I have a shed style greenhouse with a single layer corrugated plastic. The winter temps are holding up and retaining heat since I put in more insulation. The next issue is the intense summer sun in the summer months. I went with dual layer 8mm corrugated plastic in opal instead of clear to block 30% and uv protection. A have a friend who is a roofer with no horticultural experience and says we can do a belt and suspenders approach by installing it under the original roof. We were going to cut the 72×72 pieces and lock them into a place under the original but I’m not a fan. The materials are expensive and not designed for that. Worried there will be a new unvented area between the roofs where critters and moisture can be locked in. Greenhouses are hostile environments with lots of challenges that people don’t understand without experience. Original corrugated roof vents need to be disabled, besides serviceability and losing a few inches of head space, what other issues are there. I have never seen this done before and trying to see what people think. Thanks for your thoughts in advance.
by t0mt0mt0m
7 Comments
Typically with double layer, soft plastic greenhouses, there’s a small fan pumping air between the two layers. That might be an option with hard plastic to prevent the issues you mentioned.
Love the setup, excited to see what people suggest.
I would remove the old roof and put the new panels up. I had a similar roof that started to degrade after 15 years that I replaced with polycarbonate panels. If you try to mount these inside, not only will it be labor intensive, you will inevitably be replacing the current roof a few years down the road.
Tell me about the (I assume) foam insulation on your back wall? I want to do something similar. Just wondering how much it extends your season.
summer heat can be controlled with shade cloth. you say you have a 30% reduction now , add 20% shade cloth or more.
Greenhouse contractor here. Just take off the corrugated and put the new polycarb on the roof. The twin wall poly will be a HUGE step up in insulation over the corrugated.
OP. I literally did this exact thing today. I was considering making a post on here to see what everyone thought. I have clear corrugated roof panels but a lot of hear escapes through them. I decided to install .32 polycarbonate panels between the rafters on the under side of the roof purlins. This basically only gives me a 3/4″ gap between the corrugated roofing and the polycarbonate sheet. I’m wondering if that is too small of a space and if I’m going to have significant moisture build up in the summer months. I was trying to figure out the ideal spacing between the two layers that would optimize temperatures. It looks like you are planning on hanging the sheets onto the underside of the rafters which I considered doing myself. I wonder if that was the better approach. I would really appreciate anyone’s advice on this too. I can post pictures if that helps. Thanks
I built my greenhouse from 2x4s. The outside ‘skin’ in twinwall. Inside, I stapled greenhouse film, giving me 3 1/2 inches of ‘dead’ air. I sealed it well to make sure no water could get in.
Those 2x4s are holding up quite well. Two worries: Moisture condenses on the inner film, and it literally ‘rains’ inside the greenhouse. All that condensation also blocks sunlight in the winter, when I need it most. I’m going to try one of those hydrophobic sprays: https://www.greenhousemegastore.com/collections/plastic-film/products/sun-clear
Might solve the problem.