Glass for diy greenhouse (forum)

Parkesy starts with ...
Hey everyone,
I’m just about to knock together a greenhouse out of some left over glass from a job I did a couple of years ago but had some questions. The glass is 10mm thick and came from ballustrading around a balcony. It should warm up nicely, but is this likely to block too many of the suns rays and stunt plant growth? I read glass stops some UV Rays which isn’t bad, but I’m more concerned it may stop the good wavelengths getting through?

Time: 13th August 2018 8:39am

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About the Author Parkesy
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Waterfall says...
Even in commercial greenhouses they use materials that 100% block UV, from what I can tell its the amount of light transmission that is more important and they usually try and diffuse the light.

What is the purpose of the greenhouse? what are you trying to grow? Heat regulation is the real difficulty in my experience, too cold at night and too hot during the day.

Time: 14th August 2018 3:03pm

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About the Author Waterfall
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Parkesy says...
Thanks waterfall.
Being so thick it was the light transmission I was unsure of. I’m wanting one for starting seedlings in, and also for keeping some potted fingerlimes warm during our winters. I was thinking of adding a heat activated strut on to a vent so it will open up on the hot days. I’ve been using the cheap bunnings greenhouses but they don’t last more than a couple of years before they weaken and strong winds start ripping the cover.

Time: 15th August 2018 8:00am

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brad16 says...
Hi Parkesy,

The visual light spectrum that we can see ranges from red to violet (blue).
UV is Ultra-Violet light. That is light frequencies higher than violet (the highest frequency of the visible spectrum.
Infra-red is light frequencies lower than red (the lowest frequency of the visible spectrum).

One of the natural characteristics of glass is that it is a low-pass light filter, which means that it lets low frequency light pass through while filtering out high frequency light. Another example of such a filter is the ozone layer.

As for filtering out UV light, glass is very effective, as a small amount of glass filters out a large amount of UV light. But take a look through your 10 mm thick sheet of glass. You'll observe that the amount of light that gets filtered out in the visual spectrum is barely noticeable. You'll also feel the heat coming through a window on a cold winter's day. Heat is generated by the lower frequency light, beneath the visual spectrum, when it is absorbed by the material it meets (like your face or the carpet on the foor). These frequencies pass very easily through glass, as you'll note the glass window still feels cold.

UV light causes damage to living tissue, and plants are no different. You may already be aware that plants also can get sunburnt. Photosynthesis is done almost entirely in the visible spectrum.

As Waterfall has already mentioned, some people constructing greenhouses diffuse the light. Diffusing light means that the light rays are deflected into many different directions, rather than all the light coming down in one direct stream. There is transparent glass and translucent glass. Transparent is the normal glass we think of that we can see a crisp image through. Translucent glass, lets the light through, but diffuses it into many directions, and we can not see a distinct image through it. Aside from frosted glass, other diffusers of light are clouds. Frosted (or diffused) glass is an excellent way to simulate overcast skies, to produce softer light for plants that would otherwise be overwhelmed by day long direct sun conditions.

Commercial greenhouse vendors use glass sheets as thin as they can get away with. This isn't for light transmission, it's for economics. Thicker glass is more expensive, and also the weight of the glass requires more expensive support in the structure that suspends it.

One very big advantage of thick glass though is its strength. Thin glass breaks easily and is more prone to damage in adverse weather. Your 10 mm glass will ultimately produce a very robust greenhouse and should be the last one you ever need to build.

Consider a large conservatory. How thick do you think the glass would be in the Brisbane Botanical Garden's Tropical Display Dome? (I don't know answer to that, it's just an interesting question).

https://www.brisbane.qld.gov.au/facilities-recreation/parks-venues/parks/brisbane-botanic-gardens-mt-coot-tha/attractions/tropical-display-dome

Also if you build an entire glasshouse with frosted glass to diffuse the light, I think you are limiting your options, in that you can always place shade cloth or netting of some sort over specific plants or areas on an 'as-needed' basis, giving you much more flexibility with what ever you want to put into your glasshouse in the future.

Build away, I say ... and make everyone else jealous :)

Time: 31st August 2018 1:17pm

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About the Author brad16
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