Glass Block Window Sizes For Basements, Bathrooms, and Buildings – 4 Steps For Success


When sizing a glass block window panel you need to throw out the old saying “measure twice and cut once.” With an interior or exterior block window project you need to measure twice and don’t cut at all (this is because you can’t slice through a block and have it retain it’s structural integrity and energy efficiency properties). Although determining the size of the block glass window does require skill and knowledge, it does not take a neurosurgeon to figure this stuff out. In this article you’ll learn the 4 step process to get the correct window size, system, pattern, color and options for your needs and tastes.

Step 1) Where do you want to use the block window? What material is (or will be surrounding) surrounding the window opening?

o Will your glass block window be used in a lower level basement, a 1st floor garage or commercial building, or a 2nd floor bathroom window that is 20 foot to the sill? If you’re putting the window on the lower level you can generally make the window panel larger since you won’t have to be lifting it in the air (most basement windows tend to use anywhere from 8 to 15 blocks in a pre-assembled section – weighing from 50 lbs. to 90 lbs per section). Most second floor bathroom windows tend to use 12 to 24 blocks so you might want to select a thinner block thickness (either 2″ or 3 1/8″) so the panel is lighter and easier to install.

o The surrounding materials the window panel will be set in (also called the head – or top, jambs – or side, or sill – or bottom) are also important. If you’re doing a basement foundation window that will be set inside a masonry wall made of concrete block, sandstone, or poured concrete you may want to size your block window using a mortar joint (to match up with your surrounding masonry materials) vs. a silicone jointing process for the window. For a bathroom window inside of a wood framed opening the silicone vinyl framed glass block window system might be preferred because this window is lighter (usually built with 2″ thick blocks) and easier to set, making an upper level installation safer.

Step 2) Measuring an existing opening or determining the size you’d like to create a new opening

o To replace an existing window you start by measuring the opening first. When measuring an existing masonry opening for block basement windows or upper floor windows you usually want to measure assuming the frame will be removed (this happens about 90% of the time), from side to side, and from the top of the sill to the top of the header plate. The block window will need to be sized smaller than this opening remembering that you can’t cut the glass, or as my Dad used to say fit 10 lbs of you know what into a 5 lb. bag. For upper floor frame openings you usually will measure the area inside of the frame after the window sashes would be removed to size your window.

o Creating a new opening for a block window If you’re creating a new opening for a block window you want to begin with the end in mind. Choose your specific block window system, pattern, design, and/or colors and find out from your glass block window manufacturer what size the window panel will be after it’s manufactured. Then you’ll create your opening usually about ½” larger than the size of the panel (for example if the glass panel size is 40″ x 40″ you’ll want to create your rough opening to be 40 ½” x 40 ½”).

Step 3) Choose a glass block window system and select your pattern, design, air vent, colors, and block sizes.

o Block window panels can be made either with or without a frame. For existing basement windows in masonry openings a frameless block window is more cost effective, can be mortared in place, and is usually the way to go. The frameless window panels can be joined together either with mortar or silicone. The advantage of the mortared windows is they are harder to break in through and match most foundations. The advantage of the siliconed windows are they are smaller in size when pre-assembled together (which works well for tighter fitting openings), lighter, and have an all glass look.

o For upper level frame openings a vinyl framed block window can be an excellent choice since these windows are available in different frame colors and can be installed just like a standard replacement window. For openings you are creating, or can modify the size of, this is the way to go.

o There are a wide selection of block sizes, patterns, design possibilities, air ventilation products, and colors for your windows. A brief overview is presented below:

o Sizes and thicknesses of glass blocks – Glass masonry units are made is both metric and American sizes. Most of the blocks available in the United States are American sized. The American sized blocks are available in 4″ x 8″, 6″ x 6″, 6″ x 8″, 8″ x 8″, 12″ x 12″ and specialty shapes to turn corners or radiuses if you’re looking to create a bay or bow block window. The blocks have nominal sizes meaning the actual size of the block units is ¼” smaller allowing for a mortar joint (a 6″ x 6″ block actually measuring 5 ¾” x 5 3’4″). There are 3 available thicknesses – 2″, 3 1/8″, and 4″ thick. Thinner blocks are lighter and generally easier for someone less experienced to work with.

o Patterns, designs and colored glass blocks – Blocks come in a multitude of patterns providing different levels of privacy, style, security, and energy efficiency. The block are now available in standard and custom colors and artistically decorated murals to match any interior design style. Check out a comprehensive block web site to see the possibilities.

o Air ventilation and acrylic block windows – Glass block windows are available with vinyl air vents, dryer vents, or power exhaust fans. There are also acrylic block casement and awning windows that give you the look and feel of real block with the ability to open completely as well.

Step 4) Don’t worry about any of the first 3 steps and call a glass block window manufacturer and installation professional – If you really don’t want to figure out what size window you need call a block specialty company who offers design, fabrication and installation services. This type of company will usually come to your home or business (without a charge), measure your openings, figure out the right window size, and install it for you with a guarantee.

Leave a Reply