Glass Block Windows in Chicago: Why They Leak, When to Replace, and How They're Set Right

Walk any bungalow block on the Southwest Side, or any street of postwar ranches in Oak Lawn, Burbank, or Evergreen Park, and count the basement windows. Most of them are glass block — that grid of thick, wavy glass squares set into the brick just above grade.

They get installed and then forgotten for thirty years, which is mostly a compliment. But when one starts leaking, bowing, or fogging over, homeowners tend to look at the glass and conclude the window has failed. Nine times out of ten, the glass is fine. The masonry around it isn't.

Paul Lally's Masonry has been setting and repairing glass block in Chicago and the Chicagoland suburbs since 1988. Here is what these panels actually are, how they fail, and what a proper installation involves. If you have one leaking right now, call (708) 448-8866 for a free on-site estimate.

What a glass block window actually is

A glass block — or glass brick — is a hollow unit made from two pressed glass halves fused together, leaving a sealed cavity with a partial vacuum inside. That cavity is what gives the block its insulating value and its acoustic dampening.

A glass block window is not a window in the ordinary sense. It is a mortared masonry panel: individual blocks laid in courses with mortar joints, usually with horizontal joint reinforcement between courses, set into a masonry opening and sealed at the perimeter. It is closer to a small brick wall made of glass than to anything with a frame and a sash.

That distinction explains everything about how these panels behave:

• It has mortar joints, so it has all the same failure modes mortar has anywhere else.
• It is rigid and non-opening, so it cannot be used where an opening has to open.
• It is tied into the surrounding brick, so the condition of that brick controls the condition of the panel.
• It relies on a sill that sheds water and a sealed perimeter, exactly like a masonry opening.

Why Chicago has so much of it

Chicagoland's brick housing stock — bungalows, two-flats, Georgians, postwar ranches, Cape Cods — was built with small wood-sash basement windows sitting right at grade. Sixty to a hundred years later those sashes are rotted, painted shut, drafty, and about as much of a security barrier as a screen door. Glass block solves all of that at once: no rot, far better security, much less air infiltration, and daylight still comes through.

It became the default Chicago basement-window upgrade for good reasons. It just is not maintenance-free, and it is not a product you install over a problem.

How glass block panels fail

1. Perimeter mortar and sealant failure

This is the leader by a wide margin. Where the panel meets the brick opening, there is a mortared and sealed joint. Sealant goes brittle and pulls away; perimeter mortar cracks and drops out. Water then runs behind the panel into the wall and shows up on the basement side — and the homeowner blames the glass.

2. Mortar joint deterioration between blocks

The joints between blocks are mortar, and mortar in the splash zone at grade takes a beating. It erodes, cracks, and lets water track through the panel. On older installations you can often push a screwdriver into a joint that looks fine from six feet away.

3. Sill problems

The masonry or limestone sill under the panel is supposed to slope away and carry a drip edge on its underside. When the wash has eroded flat or the drip has worn off, water sheets back into the wall directly under the panel. Repair the panel and skip the sill, and it leaks again.

4. Bowing or displacement

A panel that is pushed out of plane usually means something else: a lintel above it that is rust-jacking and driving the masonry down, ground pressure and saturated soil in a window well, or an original installation that lacked reinforcement. A bowing panel is a symptom worth taking seriously, because it is telling you about the wall.

5. Cracked or broken individual blocks

Impact, movement in the opening, or a settled lintel above can crack a unit. Single blocks are replaceable in a mortared panel — but if several have gone, the panel itself is being stressed by something.

6. Failed unit seals — fogging

If a block's factory seal is compromised, moisture works into the sealed cavity and stays there. That is a permanently fogged unit and the only fix is replacing the block. It is genuinely rare. Condensation on the inside face of the panel is far more common and is a different problem entirely — that is humid basement air hitting cold glass, and it points at basement humidity and ventilation.

7. The opening itself deteriorating

Basement openings sit where the masonry has the hardest life on the entire building: splash-back from the ground, buried in snow all winter, absorbing de-icing salt from adjacent walks and drives, and staying wet longest. That is why the brick around a basement window is so often the worst brick on the house, with spalled faces and washed-out joints — and why setting a new panel into unrepaired brick is throwing money at it.

How a glass block window is set properly

1. Assess the opening first. Before anything is ordered, the brick, the sill, and the lintel above the opening get inspected. Failing masonry gets repaired now, not around the new panel later.
2. Measure and lay out. Openings on older Chicago homes are rarely square or standard. The panel is laid out to the actual opening, and vent or dryer-vent unit placement is decided before layout — not cut in afterward.
3. Remove the old window and clean the opening. Old sash, frame, and fasteners come out; the opening is cleaned to sound masonry.
4. Repair the opening. Spalled brick replaced, joints repointed, sill wash and drip edge restored, lintel treated or replaced where it has rusted.
5. Set the panel in mortar with reinforcement. Blocks are laid in courses with consistent joints, horizontal joint reinforcement between courses, and spacers to hold alignment while the mortar sets.
6. Point and tool the joints. Joints are struck cleanly so they shed water rather than holding it.
7. Seal the perimeter. Backer rod and the correct sealant at the panel-to-masonry joint, in proper bead geometry, so the joint can move without tearing.
8. Address the water around it. Window well cleaned and draining, grade sloping away, downspouts discharging away from the opening.

Steps 1, 4, and 8 are the ones that get skipped by outfits selling panels by the each. They are also the ones that decide whether you are dry in five years.

