The first greenhouse with a glazed roof was built in Rome, in 30 A.D., when Emperor Tiberius got sick and was instructed to eat a cucumber every day. By the late 17th century, fully glazed greenhouses were popular amongst the aristocracy in England. Andrew Faneuil, a wealthy Boston merchant, is said to have built the first glass greenhouse in the U.S.
Double-glazing is a 20th-century invention that will improve the energy efficiency in glass greenhouses. It will ensure you have warm winter temperatures and cool summer temperatures. It also allows light transmission of high visible light and high solar heat gain, which is beneficial for plants.
What is double glazing?
Double glazing, as its name suggests, comprises two panes of glass rather than just one. The space between the twin wall of glass is filled with gas, usually argon, or dehydrated air.
A double-glazed window has two panes of glass in the same frame. Sometimes, when people don’t want to replace a single-pane window, they will install secondary glazing in the form of a second frame installed to replicate the benefits of double-glazing.
Secondary glazing, also known as storm windows, doesn’t have the sealed air gap that defines double glazing. For this reason, they are not nearly as effective in terms of insulation or soundproofing.
An American inventor, Charles D. Haven is recognized by the Guinness Book of Records as being the inventor of double-glazed windows. He filed a patent for the design of what he called “insulating-glass windows” on October 12, 1934.
Haven’s double-glazed windows were made using what he termed a Thermopane system. It worked but wasn’t 100% effective.
The two glass panels were separated by a gap filled with dehumidified air. The edges were sealed with strips of rubber.
The Guinness Book of Records points out the Libby-Owens-Ford (LOF) Glass Company partnered with Haven in the late 1930s, and they introduced double glazing commercially in 1938.
Sealant failures resulted in condensation, and it was in 1946 that it was reintroduced to the commercial market.
Interestingly, nowadays, you don’t have to use glass for double-glazing. Often perspex or plastic is used, especially for glazing greenhouse structures.
Polycarbonate greenhouses are particularly popular.
Effectiveness of double glazing
The effectiveness of double glazing is variable, depending on the space between the double glass panes and what separates this twin wall. Typically the space between double pane glass is between ¼ and ¾ of an inch.
When argon is used to fill the space, the thermal efficiency of a double-glazed window or panel will be as much as 30% more than when air is used to fill the gap.
Types of double glazing for greenhouse
So what types of double glazing can you use for your greenhouse?
What you’re going to need to consider is light transmission, thermal conduction, the strength and weight of the glazing, scratch resistance, and cost. Glass, plastic film, and polycarbonate are the most popular options.
Double Polycarbonate Glazing
When it comes to a polycarbonate greenhouse, there are several different material options. A lot of people opt for single polycarbonate panels, but there are double or bi-wall options as well.
The thickness of the panels varies depending largely on the air space between the polycarbonate material. This will affect everything from weight, strength, and flexibility, to light transmittance, thermal conductance, and cost.
Like glass, polycarbonate glazing allows more light transmission when it is in single-panel form. But generally, they are resistant to UV degradation and they are quite strong and rigid while being relatively light in weight.
Double Acrylic Glazing
Like polycarbonate glazing, acrylic glazing is available in single or bi-wall (double) panels. The air space between the glazing panels also varies.
While acrylic panels, like virtually all plastic panels, are easily scratched, there are treatments that will minimize scratching.
Double Glass Glazing
Going back centuries, glass is the traditional material for greenhouses. But as stated earlier, double-glazing is a 20th-century invention.
The thickness of the glazing will affect a myriad of issues, from the light transmission to strength. It also affects the cost factor.
In general, glass is UV light abrasion, and heat resistant, but it has quite a low impact resistance. In other words, it breaks relatively easily, although double-glazing is stronger.
It appears that while many botanical centers use glass, particularly safety glass, many commercial greenhouses opt for other glazing methods. This is primarily because of weight and cost.
Double Plastic Film Glazing
Double plastic or polyethylene film has become an increasingly popular glazing option. It is lightweight, flexible, and easy to install, and it costs a lot less than other glazing types.
It has its benefits, but it also has problems including condensation and consequent dripping. Plastic film is hydrophobic and so the water collects on the surface and beads until large drops form and drip onto the plants in the greenhouse.
A major concern is that this can result in increased plant diseases.
Another type of plastic film is fiberglass-reinforced polyester. This is also low in cost, lightweight, and it’s quite strong.
It has a high light transmission rate of up to 90% (like glass) but it only has a lifespan of about two to three years. This type of plastic film glazing used to be highly flammable, but the newer panels are treated with a flame retardant.
Why you should consider double glazing
Double glazing is energy efficient and environmentally friendly. It keeps excess heat out in summer and extreme cold out in winter.
This cuts down on the need for any type of heating or cooling system in your greenhouse, maintaining a balanced temperature for your plants all year round. It also minimizes mildew and mold growth.
Because of the structure of double glazing, it is heavier and stronger, making it more secure than single-pane glass. Double-glazed glass is also easy to maintain, but like all types of glass, it needs to be cleaned regularly.
Of course, one of the most important benefits of using a double pane for greenhouse glazing is improved light transmission. Plants can only benefit from this.
What is the best glazing for a greenhouse?
Since it is now considered “okay” to glaze greenhouses with other materials including perspex, polycarbonate, and other related plastic-type products, we need to compare these to the real McCoy… glass. Generally, glass holds more light than plastic and its light transmission qualities are better.
Glass in a single layer allows between 88% and 94% of light to go through it. When you have a double layer of glass, as you do with double glazing, it only allows about 77% to transmit through the glass.
Glass, in general, doesn’t degrade in sunlight as plastic materials do, and it radiates from within the greenhouse back into the structure.
There are various options when it comes to glass. If your budget allows, double glazing is an option, even though it isn’t universally accepted as being the best one.
The British Royal Horticultural Society (RHS) advises against using double glazing or other special types of glass. These, they say tend to alter the “spectral composition” of natural sunlight and reduce light quality which adversely affects plants.
Apart from ordinary single-pane glass or double glazing, another greenhouse glazing option is to use laminated glass. This is when you have double-pane glass, but it is stuck together with a layer of plastic in-between.
But laminated glass is still single-glazing, not double-glazing. Another option is to install tempered glass, which is a type of safety glass.
Whichever glazing options you consider, you need to consider light transmission and other related issues.
For example, if you are going to be growing seedlings or propagating seedlings, you will need high light levels. Also, reduced light levels tend to affect the performance of plant growth both in terms of quality and quantity.
Plastic Glazing Options
Other good glazing options include polycarbonate, which is a twin-wall plastic. It’s lightweight, durable, and doesn’t break easily.
One of the other similar glazing options is twin-walled acrylic plastic. This transmits about 85% of light while polycarbonate emits a bit less, about 83%, but is more brittle than polycarbonate.
In terms of plastic, polycarbonate is the best bet. It holds in heat well and it’s more durable than other types of plastic.
Irrespective of the type of greenhouse you have, bubble wrap is another great insulation tool. It’s a natural insulator that will keep warm air inside when it gets really cold in winter.
The greenhouse has come a long way since it was first imagined by Emperor Tiberius thousands of years ago. In particular, there is a multitude of glazing options we can choose from.
Glass remains the traditional material and double-glazing is a definite option. But all sorts of other factors come into play, including cost.
While you can certainly use double glazing for your greenhouse, you don’t have to. You can buy a polycarbonate version off the shelf if you like!