Dear Jim: I am concerned about the security and efficiency of some of my old single-pane first-floor windows. Are glass block windows efficient and can I install them myself?

-- Kathy W.

Dear Kathy: As people become more concerned about security at home, they are installing more glass and plastic block windows. This is particularly true for first-floor and basement windows where a would-be thief can hide and take time to quietly pry open or break a standard window.

It is possible to break through a glass block window, but it would be difficult and noisy. Even though most glass blocks are hollow and someone may be able to break one, the opening is too small to crawl through. Solid, high-security glass blocks are available, but they are very heavy and difficult to install yourself.

Plastic block windows are also available. Most are made from acrylic plastic, which is hard to break. These windows are often installed as completed window panels to fit the wall opening. When installed, they look as though each block was installed individually.

Glass and plastic block windows can be energy efficient for several reasons. There is a sealed insulating air space inside of each block. This is particularly efficient in glass blocks because the two halves are fused together under heat. When the blocks and the air inside the sealed space cool, a slight insulating vacuum is created inside. Some plastic blocks have an efficient, low-emissivity coating on the inner surface.

Another reason block windows are efficient is they are very airtight and remain that way throughout their life. There is very little, if any, air infiltration when the blocks are assembled properly in mortar or clear silicone. When installing a completed block panel, make sure to caulk well around the frame.

If you want the option of natural ventilation with your block windows, several of the blocks can be replaced with a small hopper window. The opening panel is made from tough polycarbonate plastic, and is too small to get through even if someone could break it. A hopper-style window closes on compression-type weatherstripping, so it is airtight. Casement-style block window panels that can open are often used in bathrooms.

It is not difficult to install block windows yourself. The large complete panels are installed similarly to any replacement window. Do-it-yourself kits for installing individual blocks are also available. Corner spacers are used to properly position the blocks. When the mortar sets up, the spacer ends are broken off and covered with more mortar. Kits for using clear silicone instead of mortar are available.

The following companies offer glass and plastic block windows:

Builders Accessories, (888) 921-7086, www.acrylicblock.com

Circle Redmont, (800) 358-3888, www.circleredmont.com

Glashaus, (815) 356-8440, www.glashaus.com

Hy-Lite Products, (888) 256-2599, www.hy-lite.com

Pacific Accent, (888) 522-4527, www.pacificaccent.com

Pittsburgh Corning, (800) 624-2120, www.pittsburghcorning.com.

James Dulley's column runs every Sunday in Extra.