Replacing Glass Block Windows: DIY Steps and Vented Options

Cracked blocks, crumbling mortar, and a musty basement often trace back to one worn-out window. Replacing glass block windows is a realistic DIY project for a basement or bathroom opening, provided the rough opening is sound and you respect the weight of the glass. Most homeowners today skip laying individual blocks and instead install a prefabricated panel that arrives already assembled in a vinyl frame or with mortar cured at the factory.

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The basic process is simple: remove the old window or failed block panel, clean and check the masonry opening, set a new prefab panel (or mortar individual blocks) on a bed of mortar or sealant, shim it plumb, and seal the perimeter inside and out. A single basement window usually takes four to six hours for a DIYer, plus mortar curing time. If you want airflow, you can order the panel with a built-in vent block so the basement is not sealed shut.

Before you start, decide on three things: prefab panel versus mortar-set blocks, whether you need a vent or dryer-vent opening, and how you will manage water at the sill. Those choices drive what you order.

Replacing Glass Block Windows: Prefab Panels vs Mortared Blocks

You have two main paths for a block window replacement.

Prefabricated Glass Block Panels

Prefab panels are built to your opening size, either with blocks set in a vinyl frame or mortared together at the shop. You set the whole unit into the opening like a replacement window. Advantages include consistent joints, faster installation, and fewer chances of a crooked result. Vinyl-framed panels usually include a nailing flange or are designed to be set in sealant, and many come with vent options pre-installed. The trade-off is weight; a typical 32 x 16 inch basement panel can weigh 50 pounds or more, so plan for a helper.

Mortar-Set Individual Blocks

Laying individual blocks with glass block mortar and plastic spacers gives you full control of size and pattern and can cost less in materials. It is also slower and less forgiving. Joints must be consistent, panel reinforcement is needed on larger openings, and a first-timer can easily end up with wavy lines. For small basement openings, prefab usually wins on time and final appearance. For custom shower walls or decorative interior partitions, individual blocks with a spacer system make sense.

Vented and Air-Vent Block Options

A standard glass block window is sealed, which is great for security and energy but can leave a basement stale and humid. Vent options include:

• Hopper-style vent insert: A small operable window, often vinyl with a screen, set into the middle of the block panel. It tilts inward to provide airflow.
• Air-vent blocks: Special units with louvered openings and a screen, replacing one or two standard blocks. They provide passive ventilation but cannot be fully closed on some models.
• Dryer vent block: A panel with a round opening sized for a dryer or exhaust duct, useful when the window is the only practical route out of a laundry area.

Check local code if the room is a bedroom; a glass block window does not meet emergency egress requirements, and a finished basement bedroom needs a code-compliant egress window. Your building department can confirm what is required in your area.

Safety: Handling Glass and Old Materials

Glass block edges and broken block shards are sharp, so protective gear is not optional when replacing glass block windows. Wear cut-resistant gloves, safety glasses, long sleeves, and sturdy boots. When removing an old sash window, handle any tempered or insulated glass unit (IGU) carefully: carry it upright by the edges, never lay a sealed unit flat on a hard surface with a point load, and keep children and pets away. Tempered glass shatters into many small pieces if the edge is struck, so tape an X across the pane before prying out the frame.

Chiseling old mortar creates fine crystalline silica dust. Mist the work area lightly and wear a properly fitted N95 or better respirator. In homes built before 1978, painted window frames and trim may contain lead; test with a lead test kit before scraping or sanding, and follow lead-safe practices if it tests positive.

Tools and Materials

• Prefab glass block panel sized to your rough opening, or blocks, spacers, and glass block mortar
• Cold chisel, brick set, and 3-pound hand sledge
• Reciprocating saw with a metal-cutting blade (for old steel frames)
• Pry bar, utility knife, wire brush
• 4-foot level, tape measure, plastic shims
• Type N or S mortar or glass block mortar, mixing bucket, and trowel
• Exterior-grade polyurethane or silicone sealant and caulk gun
• Masonry waterproofing coating (optional, for the sill)
• Cut-resistant gloves, safety glasses, respirator, drop cloths

Step-by-Step Block Window Replacement

1. Measure the rough opening. Measure width and height at three points each and use the smallest numbers. Order a panel about 1/4 to 1/2 inch smaller than the opening in each direction to allow room for mortar or sealant and shims.
2. Remove the old window. For an old steel basement window, cut the frame at the corners with a reciprocating saw and pry it out. For a failed mortar-set block panel, chip out the perimeter mortar with a cold chisel, then remove blocks one at a time from the top down. Work carefully; you want the surrounding foundation intact.
3. Clean and inspect the opening. Chisel off loose mortar, wire-brush the sill and sides, and vacuum the debris. Look for cracks, spalling concrete, or a sagging lintel above the opening. Minor chips can be patched with hydraulic cement; anything more serious needs a pro.
4. Prepare the sill for water. The sill should slope slightly outward so water drains away. Build up the slope with mortar or patching cement if needed, and apply masonry waterproofing coating once it cures.
5. Dry-fit the panel. Set the panel in the opening on shims to check the fit. Confirm it sits plumb and level with even gaps on all sides.
6. Set the panel. For mortar installation, spread a 1/2-inch bed of mortar on the sill, set the panel onto it with plastic shims to hold the height, and pack mortar into the side and top gaps with a trowel. For vinyl-framed sealant installs, run continuous beads of sealant on the sill and jambs as the manufacturer directs, then set the panel and shim it tight.
7. Check plumb and level again. Adjust shims before the mortar sets. Wipe excess mortar off the glass promptly with a damp sponge.
8. Tool the joints and cure. Strike the mortar joints smooth once they are thumbprint-firm. Keep mortar lightly damp and protected from rain and freezing for at least 24 to 48 hours.
9. Seal the perimeter. After curing, remove shims that would show, fill the voids, and run a bead of exterior-grade sealant around the outside joint between panel and foundation. Caulk the interior joint as well.

Sealing Basement Moisture Around Block Windows

Glass block itself does not leak, but the perimeter and window well often do. Make sure the exterior grade slopes away from the house and the window well has a gravel base and ideally a drain tied into the footing drain or a dry well. A clear window well cover keeps rain and leaves out while letting in light. Inside, a dehumidifier holding the basement around 50 percent relative humidity helps prevent condensation on the cold glass. If condensation drips down the panel in winter, a vent block improves air exchange.

Troubleshooting

• Water at the sill after rain: Check the exterior caulk joint and the window well drainage first.
• Hairline cracks in the mortar joints: Common with movement; seal with a clear masonry sealant or repoint small sections.
• Panel sits crooked: Remove and reset before the mortar cures; once hard, correction means removal.
• Condensation between blocks: Individual blocks are sealed units; a cracked block lets moisture in and should be replaced.

Typical Costs

A prefab basement glass block panel commonly costs from around $100 to several hundred dollars depending on size, pattern, and vent options, with vented panels at the higher end. Mortar, sealant, and supplies typically add $30 to $75. Professional installation of a single basement block window usually runs a few hundred dollars per opening, more if the opening needs resizing or masonry repair. Get at least two quotes for multiple windows.

When to Call a Pro

Replacing glass block windows is within reach for many homeowners, but hire a mason or window contractor if the opening needs to be enlarged, the lintel above the window is cracked or sagging, or the foundation shows significant cracks, bowing, or active water intrusion. Cutting into a foundation wall or altering a structural opening should be evaluated by a contractor or structural engineer. A pro is also the safer choice for large or high openings where the panel weight makes handling risky.