Dear Jim: I want security and efficiency, so I might replace some old leaky windows with glass blocks. I also want to use them for decorative interior accents. Is this a good idea and are plastic ones as good? – Bud W.

Dear Bud: New glass or plastic block windows are easy to install and efficient. They are more secure than standard windows against break-ins and the insulation value of some surpasses standard double-pane windows.

“Glass block” window has become a generic term for block windows made of glass or plastic. People think block windows are solid when, in fact, the majority are hollow. There are some solid ones for very high security.

Hollow blocks improve the efficiency just as the air gap does for standard thermal windows. When the two hollow glass block halves are fused together under high heat, the air inside contracts as it cools to create a partial vacuum.

Acrylic block window halves are not fused under high heat, so the insulating vacuum is not created. To improve efficiency, some have a very thin invisible layer of metal atoms on an inside surface. This reduces the direct sun’s heat flow during the summer and keeps heat indoors during the winter.

Some block windows offer the option of ventilation for efficient cooling and for egress during emergencies. One option is using casement or hopper plastic block windows that crank open. The plastic blocks are lightweight.

Another option is hinged hopper window panels that replace several blocks in the center of a window. These use clear tough Lexan plastic (bulletproof glass). Its impact strength is more than 100 times greater than glass.

Block windows function as natural shades to block the intense sun’s rays during the summer. When the sun shines down from a steep angle at midday, the horizontal mortar or tinted sealant joints become a louvered shade.

There are several options for installing block windows. The simplest method is to buy complete glass or plastic block windows which install similar to any replacement window. They can be custom sized to your window opening.

Several installation kits are available to stabilize and space the blocks. One method uses hidden spacers between the corners of adjacent blocks. Another kit uses tracks and clear sealant between the blocks. These kits include everything needed and are ideal for the inexperienced do-it-yourselfer.

Interior decorative block walls allow natural light to pass through them, yet still provide privacy. An attractive touch is to install efficient fiber-optic lighting in the joints so the blocks themselves appear to glow.

Write for (instantly download – www.dulley.com) Update Bulletin No. 497 – buyer’s guide of nine efficient designer glass-plastic blocks and do-it-yourself kit manufacturers listing sizes, colors, patterns, features, prices, and installation details. Include $3 and a business-size SASE, and send to James Dulley, 6906 Royalgreen Drive, Cincinnati, OH 45244.