Insulation question

Do-it-yourself projects such as greenhouse or shadehouse builds and related topics.
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Carbo
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Insulation question

Post by Carbo »

If one wants to use 2 layers of foil in his GH for better insulation during cold months, does the distance/volume of air between 2 foils affect how well the heat is kept in? This is what I mean:
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jerrytheplater
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Re: Insulation question

Post by jerrytheplater »

The more airspace the better. You say "foil". I'm assuming you don't mean something impervious to light.

In some commercial greenhouses I've been in, hoop houses at least for the roof, they have an airspace of maybe 6 inches with fans blowing air in to keep them inflated. They also have an inner shade that slides over the bays inside to seal off the arch. Hard to write it, but if you look at your drawing, the shade would be the bottom straight line. That is increasing the airspace. The shades automatically deploy right around an hour before dusk. I'll have to ask my friend if he knows how much heat it saved, although his greenhouse used tempered glass rather than plastic film. He managed a 15 acre greenhouse with capacity for about 250,000 flats, and with roll out trays that put the plants in full sun outdoors in the wind on suitable days. They go out the sides of the greenhouse. The roof of that greenhouse opened up vertically to allow air and sun to come in unfiltered. Really grew hard plants. He owns his own greenhouses now that use plastic film.
Jerry Smith
Bloomingdale, NJ
45 inches (114 cm) rain equivalent per year, approx. evenly spread per month
2012 USDA Hardiness Zone 6b: -5F to OF (-20C to -18C) min.
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Carbo
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Re: Insulation question

Post by Carbo »

jerrytheplater wrote: Thu May 06, 2021 3:41 am The more airspace the better. You say "foil". I'm assuming you don't mean something impervious to light.

In some commercial greenhouses I've been in, hoop houses at least for the roof, they have an airspace of maybe 6 inches with fans blowing air in to keep them inflated. They also have an inner shade that slides over the bays inside to seal off the arch. Hard to write it, but if you look at your drawing, the shade would be the bottom straight line. That is increasing the airspace. The shades automatically deploy right around an hour before dusk. I'll have to ask my friend if he knows how much heat it saved, although his greenhouse used tempered glass rather than plastic film. He managed a 15 acre greenhouse with capacity for about 250,000 flats, and with roll out trays that put the plants in full sun outdoors in the wind on suitable days. They go out the sides of the greenhouse. The roof of that greenhouse opened up vertically to allow air and sun to come in unfiltered. Really grew hard plants. He owns his own greenhouses now that use plastic film.
Yes I mean the clear stuff, polyethylene. So there is a trade-off, either you get more space or more insulation. My friend has a GH with that setup it's like another GH inside a bigger one, the internal one doesn't have to have tough construction. Works great for him, gets toasty in there when he fires up his furnace in winter. Lexan polycarbonate sheets are the best but they're 5x more expensive (in my area) even compared to this double-foil.
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greenknight
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Re: Insulation question

Post by greenknight »

More is better only up to a point - dead air spaces of more than 50 mm allow air to circulate more freely, which increases heat transfer by convection, so 50 mm should be considered the maximum. An air space inflated by fans, such as Jerry describes, is a very different situation.

In the US, the term "foil" is used only for metal foil, unlike many other countries where it can also refer to plastic - thus the confusion.
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Carbo
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Re: Insulation question

Post by Carbo »

greenknight wrote: Thu May 06, 2021 8:33 am More is better only up to a point - dead air spaces of more than 50 mm allow air to circulate more freely, which increases heat transfer by convection, so 50 mm should be considered the maximum. An air space inflated by fans, such as Jerry describes, is a very different situation.

In the US, the term "foil" is used only for metal foil, unlike many other countries where it can also refer to plastic - thus the confusion.

Aaah I see, well 50mm isn't much of a loss in space. I've seen inflation systems but I really don't understand how they work once your GH is full of holes from birds, hail etc.
I gues the term "polyethylene film" is more clear - pun totally intended :D
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jerrytheplater
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Re: Insulation question

Post by jerrytheplater »

Carbo, I never asked about puncture damage. I know they turn up the heat when it snows so it melts the snow off. I know they have gutters between bays to allow walking on the roof. Maybe they patch. I don't know.
Jerry Smith
Bloomingdale, NJ
45 inches (114 cm) rain equivalent per year, approx. evenly spread per month
2012 USDA Hardiness Zone 6b: -5F to OF (-20C to -18C) min.
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Carbo
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Re: Insulation question

