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Glass Block Installation

Glass Block Window Installation Tips

  • Take great measurements - mortar gap on sides and top can be up to 3/4 inch
  • Use ingenuity to remove old window frame
  • WATCH glass block videos below
  • Wood shims hold new pre-made glass block window in place
  • CLICK HERE to Get Tim's FREE & FUNNY Newsletter!

Glass block windows are a fun project to tackle. Minimal tools are needed. If all goes well even a "newbie" DIY'r can install four glass-block windows in a day. Here is what I have learned over the years about installing these neat windows.

Measure Twice and Draw

Make a simple sketch of each window opening you intend to transform to glass block. Measure the narrowest point side to side and up and down to the masonry or where you "think" the masonry will be once you remove the frame.

If you're in doubt, take a photo of different windows. You will need to show these to the company that will make your glass block panels. Check your measurements.

Go to the fabricator and ask if everything is in order. Show him/her using your photos and drawings where the measurements were taken. Getting the right size is the most critical aspect of the ENTIRE job. Let's take our time and do this right.

Free & Fast BIDS

CLICK HERE to get FREE & FAST BIDS from local glass-block window installers. Ask about venting windows.

Big Slabs of Glass

When the windows are built, they will be a giant slab of glass. Some fabricators put a band clamp around the windows so they can't break apart. If you don't have a clamp, be CAREFUL! If you bump a corner, you can pop a glass block off. Ask the fabricators to wrap the windows with a band clamp if possible.

Glass Block Videos

Watch these two videos to give you an idea how easy it can be to work with glass block. In these, I put together panels with single block.

You can BUY pre-made windows so you don't have to do what I did in the videos.

Removing the Old Frame

This is the hardest part of the job. If you get the frame out easily, celebrate! If your window frame is a full width metal frame that was cast with the concrete, then you better ask the fabricator for his recommendation. I always leave those frames in place. It is a NIGHTMARE to remove them.

Once the old frame is out, use a brush to clean away dust and debris. I also recommend that you sweep up the floor and get all unnecessary stuff out of the way.

If you are positive that all of the new glass block panels are the right size and will fit, then remove as many frames and old windows as you think you can deal with in a day. I prefer to get all the demo work done and do things in stages. I don't like to do these windows one at a time. It really breaks up your momentum.

Setting the Panels

Now for the fun part. I hope you bought several bundles of cedar shims, as we are going to need them. I also assume that there will be a 1/2 inch gap of mortar around the window. If so, then place several shims on the bottom frame that extend up 1/2 inch. Place these in from the corners about 3 inches or so. Slightly dampen the exposed masonry jambs with a spray bottle of water.

Trowel some of the cement mortar onto the sill. Don't use bricklayers' mortar. Use Portland cement and sand. It will be stronger and more waterproof. If mortar is your only alternative, it will work, but I prefer cement.

Now tilt or slide the window onto the shims. Take two other shims and place them between the top of the window and the jamb. Slide them until they are just snug. Check the window for plumb and square. You are also looking for equal reveal side to side. You want the window parallel with the inside and outside walls, not twisted.

CLICK HERE to get FREE & FAST BIDS from local glass-block window installers. Ask about venting windows.

Massive Quantities of Mortar

Use rubber gloves, small trowels, sticks, whatever it takes to pack the cement mortar around the window. Fill any pesky gaps at the bottom. Don't put any mortar on the top of the window. Downward pressure or stress over time can crack these windows. The top gap can be successfully filled with caulking tomorrow.

Your mortar may be too wet to tool as soon as you install it. Don't worry. Within a short period of time the mortar will begin to harden. You can tool it with an old spoon, a stick, or with your fingers. Use a damp sponge to remove mortar smears from the glass.

Shim Removal

Usually within about two hours, you can gently remove the shims. Apply pressure against the glass as you pull the shims. Immediately, fill the bottom holes with cement mortar. Blend it in to match the texture and tooling of the rest of the joint.

Caulk the Top

Don't try to caulk the glass on the same day it is installed. I get the best results waiting 24 hours. Don't try to fill the entire space with caulk! Use a foam caulking backer rod to get within 1/2 inch of where the caulk joint will finish out. Use 100 percent silicone caulk. Wipe up smears immediately with lacquer thinner.


"Dear Tim,

I was looking at info about building glass block windows and you were talking about using lacquer thinner to clean up silicone residue. I thought I'd e-mail you to offer the suggestion that the next time you are using silicone you try denatured alcohol. My tile man taught me this and it works really really good! Probably less toxic too! I share your interest in building...I live in what was the Hartwell fire department at the turn of the last century and remodeled it into something a little more modern. " - John Figueras, Cincinnati, OH

Dear John,

Thanks for this tip.

Author's Note: We've received other questions with similar problems or questions. Here's one from Lena K. of Rockville, MD, regarding basement replacement windows.

"Our basement windows leak air as well as water when snow melts. The house was built in 1965 and the windows seem to be from that time. They are below the ground level with wells dug out around them. They are set directly into concrete blocks. I looked at different replacement window manufactures and installers, and none of the ones I saw offer specific windows for basements. Do you have any suggestions? (We aren't planning on doing the replacement ourselves). Thank you."

CLICK HERE to get FREE & FAST BIDS from local glass-block window installers. Ask about venting windows.

Companion Articles:  Glass & Acrylic Blocks Do Many Things, Glass Block, Glass Block Manufacturers, Glass Block Greenhouse

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7 Responses to Glass Block Installation

  1. I'm going to be doing glass blocks for my basement windows and it's my first time doing so. The question that I had was that the mesurements for the windows is 31 and 1/4 height and 31 and 3/4 width when I mesure them to put in they seem to be short on the height and I was wondering on how to fix that issue.

  2. Hi, Is there a guide on how long such glass block windows can be? I have a 3355mm long x 625mm height opening currently fitted with rotting windows and frame. I like the idea of glass blocks but I know that they are not load bearing. There is a large concrete lintel but I guess that there is always a bit of downward load particularly over such a distance. If it is too long I could break it up with a brick pillar 1 in the middle of 2 at 1/3-2/3. Is there any guidance please?

  3. I had my basement windows (above grade) replaced in 2/2014. They left the complete metal frame in the wall and placed the glass block windows inside the frames and mortared them. I am now in the process of finishing my basement and noticed that the metal frames have condensation (water and also ice closest to the window). I thought it was standard practice to remove the metal frame when installing. I hadn't paid attention to this detail until now (Due to the basement finishing project). Were these windows installed correctly or should have the metal frame been removed? What can I do now to prevent the condensation and ice as I have had my entire basement insulated with spray foam and had drywall walls put up and around these windows...I am afraid the condensation will cause water and/or mold issues. Any suggestions?

    • Ouch! You should have thought about this before the job was bid. There's no easy fix at this point. What a darn shame.

      Seriously, I can't think of an easy way to remove the inner steel frame without a serious amount of work. Yes, the steel should have been removed BTW.

      Do me a favor please. Before you ever start to do any future job, no matter what it is, please come here and pound on my search engine. Read as many columns as possible. Watch any relevant videos.

      If you can't find what you need, I can call you on the phone. This product I have will turn out to be the BEST MONEY you spend on the project. Guaranteed.

      http://shop.askthebuilder.com/products/15-Minute-Phone-Conversation-with-Tim.html

  4. I had four glass block windows installed in my basement and there was supposed to be caulk around the perimeter. I noticed that when I touch the mortar on the Windows dust comes off and the mortar is starting to separate. I am fairly certain that there was no caulk. There was also supposed to be backer rod, is there a way for me to visually tell if that was installed or would it be underneath/behind the mortar typically?

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