# 4mm vs 6mm polycarbonate



## s1214215 (Dec 15, 2011)

Hi.. I know this is a silly question, but I dont know the answer and failed with a google.

I am going to buy a greenhouse, and I have the option of 4mm vs 6mm polycarbonate. What benifits would one have over the other? Is one better for insulating? My aim is to keep cooler in the summer (may need aircond) and warm a little in the winter.

Brett


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## likespaphs (Dec 15, 2011)

uv stabilized, right?


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## paphreek (Dec 15, 2011)

The greater the air gap between the walls, the better the insulation value, which is probably more important for heating as cooling can be accomplished through shade cloth and an evaporative cooler. Here's a link showing the insulation value (R factor) for various polycarbonates: https://www.sundancesupply.com/index2.html


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## Ray (Dec 15, 2011)

Ross is right, up to a limit.

Heat loss through a wall is due to conduction of the heat from one point to the next, then to the next, etc., etc. Solid materials conduct better than do gases, so by having a larger gap between the solids, the rate of heat flow is reduced. However, transfer of heat from one phase to another is even slower.

Insulation works by having noncommunicating zones. The less the communication - transfer of heat to another zone - the slower. Polystyrene foam is so good because it has millions of tiny bubble separated from each other by the polymer. Hence the air-polymer-air-polymer transfer is slower than that same thickness of a single polymer bubble.

Likewise, if you have the option, a multiwall sheet is better than twinwall, although those are typically not available until at least 8 mm thickness


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## jtrmd (Dec 15, 2011)

I was going to pretty much say the same thing as ross.


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## biothanasis (Dec 15, 2011)

Having searched this myself (but it is very expensive here... ) the 6mm is much better....for thermal isolation. Also remember to put the uv protection (I think that the 4mm does not have it) from the outside. Good luck...!


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## gonewild (Dec 15, 2011)

Is 2mm more air space actually a lot more insulation value?


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## ehanes7612 (Dec 15, 2011)

gonewild said:


> Is 2mm more air space actually a lot more insulation value?



when you increase the airspace by 50 % (4 to 6) or by 100 percent (from 4 to 8) i would think its significant...if you increase from 10 to 12 (20%)its less significant and so forth and so on


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## gonewild (Dec 15, 2011)

ehanes7612 said:


> when you increase the airspace by 50 % (4 to 6) or by 100 percent (from 4 to 8) i would think its significant...if you increase from 10 to 12 (20%)its less significant and so forth and so on



I realize there is a 50% increase in space but I wonder if that actually provides enough more insulation to save much energy?

Increasing from 2mm to 4mm is 50% and increasing from 10mm to 12mm is 20%.
But wouldn't the increase of insulation actually be the same since both add the same extra 2mm of air space? How much insulation value does 2mm of dead air space add?


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## ehanes7612 (Dec 15, 2011)

you get diminshing returns for increasing airspace..but whenn you consider how large your greenhouse is, the savings could be much greater than not adding airspace..but at some point its just best to add a different medium ..as Ray talsk about....i have several layers of 6ml plastic 2 inches apart(plastic/air/plastic air plastic added a layer i noticed a drop in needed heating...r values do increase as you increase airspace.


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## gonewild (Dec 15, 2011)

4mm Twin Wall Polycarbonate R Value is 1.43
6mm Twin Wall Polycarbonate R Value is 1.54

So 6mm has a 7.5% higher R Value than 4mm.
But does that mean it would reduce the heating expense by 7.5%? I don't think so?

How much more cost is added to build with 6mm instead of 4mm?
Somewhere I read that completely dead air space has an R Value of "R1" per inch of space. So adding a small amount (a few mm) of extra air space will not make much difference in the heating and cooling costs.

Probably more significant is the durability of the choices. Maybe the 6mm material is actually thicker acrylic and will last more years and resist damage better?

Just saying!


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## ehanes7612 (Dec 15, 2011)

this is better at answering question

http://www.littlegreenhouse.com/heat-calc.shtml


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## ehanes7612 (Dec 15, 2011)

but they use invers R value


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## emydura (Dec 16, 2011)

So for those using twin-walled polycarbonate, what thickness are you using? In Australia you seem to be able to mainly get 4 or 6 mm. But looking at the site Ross provided, you can go much thicker - up to 25 mm. 

