Silicone Versus Mortar in Pre-Assembed Glass Block Panels

Today I'll compare the differences between Pre-Assembled Glass Block Windows using Mortar and Silicone between the block. This information should not be considered if you are constructing a dividing wall, bar or other glass block "on-site" construction.

Mortar -V-Silicone is a hot topic between competing companies. Each claims their application is better and each company manufactures evidence to back their claim, usually ignoring the benefit of the other.

If I seem a bit biased towards one application, I apologize in advance. I will further explain myself in closing.

I'm not going to compare the two applications side by side. I will describe each separately and their positives and negatives so that you can form your own opinion.

Pre-assembled glass block windows have been available to the "do-it-yourselfer" for as long as glass blocks have been used in windows. The traditional method of assembly was to build each window in a controlled environment using mortar between the blocks. The assembly was performed by a brick layer and the windows were assembled vertically as if it were built on site.

Today's Mortar Assembly

In the late 1980's, the method of assembly evolved. A mason was no longer needed to assemble these windows. Windows were assembled flat on a table w/ acrylic dowels or other spacer materials on the table side, leaving the joints between the blocks open. A plasticizer was added to the standard mortar mix to "loosen" the mortar to a near liquid consistency and an accelerant was added to counteract the effects of the plasticizer. The mortar could now be poured between the blocks and in just a few minutes, the mortar was hard enough that you could dress the joints and handle the window. This decreased the labor costs involved and increased the per-person production by nearly 10 times. Despite the effects the added chemicals had on the mortar, the windows were still strong and durable and suitable for nearly any window opening in the home.

The biggest "pro" about mortar assembly is the size limitation, or lack thereof. Because it is a solid material between the blocks, weight bearing down to lower level blocks is rarely an issue in a residential application.

The blocks themselves are nominally sized one quarter inch smaller than their advertised dimensions to allow for a one-quarter inch mortar joint. An eight by eight glass block actually measures seven and three-quarters square. Despite being made of glass, the blocks are considered a masonry product. They're sized this way intentionally so that in a concrete block wall, the mortar joints should line up from concrete block through the glass block. The problem with this is that the blocks were intended to be assembled "on-site" and though they are fairly consistent in size, the actual size could vary by as much as one-eighth inch. This means when they are assembled lying flat with a consistent spacer, the windows could be slightly out of square. This issue isn't noticeable in the vast majority of openings where a single unit is installed. When the opening is large enough that it requires multiple sections, this issue could become more noticeable.

In openings where multiple sections are needed, the joint between the sections rarely matches the window. The installation technician can not use the plasticizer or the accelerant on sight and so the color difference will be noticeable. Additionally, matching the finish of the mortar joints could be a problem if the installer does not take the time to dress the entire window.

When putting together multiple windows in the field, despite the color and possible finish differences, without horizontal reinforcing from one section to the next, the joint between the sections becomes the weak part of the window. Since rarely are multi-section windows needed in residential applications, this issue is for the most part, not an issue at all.

Mortar does not bond to the glass; it bonds only to itself. Mortar's strength is a sheer bond and it needs a rough or porous surface to create its bond. The glass block manufactures use a wide range of edge coatings and the intended purpose is an isolation barrier so that if there is movement, it will sheer the edge coating before destroying the block. So long as the foundation is without cracks or other issues, the window should last near forever. If there is any movement at the foundation, cracks will develop. Usually along the point where the mortar and the glass come together. This is the purpose of the edge coating. Cracks no matter how small could leak water. This same issue exists at the perimeter of the window where the mortar is used for the installation.

Mortar is porous by nature and if exposed to standing water for a lengthy period of time, will seep water. Most reputable companies use a Type S masonry mortar in their assembly and installation and will often add a bonding and / or waterproofing additive to their installation mortar. This means with the exception of applications where there are cracks in the wall or window wells that fail to evacuate standing water in a reasonable amount of time, your windows should remain water tight.

Because of the porosity of mortar, it is not uncommon to find mold or mildew on either surface of the window. It would take an additional grout sealant and routine maintenance to prevent this completely.

I discussed mortar failure above in regards to the bond between the glass blocks. Because mortar is a rigid material, it will take very little abuse before it shows signs of failure. Usually the mortar failure itself will not cause a structural issue within the window. Typically it requires routine maintenance to repair these cracks. Once a crack, always a crack. That is unless the issue forcing the failure is addressed or an expansion joint is added.

Aesthetics are another reason for mortar assembled windows. Some folks like the traditional or institutional look of glass block. Since it is a masonry product, it should look like one... Though some people use this logic, it's hardly a comparison point.

