Twin wall polycarbonate - Question

Julia NY(6)November 14, 2011

As I'm doing my research on various greenhouses and what they include and don't include, I noticed the differences in the number associated with the manufacturers Twin Wall Polycarbonate. Some say 4mm, 6mm, 8mm and 10mm so far.

My understanding is that the higher the number, the more insulated the greenhouse is for growing plants in the winter months. If folks don't plan to grow anything in the coldest months where temps dips down to 0 degree F, would this really make a difference in their choice of which greenhouse is more suitable if all other construction, set-up,wind, snow load and warranty factors were equal?

Julia

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karin_mt(4 MT)

Hi Julia,

Are you going to use the greenhouse during cool weather where you'd prefer to keep things warmer at night? If so, the thicker panels will help. My take is anytime some nighttime warmth is desired, then more insulation is better. What times of year are you going to be growing things and what are you growing? Do you plan to heat your greenhouse?

Karin

    Bookmark   November 15, 2011 at 1:32AM
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Julia NY(6)

Hi Karin:
Right now, I have no plans to heat the greenhouse but may change this once I get myself more knowledgeable about exactly how much fuel( propane/electric) is needed to keep things sustained through most of the winter. My idea was to keep 2 areas with conduit pipe installed for future changes.

I grow tomatoes,peppers, eggplant, squash, cantaloupe and lots of herbs. I want to add more but no room inside to do all of it and I have no way of knowing if our spring will be so rainy and cold we are delayed planting out. I also am an avid perennial/annual seed starter as well.

I start putting out seedlings mid-April or mid May depending on our anticipated last frost date and the weather.Right now I start all my seeds indoors and then transfer them out to the garden. I'd like to be able to start seeds in the greenhouse but not sure about just how much heat I'll need to achieve this. Newbie to all of this so researching all the issues.

I want to use one side of the greenhouse as a planting bed or make a soil bench to start broccoli, spinach, etc mid season so I have these still growing into late November/early December.

My thoughts were to place the greenhouse in a north - south direction but will be exposed also to east-west as well. Very open area and we do have high winds from time to time. We don't get as much snow as folks in the Buffalo NY or Binghamton/Syracuse area. We are in between those cities here in Upstate NY, but do get a least one to two snow storms were we may end up with 10 inches of snow but because of the winds, we get alot of drifting. Our temps do not get very often get below zero however windchill may feel like below zero.

Julia

    Bookmark   November 15, 2011 at 10:52AM
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Slimy_Okra(2b)

Hi Julia,
I agree with Karin. If cost is not an issue, get the thickest grade. Night-time warmth is always beneficial in our zones, except for cool-season crops growing in late spring-early fall.
I would try to orient the greenhouse E-W such that a long side (if your GH is rectangular) is facing south.

    Bookmark   November 15, 2011 at 1:28PM
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karin_mt(4 MT)

Hi Julia,

You sound like the ideal greenhouse owner to me! I do all of those same things and I get better results with the GH than I ever did indoors. In general you can start things quite early in the GH in springtime. It gets very warm during the daytime, even by March it is toasty in there and with the bright light you get very strong seedlings. Of course it is cooler at night and there are ways to deal with that, such as covering your seedling trays with frost blankets, using a heat mat, bringing seedling trays inside if it's exceptionally cold, or using a heater.

Using today as an example, it's a raw 26 degrees outside and mostly cloudy. Not a nice day. But in the greenhouse it's 58-65, depending on the location, and the lettuces all look pretty happy.

Our GH is oriented north-south, which I really like. That way neither side is shady and it provides maximum growing space. Planting things in the ground is a wonderful means to have a very long growing season. I think you'd be able to get cool weather crops to go all winter long, although they won't put on much new growth in Dec-Jan.

Back to your original question, I'd go with the 8-10 mm panels if you can. Having more insulation is never a bad thing if it is not cost prohibitive.

While you are in the research (aka dreaming) stage of the process, you may want to read The Greenhouse Grower's Companion by Shane Smith, and/or Elliot Coleman's books.

Good luck with your research and planning!

Karin

    Bookmark   November 15, 2011 at 1:38PM
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kudzu9(Zone 8a - PNW)

Let me point out -- without going into the thermodynamics -- that, while thicker twinwall does produce more energy savings, it may not be as much as you are assuming. For example, 8mm twinwall is only about 5% more efficient than 6mm, and 10mm is only about 10% more efficient than 6mm. So don't make the false assumption, for example, that a twinwall panel twice as thick as another one is twice as efficient; it's nowhere near that. In addition, unless you have your greenhouse tightly sealed to prevent air leakage, the heat losses from air infiltration/exfiltration can easily swamp out anything you gain from having the thickest twinwall. By all means get the thickest twinwall you can afford, but don't pay a huge premium for it in the expectation of huge energy savings.

    Bookmark   November 15, 2011 at 9:05PM
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Julia NY(6)

Slimy Okra: I guess I got lost on that point about orientation of the GH. If I placed it so the door is facing north and the rear facing south, wouldn't it still get enough light from the east - south - west sun? If I have a long side facing south then the other long side will be facing north which doesn't get as much light. Or maybe I'm missing something on the concept.

Advice?

Julia

    Bookmark   November 15, 2011 at 9:39PM
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Slimy_Okra(2b)

Yeah I guess you're right. I didn't think my answer through properly.

    Bookmark   November 15, 2011 at 10:34PM
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Julia NY(6)

Slimy Okra: Thanks for clarification. I was thinking I needed to rethink my site plans.

karin: Those books are on my Amazon Wishlist already. Checking libraries to see if they have them available before I buy my own.