I too am looking to upgrade to a polycarbonate greenhouse. I currently have glass but the heating costs are killing me in winter.


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## labskaus (Dec 16, 2011)

You need to consider the greenhouse construction as well. A frame with non-insulated/separated metal parts wastes much more heat than you can win with thicker polycarbonate panels.

Where do you live/what's the absolute minimums you expect and for how long and how often? Much wind where you live? Do you have trees next to your GH?

Apart from the cost factor (heat saving vs. actual cost of the panel), stability was a factor for me too. I have 16mm twin walls, and I can lean or stand on my panels for cleaning and maintenance. Also, these panels stand hail and the occasional twig falling off a tree.

I would have gone for 20mm or thicker if the manufacturer of my GH had offered it. 16mm really is the low limit in my climate if I had known 10 years ago how fast energy prices rise, I would have made a different decision. I seriously consider to wrap my GH in bubble foil from next winter on. Which might be an option for you as well.


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## ehanes7612 (Dec 16, 2011)

I think the most economical option (for insulation) is to go with a 4ml polycarbonate with a layer of 6ml plastic (spaced an inch or two from wall) inside the greenhouse..what i plan on doing eventually


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## Ray (Dec 16, 2011)

gonewild said:


> 4mm Twin Wall Polycarbonate R Value is 1.43
> 6mm Twin Wall Polycarbonate R Value is 1.54
> 
> So 6mm has a 7.5% higher R Value than 4mm.
> But does that mean it would reduce the heating expense by 7.5%? I don't think so?



Those numbers indicate that the thicker material will lose heat 7.7% slower, but as loss though the glazing is not the only way heat is lost from a greenhouse, of course it is not a direct translation to savings.

8mm twinwall has an R-value of about 1.7, making it 19% better than the 4mm stuff, but if you look at it from an energy loss perspective, it helps.

Let's say we have 1000 square feet of glazing, our minimum interior temp should be no lower than 50°F, and our expected outside low is 0°. With the 4mm covering (assuming it's 100% of the heat loss), you'd need 35,000BTU heater output to maintain the temperature. for the 6- and 8mm glazing, respectively, that would drop to 32500 and 29400 BTUs.

Thicker materials tend NOT to have thicker plastic, but more air space.


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## gonewild (Dec 16, 2011)

Ray said:


> Those numbers indicate that the thicker material will lose heat 7.7% slower, but as loss though the glazing is not the only way heat is lost from a greenhouse, of course it is not a direct translation to savings.
> 
> 8mm twinwall has an R-value of about 1.7, making it 19% better than the 4mm stuff, but if you look at it from an energy loss perspective, it helps.
> 
> ...



Then 6mm vs 4mm saves 2500 BTUs... How much in dollars is 2500 BTUs worth?


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## cnycharles (Dec 16, 2011)

If you're going to have a small greenhouse, you could have twin flexible poly and an inflator to keep it up. this could also be used over polycarbonate, and again if a small house (or you have help), you could take it off in the spring if you think you won't get enough light. many of our greenhouse roofs at work are inflated double-poly, even some walls


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## gonewild (Dec 16, 2011)

ehanes7612 said:


> this is better at answering question
> 
> http://www.littlegreenhouse.com/heat-calc.shtml



Based on this calculator with a hypothetical temperature range/span using 6mm acrylic instead of 4mm will save you 6.7% on your heating cost.


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## gonewild (Dec 16, 2011)

cnycharles said:


> If you're going to have a small greenhouse, you could have twin flexible poly and an inflator to keep it up. this could also be used over polycarbonate, and again if a small house (or you have help), you could take it off in the spring if you think you won't get enough light. many of our greenhouse roofs at work are inflated double-poly, even some walls



You could use the inflated double poly cover on top of your glass and have great insulation. Then if you wanted to you could remove it during the warm season.


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## Lycaste53 (Dec 16, 2011)

There is a relation between the volume, which has to be heated, and the surface (walls and roof) which loose energy. The smaller the volume, the biger the surface and the moore energy is lost by transmission.
It´s not possible to calculate it by the material only
Best regards, Gina


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## s1214215 (Dec 17, 2011)

Thanks everyone

I havent had time to reply to this thread this week. I appreciate all the information. I think I will go with a 6mm twin wall in the end.

Brett


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