Materials and options worth knowing about

• Block patterns. Wavy, diamond, ice, and clear-view faces trade privacy against light. Basement panels usually favor obscured patterns; anywhere you want a view takes a clear-view block.
• Thickness. Standard-thickness blocks are the norm for residential basement panels; thinner blocks exist for lighter applications and are not the same product.
• Vent units. A hinged or sliding vent block gives you controlled ventilation in an otherwise sealed panel. Worth it in a basement with laundry, a workshop, or humidity issues.
• Dryer vent blocks. A dedicated unit built into the panel for a dryer exhaust — infinitely cleaner than coring a hole later.
• Mortared vs. silicone-jointed systems. Mortar ties the panel into the masonry wall and is what we use in a brick opening. Silicone-jointed and pre-assembled framed panels exist and have their applications, but in a Chicago brick opening a mortared panel with a properly sealed perimeter is the durable answer.
• Mortar. Joints between blocks and at the perimeter are matched to the surrounding masonry for color and, on older soft brick, for hardness — the same rule that governs tuckpointing applies here.

One thing to check before you close up an opening: egress

A mortared glass block panel does not open. Emergency escape and rescue requirements exist for sleeping rooms, including basement bedrooms, and a fixed panel cannot satisfy an opening that has to function as an escape route.

If your basement has a bedroom now, or you are finishing it and might add one, confirm the requirements with your local building department before that opening is closed up. Requirements vary by municipality across Cook, DuPage, and Will Counties, and it is a far cheaper conversation to have before the work than after.

What drives the cost of glass block work

We do not quote a per-panel rate sight unseen, because two openings on the same block can need completely different work. What moves it:

• Size and number of openings
• Condition of the brick opening — how much repointing, brick replacement, or sill repair is needed first
• Lintel condition above the opening
• Vent or dryer-vent units included
• Block pattern and availability — matching an older panel is harder than a new install
• Access — a tight window well or a below-grade opening takes more work than an open wall
• Repair vs. full replacement — cutting in a single cracked block versus resetting a panel
• Season — mortar needs above-freezing temperatures to cure properly

The estimate is free, and we will tell you when the honest answer is repairing the panel you have.

DIY vs. hiring a mason

Glass block panels are sold as a homeowner project, and the block-setting part genuinely is not the hard part. The hard parts are the ones that decide whether it leaks:

• Reading whether the opening is sound before you set anything in it
• Getting a panel plumb, level, and aligned in an opening that is none of those things
• Getting reinforcement and joint consistency right so the panel holds together
• Sill and drip-edge repair — the most commonly missed leak source
• Perimeter sealant with proper backer rod and bead geometry
• Recognizing when a rusting lintel above is what actually broke the last panel

A panel set into deteriorated brick will leak no matter how neatly the blocks were laid. That is the failure we get called to fix most often.

Chicagoland context: why these openings take the worst of it

Basement window openings sit in the toughest microclimate on any Chicago-area building.

They are in the splash zone, catching rain rebounding off walks, patios, and soil. They spend winter buried in snow that melts, refreezes, and melts again. They absorb de-icing salt from adjacent walks and driveways, and salt crystallizing inside brick pores blows the face off masonry faster than water alone. They sit at the point where grade, gutters, and downspouts all fail first. And in a region that runs through dozens of freeze-thaw cycles a winter, the wettest masonry on the building takes the most damage.

On a century-old bungalow with soft common brick, that combination is why the courses around a basement window are often visibly worse than brick eight feet higher on the same wall. Any glass block work at grade should be treated as masonry repair with a window in it — not as a window installation.

Maintenance: what to actually watch

• Look at the perimeter joint every couple of years — cracked, pulled, or missing sealant is your early warning.
• Check the mortar joints between blocks for erosion or cracking.
• Keep window wells clear of leaves and debris so they drain instead of ponding.
• Keep soil, mulch, and snow piles off the masonry around the opening.
• Make sure downspouts discharge well away from basement openings.
• Note whether interior wetness is at the panel (a leak) or on the glass face (condensation) — they lead to completely different fixes.

Related masonry work

Glass block rarely turns out to be a standalone job. Openings that need panels usually need some of this too:

• Brick repair for spalled and cracked brick around the opening
• Tuckpointing and repointing for eroded joints in the surrounding wall
• Limestone and sill repair to restore the wash and drip edge under the panel
• Lintel replacement where rusting steel above the opening is driving the damage
• Foundation masonry repair where deterioration runs along the whole base of the wall
• Masonry waterproofing once repairs are complete and mortar has cured

The short version

Glass block is one of the best upgrades available to a Chicagoland brick home — secure, durable, low-maintenance, and a real improvement over a rotted basement sash. But a glass block panel is masonry, and masonry lives or dies on the joints, the sill, the lintel, and the water around it. A panel dropped into a failing opening will leak, and the glass will get blamed for it.

Paul Lally's Masonry Inc. is a family-owned, insured masonry contractor serving Chicago and the Chicagoland suburbs since 1988 — tuckpointing, brick repair and replacement, chimney repair and rebuilds, lintel replacement, masonry restoration, and waterproofing for residential and commercial properties. Built on Craftsmanship. Backed by Experience. Free on-site estimates — call (708) 448-8866.

If a glass block window at your house is leaking, bowing, cracked, or sitting in brick that has seen better decades, we will look at the whole opening and tell you what it actually needs. Request a free on-site estimate or call (708) 448-8866.