We're currently planning a 25sqm freestanding greenhouse, and a 40sqm conservatory/attached greenhouse (with ground insulation and possibly climate battery) for building in the spring (April/May here in south central Sweden). Our instinct is to go for glass, because we want something that doesn't need replacing every 10-20 years like polycarbonate. However the only glass that's used here is safety glass, partly because of the snow load in the winter , and I know that that contains a PVB mid layer, which I can't find any information on the lifetime of - the PVB film when sold separately only has a lifespan of 15-20 years according to 3M, which puts us back to the same time frame as polycarbonate! An alternative would be double glazing, but that's prohibitively expensive and also has degradable parts. Does anyone have information on the lifespan of security glass? Perhaps someone can convince me it's ok to replace your polycarbonate glazing every 15years? Because as far as I can tell, that's going to put a significant dent in the sustainability of the enterprise, not to mention the unknown availability of polycarbonate that far into the future. All comments welcome:)
Mycorrhizae for permaculture - chaosfungorum.co.uk
Good catch Jake. I think the plastic in the laminate glass will eventually fail from UV degradation. I figure "UV stabilizers" can only go so far and still remain translucent. I think other types of "safety glass" are available that don't contain plastic. Tempered glass. Also that glass with the metal wire mesh in it. Tempered glass is definitely used in greenhouses. My guess is it would be preferable for this very reason.
Another option would be to try to enhance the UV protection. Smear sunscreen all over your greenhouse...I'm kidding, but this is sort of the idea. One way to use chemistry to manufacture a UV stabilizer might be to start with a zinc salt that decomposes at increased temperature. Dissolve it in water. Then heat it to evaporate the water and calcine/decompose the salt in air. Presumably something like translucent zinc oxide nanoparticles remain, and presumably this powder can be added to any sort of coating material or paint, or melted into a plastic surface.
Isn't glass opaque to UV light? At least the normal window glass is, as opposed to quartz glass (which is more expensive).
Unless I am thinking about something completely different, the plastic layer between the two glass layers should not see any UV light.
Common soda glass blocks most UVB but transmits most UVA. source It's harder to get a sunburn through, but if you've seen dyed materials color-fade over time, that's ionizing radiation getting through.
We had a greenhouse where the roof panels had some kind of solar reflecting plastic film in them, and after 15-20 years it completely disintegrated. The double panes looked tightly sealed with tough material but eventually water got in. Metals corroded. It looked so terrible. It all had to be torn out. The entire thing had to be rebuilt. :/
In comparison the wood hoop house structure held up rather well. Sure the plastic had to be replaced. But considering that any greenhouse will need an overhaul, maybe going with the cheaper hoop house is a reasonable strategy. It's like trying to make a long-term investment in a computer...it doesn't work. Sadly, anything that will go obsolete eventually isn't really a good long-term investment. Even glass can crack, break, or be etched by acids in the rain and in the air and environment. :( oh well.
On the other hand, if you do it right, you can grow plants in the greenhouse that *increase* in value overtime. But maybe that suggests investing in the plants more than the walls around it.
Sebastian, that's a cool material. Too bad it's not more affordable.
Yeah, I'm unconvinced that UV light makes much difference to plants, so in my opinion blocking UV is definitely still an option. The company you linked to that sells ETFE makes bold claims. I don't know if their claims are true or not. Even supposing the claims had merit, unfortunately the material appears to be around 30x more expensive than the more common greenhouse film, so I don't see any way to justify that cost. Maybe someday it will be only 5x more expensive or less, and then maybe a better case could be made for its durability.