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September 2011 Energy Solutions
Block up the window … with glass
 by James Dulley
Engineering Consultant
Glass and acrylic blocks are safe, efficient option for windows
Q: I am concerned about the security and efficiency of some of my old single-pane first-floor and basement windows. I think glass block windows would be more secure. Are they efficient, and can I install them myself?
A: As people become more concerned about security at home, they are replacing more of their old, inefficient windows with 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.
In addition to security against break-ins from thieves, protection from severe weather is now also becoming a more widespread concern. Some standard pane-glass windows can withstand the force of the wind during a violent storm, but damage from flying debris, which can break through most pane glass, often does much of the damage.
It is possible for an intruder to break through a glass-block window, but it would be very difficult, take quite a bit of time, and create a lot of noise. Even though most glass-block windows are hollow and someone may be able to break through a single block, the opening is still not big enough to crawl through. Solid, high-security glass blocks are also available, but they are very heavy and more difficult to install yourself.
Plastic-block windows are also available and look identical to true glass blocks. Most are molded from acrylic plastic, which is fairly impact-resistant, much more than standard double-pane glass windows. Acrylic plastic is often used for window application, such as blocks, storms, etc., because it does not yellow over time as other plastic may do in the sun.
Glass- and plastic-block windows can be energy efficient for several reasons. There is a sealed insulating air gap inside of each block. This is particularly efficient in glass blocks because the two halves are fused together under heat. When the glass blocks and the air inside the sealed gap cool, a slight insulating vacuum is created inside the block.
Just as most replacement windows now use low-E (low-emissivity) coating on the glass, so do glass and plastic blocks. Some Hy-Lite acrylic plastic blocks have an efficient low-emissivity coating on a third pane inside the block, providing an R-3 insulating value. This can be combined with a tinted block for summer heat rejection of SHGC (solar heat gain coefficient) = 0.27 plus winter heat savings.
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 perimeter.
If you want the option of natural ventilation from your block windows to reduce air-conditioning costs during mild weather, several of the blocks can be replaced with a small hopper window. The opening panel is made from tough polycarbonate plastic. It is too small to get through even if someone could break it. A hopper-style window closes on compression-type weatherstripping, so it is very airtight.
Opening casement-style block window panels with privacy style blocks are often used in bathrooms and basements. Always check your local building codes about egress (escape) requirements for various rooms in your house. If there is a fire or a roof collapses during a tornado or hurricane, you need to have a window that will open wide enough to crawl through. Many casement style block replacement windows should meet these requirements.
It is not difficult to install block windows yourself. There is quite a learning curve to install individual blocks yourself, so if you are inexperienced, select preassembled panels. These large complete panels are installed similarly to any replacement window. Some of the strongest glass block panels, which meet IBC (International Building Code) and Dade County, Fla., hurricane impact tests, are framed by 2x6 pressure-treated lumber.
Glass block panels, such as from Pittsburgh Corning, are available in 60 sizes and three block patterns. Some minor framing of the rough opening is required to fit the panel. Plastic blocks, such as from Hy-Lite, can be custom-sized to fit your existing window opening.
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 also available for a more seamless appearance.
For more information:
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 is a nationally syndicated engineering consultant based in Cincinnati.
If you have a question about energy use or energy-efficient products, send it to: James Dulley, Electric Consumer, 6906 Royalgreen Drive, Cincinnati, OH 45244; or visit www.dulley.com.
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