Today's Silicone Assembly

In the late 1990's the Pre-Assembled Glass Block Window evolved even further. Commercial grade silicone became the new bonding agent between the glass block. Thin beads of silicone on the inside and outside edge of the block was all that was needed to copy the characteristics of the mortar assembled window. This further decreased the production costs and increased the productivity.

The silicone used to bond glass blocks together is a specialized material. It's not something you can buy off the shelf at your local home improvement store. These sealants are a commercial grade glazing compound that has a higher bonding strength than "off the shelf" materials and have a lower deflection rate. Most products used for this application have additives to prevent UV deterioration or discoloring and often have anti-microbial or anti-fungal additives to prevent the growth of mold and mildew or resist the bond of the mold and mildew.

Silicone is an adhesive. Each block is literally glued to the next. Most of these commercial sealants are guaranteed to maintain their bond and flexibility for 25 ore more years. Since the silicone remains flexible, it will flex or compress under pressure and take the abuse from minor cracks in a foundation or other movement.

When multiple sections are needed to fill an opening, the additional sections are siliconed or glued to each other. The sealant used to bond these sections is the same material used to assemble the windows at the shop. This guarantees consistency in appearance but more importantly, guarantees the bond between the sections.

Because the silicone will compress, the total height is somewhat limited. Unless you have ten-foot ceilings and your window sill is at floor level, these limitations will not be an ussue in most residential applications. I did not say that the limit is ten-feet, it is a bit less than that but we have yet to find a typical residential window too wide or tall for a silicone window.

The silicone used for assembling glass block is clear. Coincidentally, the glass block itself is clear. The variations in the block sizes and potential inconsistencies in the joints of multi-section windows all but disappear. The "all glass" look of a silicone assembled window hides many of those inconsistencies.

Silicone is waterproof. The only opportunity for a silicone window to leak or weep water is at the perimeter where the mortar was used to install the window. Should this leak occur, maintaining the perimeter mortar joint is much easier than maintaining the whole window.

I would be lying if I said that a silicone window can not fail. If there is enough movement, even a silicone assembled window can fail. However, because there is compression space available between the blocks, the silicone assembled window stand a better chance of surviving situations where the mortar assembled window will fail.

Aesthetics.... again, it's about personal preference. The silicone assembled windows are brighter because of the clear edge adhesive as opposed to a full grey mortar joint. The "all glass" look of a silicone window reminds one of an actual window and not a masonry product.

Criticisms I've heard about silicone:

"...mold and mildew will grow between the block". In the tens of thousands of silicone assembled windows we have installed, I have not once heard of this happening. The windows are assembled so that the openings and air passages between the blocks are open to the perimeter mortar. Mortar has a drying effect and will eliminate the moisture between the blocks.

"...silicone windows can be cut apart with a razor knife and a burglar can get in". First, these windows are three inches thick. No one make a razor knife blade long enough to reach from one side to the other to "cut apart" the window. Secondly, the silicone joint between the blocks when compressed at installation is in most cases, too small for a razor knife to penetrate completely.

"...my mortar windows have full joint which makes them better weather barriers". If that were true, wouldn't all prime windows be seven-eighths of an inch solid glass? Air space between panes of glass is what makes windows thermal barriers.

"...someone can break in to your silicone windows easier". Kick a mortar window once and you create a crack, each additional time you kick that window, the crack gets bigger until ultimately the window will fail. Kick a silicone window, the sealant will flex, then reflex to it's original position. Kick it all you want... silicone is fatigue proof.

Which application should I choose?

If you are purchasing for security, no burglar in their right mind would attempt to break through a glass block window whether it was assembled with silicone or mortar.

If you are purchasing for privacy, regardless of what material is used to assemble the blocks, the same patterns are available.

Thermal Effeciency? The measurable performance of the window is of the glass itself not the material used to put the blocks together. The benefit of installing glass block windows in the basement is that it replaces the drafty or broken existing windows. The difference between the mortar and silicone on an inside versus outside surface temperature is not measurable.

It boils down to what you want your window to look like. Mortar for it's traditional look or silicone for the all glass look. The clear silicone windows do allow more light through because it is entirely clear where the mortar window does have mortar joints that light will not pass through.

Still need help deciding?

Go to any home improvement store who retails glass block windows. Look on the shelf at what they're selling. You will find silicone assembled windows. You're not going to find any mortar windows. Would Lowe's or Home Depot sell something that they're not confident in?

Pittsburgh Corning (The only domestic manufacture) sells a product called Guardwise. This is a pre-assembled glass block window, assembled with silicone between the blocks. Would the only domestic glass block manufacturer sell a product that they weren't confident in?

Evolution: A process of continuous change from a lower, simpler, or worse to a higher, more complex, or better state. (Merriam-Webster)