What’s Wrong With My Glass Block? Tricks of the Tradesman

Glass block is a beautiful architectural detail, often found in bathrooms 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 tub, where privacy is essential, yet light is desired.

I have installed thousands of windows and showers in the last seven years in Florida, and have seen several puzzling occurrences. Let me reiterate that of the thousands of installs, I have only seen a handful of issues, and have been able to easily remedy them. The following are my favorite:

Foggy Block

This appears like a opaque or hard to see through block, unlike those around it. Sometimes moisture can be seen condensed inside. The cause is simple. Glass block is formed by casting molten glass into two forms, then sealing those two forms together, creating one block. During the sealing process, the block is typically vacuum sealed, preventing moisture to get into the block. Occasionally the seal is not complete, or sometimes movement once installed will break the seal, which lets air (and humidity) into the block. This moisture will eventually fog and condense on the glass. The only solution is to remove and replace the block, best done by a hammer and a chisel (with eye protection) or even better, pick up the phone and hire a professional!

Black spots inside the black

Another result of the sealing process is the occasional presence of ash inside the vacuum sealed block. This appears 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 directed above.

Spider lines on the inside of the block face

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

Leaking grout joints

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

  • The first and most benign is that the moisture in the air is condensing on the window. Grout is a cementacious product, which is porous by nature. While glass block mortar does have a waterproofing agent present, moisture will sometimes come through the grout. If the moisture is present regardless of rain or wind, and 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 mortared window (the joint may have mortar on the inside and outside, yet be empty in the middle, providing little matter to block the water from wicking inside). This is often seen in instances of heavy rain, or rain from a particular direction. The grout needs to be cut back with a 4″ electric grinder, packed full with mortar, and regrouted. Seal as indicated above.
  • The third and most difficult to resolve option is that of a leak somewhere else in the house, that is finding its way out through the glass block window’s porous joints. First, this leak must be detected, which can be done by visual inspection, water test (with a hose spray water continuously on the side of the house) or a moisture meter. A good mold remediation company can do the water test and will have a moisture meter if you need assistance. Things to look for–cracks in the stucco above the window. Voids in the stucco banding or joints above the window. Holes where two stucco beads or bands come together. Roof junctures that have no flashing behind the stucco or siding material. A good home inspector can resolve these issues. Note that this last issue has nothing to do with the glass block, and needs to be resolved at the source of the leak.

Mysterious cracking glass block

This is my favorite, because it is so unusual. You are asleep in the middle of the night and you hear breaking glass. You search the house and find that your glass block window is cracked. Or you go to take a shower and notice broken glass in the floor of the shower, only to discover that your glass block window is broken! As time goes by more and more cracks appear and glass blocks break. I fixed quite a few of these before I realized what was happening (as a disclaimer, I did not do the original install!) The original installer decided to save a few dollars and forego using glass block panel anchors as directed by the manufacturer. Instead he used thick roofing straps. Eventually these straps rusted, which caused them to expand in the mortar joint. Something had to give to accommodate the expansion. Mortar has great compressive strengh, and is dense in composition, so it had no room to spare. The only other option was the glass, which finally gave under the constant and increasing pressure of the expanding metal. Amazing that rusting metal can break glass!

As you proceed to resolving any of these issues, please be careful! Removing glass block is tedious, and dangerous. Always wear eye protection, and be advised that it is very difficult to remove one block without breaking the one next to it. Clean all broken glass meticulously, to prevent future harm. It would probably be well worth a few dollars to hire a professional to either inspect or resolve any of these issues, as this is beyond the basic home maintenance project.

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

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