Glass orangeries

Conservatories were invented in the 17th century to conserve newly imported citrus plants in the winter in northern climates. They were masonry buildings with large windows, also known as orangeries.

As glass, steel and timber technology developed in the 18th and 19th centuries, the walls and roofs became framed structures infilled with glass panes which got bigger and bigger as the technology improved. Because large panels of glass were expensive to produce, only public buildings and grand houses of the very rich contained the exotica of far-flung jungles. In the late 19th and early 20th centuries the price of glass fell and this coincided with the expansion of suburbs, which saw a growing demand for smaller conservatories for those houses.

Customising

You can't buy frameless silicone sealed systems off the shelf, so if you want one of these you'll need to employ an architect or structural engineer. This option needn't be more expensive - if the glass is adhered to a steel or aluminium structure, the superstructure costs are about the same as off-the-shelf ones.

More complex systems in which the glass is adhered to glass beams and columns, or the double glazing is bolted to a steel structure, are considerably more expensive, but they are far more elegant.

Is the heat on?

Even if it's heated and double glazed, a conservatory can feel cold at night or in overcast conditions. This is partly because of the downdraughts resulting from air cooled by the surface of the glass overhead. The most comfortable way of preventing downdraughts in a conservatory is to use underfloor heating. Although it's expensive to install, the running costs are lower than for a radiator system. A cheaper compromise is to use the combination of a radiator and low-level perimeter heating along the glazed walls - you can get radiators that sit in trenches or tiny radiators at low levels.

In the frame

The framework of a conservatory can be made from timber, steel, aluminium, uPVC or even structural glass.

It can be single, double or triple glazed, or made from polycarbonate sheet (although this scratches quite quickly, tends to creak when the sun comes out and makes a racket in the rain - which you won't really hear with double glazing).

A single-glazed conservatory won't keep out much cold on overcast days or at night, but it will heat up quickly in the sun even in mid-winter.

If you have double or triple glazing and underfloor heating you will be able to use the conservatory all year round.

Growing panes

Because the main purpose of having a conservatory is to maximise the amount of light, you should have as few glazing bars as possible and use the largest panes of glass you can. This used to be achieved with very thin steel or timber mullions but the weight of double glazing needed to make year-round habitable conservatories makes this difficult, as the relatively thick sections of aluminium patent glazing testify.

Seal with silicone

Timber framing in conservatory roofs and the putties used to keep the glass in place wear away quite quickly, and if they're not repaired will cause the timber to rot.

The seal in metal or uPvc framed roofs can be lifted after a while by moss or lichen and will make the roof leak. Often criticised for its environmental impact, uPVC goes brittle with age. Aluminium and uPvc frames need to be quite thick, which cuts down the transparency of the conservatory.

The correct type of high specification silicone seal between each glass pane is not prone to rot, and doesn't provide a place for moss. It also allows some thermal movement of the glass because silicone seal expands and contracts so you'll get fewer leaks.

The two layers of double glazing used with this system should be stuck to each other using a similar silicone seal.

Off the shelf

Conservatories can be bought off the shelf from the local DIY superstore, or from a specialist supplier (who will generally include a made-to-measure element), or it can be custom designed by an architect and/or certain specialist structural engineers.

All types need foundations, ground slabs and rainwater disposal. People often make the mistake of seeing conservatories as a cheap way of having an extension and the cost of a conservatory in a DIY store or brochure can be misleading because it won't usually include the cost of heating, foundations, flooring and drainage - which are about the same whatever type of conservatory you choose. These can amount to as much as half the overall cost of the extension. The glazed part is usually cheaper if it's bought off the shelf, and timber, aluminium and uPvc framing cost about the same as each other. All in all, conservatories cost about the same as a brick extension (around £750 to £2,000 a sq m).

Open the wall

Although most standard conservatories have only one exit into the garden, they don't have to be like this. They can be designed so the side walls can be opened up. This way you can create a shelter out in the open and be much more flexible in how you use the space. For instance, south-facing conservatories need blinds in summer to reduce excessive heat gain and an open-sided room with the roof blinds drawn will create a shaded awning.

Blinding sun

In summer a conservatory can get uncomfortably hot if it's south or west facing. This solar gain can be reduced with blinds and plenty of ventilation, preferably at high level. Tinted glass is one way of reducing solar gain but is not very pleasant. Dappled sunlight through timber slatted blinds creates a beautiful effect.

Streamlining

There are now adhesives which are strong enough to fix the glass to other glass or to smooth materials such as steel or aluminium. So conservatories can be made by glueing the glass to metal or glass beams and columns, and sealing them with silicone. This way the streamlining of the structure and framing, striven for in the 19th century, can be achieved, and all-glass conservatories are now being made.

Because the Victorian conservatory represents pseudohistory, it is arguable that the more transparent the structure attached to a 18th or 19th century house is, the more the integrity of the home's original design will be respected.

Tom Fitzsimmons runs his own architectural practice and has designed a number of conservatories. 0044 207 700 7970 or tomfitzsimmons@freeuk.com