Glass Block Windows

Glass Block Window Wall and Door SurroundModern glass blocks, also known as glass brick, are manufactured by bonding together two molded pieces of glass under high temperatures (around 800 degrees Celsius), leaving an insulating air cavity of partial vacuum in the middle. After being fused, the blocks are annealed in a cooling oven for strength.

This type of glass block dates back to the 1930’s, when Pilkington Brothers Ltd. patented a technique for mass-producing these. Various forms of prismatic glass had been in use long before this. They were used in shipbuilding and cellars as far back as the early 1800’s.

Glass block for windows are an interesting material, both from a structural and decorative point of view. They are useful for ensuring privacy in bathrooms or other rooms, as well as for blocking unwanted exterior views, while at the same time allowing sunlight in. You can also use glass blocks for partition walls, screens and shower enclosures and maintain an open, airy effect.

Glass Block Advantages

A properly constructed glass block window can give superior resistance to forced entry, and where breakage does occur, replacement of single blocks is less expensive than replacing an entire glass pane.

Typical R-value for today’s 3 7/8” thick glass block is around 1.75, which is roughly equal to the R-value of a standard double-pane window. More recently, acrylic block, and low-e glass block with higher insulation capabilities have become available.

One of the biggest advantages of acrylic block is low weight compared to glass block- up to 70% lighter. This allows architects to use larger windows without structural limitations (larger support framing and sills etc.), keeping down building costs. Also, acrylic block is available in curved styles that let you build windows along a curved wall portion or introduce an arch-top design.

Standard sizes for 3 7/8 inch thickness glass block include 4x8, 6x6, 8x8 and 12x12 inches. “Thinline” glass blocks that are 3 1/8 inch thick and weigh 20 percent less are available in the same sizes. Also available are rectangular blocks such as 4x8-inch and 6x8-inch, and bull-nosed edge blocks for finishing vertical and horizontal dividers, plus a variety of corner and angled blocks.

Design Considerations

Glass Block Exterior DesignLargest allowable window sizes are based on wind pressure loads. Wind resistance for walls is usually calculated at 20 pounds per square foot. This works out to a maximum area for exterior glass block windows of 144 square feet, with max height of 20 feet or a max width of 25 feet. Anything larger than this will require incorporation of vertical or horizontal stiffener members.

Interior glass block panels such as partitions or shower walls have less rigorous requirements; they are allowed to be up to 250 square feet due to the lower design loads (only 5 pounds per square foot).

Although they are known as bricks or blocks, these units should not be thought of in the same class as masonry materials such as concrete blocks or bricks, in that glass blocks are not load-bearing, i.e., able to support structural weights.

Because of this, the window support structure for glass block window panels is designed to isolate weights and other loads of the surrounding walls from the glass blocks. Window framing also incorporates expansion strips of resilient material to accommodate the thermal expansion properties of the glass. (47 x 10 to negative seventh power per degrees F)

Installation of Glass Block

Detail of Glass Block Window Mortar JointAs noted above, glass block cannot be installed in standard window frames due to its unique structural requirements. The actual installation of the blocks is not difficult; only regular masonry tools are required.

Because of the additional weight of glass block window panels (when installed with mortar, they weigh about 20 pounds per square foot), structural reinforcement to the existing wall or floor joists may be required. Consider using acrylic block, which is lighter.

If you do need to perform reinforcement or removal of structure, consult an architect or general contractor to make sure the structural integrity of your home will not be affected.

Size and Prep the Window Opening

    1. Once you have completely removed the existing window frame, check the size of the opening against your glass block size. Multiply the number of block units by nominal block size and then add an allowance for mortar (1/4 to ½ inch, depending on manufacturers instructions). For example, If you want a window panel using 8 inch square blocks, which require 3/8 inch mortar joints, 4 blocks wide and five blocks high, your calculations would be: Width = 4 X 8 + 3/8 = 32 3/8 inches wide. Height = 5 X 8 + 3/8 = 40 3/8 inches high. So you would need an opening 2 foot eight and 3/8 inches wide by 3 foot 4 and 3/8 inches high.

    2. Using a spirit level, ensure the opening is level and plumb. Also, measure the opening diagonally corner to corner both sides and compare the measurements. The two measurements must be equal within an eight of an inch to ensure the squareness of the opening. If the opening is unsquare or not level, adjustments must be made to the opening or the glass blocks will not lat properly aligned.

Prepare Block Spacers, Panel Anchors and Expansion Strips

    3. Spacers are assembled by snapping together two halves such that they span the width of the block. You will need 3 different types. With tin snips, cut two "L"-shaped spacers for each corner and "T"-shaped spacers for each joint on the bottom level of blocks. The third type is used as is for joints where four blocks come together. Spacers should be prepared ahead of time, since you don’t want to take time to do it once the mortar is mixed.

    4. Each panel anchor should be trimmed to 12" in length, and bent with a 90 degree angle to give a 4" vertical arm.

    5. Trim the expansion strip lengths based on distance between panel anchors. Width should be trimmed to 2 1/8 inch for Thinline blocks and 2 7/8 inch for standard blocks. (The strips go between the glass blocks at the jambs and head, in between the panel anchors, and allow the blocks to expand without cracking mortar.)

