Install a Security Glass-Block Window Yourself

Dear James: It would be easy for a thief to break through our utility-room window. I think a glass block window would be better. Is it difficult to install a mortar-type glass block window or is a kit better? – Steve H.

Dear Steve: Installing a glass block window from scratch with the standard mortar method is not a difficult job. Since you are installing it in a utility room, a perfect, professional-looking job is not critical. As long as the blocks are set in the mortar, a thief will have to work long and hard to break through them.

There are glass-block “kits” available that require varying levels of work to install. A preassembled glass-block window panel is probably the easiest to install yourself. The complete glass-block window panel is made to fit your window opening size. The most difficult aspect of the installation job is removing the old window and picking up the heavy panel.

Other kits, which assist you in building the glass-block window, are available in several types. For a truly professional look, select a kit that has spacers to locate each block in the opening. During the installation, the ends of the spacers are broken off and hidden under the mortar joints.

Complete, clear, glass-block joint kits use clear silicone instead of mortar between the blocks. Clear, horizontal spacer strips are laid between the blocks for a uniform gap. Although it is not as strong as a mortar joint, most thieves will still avoid them.

If you are interested in some of these do-it-yourself glass-block kits and have trouble finding them, try some of these manufacturers: Pittsburgh Corning at (800) 624-2120; Circle Redmont at (800) 358-3888; and Pacific Glass Block at (888) 522-4527.

To install the glass blocks individually with mortar joints, you must first select the blocks that you want. For a utility room, blocks with a moderate surface pattern are often used. These will provide plenty of light and privacy, but the partially distorted view will still allow you to distinguish objects and motion outdoors.

Be brave and give the standard mortar method a try. Plan on it taking about four to six hours once you have removed the old window and cleaned up the opening. First, coat the sill where the glass blocks will rest with a thick coating of water-based asphalt emulsion. Give it about two hours to dry. Attach expansion strips to the header and window jambs, (polyethylene foam works great for this).

Measure the block and opening widths to determine the proper width of the gap between blocks. Your target gap should be about 1/4 to 3/8 of an inch. Set a mortar bed on the asphalt emulsion-coated sill and place the first course (horizontal row) of glass blocks on the mortar bed. Tap them down into the mortar bed with the rubber hammer or trowel.

Spread another mortar bed on top of the first course of glass blocks and set the second course of blocks. If it is a large window opening, you may have to stop at about four or five courses. This allows time for the mortar to set a little or the weight may cause the lower mortar joints to settle.

On every second course, install steel reinforcing rod (rebar) and panel anchors to later be attached to the jamb. Spread a half-thickness of mortar on the course of blocks. Lay the rebar and anchor on this and then cover them with more mortar and set the next course of blocks.

Before the mortar totally sets, strike all the joints smooth and clean out the perimeter gaps. Insert packing material in these gaps and finish with trim.

Send your questions to Here’s How, 6906 Royalgreen Dr., Cincinnati, OH 45244 or visit