Installing Glass Block Windows in Basement
The most practical basement window—glass block windows—is also the simplest to install.
A full day
IntroductionWe'll show you how to install a preassembled glass block panel for a basement window. This is much easier than installing a traditional window. In fact, it's nearly foolproof. All you need are a few tools and a few hours and you'll be done!
- Caulk gun
- Circular saw or reciprocating saw
- Cold chisel
- Hearing protection
- Margin trowel
- Pointing trowel
- Safety glasses
- Striking tool
- Tape measure
- Wrecking bar
- Acrylic caulk
- Glass block window
Cutaway of the glass block windows panel
The glass block window panel sits on the block or concrete wall and is held in place with mortar and caulk.
When it comes to basement windows, there’s nothing more practical than glass block windows. Glass block windows are weather-tight and maintenance-free. They let in the sun but keeps burglars out.
And installing glass block windows (or glass block window replacement) is nearly foolproof—if you use preassembled glass block panels. Glass block window panels come ready to install, with the blocks mortared together and secured with a metal band. All you have to do is set the panel in place and pack mortar in around it.
This isn’t exactly easy—large glass block window panels might weigh 100 lbs. or more, and filling in alongside the panels is tedious business—but it is simple. Aside from ordering a panel that won’t fit into its opening or dropping the panel, there’s not much that can go wrong. Installing glass block window panels is fast, too: You shouldn’t have any trouble installing two panels in one day.
This article will walk you through the process of ripping out old windows and installing glass block window panels in a basement made from poured concrete or concrete block. Installing glass block window panels in a wood-framed wall or basement is a bit different and not covered here.
The only specialized tools you’ll need are the masonry tools shown in our photos: a masonry or cold chisel, a pointing trowel, a margin trowel and a striking tool.
Measuring and Ordering
Some home centers carry glass block window panels in standard sizes like 14 x 32 in. and 18 x 32 in. You can also have panels custom-made by a fabricator (search online). With custom-made panels, you can choose from a variety of glass block sizes, colors and surface textures. You might also want glass block window panels with small operable windows built in to allow ventilation.
NOTE: Some building codes require that basement windows allow for ventilation. Call your local building inspector.
Fabricators can make glass block window panels any size in 1-in. increments. Panels usually can’t be returned, so it’s vital that you give the fabricators correct measurements. Most fabricators simply ask for the rough opening measurements, then figure the size of the panel.
To determine the size of the glass block window panel yourself, just subtract 1/2 in. from both the length and the width of the rough opening. This will allow space to build up a curb under the panel and provide gaps at the sides, which will be packed with mortar. Remember, your measurements must be in whole inches, not fractions of an inch.
Project step-by-step (7)
Removing the old window: Rip out the old jamb
Pry out the old basement window jamb with a wrecking bar. First cut the wood sill with a handsaw or circular saw. Be careful not to cut all the way through to the concrete or you’ll ruin the saw blade. Then rip out the sill, the side jambs and the head jamb. Our sill sat against a sloped mortar “curb,” which we had to chip away with a cold chisel before we could cut and pry the sill.
Old sashes often won’t come out of their jambs without a fight, so it’s a good idea to wear gloves and eye protection in case you break glass as you tug and pry at the sash. If caulk or paint is holding the sash shut, cut through it with a utility knife.
Some jambs are set in mortar or concrete at the sill, so you may have to chisel away part of the curb before you can cut and pry the sill. After that, the side jambs and head (top) jamb will pry off easily. Then finish chiseling away the curb.
Attach a temporary block to hold the glass block window upright
Screw a wood block to the underside of the sill plate. This temporary block will keep the glass block window panel from tipping inward, so make sure it extends down far enough to catch the upper edge of the panel. Many older homes have no sill plate. In that case, just screw the block to a joist.
Install the glass block window
Set the glass panel in place with the bottom resting on two wedges cut from a 2×4. The bottom of the panel will tend to slide inward, so you must have a helper inside the basement to steady the panel as you position it.
Level the window in the opening
Center the glass block window panel in the rough opening. Then raise the panel until it’s 1/8 to 1/4 in. away from the sill plate by tapping the wedges inward. Level the panel by laying a level across the wedges and adjusting them. Measure at all four corners from the face of the panel to the outer edge of the rough opening. When the panel is positioned, have your helper insert a third wedge near the center of the panel from inside.
Before you set the glass block window panel in place, screw a block to the underside of the sill plate. We placed our block 5 in. from the outer edge of the sill plate so that the 3-in. thick panel would stand 2 in. from the outside of the foundation. But there are no rules here; your panel can stand far inside the rough opening or nearly flush with the outside of the foundation.
You’ll need three wedges to position each panel. Make the wedges thicker and longer than necessary—you’ll have more handle to grab on to when you yank them out from under the panel.
Our glass block window panel had to be raised nearly 2 in. off the bottom of the opening, so we made our wedges 2-1/2 in. thick at the blunt end and about 8 in. long. Sweep the rough opening clean before you set the panel in place. With a helper, set one end of the panel in the rough opening on top of two wedges and lay the other end on the ground. Then, with your helper inside the basement, tip the panel up into place and adjust its position.
Mortar along the sides of the panel
Pack mortar into the gaps under and alongside the glass block window panel. Pack the bottom and smooth the curb first, staying away from the wedges. Give the curb time to harden to the touch before you begin on the sides. That way, crumbs of mortar you drop can be brushed off the curb. Fill the gaps alongside the panel using your margin trowel and a pointing trowel.
After the mortar has stiffened a bit, smooth the joints with a striking tool.
Fill the gaps left by the wedges
Remove the wedges and fill in the gaps in the curb after the curb has set for at least two hours. In cool weather you may have to wait up to five hours. Also wait at least two weeks before painting the new mortar.
Caulk around the window
Seal the gap between the glass block window panel and sill plate—both inside and out—with silicone or acrylic latex caulk.
It took about half a 60-lb. bag of mortar mix to install our panel. We mixed small batches—four or five trowel loads at a time—in a small bucket and then dumped the mixed mortar onto a scrap of plywood so that we could easily scoop it up with a trowel. Spare the water as you mix the mortar. It should be stiff rather than sloppy, about the consistency of wet sand.
Caution: Wear gloves when working with mortar. Like any other cement product, it can burn your skin. Shove mortar under the panel and build up the curb first.
Then wait until the surface of the curb has hardened to the touch before you begin to pack the sides. On a hot, dry day, this may take only 15 minutes. In cool weather it may take more than a half hour. Packing in alongside the panel is slow-going. It took us about 15 minutes per side. Inside the basement, you’ll find clumps of mortar pushed far past the panel, and empty spots under and alongside the panel. Simply slice off the clumps with your trowel and fill the voids.
You can build up a sloped curb inside the basement just as you did outside. Or you can cut the curb flush with the glass block window panel, leaving a flat surface for a trim board or “stool” if you plan to finish the basement.