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When building or buying a greenhouse, it seems to me that the material to use is one of the first concerns. The two options I'm trying to compare right now are glass and plastic films. The characteristics that seemed important to me were transmittance, insulation, maintenance and cost (both purchase- and maintenance-). What I could find on the net was the following.

  • Transmittance

    As Wikipedia shows, plants need mostly light from the visible spectrum:

    Active radiation,

    at which range both glass and most plastics transmit most light. A quick scholar.google search gave me The Radiative Effectiveness of Plastic Films for Greenhouses, with the following info.

    enter image description here

    The mil used here was strange to me, which I converted using the first converter Google gave me. It seems that there is hardly any difference between glass and plastic here, although there are special glasses designed to transmit >99% of the sunlight, as described in Transmittance of optical glass, which are out of reach for most people due to their cost.

  • Insulation

    As can again be read on Wikipedia, it is mostly (the lack of) convection that accounts for the temperature, and both glass and plastic stop this. Of course, glass insulates better than plastic, and double glass would do so even better (again due to the lack of convection in between these layers of glass), but I couldn't find any literature on this.

  • Maintenance

    Plastic changes colour over time and needs to be changed every few years (see The Radiative Effectiveness of Plastic Films for Greenhouses again for precise numbers), while glass lasts ages (look at the royal greenhouse of Laeken). I would thus say glass needs less maintenance.

  • Cost

    Plastic is cheap to buy, but needs to be changed every few years, as noted at the maintenance point. Glass is more expensive at first, but needs less maintenance. I could not find any thorough comparisons of the costs on the web.

  • Other considerations

    Glass just looks better. It does. It gives a different atmosphere, and you can actually see the rest of the garden when you're for example using it as a hangout spot in the winter.

From this intensive search of the web I can only conclude that glass is a little bit better at some points, but I have no idea if it really differs that much. The only point where I think the glass really wins is the maintenance, but that isn't really such a breaking point (I care more about which of the two is best for the plants). Is glass worth the investment? Did I overlook something? Does any of you have experience with both, or argumented reasons for why one of the two is better?

  • what about polycarbonate sheets ? Looks like the best overall option, all things considered. From a bit of googling around, it seems like glass has no major advantages, aside from aesthetics. – Mihai Rotaru Feb 17 '18 at 23:55
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The answer is the one I hate the most: it depends...

Factors influencing your decision include:

  • aesthetics: glass looks better than plastic if you are putting a greenhouse close to your house
  • local climate: if it gets cold where you are then the material best suited for the snow load and that provides the most heat retention will be a better choice even if it costs more
  • what you are trying to grow: if you just want to get a head start on spring and sow vegetable seedlings then plastic might be a better choice due to lower cost. Growing orchids in Hawaii is a different set of factors
    • length of time the crop or plants will remain in the greenhouse. If you intend the greenhouse to be in use year round and have perennials or trees then glass is way to go as it has a longer lifespan and if you need to replace a pane or two the environment of the greenhouse is not compromised. Imagine that it's the middle of winter and a fierce storm does some damage to the greenhouse. Would you rather cover up a broken pane of glass or try patching a tear in the plastic?
    • your choice of covering influences what you can grow. Plastic is normally used to cover a series of metal hoops giving a quonset hut shape. Due to the shape of the greenhouse this lends itself well to growing short plants on the edges with a double row of hanging plants in the middle. This does not work well with trying to grow a tree or shrub that could be 2 or 3 meters tall. There is no place for it except in the middle which is normally an access area.

Where I live has a similar climate to yours. Commercially, plastic is used for vegetables and flowers. Every year a larger percentage of crops are grown using plastic. Glass remains the choice for consumers and show greenhouses.

  • Considering the fact that I plan on having perennials (you might have noticed my question on fruit trees), and that we have about a month of >20cm (~0.7') snow a year, your answer makes me choose for glass. – Betohaku May 5 '13 at 17:44
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Gabled glass if you're in a snowy part of the world. Glass is simply better: higher in capital investment offset by lower maintenance costs and a longer lifespan. If it's part of your business' overall aesthetic invest in it. Balance that reality against brute force theft if you lack 24/7 defense against a crowbar.

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Another factor: Glass houses leak. A plastic house has fewer material transitions, and is much easier to keep close to airtight. This is a major concern in heating the greenhouse.

If you want a greenhouse as a hangout, focus inward, instead of outward. Planters, water fountain etc. Make the walls invisible.

A compromise is to make two greenhouses end to end. One is glass, is sparsely populated most of the year, and is suitable for hangouts. The continuation is plastic.

Transmission isn't as big a deal as you think. Few plants are limited by the intensity of light. Indeed, if you get even half the light indoors you will be fine for most plants. Hours of light are more important, and the amount of light on heavy overcast days can be limiting. (E.g. Heavy overcast cuts light by 60-70% Multiply that by 70% transmission house cover, and you are down to 21-28%. This can be critical if you are in a very cloudy climate such as in the Pacific Northwest.

The biggest problem in most greenhouse operations is overheating. In full sun, you can need an air change per minute to keep the plants from shutting down. (Sunlight on a clear day is 1000 W/m2. A greenhouse with 4 meter high ceilings has roughly 4 cubic meters of air per square meter of floor. Specific heat of dry air is about the same as water -- per weight. So 1.25 kJ/m3 4 meter high column is 5 kg of air. 1000 W is 1000 J/s or 1 kJ/s So that column of air warms up by 1 C every 5 seconds. 12 degrees per minute.

Humid air gets messier, as it depends on starting and ending temps. And a LOT of the heat supplied will be used to transpire water. If you have misters that spray water into the air, you can keep the plants from doing it, and the plants will grow faster.

(More complicated equipment to monitor and check....)

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From a website that sells the three kind of greenhouse glazings, I found a good list of pros and cons.


1. Glass (single glazed)

Advantages

  • Longevity
  • Traditional look
  • Light clarity

Disadvantages

  • Fragile and less forgiving to knocks
  • Poor thermal efficiency (3mm glass R=0.95 and 4mm glass R=1.0)
  • Potential to burn plants due to the level of clarity

2. Polycarbonate (hard plastic)

Advantages

  • Good thermal efficiency (6mm Twinwall R=1.54). Keeps the warmth in longer into the night and offers better frost protection
  • Very tough and durable
  • Good longevity (provided it is a premium grade polycarbonate)
  • Produces a slightly diffused light which helps prevent burning/scorching the plants

Disadvantages

  • Prone to scratching
  • The flutes in the twin-wall can attract moisture, mould and bugs - if not sealed sufficiently in the frame.

3. Plastic film

Advantages

  • Low cost per square meter
  • Takes the knocks
  • Diffused light - prevents burning and aids photosynthesis

Disadvantages

  • Relatively short lifespan and requires replacement
  • Prone to rips and tears
  • Poor thermal efficiency (R=0.83), unless double skinned

Finding a solution to glass glazing

The aforementioned list does not take into account the possibility to use double glazed glass but according to wikipedia, doing so would double insulation efficiency (R≥1.9 based on the number given above for 3mm glass of R=0.95), or even triple it using Argon gas filled double glazed glass (R≥2.85).

To prevent burning/scorching, it is possible to make use of several techniques to shade your plants from the sun. Using internal/external blinds, whitewash (shading paints diluted in water and painted onto the outside of the glass), or shade netting (using shade cloth mesh).

Reinforcing glass using thick and/or tempered glass should make the panels sturdy enough for their "fragility" not to be an issue anymore.

EDIT: this article might give you some different numbers, very interesting read pro-polycarbonate.

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