Mortar Mixing

    6. Following the instructions on the bag of mortar, mix enough mortar for one hour’s work. When mixed to the right consistency, mortar should have the thickness of bread dough; it should stick to a glass block’s side. Use caution with premix mortar, as it is caustic to the skin- follow all manufacturer’s precautions.

    One 50 pound mortar bag should be enough for installation of about 25 8 x 8 blocks, or 35 6 x 6 blocks.

Laying the First Row

    7. Using trowel, apply a 1/2" deep layer of mortar on the surface where you will place the first row of blocks.

    8. Place an "L" spacer at the lower outside corner of the first block such that spacer cross-arm sits at the bottom of the block. At the other lower corner, place a “T” spacer with the spacer cross-arm at the side of the block.

    9. Between the block and the side jamb, slide a 16” long piece of expansion strip, then push the glass block into place firmly.

    10. Place a “T” spacer on the top inside corner of the block such that the cross-arm sits on the top of the block.

    11. Apply ½” layer of mortar to the second block, on the vertical side that will contact the first block. On the second block, place a
    Regular spacer on the top corner nearest the first block and a “T” spacer on the bottom corner furthest from the first block. Lay the block on the lower surface and push it against the first block firmly.

    12. Repeat step 11 for all the blocks in the first row except for the last one.

    13. The last block is installed like the first in the row, with an “L” spacer at the lower outside corner, a “T” spacer at the top outside corner, and a Regular spacer at the top corner nearest the next to last block.

    14. Visually check for level, plumb and evenness of the blocks, adjusting their placement and lay by tapping gently with a rubber mallet if required. Remove any excess mortar with a polyfoam brush. Fill in any voids or gaps with the mortar and trowel.

Laying Subsequent Rows

    15. Using trowel, apply a 1/2" deep layer of mortar on the top of the first row of blocks. Avoid spreading mortar on top of any spacer, as this will cause blocks to lay uneven.

    16. Using the spacers already in the first row, place the first block without mortar against the side jamb, on which there should be a previously installed length of expansion strip as well.

    17. Place a “T” spacer on the upper corner of this block, against the side jamb.

    18. Continue with blocks as per the first row, but without placing spacers on the bottom of the blocks.

    19. After completing the second row, remove the “T” spacers from the top of the 2 outside blocks which are against the side-jamb. This is to accommodate the panel anchors.

    20. Apply a ½ inch layer of mortar to the top of this row of blocks. Avoid spreading mortar on top of any spacer, as this will cause blocks to lay uneven.

    21. Panel anchors must be installed at the top of every second row. Install panel anchors as follows: Attach the 4" end of the panel anchor to the side jamb using two #12 1" zinc-plated pan head screws per anchor. Push the long end of the panel anchor in the mortar on top of the blocks so that it is completed covered. Repeat for the opposite side jamb, then replace the two “T” spacers removed in step 19. Additional anchor strips may be required to ensure the entire length of the row is covers; if so, overlap the strips by at least six inches.

    22. Continue with the next rows. Remember to keep inserting expansion strips between the blocks and side jambs, and to install panel anchors every second row.

Laying the Topmost Row

    23. Using trowel, lay a 1/2" layer of mortar to the next-to-last row.

    24. So that you can slide the blocks in between the head piece and the next to last row, remove the spacer tabs on the top of the next-to-last row’s spacers, by twisting them.

    25. Place a block (unmortared) against one of the side jambs, with the expansion strip between it and the top of the wall opening. No spacers are required on the top of the top row.

    26. Apply ½ inch of mortar to the vertical side of the next block; place it against the first block.

    27. Install a vertical panel anchor, with the short length leg attached to the top head (with zinc-plated pan head screws) and the vertical leg facing down, against the vertical side of the second block.

    28. Cover the vertical leg of the panel anchor completely in a layer of mortar.

    29. Repeat this two block procedure for the remaining space in the last row.

    30. Install expansion strips across the top by sliding them in between the blocks and the head. No mortar is needed on the block tops.

    31. Allow mortar to set about one hour before next steps.

Mortar Striking and Setting

    32. Twist off all spacer tabs on both sides of panel.

    33. In order to remove excess mortar and create a smooth seal, a striking tool is used. Run the tool with medium pressure over all joints, beginning with horizontal joints, then the vertical joints. Re-fill any gaps and voids with mortar.

    34. Remove excess mortar from the face of the blocks with damp sponge. Any dry film remaining will be cleaned in a later step.

    35. Allow two to three hours for mortar drying before next step.


    36. Remove any remaining dry film excess mortar from the block faces with a soft dry cloth, or household plastic scouring pad.

    37. Apply a coating of clear acrylic or silicone sealer around the perimeter of the panel. Tile grout sealer can be applied over mortar joints as well. This helps prevent mold and mildew.

Photos by redband roaster, Caylee Christine Photography, and Modified Enzyme- Creative Commons Attribution License