Windows And Doors

The word window originates from the Old Norse vindauga, a composition of vindr 'wind' and auga 'eye'. Windows bring in light, and views, and act as a protection from extremes of climate. A wide range of glass types has been used through history, from cut mica to float glass. A modern window is usually glazed with one large opening, perhaps in several layers, whilst windows in the past were glazed with multiple smaller panes separated by glazing bars due to the unavailability of large sheets of glass. Glazing bars were once made of lead, often strengthened by iron, within a main frame of timber. From the beginning of the eighteenth century, wooden glazing bars were used, and glass was kept in place with putty. Today there are four main types of window frame: timber, aluminium, steel and plastic. These are also used in different combinations.

The word door comes from Sanskrit and means 'the shutting of an opening'. The entrance door to a house was traditionally formed in a very special and careful way. The door was for receiving guests, as well as for greeting greater powers, both physical and supernatural, or for keeping them out. The material most often used is wood, but steel, aluminium and plastic doors are also made.

Both windows and doors can be seen as movable or fixed parts of the wall. They face the same demands as the external or internal wall: thermal insulation, sound insulation, resistance to the elements, security, etc. Not least, both windows and doors must be able to withstand mechanical wear and tear and keep their form and strength through varying moisture conditions. It has proved difficult to satisfy all these conditions. The thermal insulation of a modern outside door is three to five times less than the external wall, and a window's thermal insulation is four to ten times worse. Reducing the areas of glass will thus reduce energy demands more than using even the most effective energy windows and insulating frames. This is primarily an issue for the architect; how much glass does one really need? For example, if a classroom has a deep plan, it will need 20 to 25% glazing as opposed to a shallower plan classroom where 15% glass will suffice for daylighting (Traberg-Borup etal., 2004). Windows placed high up in a wall also give more light than low windows.

16.1

A fortocha-window.

Source: Sergio Fox and Morten Kjœrgaard.

THE FORTOCHA-WINDOW

The'Fortocha'-window is a multifunctional window based on old Russian building traditions with two layers of glass in separate frames at some distance apart (Figure 16.1).The outer frame is equipped with a single glass to provide basic weather protection, and the inner frame with high performance double glazing to provide thermal comfort. There are openings at the bottom of the outer window and at the top of the inner window. Cold ventilation air is thus drawn in and preheated by the transmission heat being lost towards the outside. This provides draught free incoming air and minimal heat loss.The air space between the two frames at the same time effectively acts as a noise barrieras well as eliminating wind gusts, and limiting intake of particles of dust and pollen.

16.1.1 Glass and methods of installation

Float glass is normally used these days, though machine glass is still in production in some European factories (see page 99). Cast glass is used indoors, often as a decorative product that does not need to be fully transparent. There are various types of energy glass, security glass, sound insulating glass and fireproof glass. Energy glass is often slightly coloured or covered with a metallic oxide, such as tin, indium, antimony, silver or gold. Security glass is specially hardened or laminated with a foil of polyvinyl butyral between the sheets of glass. Sound insulating glass is also laminated in two or more layers. Fireproof glass usually consists of several layers laminated with sodium silicate. Glass that is self-cleaning by the addition of nanoparticles is now available.

Depending upon the level of insulation required there will be one, two or more layers of glass in windows (Table 16.1). There are several ways

Table 16.1 Properties of window glass

Types

U-value (W/m2 K)

Transmission of daylight (%)

Single glazing

4.6-5.0

89-90

Double glazing

2.6-2.9

80-82

Double glazing with one low emissivity coating

1.4-1.B

74-78

Double sealed glazing with argon filling

2.5-2.6

80-82

Double sealed glazing with argon filling and one low emissivity coatings

1.2-1.4

74-78

Triple glazing

1.9-2.0

74-76

Triple glazing with two low emissivity coatings

1.0-1.1

62-67

Triple sealed glazing with argon filling

1.7-1.9

62-67

Triple sealed glazing with two low emissivity coatings and argon filling

0.B-0.9

62-67

Note: To maintain interior daylight quality, loss in transmission must be compensated by larger windows.

Note: To maintain interior daylight quality, loss in transmission must be compensated by larger windows.

of achieving this. The easiest is to hinge two timber windows together, a traditional way of constructing windows in Scandinavia. A common way to fix the glass sheets in the frames is with putty based on acrylic polymers or linseed oil. Internal glazing can be mounted with special beading of wood or aluminium. Before using linseed oil putty on a window frame, the timber must be treated with oil or paint, otherwise the linseed oil will be absorbed by the window frame and the putty will crack.

Sealed units have become the most common type of glazing in the building industry. These consist of two or three sheets of glass with a layer of air sealed between them. The air can be replaced with an inert gas, such as argon, krypton and sulphur hexafluoride, which improves the thermal and/or sound insulation of the window because it is heavier or circulates slower than air. The sheets of glass are connected by plastic or metal spacers and sealed with elastic, plastic-based mastic. Until the late 1980s, polychlorinated biphenyls (PCBs) were widely used, but are now prohibited in most places because of their high toxicity. Most common today are products of silicone rubber, polysul-phide rubber or polyurethane. Sheets of glass can also be welded together. The sealed units are usually fixed into a window frame with beads of wood or aluminium, together with rubber packing. It should be noted that the requirements for thermal insulation and noise insulation are completely different. For sound insulation, the glass layer should be as far apart as possible; for thermal insulation they should be a centimetre or less apart. This is because if the air gap is wider, the air between the panes will circulate, and the heat transfer from one side to the other will be significantly greater.

