How to Cut HVAC Operating Costs Using Spectrally Selective Applied Window Film by Marty Watts, President & CEO, V-Kool, Inc., Houston, TX (www.v-kool-usa.com)

Solar heat gain through south and west facing windows is a serious problem for apartment facilities in Orange County. Many properties experience costly overheating problems. Nation wide, uncontrolled heat gain is one of the most significant factors in decreased tenant retention.

In California the use of heating, ventilating and air conditioning (HVAC) systems to mitigate solar heat may cause power blackouts this summer. In the rush to conserve energy, few realize that a facilityâs windows are the biggest contributor to heat gain and the need to run HVAC systems. Block heat from entering windows and it is possible to run HVAC systems less, reduce energy bills and perhaps avoid power blackouts.

Large expanses of glass raise temperatures and air conditioning bills. Solar heat gain can lead to the following problems:

- Inability to use space near windows;
- Frequent use and replacement of HVAC equipment;
- Fading and heat damage to furnishings and carpets;
- Uncomfortable tenants.

Solutions To Overheating

A window should let in desirable energy (daylight) and block unwanted solar energy (near infrared heat) which causes overheating. Heat gain can be mitigated by replacing existing glass with expensive solar control glass. At far less cost, solar control films can be applied to existing windows.

Tinted or mirrored window films block unwanted heat but reduce visible light and darken interiors. Buildings with dark films on their windows may have to increase lighting to compensate for the decrease in visible light. The additional use of electricity for lighting (which increases energy costs) may actually increase temperatures requiring more air-conditioning. Increased utility costs will be passed on to tenants which subsequently defeats the major benefit of the film - cost savings.

The best solution to overheating - spectrally-selective clear film.

Transparent, almost colorless spectrally-selective applied window film offers the best ratio of visible light transmission to heat rejection. Clear and almost colorless films do not change the aesthetics of existing glass be it clear or tinted. These films are constructed with a scratch resistant coating on one side and an adhesive coating on the other, allowing for retrofit application to existing glass.

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

Type of Glass or Applied Film	% of Daylight Through Glass	% of Solar Energy Through Glass	* Shading Coefficient	% of Visible Light Reflectance Interior/Exterior
1/4" Clear Glass	89	77	.96	7/7
1/4" Clear Glass With Tinted Film	37	64	.74	6/6
1/4" Clear Glass With Reflective Film	37	44	.51	18/28
1/4" Clear Glass With Clear Film	70	45	.50	8/8

How different types of glass and applied film deal with heat.

Questions to Ask About Applied Window Film

How clear is it?

The ideal film would be clear, yet able to significantly block unwanted solar heat. Look for a 70 percent daylight transmission.

How much heat does it really block?

The manufacturer should be able to provide proven performance numbers. Look for a shading coefficient of 0.50 or lower.

Does it effectively block unwanted ultraviolet (UV) radiation?

An applied window film that blocks heat but not UV radiation will permit fading damage to furnishings. The best clear films block 98% of UV radiation.

Does it require special care?

The best applied films require no special care. They can be washed just like the surface of glass.

Where can it be applied?

Look for films that can be applied to operable windows and doors as on both insulating as well as single pane glass.

What is the price?

The price of applied film is a function of the degree of difficulty of the particular job. This is determined by the local applicator. Depending on the particulars of the job and the geographic area, the best clear applied window film ranges in price from approximately $9 to $12 a square foot installed. Installed prices are volume dependent, therefore on larger projects such superior performing films may be installed for less.

What is the pay back?

Given a buildingâs footprint, orientation, HVAC system, and using local weather patterns, computer simulations can provide a relative pay back for applied film.

What is the guarantee?

The best applied films are guaranteed not to peel, discolor, blister, bubble or demetalize for at least 10 years. Look for a guarantee from the manufacturer, in addition to any by the installer.

*The lower the shading coefficient, the lower the solar heat gain.

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