By Mike McClintock
Special to the Tribune
February 28, 2008
Traditional block is the thickness of a standard wall and sold in units from 6 to 12 inches square. Thinner glass block (only two and three inches thick) is often used in kits and interior assemblies.
Each unit is, in fact, a block made with glass surfaces around a core of insulating airspace. There are many finishes. For instance, the glass can be almost as clear as a window, textured, or made with an opaque finish that lets in light while preserving privacy.
On the plus side, glass block doesn't split or warp or rot or need painting the way a wood window does (the frame at least), and holds up well in exterior walls with little or no maintenance.
You can also use glass block inside, of course, to create room dividers, partitions and even full-height walls.
The traditional installation has mortar between units laid up pretty much the way you assemble a masonry block or brick wall. Projects requiring many courses (jobs best left to a pro) must be built in stages to keep the accumulating weight of blocks from squeezing mortar out of lower seams. Many larger installations also require reinforcing mesh in the joints and strapping tied to adjacent walls or frames.
But there are shortcut systems, often sold in kits, that take the most difficult part of the job, mortaring, out of the mix. Typical non-mortar kits use interlocking plastic strips and spacers that align and hold the blocks.
These systems make building with glass block a job almost any do-it-yourselfer can handle. Working with a typical kit, you need to frame an opening that accounts for the glass block and joints, and install a structural channel around the perimeter. Then it's a matter of building up courses around the flexible connectors. One of the largest block manufacturers, Pittsburgh Glass Block Co., includes details of several types and sizes of kits with a few installation photos on the Web at pittsburghglassblock.com.
One of the most practical applications of glass block is providing light and security in foundation windows. These openings are normally filled with narrow hopper- or awning-style windows that tip open. The idea is to let in light year-round and allow seasonal ventilation, including snap-in screens if you use the space a lot during the summer.
The catch is that foundation windows are among the easiest entry points for burglars. They are often tucked behind shrubbery and concealed in ground-level shadows around the house where burglars can force the locking hardware and go in and out unobserved.
Glass block isn't impregnable. But each surface is thicker than standard window glass and doesn't easily fracture. There are heavy, solid glass, high-security blocks normally reserved for prison walls. But you don't need them for security in a house. Residential-grade glass blocks joined with beds of mortar in a foundation wall would have to be pounded with a sledgehammer before a burglar could get through.
Another option is to use DIY window kits that make installation easier for this common application. Some kits even include a small vinyl flap in the center to provide at least a bit of ventilation. Others offer an insert to accept a dryer vent. But most block kits are considerably thinner than blocks designed to fit nearly flush in a 4-inch stud wall. This saves weight, but cuts down on security and energy efficiency.
Some of the lighter glass block kits even come preassembled with flanges and frame attachments that allow for adjustments into existing openings. But if you're concerned about security, there is not much point in changing a standard foundation window for a lightweight glass block kit that's thin enough to kick in.
If you don't want to tackle a mortar installation, a good compromise is full-thickness blocks assembled with plastic inserts in the joints. The interlocking plastic guides are easy to install and keep the blocks aligned during assembly. They aren't quite as strong as mortar on their own, but are sealed with silicon caulk, which hides the plastic, keeps out the weather and blends in with the glass to create a nicely finished, low-maintenance surface.