Glass block is a beautiful architectural detail, often found in bathrooms such as windows or shower walls. It is a very durable and 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 several puzzling events. Let me reiterate that out of the thousands of installations, I have only seen a handful of problems and have been able to fix them easily. The following are my favorites:

Misty block

This looks like an opaque block or hard to see through, unlike those around it. Sometimes you can see condensed moisture inside. The cause is simple. The glass block is formed by melting molten glass in two shapes, then sealing those two shapes together, creating a block. During the sealing process, the block is usually 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 replace the block, best done with a hammer and chisel (with eye protection) or, better yet, pick up 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 on the inside of the block face

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

Leaky grout joints

This is a particularly troublesome event, as you will undoubtedly experience water running down the block and sometimes collecting water. There are several possibilities in this case.

  • The first and most benign is that the moisture in the air condenses on the window. Grout is a cementitious product, porous by nature. While glass block mortar has a waterproofing agent, moisture will sometimes pass through the grout. If moisture is present regardless of rain or wind, and it results in faint white streaks running down the glass, simple condensation is probably your problem. Buy 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 mortar window (the joint may have mortar on the inside and outside, but be void in the middle, providing little material to prevent water from seeping into the inside). This is often seen in heavy rain or rain from a particular direction. Grout should be cut with a 4 “electric grinder, backfilled with mortar and re-routed. Seal as above.
  • The third option and the most difficult to solve is that of a leak. somewhere else in the house, which is finding its way 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 continuously spraying water on the side of the house), or a moisture meter. A good mold removal company can test the water and will have a moisture meter if you need help. Things to look for: cracks in the stucco over the window. Gaps in the stucco bands or gaskets over the window. Holes where two stucco beads or bands meet. Roof joints that have no flashing behind stucco or siding. A good home inspector can solve these problems. Note that this last issue has nothing to do with the glass block and should be resolved at the source of the leak.

Mysterious block of cracked glass

This is my favorite, because it is so unusual. You are asleep in the middle of the night and you hear broken glass. He searches the house and finds that his glass block window is broken. Or you’re going to take a shower and notice broken glass on the shower floor, only to find that your glass block window is broken! As time passes, more and more cracks appear and the glass blocks break. I fixed quite a few of these before I knew what was going on (as a disclaimer, I didn’t do the original installation!) The original installer decided to save a few bucks and forgo using glass block panel anchors as indicated by the maker. Instead, he used thick roof straps. Eventually these belts rusted, causing them to expand at the mortar joint. Something had to give in to accommodate the expansion. The mortar has a great resistance to compression and its composition is dense, so it does not have space to spare. The only other option was glass, which eventually gave way under the constant and increasing pressure of the expanding metal. It’s amazing that rusty metal can break glass!

When 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. Clean all broken glass thoroughly to prevent future damage. It would probably be worth investing a few dollars to hire a professional to inspect or troubleshoot any of these problems, as this goes beyond the basic home maintenance project.

Don’t let any of these problems 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 cannot find someone in your area, please feel free to email me and I will do my best to locate a qualified installer in your area.

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