Julia

    Bookmark   November 16, 2011 at 8:28AM
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cuestaroble

In addition to the relatively small heat retention differences( K factor) noted by kudzu9, another factor is the increased weight among the various sizes. 10 mm is more than twice as heavy as 4 mm.
size, g/m2, K factor :
4 mm 800 3.9
6 mm 1300 3.6
8 mm 1500 3.4
10 mm 1700 3.2

    Bookmark   November 18, 2011 at 7:40PM
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Julia NY(6)

cuestaroble: What is the significance in weight when considering a greenhouse polycarbonate? I'm a newbie so not sure how I need to factor that in to my overall considerations.
Thanks
Julia

    Bookmark   November 18, 2011 at 8:48PM
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cuestaroble

juliany,
Once you start building your greenhouse, lifting and supporting the panels during construction is a consideration. Perhaps more importantly, the support structure of the greenhouse itself has to be twice that for 10mm vs. 4 mm.
With the relatively small heat factor differences mentioned by kudzu, and the added weight with the 10 mm panels, it is just something to consider when designing a greenhouse.

    Bookmark   November 18, 2011 at 9:56PM
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barrie2m_

The support of the weight is negligable compared to wind and snow/ice load. What is probably the biggest issue is the installation work (on you) caused by the added weight and thickness.
But I would still opt for thicker mainly because of useful life expectancy.

    Bookmark   November 21, 2011 at 6:36PM
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cuestaroble

bmoser- Is there a difference in product useful life expectancy between the various sizes of polycarbonate panels- i.e. 4mm, 6mm, 10mm ?

    Bookmark   November 21, 2011 at 7:15PM
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Julia NY(6)

I have been doing extensive research on greenhouses and what companies are giving as warranties. It is enough to make a persons head spin.
So far, I've only found Riga to be giving the longest number of years:15 on the frame and 10 on the covering(polycarbonate). However, I haven't found in writing yet what the 10 year covering warranty actually covers. But I'm sure I'll find it. I seem to recall they did have a wind and snow load posted .

Back to my original question though, it seems the higher the number the heavier and more efficient the polycarbonate is.
I don't want to spend X amount of dollars and within a year or two, have holes, yellowing and cracking occuring because I choose a 4mm or 6mm versus an 8mm or 10mm. I am in a high wind area so stability is a number 1 issue.

So if anyone has a greenhouse with long term warranty and high polycarbonate rating, I'd appreciate reading your feedback other than a Riga GH.

In the same token, I'd appreciate hearing from anyone who has a shorter term warranty, lower polycarbonate rating, that the GH is still in great shape after several years of use.

Julia

    Bookmark   November 22, 2011 at 12:36PM
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cuestaroble

My non-riga, non-hf greenhouse, 6 mm polycarbonate, has a "10 yr manufacturers warranty against breakage, yellowing, and loss of light transmission". It is almost 8 yrs. old and shows no sign of any degradation, in a California location.The name of the manufacturer of the panels is probably available from the greenhouse supplier.

The difference in longevity of panels is not based on the size of the air pockets (4mm,6mm, etc). The quality of the plastic used in the panels and the UV protection is what counts.
So, getting feedback, as you have done, is probably the best method to find a good supplier.

    Bookmark   November 22, 2011 at 7:53PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

Glad someone finally mentioned light transmission. With the thicker twin-wall you are trading off light transmission for a minimal amount of insulation value. As thickness foes up, light transmission goes down.

Cost also rises with the thicker twin wall of course, life expectancy 'supposedly' remains the same. Our old greenhouse had 8mm on the roof and after 15 years it was still just fine (until it burned down). We went with 6mm on the new one specifically to increase light transmission because insulation (if needed) is easy to do and is rarely needed in our zone.

Lastly, the multi-channel 10mm and higher just has more channels for moisture to get trapped in and for algae to grow in which is a real pain.

Dave

    Bookmark   November 23, 2011 at 10:41AM
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Julia NY(6)

cuestaroble and digdirt: What GH's did you buy?

Julia

    Bookmark   November 23, 2011 at 4:03PM
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penfold2(4b, MN)

"Glad someone finally mentioned light transmission. With the thicker twin-wall you are trading off light transmission for a minimal amount of insulation value."

The specifications I've seen suggest the opposite. Here's some numbers for Green-Tek's twin wall polycarbonate. Light Transmission R-Factor 4mm 82% 1.49 6mm 80% 1.62 8mm 80% 1.72 10mm 79% 1.89

With increasing panel thickness there seems to be a minimal loss of light transmission with a more substantial increase in insulation. I use their 8mm triple wall which still has 76% light transmission and an R-factor of 1.99. I don't see how panel thickness would have any impact on lifespan, so I don't think that's a concern. I'd go for thicker panels unless cost becomes an issue.

One more benefit is that thicker panels are stiffer and don't require quite as much framing to support them. And less framing increases your light transmission.

    Bookmark   November 24, 2011 at 2:09AM
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cuestaroble

Whether 6 mm vs. 10 mm is the most cost effective would depend on your outside temps. and the temp. you want to keep your GH. Using an example for my area of 20 deg. outside and 50 deg inside, for a 10 x 12 GH, using electric heat at $0.10/kwh, the difference is $37 per year. ($202 vs. $165).
Try the calculator below with your local figures to get an idea of the potential savings with more expensive polycarbonate panels.

Here is a link that might be useful: greenhouse heat calculator

    Bookmark   November 25, 2011 at 12:56PM
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Julia NY(6)

cuestaroble: Thank you for posting the calculator. I only browsed it briefly but the information should provide me with additional information to think about when choosing a GH and how we will heat or not heat it.

Julia

    Bookmark   November 30, 2011 at 8:48AM
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