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I bought a cheap grow house with 4 shelves for growing herbs but the wind has knocked it about and it is starting to lean to one side a bit more than it should.

I plan to build a wooden frame, to remount the shelves on and use twinwall polycarbonate for the panels, instead of the plastic that was over the grow-house frame.

The final unit will be 1.8m high and 40cm by 60cm shelf size. I was wondering what thickness twinwall polycarbonate to use for the panels, the local DIY shop only had 1 thickness but when checking online there is quite a variety: 4mm, 6mm, 10mm, 16mm, 25mm, 32mm and 35mm.

I will likely use the same material to build some cold frames later on.

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4mm twinwall polycarbonate is the best choice, for reasons which I'll explain. Where you live in Edinburgh, Scotland, the days are shorter and the light less bright - 4mm twinwall polycarbonate allows 80-84% light through, compared to single wall polycarbonate, which allows 94-96% (note that 3mm glass allows 97-98% light). This might sound like a big reduction in light transmission, but it is diffused by the polycarbonate, which means light reaches what would be darker areas were you using glass in your greenhouse. It's also obvious that if you used even 6mm, never mind 10mm, the light transition would be even less, and where you are, you don't want that.

The thermal ('R') value of 4mm twin polycarbonate is 1.42, compared to 0.83 for single, so its insulating properties are pretty good - 4mm is the one most commonly used in Germany, where it gets pretty cold. A higher mm would give more insulation, but you'd lose more light, so on balance, 4mm seems the best choice.

Note that you will need to tape over the cut ends of the sheeting to prevent moisture infiltration and subsequent algal proliferation. Note also that it excludes harmful uv light, so you won't get sunburnt if you spend a lot of time in there, and neither will your plants!

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    Also seal the seams to reduce air infiltration. This will give you more control over ventilation. – OrganicLawnDIY Sep 11 '13 at 12:42
  • 4mm is also the cheapest, so I really like this. Great info had not even considered the loss of light transmission, and did not know 'R' values were used to measure the thermal value. – Morgan Sep 11 '13 at 13:12
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Just like double paned, insulated glass, different thicknesses provide different R-Values. Manufacturers/retails will sometimes list the R-value for their products. Here you can find some R-Values for twinwall.

The greater the thickness, the more effective it is as an insulator. Determining which thickness to get will be based on your climate, what you plan to use the green house for (ex growing in the winter) and any heating you may provide to the greenhouse.

The first link also contains R-values for 4mil and 6mil poly sheeting, which is what your original greenhouse likely used. You can compare that to the R-Values of twinwall. If the greenhouse you were using provided sufficient insulation, even the thinnest twinwall will work.

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It seems to me 4 mm and 6 mm sheets are mostly not very sturdy? You need quite some framing to get it halfway stable and even then they can be destroyed quickly.

Most 10 mm sheets are stable enough, isolate a bit and let enough light through? So they seems for many places the best choice.

I am myself working on such a solution, but using wood for framing, the offered aluminium framing seem much to expensive. There must be a way to make those thing yourself?

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