Post by Carbo »

jerrytheplater wrote: Thu May 06, 2021 10:50 pm Carbo, I never asked about puncture damage. I know they turn up the heat when it snows so it melts the snow off. I know they have gutters between bays to allow walking on the roof. Maybe they patch. I don't know.
I worked at a local plant nursery and they have that exact type with inflation and gutters. They turned off the air pump after just 2-3 seasons because the birds made too many holes. I don't think it's a very good system imho, double-film seems to do the same job and it's cheaper
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jerrytheplater
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Re: Insulation question

Post by jerrytheplater »

Carbo wrote: Fri May 07, 2021 8:16 am
jerrytheplater wrote: Thu May 06, 2021 10:50 pm Carbo, I never asked about puncture damage. I know they turn up the heat when it snows so it melts the snow off. I know they have gutters between bays to allow walking on the roof. Maybe they patch. I don't know.
I worked at a local plant nursery and they have that exact type with inflation and gutters. They turned off the air pump after just 2-3 seasons because the birds made too many holes. I don't think it's a very good system imho, double-film seems to do the same job and it's cheaper
Carbo, I spoke with my friend last Saturday. He's worked in a greenhouse for at least a decade here in NJ which uses plastic sheeting inflated by fans. Three of his current greenhouses that he owns out of 4 use the plastic sheets inflated by fans. I didn't think to bring my camera, but it is a fan on the vertical wall with ducts leading to each bay. I didn't look at the other end. His roof opens up hinging at the ridge and lifting up about half way across the span. The side walls open out as well. Plenty of ventilation in the warm days. He will change the plastic every 4 years as that is what the growers in this area do if they are not using glass or double wall polycarbonate.

He has never seen birds damage any greenhouse he's been in and that is at least 25 years or 30 in the industry. We seldom get hail out here, and when we do it is minor. I have heard stories of extremely strong winds ripping open a bay on a greenhouse causing huge damage to the whole greenhouse as it lost heat. Since most of the growers around here are related to one another in one way or another, they all join in in an emergency repair job.

Do you know what kind of birds are doing the damage? I was looking at the greenhouse Saturday and really didn't see many places a bird could perch in order to peck at the plastic sheeting.
Jerry Smith
Bloomingdale, NJ
45 inches (114 cm) rain equivalent per year, approx. evenly spread per month
2012 USDA Hardiness Zone 6b: -5F to OF (-20C to -18C) min.
DaveW
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Re: Insulation question

Post by DaveW »

As Greenknight says there is a certain limit in double glazed glass windows where if the gap gets too great you get convection currents in the gap that convect the cold from the outside pane to the inner and vice versa. If the double glazing had a vacuum between them there would be no convection, but most have air between them usually in the UK.

https://ecoeco.com.au/double-glazing-ai ... -vs-truth/

If it is fan blown it is not really double glazing since you are introducing air constantly into the gap, which if warm will warm it up just as with a fan blowing warm air is used in a single skin polythene greenhouse.

How long does the polythene or polyethylene skin on his greenhouse last? We have used polythene as an insulation inside glass greenhouses here for years and ordinary polythene sheeting only lasts a few years since its not Ultra Violet (UV) stabilised. I lined my present greenhouse with UV bubble wrap stabilised polythene and that has lasted about 10 years, but is now disintegrating and falls to bits if you touch it.

Most of us in the UK stopped using flat polythene years ago and use bubble wrap polythene since the closed bubbles form better insulation. Bubble wrap is usually used to pack things in to prevent breakages but now also sold for greenhouse insulation. The best is UV stabilised if you can get it, but it is not as common as only needed for greenhouses and not packing uses.

https://www.bing.com/images/search?view ... ajaxserp=0

There is an even better form called double wall bubble wrap. The single is like a flat sheet of polythene with bubbles on one side, whereas the double walled then has another flat sheet on top of the bubbles sandwiching them in between (like double glazing! LOL)

https://www.bing.com/images/search?view ... ajaxserp=0

Re clearing out the greenhouse to fix it. Some take it down in summer but you can see why I left it up summer and winter to save moving all the plants out. See:-

https://www.youtube.com/watch?v=LHwVQbnOYJE

With a wooden greenhouse you can just use Drawing Pins or Thumbtacks to fix it. I also find sometimes stringing cord along helps to hold it up at the eaves etc.
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