Читать книгу Creating an Eco-Friendly Home & Workplace - Paul Hymers - Страница 13

Of course the very best form of light is natural light, and to be truly green you would have to live by it alone, but that would be a little restrictive. For the times when natural light is insufficient for our requirements, we have to create light artificially, and it seems we need a lot of artificial light these days. But before we get into that, we can look to see if we are making the best possible use of the daylight that is free to us all.

When the first glazed windows were used in the 16th century, they were formed from tiny panes that were joined together with thick strips of lead. Daylight had a hard job penetrating them. The Georgians made huge windows to accommodate the fact that small panes of cast glass were still being used between substantial timber bars, and it wasn’t until 1838, when the rolling process of glass manufacture was invented, that windows had more glass than frame. Rolling made it possible to make larger panes. The Victorians gradually increased their size in the styling of their sash windows, which once had contained as many as eight panes in each sash, leading to just two in the Edwardian era. Edwardian homes that haven’t since been modernized with plastic double-glazed units are flooded with daylight because of them.

Windows are still large to this day, but by a strange twist of fate, the method of supporting the glazing is still reducing the amount of light they let through. To avoid the chore of repainting window frames, we have switched from wood to plastic, and to improve insulation, double-glazing has become standard. The gap between the panes has grown progressively wider, and now two panes of 4 mm glass are used in most, with a 20 mm cavity between them.

Low-e glazing heat

Floor plans of two identical terraced homes showing the extent to which daylight can penetrate rooms when different windows are installed

PVC-u replacement windows have been installed at a phenomenal rate since the 1980s, but plastic isn’t as strong as timber, and to compensate for that the framework has to be thicker. The sections can be deep and chunky, and on a standard double-casement window measuring 1.2 m wide by 1.05 m high, the glass can be account for less than 75 per cent of the whole area.

It is worse than you think. The single thin sheets of glass used by the Edwardians might have been terrible for keeping in the heat, but they were excellent for letting in daylight. Almost all ultra-violet light can pass through single glazing, but only 75 per cent can pass through a 20 mm double-glazed unit, while a triple-glazed unit offers significantly worse performance. Consequently, larger areas of glass are needed to compensate for that light loss.

To put it simply, we need bigger windows if we are to enjoy the benefit of daylight fully. The problem is that even the best triple glazing can lead to a significant heat loss compared to an insulated wall. At best, it offers an insulation value that is slightly worse than a cavity wall comprising two skins of bricks and no insulation. The best double-glazed windows are comparable to a solid one-brick wall in the rate of heat loss.

There is some balancing to be done here: as well as sizing windows, their orientation lies at the core of the problem.

In spite of the fact that light travels at a constant and incredible speed, it is easily deflected. Daylight arriving on vertical walls does so at an angle; some of it is reflected, some becomes distorted by double glazing and some makes it into the room just behind the window. High-level windows and windows that are tall rather than wide are much better for lighting a room, although ultimately a combination of both is best. Patio doors are great for letting natural light into a room, but traditional thinking in house design has restricted most homes to one pair, which tend to be stuck in the back somewhere and covered with a conservatory later.

My guess is that in most homes there is at least one window that could be replaced by a pair of glazed doors, without having to widen the opening or disturb the lintel over it. All that’s required is to cut out the wall beneath it, install the doors and make good the reveals. Of course if extra doors will prove to be an unwanted security risk downstairs, full-height glazing can be used instead with passable vents as required. Full-height glazing is in architectural fashion at the moment, and many new-home builders are installing glass that stretches from floor to ceiling. To do this in an existing home requires some structural alteration. Better to stick with your existing window openings and extend beneath them. Upstairs, you can do the same with fixed laminated glass or opening doors and a ‘Juliet’ balcony. If you don’t want to run to the expense of a balcony railing, a pane of 10 mm laminated glass fixed 1100 mm off the floor will keep you safe.