Magazine article Journal of Property Management

Sun Block: Window Film Lets in Daylight

Magazine article Journal of Property Management

Sun Block: Window Film Lets in Daylight

Article excerpt

Solar heat gain through east, south and west facing windows is a serious problem for buildings in Sun Belt climates and even in cities such as Seattle, Chicago and Boston. Computers, lighting and people are already generating heat. Large expanses of glass can raise temperatures and air conditioning costs. In addition to high energy bills, solar heat can lead to an inability to use space near windows, frequent use and replacement of HVAC equipment, fading and heat damage to furnishings, carpets and office equipment and uncomfortable and unproductive employees.

Profile of Solar Overheating

Ideally, a window should let in desirable energy (daylight) and block unwanted solar energy (near infrared heat) that causes overheating. Some buildings have mitigated solar exposure by replacing existing glass with expensive solar control glass. More often, at far less cost, building owners may apply solar control films to existing windows to reduce unwanted solar heat.

Most attempts to reduce solar heat involve the use of tinted or mirrored window films that reduce visible light and darken interiors. Most of these films are highly reflective, giving them a mirror-like appearance externally in daylight. Internally, in artificial light and at night they appear mirrored. These conventional films may block unwanted heat, but they also block desired visible light and change the appearance of the building.

Buildings with dark films on their windows may have to improve lighting to compensate for the decrease in visible light transmission. This leads to higher electricity consumption and may increase inside temperatures requiring more air-conditioning. Increased utility costs defeat the major benefit of the film-cost savings.

One solution to overheating is the use of clear spectrally-selective applied window film that offers the best ratio of visible light transmission to heat rejection. Spectrally-selective refers to the ability of the film to select or let in desirable daylight, while blocking out undesirable heat.

The best spectrally-selective clear window film is constructed with a scratch-resistant coating on one side and an adhesive coating on the other, allowing for retrofit application to existing glass.

The table below shows how different kinds of glass and applied films transmit light and heat.

Evaluating Window Films

Performance numbers change when comparing films applied to single pane vs. insulating glass. With single pane glass, applied films shpuld transmit more than 65 percent visible light, reflect 98 percent of the UV, and have a shading co-efficient of 0. …

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