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Abcs Of Window Blocks

I might replace some old windows with glass blocks for security and efficiency. I also want to use them for decorative interior accents. Is this a good idea and are plastic ones just as good?—Bud W.

Glass or plastic block windows have come a long way in installation and energy efficiency. They are more secure than standard windows against break-ins, and thieves seldom even try to enter through a block window. The energy efficiency of some designs surpasses standard double-pane windows.

“Glass block window” has become a generic term for block windows made of either glass or acrylic plastic. People often think that block windows are a solid block of glass or plastic when, in fact, most are hollow. There are some solid glass block windows for high security, but they are expensive and very heavy.

In addition to being lighter weight, hollow blocks improve the energy efficiency just as the air gap does for standard double-pane thermal window glass. When two hollow glass block halves are fused together under heat, the air inside them contracts as it cools. This creates a partial vacuum to increase the insulation.

Acrylic block window halves are not fused under heat when assembled, so the partial vacuum is not created. To improve their energy efficiency, some solar plastic blocks have a microscopically thin layer of metal atoms deposited on one inside surface. This coating, similar to a low-e coating on standard windows, reduces the sun’s heat flow through it during the summer. This also helps keep heat indoors during the winter.

Some block window designs offer the option of ventilation for efficient cooling and for exit during emergencies. Each room in your home should have a window that will open wide enough for someone to escape through it.

One ventilation option is using casement or hopper plastic block windows that crank open like ordinary windows. Since plastics are less dense than glass, these windows often weigh less than standard double-pane glass windows. Most glass blocks are too heavy to be used for windows that open.

Several companies produce hinged hopper-style window panels that replace several of the glass blocks in the center of a window. These use double-pane polycarbonate plastic (bulletproof glass). For security, its impact strength is more than 100 times greater than glass.

Glass or plastic block windows function as natural shades to block the intense direct sun during the summer. This is particularly true of block windows with real mortar or other tinted sealant between the blocks. When the sun shines down from a steep angle, as it does during the summer, the horizontal mortar joints between the blocks act as a louvered shade.

There are several options for installing block windows. The simplest method is to buy complete glass or plastic block windows in a vinyl or insulated frame.

There are many standard sizes of block window frames available. Custom sizes can be made to within one-quarter inch of your window opening.

Interior decorative block walls can be a stunning addition to any home. There are many patterns, angles, and colors to provide from a clear view to total privacy. You may want to use a combination of glass and plastic blocks because they look and feel almost identical.

Block walls offer the possibility of using natural lighting in interior rooms because the light from a window will pass through them. An attractive touch is to install fiber optic lighting in the joints between them so the blocks themselves appear to glow.

Write for Utility Bills Update No. 497—buyer’s guide to window block and kit manufacturers. Include $3.00, a business-size SASE, and Update number. Mail requests and questions to James Dulley, Kentucky Living, 6906 Royalgreen Drive, Cincinnati, OH 45244. Go to to instantly download.

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