'x8' twinwall polycarbonate panels surrounded by plywood shingled with asphalt;

- standard size house doors on the west and east end, each with a large, screened sliding window ~25" square;

- a solar-powered vent opener to open a ~20"x20" window in the east-side peak/gable.

Four of the windows in the workshop wall can be opened as well, and a couple of the south-facing GH windows can be opened.

Enough ventilation you think? It's mainly the heat potentially collecting in the peak I'm worried about. I thought of also installing a turbine or a stationary attic vent in the most eastward solid roof panel.

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Comments (6)

oakhill (zone 9A, Calif.)
17 years ago
paulns,
Most passive ventilation designs call for at least 10% of the floor area as roof vent area, and at least 5% in bottom side vents. That would mean 24 sq.ft roof vent and 12 sq. ft bottom vent area. However, your relatively cool summer may allow you to reduce this some. one 20 inch X 20 inch roof vent will probably not be enough for you, unless some of your side wall windows are placed fairly high up.
The link below recommends 17-20% venting area in total.

Here is a link that might be useful: passive ventilation
paulns
Original Author
17 years ago
I should have known ATTRA would have excellent information - it didn't occur to me to check them out re: greenhouses. Thanks.

That's a lot of venting area! So far, the wood frame is built. We could set more vents into the bottom and top of the side walls...And I figured that since it's almost always windy here, and a turbine vent acts somewhat like an exhaust fan, it would decrease the amount of venting area needed...

We won't be using this greenhouse in winter - if that makes any difference. And the workshop is wired for electricity (by the previous owner), we just haven't hooked it up - hope to in a year or two.

Our local hardware store sells a 'sloped roof vent', "picks up wind from all directions" and is more expensive than a turbine vent. Does anybody have experience with these? Is it as effective as a turbine?
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nathanhurst
17 years ago
Putting shade cloth over the top of the greenhouse in summer will be more effective, cheaper, and protect your polycarbonate from UV, perhaps tripling its life. I hold mine down with cord sewn through the bottom edges of the shade cloth and some 'drawbridge' anchor points. Total cost for a 10'x24' roof might be $25AU. In winter you roll it up and velcro it out of the way. I use 50% cloth, which cuts plant available light by 50%, but claims to cut UV by around 95%.

The plants will thank you too. (you'll need less water, and they are less likely to suffer heat stress)
stressbaby
17 years ago
Paulns, I think it is going to be a struggle to keep the temps down. I agree, shade cloth will be necessary. But it might be worth your trouble to get power out there and put in an exhaust fan.
paulns
Original Author
17 years ago
Alright, we'll certainly get shade cloth. Will the cooling lessen the need for air circulation, at least a little? I understand a breeze is particularly important for the tomatoes we hope to grow in the GH. And if the roof is going to be 50% glazing, and we subtract 50% more light because of shade cloth....

We''re gradually jacking up the ventilation factor. Turns out one of the 2.5'x5' windows has an opener and can go in the east wall.

We will go ahead, see how all the variables, esp. climate, combine and play out. We do run an extension cord to the workshop for power tools; we could operate a fan on it I suppose - though reluctant to do so.

Thanks for all ideas and advice.
paulns
Original Author
17 years ago
The roof will be 30% glazing, that is.

Passive venting scenario - attached, recycled GH

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oakhill (zone 9A, Calif.)

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