Sources for 4mm twinwall mouldings?

17 years ago

Hi everyone --

I'm in the planning/design stage for a greenhouse that I'd like to cover with 4mm twinwall panels. Trouble is, I can't find the nice mouldings that are available for 6mm, 8mm, and 10mm. The following two examples are of the kind of H-channel I'd like to find for 4mm:

But this is all I can find:

As you can see, the 4mm H-channel can't be mounted directly over the studs. It has to be "free floating" next to a stud, which may not necessarily be a bad thing, but it's just not a clean installation and is potentially prone to problems. Also, I realize it's probably a relatively minor issue, but having the mouldings off to the sides of studs cuts down a bit on the amount of light able to shine through the panels.

If no one knows of better 4mm mouldings, might it be possible to use 6mm mouldings -- or would there be too much slop in the grooves?

Also, I'd really like to find nice corner profile for 4mm like this:

Same problem -- they make this for the bigger thicknesses, but not for 4mm. If it's not available, what I'll have to do is use two U-profiles for each corner, then cover with Peel & Seal or something. As you know, the cost for mouldings can add up over the entirety of a project, and if I have to double the usage of mouldings on corners with another product to seal them tight, it adds to the bottom line.

Thanks in advance for your help!

Bruce C.

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Comments (5)

mollyd
17 years ago
Why don't you use wood instead of the plastic?
I'm going to build a GH using Solexx panels and I'll be using strips of wood and caulk to seal the panels in place.

MollyD
maineman
17 years ago
Bruce,

How about just going to 6mm panels? That isn't very thick, about Â¼" as I recall. The 4mm is pretty thin. Incidentally, did you find those top two channel profiles in 6mm or up and, if so, where?

You might check you local hardware store or building center for alternative shapes that could be combined to get the same effect. For example, channel with a profile like Â¯:_:Â¯ and a flat channel ___ or a pair of the "winged U's" could be combined to get an H channel effect that would have space for screw holes for attachment to a framework member.

MM
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Corrugated polycarbonate (wavy plastic?) is most commonly found in either 3x8 or 3x12 sheets. However, it makes for a poor covering in most gh designs. There is moulding designed to fit the corrugation, but this still fits poorly, with cracks galore. Trying to seal with caulking is a temporary solution at best, once there are a few expansion/contraction cycles and drying out from heat. In your next post you asked about "corrugated plastic" that might be available in rolls. This has me wondering if you may be talking about twinwall, which comes in either polyethylene or polycarbonate. Polyethylene twinwall is available in rolls, but polycarbonate is not. Solexx is one manufacturer of polyethylene twinwall, however, it is not cheap. If you are looking for an inexpensive solution that is durable, you will have to consider reinforced UV fabric. -Bruce
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Some good terms to use when searching (here in the archives, or on Google for suppliers) are polycarbonate, or twinwall polycarbonate. It comes in various weights. Mine came in pre-cut panels with my kit GH, and is only 4mm thick (the thinnest you'll find sold.) Heavier weights will retain more heat, but cost more. Here is a pic of the end of a 4mm thick polycarbonate panel, to help you judge thickness. I have found it to be pretty tough and very lightweight. It's somewhat flexible (not brittle.) I haven't built from scratch like you are, and my polycarbonate panels attach with spring clips (Harbor Freight greenhouse.) However, we also put self-drilling screws (with rubber-faced washers) through it, into the aluminum frame, with no problem.
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Hi steve333, good question. I always said I'd never buy HF panels again, but when I was faced with the purchase, I caved! It was a combination of price, and convenience. At the time, I priced 4mm twinwall polycarbonate at a number of greenhouse supply companies. The best price at the time was through FarmTek (cheaper than Charlie's or any other source I tried.) Their product had a ten year warranty, and was truly UV-protected. However, the cost to purchase enough material (just for the roof) with crating and shipping fees to my address, was $410. The cost to order more Harbor Freight roof panels, with shipping, was $135. That difference was enough to give me pause. Hmmmm. Note, it's possible folks living closer to urban areas with more businesses would have found better deals on shipping, but down here in southern NM, the only things close to us are lizards and cactus. Also, the HF panels arrive sized to fit; if you buy better polycarbonate from a greenhouse supply company, you'll have to cut the material to size yourself for each part. Not difficult, but it does take time, so I factored the convenience in as well. Also, occasionally I consider upgrading to a larger home-built greenhouse, so kicking the can down the road doesn't bother me as much as it would if I was positive I wanted to keep this small greenhouse another ten years. I think an argument could be made for both sides. If rtyu's and my experimentation with the Top Secrets clear epoxy product pans out, though, it will definitely tip the scales towards using the (clearly lower quality) HF panels and protecting them with the coating, especially if folks can apply it to their new greenhouses, and avoid the need for panel replacements. Fingers crossed. I think much of the appeal of the Harbor Freight greenhouses is cost/value. They appeal to folks who want to try greenhousing but don't want to make a major investment. I haven't tallied it up exactly for a while, but scrounging supplies, materials, and labor, I figure we have around $2500 in ours total, including kit, benches, sink, plumbing, electric, heaters, thermostats, shadecloth, pavers, and the new roof panels. Over six years that's roughly $400 a year (not including heating costs, which aren't too bad in my climate.) So I think many folks who are attracted to the HF structures also enjoy keeping costs low. It's just one option in the world of greenhouses, and not the right fit for everyone. Happy New Year!
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Stressbaby, I was thinking along the same lines. I've even had a contractor help me out a little bit. I'm trying to find out if its okay to use a silicon caulk instead of the rubber strips that came with the kit that go around the polycarb panels. (The rubber stuff makes the panels buckle) I think caulk would work. I'll probably get some Vulkem or something similar to it to put on all the 'cracks' and seams. The only problem is if I want more than a few hundred watts of electricity in the GH, I have to spend over a thousand dollars getting a line run from the house. At least that's what the electrician said because there is only a small amount of volts going into my garage ( which is right next to the greenhouse.) I wish I could just use a big heavy duty extension cord from the house for this winter and see if the thing atleast holds its heat. Part of me feels like two space heaters would totally heat this thing! But another part of me thinks I would need a huge heater and even then it might not work. I mean if I get the darn thing sealed and covered with a pool cover I don't see why it won't work which like an appropriate heater. Plus it isn't a huge GH, it's 8.5ft by 12.5 ft.
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maineman
17 years ago
Bruce,

