Acrylic is a trade name for Polymethyl methacrylate or PMMA. Once upon a time, PMMA was called 'Perspex' and 'Plexiglas'. People have been making perspex greenhouses since before I was born.

Having said that, it is possible that newer PMMAs have better UV protection and might last a little longer. PMMA glazing is a lot heavier than Polycarbonate and PETG and I don't believe available in twinwall.

You can get acrylic twinwall and I've seen greenhouses that use it but in this case it is solid clear sheets.

Acrylic has better natural resistance to yellowing than polycarbonate. This means that polycarbonate greenhouse glazing is always UV treated to virtually eliminate yellowing. Acrylic can be UV-stabilised and most modern products are, but it is interesting to note that the link only mentions a discolouration guarantee for the polycarbonate. This is something I would certainly confirm before purchasing acrylic glazing.

Acrylic is almost exactly the same density as polycarbonate, which is about half that of glass. Solid acrylic will be considerably heavier than twinwall acrylic or polycarbonate. Acrylic is not as strong as polycarbonate but it is much cheaper. It is still essentially non-breakable as a glazing sheet but can be scratched or damaged by certain cleaners.

In summary, modern acrylic is a viable greenhouse glazing but perhaps its past reputation is hurting it. Buyer beware that you are actually getting a quality UV stabilised acrylic sheet.

Here is a link that might be useful: Full acrylic greenhouse description

SnB,

"Acrylic is not as strong as polycarbonate but it is much cheaper. It is still essentially non-breakable as a glazing sheet but can be scratched or damaged by certain cleaners."

I think polycarbonate is much stronger than acrylic. Incidentally, bullet proof "glass" is actually thick polycarbonate.

The Rion 30 Acrylic Greenhouse looks pretty good, but it has resin PVC framing. To some people, "resin" may sound better than "plastic". The Rion webpage doesn't hesitate to refer to it as a "plastic greenhouse."

On the webpage you linked to, despite the picture of people perched on top of the Rion greenhouse and the "test" of the roof supporting an "incredible" 1,100 pounds of snow, that webpage warns,

"Despite this testing, please note that Rion Greenhouses are NOT load-rated structures. Before purchasing your Rion Greenhouse, be sure to check with your local building department to find out any structural requirements."

I am not certain that we could get a building permit for a resin greenhouse here in our town in Maine.

All that said, the Rion's green framing and clear acrylic glazing look great, and the price is quite reasonable. I think it looks better than a Harbor Freight. It might even be easier to put together. If they had an attached or lean-to model, I might be willing to give it a try.

MM

Very approximately, the impact resistance of standard acrylic is 20-30 times lower than polycarbonate (not good for stopping bullets!). "High impact" acrylic is a range of slightly modified acrylics that are 5-10 times stronger (so still less than polycarbonate) by this measure, but impact modified acrylics go yellow where standard acrylic is virtually immune to yellowing. Structurally, acrylic behaves very much like polycarbonate, same tensile and flex strength, but it gives way much sooner. Acrylics are also softer than polycarbonate and will scratch easier.

For comparison, glass is stiffer and has higher compressive and tensile strength than any of these plastics, but impact strength is way lower than even standard acrylic. Actual "bullet proof glass" and even car windscreens combine the properties of plycarbonate and glass in layers to try and achieve extremely high strength with high impact resistance. Laminated greenhouse anyone?

Acrylic compares favorably to polycarbonate on everything except impact resistance. It transmits more light, it's a touch less dense, it blocks IR about the same**, and it's more UV stable (I'm not sure where the comments about it being less UV stable came from. It transmits UV instead of blocking it like polycarbonate does, but this can be changed with coatings).

On the other hand, if you're worried about hail, it doesn't get much better than polycarbonate.

**This is what I've heard, but I've been unable to find a graph or raw numbers, so if someone had that available, it would be appreciated. :) Multiwall polycarbonate transmits ~8% in the thermal IR range, which compares favorably to glass's ~18%.

In my search, I did run into a paper on IR transmittance that mentioned an interesting tidbit: mylar and kapton are also good thermal IR blockers, better than PVC (which is, in turn better than PE), and were rated as being in the same category as acrylic. Might be interesting to try an experimental film-glazed greenhouse with them.

PMMA blocks TIR longer than 2200nm until you get down to terrorhurtz waves. The biggest advantage of PMMA is that it is very clear - even a metre thick piece of optical grade PMMA is clearer than a normal 3mm piece of window glass. Until it gets scratched anyway :)

This is not very useful for greenhouses, but quite significant for aquariums.

The UV transmission is a very interesting feature that rarely gets mentioned. It has been used for reptile enclosures because it allows UVB to pass (unless you get a grade which has been specifically modified to block UV), which the lizards need. You can get a suntan behind acrylic but not behind glass or polycarbonate. Anyway, for a greenhouse this can be a valuable feature, or it could be considered harmful, depending on what you want to grow and how intense your sun is. With my hobby of cacti and succulents, and my current home in England, extra UV is very valuable.