Glass block is a beautiful architectural detail, often found in bathrooms as windows or shower walls. It is a highly durable, low-maintenance product that diffuses light and adds value to any home. One of the best uses is as a window in a shower or over a bathtub, where privacy is essential, but light is desired.

I have installed thousands of windows and showers in the last seven years in Florida and have seen some disconcerting occurrences. Let me reiterate that out of thousands of installations, I have only seen a handful of issues and have been able to easily fix them. The following are my favourites:

fog block

This appears as a dull or hard-to-see block, unlike the ones around it. Sometimes condensed moisture can be seen inside. The reason is simple. The glass block is formed by melting molten glass into two shapes, then sealing those two shapes together, creating one block. During the sealing process, the block is typically vacuum sealed, which prevents moisture from entering the block. Occasionally the seal is not complete or sometimes movement once installed will break the seal allowing air (and moisture) to enter the block. This moisture will eventually fog up and condense on the glass. The only solution is to remove and reattach the block, best to do it with a hammer and chisel (with eye protection) or even better, get on the phone and hire a professional!

black dots within black

Another result of the sealing process is the occasional presence of ash within the vacuum sealed block. This looks like black matter, and is not mold! This is harmless and the best solution is to ignore it! If it really bothers you, remove the block as above.

Spider lines inside the block face

These are birthmarks that result from heating and cooling the glass. The best solution in this case is also to ignore them or, if it really bothers you, remove them as directed.

Leaking grout joints

This is a particularly troublesome occurrence, as you will no doubt experience water running down the block and sometimes puddling. There are several possibilities in this case.

  • The first and most benign is that the humidity in the air condenses on the window. The grout is a cementitious product, porous by nature. Although glass block mortar does have a waterproofing agent present, sometimes moisture does get through the grout. If moisture is present regardless of rain or wind, resulting in faint white streaks running down the glass, your problem is probably simple condensation. Purchase a grout sealer and seal the grout on both sides of the window.
  • The second possibility is a broken seal in the grout or a poorly grouted window (the joint may be grouted inside and out, but empty in the middle, providing little material to block water from entering). This is often seen in cases of heavy rain or rain from a particular direction. Grout should be cut with a 4″ electric grinder, filled with mortar and re-grouted. Seal as above.
  • The third option and the most difficult to solve is that of a leak. elsewhere in the house, which is coming out through the porous joints of the glass block window. First, this leak must be detected, which can be done by visual inspection, water testing (with a hose that continuously sprays water on the side of the house), or a moisture meter. A good mold remediation company can do the water testing and will have a moisture meter if you need help. Things to look for: Cracks in the stucco above the window. Vacuum in the stucco bands or joints above the window. Holes where two beads or bands of stucco meet. Roof seams that are not flashed behind the stucco or sheathing material. A good home inspector can solve these problems. Note that this last issue has nothing to do with the glass block and must be resolved at the source of the leak.

Mysterious cracked glass block

This is my favorite, because it’s so unusual. You are asleep in the middle of the night and you hear glass breaking. You search the house and discover that your glass block window is broken. Or you go to take a shower and notice broken glass on the shower floor, only to discover that your glass block window is broken! As time goes by, more and more cracks appear and the glass blocks break. I fixed quite a few of these before I realized what was happening (as a disclaimer, I didn’t do the original install!) The original installer decided to save a few bucks and forgo using glass block panel anchors per the instructions manufacturer. Instead, he used thick roofing straps. Eventually these straps rusted, causing them to expand at the mortar joint. Something had to give to accommodate the expansion. The mortar has great compressive strength and is dense in composition, so I had no room to spare. The only other option was glass, which eventually gave way under the ever-increasing pressure of the expanding metal. It’s amazing how rusty metal can break glass!

As you proceed to solve any of these problems, be careful! Removing the glass block is tedious and dangerous. Always wear eye protection and keep in mind that it is very difficult to remove a block without breaking the one next to it. Thoroughly clean up all broken glass to prevent future damage. It would probably be worth hiring a professional to inspect or fix any of these problems, as this goes beyond the basic home maintenance project.

Don’t let any of these issues stop you from using glass blocks. As you will notice, most of these problems can be avoided by using an experienced installer who follows the manufacturer’s instructions. If you can’t find someone in your area, feel free to email me and I’ll do my best to locate a qualified installer in your area.