Another approach would be to mount the H channel in your third picture directly over the studs and to alternate your screw holes on either side of the central spine. The spine isn't very thick, so there would be room for the screws to engage the stud. Some people have been known to deliberately drill through the polycarbonate in order to help keep them from popping out of the channels in a high wind.

MM
bcfromfl
Original Author
17 years ago
Thanks everyone for your thoughts and suggestions!

I'm on a very tight budget, but am trying to do this "right", within my means. I grow orchids, and have plans to design the greenhouse with an attached "chiller", which means the greenhouse has to be sealed airtight in order to achieve the vacuum I need to help draw the incoming air through the chiller and ducting. I found a source for 4mm polypropylene that's $15 a panel -- if I go to 6mm I have to get polycarb, which bare minimum is $45 a panel. That would be enough to make the project unaffordable.

I don't know how wide the H-channel in the third photo is -- if I drilled through it might there be a risk of splitting it, or the panel underneath since the screws would be so close to the edge? I realize the difference between 4mm and 6mm is pretty miniscule -- how close are those tolerances with the H-channels? It would be a pain, but I could take strips of reinforced PolyMax greenhouse fabric I have and thicken the edges of the 4mm panels with the fabric and a bit of glue, then use the 6mm mouldings. A viable alternative, or a royal pain with questionable results?

I'm afraid that wood moulding wouldn't withstand the rigors of weathering without expansion/contraction, plus, I don't think there's a caulk that will stick to both wood and plastic.

The link below is one of the places where I found the various profiles I posted above. I'll have to wander the aisles in a hardware store and see if there's anything else that might work, although I think most construction mouldings are designed for products that are much thicker. Perhaps shower panel moulding?

Thanks again!

Bruce C.

Here is a link that might be useful: mouldings
bcfromfl
Original Author
17 years ago
I just checked my notes -- there is also 6mm polyethylene, but it's $24 a panel. I need 21 panels total, so that'd mean an extra $190 plus additional shipping/crating.

Sources for 4mm twinwall mouldings?

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Sources for 4mm twinwall mouldings?

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