I am making a greenhouse from old aluminium beams designed to take 25mm multi-wall polycarbonate sheeting. Will a sheet thickness of 25mm leave in sufficient light? Or would I have to buy thinner sheets? I live on the South coast of Ireland.
This stuff would be great for the north wall, perhaps part of the north slope of the roof and the end walls. Do not use this for the south slope of the roof!
You need to find the proper greenhouse skin that will not disintegrate in the UV of the sun. They have this stuff warranted for 4 years and upwards. One needs two layers if you've got a heated greenhouse. 2 layers of proper greenhouse skin with air in between. Some even pump air to keep those two layers separated.
No way could this be thin enough to allow enough light and spectrum from the sun into your greenhouse.
When you use a proper greenhouse skin, plastic covering, make sure you know which side is supposed to be on the outside, which side is supposed to be on the inside. Don't trust the manufacturer so much. We've got 4 year warranted skin on our greenhouse where the words on the skin were backwards! So we've got another skin coming for another 4 years?
Use a light meter! Go rent or borrow one! Do you have ventilation handled? Big fans that run 24/7 when growing plants. Do you have ventilation pulling air out of the greenhouse and one bringing air into the greenhouse? Are you growing in pots or in the soil in properly constructed beds? Hopefully, all pots have sterilized potting soil. Is this your first greenhouse?
I have a modest green house with only a 8' X 4' clear roof. I have used a few kinds of plastic sheets ; they blocked some light, deteriorated in UV, cracked in hail , and were unsatisfactory . I bit the bullet and put in 1/4 " ( 6 mm) plate glass ; four pieces 2' X 4'. Expensive but worth it. The glass has been through a hail storm that damaged my asphalt shingles so badly the insurance replaced them . And a 100 foot pine fell on the glass , well only the top 15 ft of the tree. So glass is strong and clear and is reasonable cost if you factor in a long life compared to plastics.