More recently, alternatives to glass have appeared on the market. These are mainly polymethyl metacrylate (plexiglass) and polycarbonate, which are often used in roof lighting, greenhouses and conservatories. The sheeting products are mounted in a similar way to the sealed units. These products are lightweight, easy to cut and mount, but are not always fully transparent. Some may discolour fairly rapidly in zones with high ultraviolet radiation, such as at high altitudes.

Glass for windows can be produced partly from recycled glass (about 20%). The rest is based on raw materials that have rich reserves, whilst production consumes large amounts of energy and generates pollution. Ingredients of plastic and metal oxides used also cause a high environmental impact both with respect to pollution and the use of resources. The same is true of most transparent plastic products that are based on fossil oils. The production of polycarbonate has bisphenol A as an important ingredient. In sealed windows, the use of krypton gas will reduce heat loss considerably compared to argon. However, the production of krypton is so energy intensive that this is more than outweighed over a building's lifetime (Krogh et al., 2003). Sulphur hexafluoride SF6 often used for sound proofing is a very potent greenhouse gas.

Glass coatings can reduce the transparency of the glass to such an extent that larger windows become necessary (see Table 16.1). Plastic and glass present no specific problems for the indoor climate, even though there may be small emissions from plastic-based putty, mastics and sealants, depending upon the type of plastic and the mounting technique.

Little is known about the durability of plastic rooflights. Normal glass has almost unlimited durability, but can be chemically broken down if exposed to lime, cement or calcium silicate paint. Coloured heat-absorbing glass can break if part of it is permanently in the shade and the rest is exposed to the sun.

Under special circumstances sealed units can have problems: at low temperatures, the pressure inside them drops, which bends the panes of glass inwards in the middle, giving a lower insulation value. If the building is not heated during the winter, the tension within the glass can be so great that it can break, especially if there is a wide enough space between the panes of glass. Otherwise, the weak link in these units is the seal or sealants. If they are not durable, moisture leaks in and condenses. If there is a vacuum or gas filling, this effect will be lost. In a penetrating durability test carried out by a Norwegian building research group in 1986, one third of metal-sealed windows were defunct after 20 to 32 years. For some of the plastic sealed types, nearly all were failing after four to five years (Gjelsvik et al., 1986).

Another important aspect of sealed units is that if just one of the panes of glass splits, usually the whole window must be changed, whether it is double or triple glazing.

Sealed windows with a gas filling and special coating achieve the highest insulation values. In terms of use of material resources, there is little doubt that coupled timber windows give the best results; especially with a window divided into smaller panes on the outside, where the chance of breakage is highest. However, when one considers the lifecycle of a building, the energy balance will probably be best for one with sealed energy windows. But this is a complex calculation, which will also depend on local climatic conditions. The demolition phase should also be accounted for. Pure glass can be recycled for the production of new windows, while glass with metal coatings can only be recycled as bottle glass. Glass containing laminations of plastic foils, reinforcements, etc. will most often have to be dumped at controlled tips. The possible environmental effects of waste glass material containing nanoparticles is shrouded in some uncertainty (Meili, 2007).

16.1.2 Timber windows

Timber frames used to be made of high quality timber with no knots -often pine heartwood. When constructing the window, the highest quality was selected for the most exposed parts, such as the sill. The components were slotted together and fixed inside with wooden plugs. These days, windows are still generally made of pine, but without the same demands on quality or preparation. Also, the proportion of heartwood used is often very low. To compensate for this, it is quite common to use pressure-impregnated timber, or to use aluminium cladding on the most exposed window parts. Adhesives or screws are used as a binder between the components. The window fittings and the hinges are usually made of galvanized steel or brass. Between the frame and the casement in windows that open there is a weather strip - usually made from polyurethane or ethylene propylene rubber, although it can also be made from silicone rubber, polyvinyl chloride, butyl rubber and chloroprene rubber. Strips of woven wool and cotton are probably the most robust. These products can, however, contain biocides.

Timber windows are mainly based on renewable resources. The energy consumption in the production phase is low, as are the levels of pollution. Pressure impregnation, weather strips of plastic and metal furniture reduce this advantage. Timber windows are well-suited for local production and create few problems in a building, except for potential emissions from some impregnated timber.

Old quality timber frames have been known to last for 250 years under favourable conditions. Until the middle of the twentieth century, a timber window was considered to have a lifespan of 50 years. However, since the 1960s, the rotting of timber windows has increased considerably. In many cases, serious damage has occurred 10 or 15 years after installation. Sweden's State Testing Station has registered that linseed oil and alkyd oil paints give timber the best durability (Phil, 1990).

Timber windows of high quality are usually well-suited for re-use (Figure 16.3). Cleaning, repairing and repainting an old window amounts to approximately 80% of corresponding new production costs (Lauritzen et al., 1991). Older windows usually need a new sill; in some cases turning the window upside down so that the previously exposed parts rest further up is sufficient. The recommended way to remove old paint is to use a blowtorch; however, the vapour from a blowtorch can cause acute allergies. Treating the paint and timber with acid or soda is also possible, but this is often quite aggressive to the wooden material. Metal ironmongery and furniture can often be re-used or recycled. Pure timber waste can be energy recycled in normal incinerators. Impregnated timber and plastic materials have to be incinerated at high temperatures with flue gas cleaning or dumped at controlled tips.

16.2

Traditional window construction for single-glazed windows. Source: Jessen, 1975.

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