Replace a Window with a Glass Block Window: 4 Tips

Replacing a single pane window with a glass block window is not as difficult as it sounds. Here are a few tips to get you started.

1. Take Measurements and Make Plans

Measure your window opening. From that, figure out what size glass block you should use and how many you will need to complete the project. Remember to take into account the 1/4- to 1/2-inch gap that is required on all sides of the glass block window for the mortar. Glass blocks cannot be cut, so you need to be exact with your measurements.

If you are replacing more than one window, you have to take the measurements of each one. They may seem the same size but they actually aren't. Over time wood shrinks and expands. If you base the plans for all of your glass block windows off of the measurements for a single window, chances are the windows won't turn out right and you will have to start over.

2. Preparing the Opening

Put on protective gear and pull out the old window using a pry bar and a hammer. There are two basic types of openings: wood frame and masonry. To prepare a wood frame opening, brush the open wood with a wood preservative. When you put in the glass block window, remember not to use mortar in between the wood frame and glass block. You will be using wood molding to fill the gaps.

For a masonry opening, place a level on the bottom of the window. Level out any high spots using a chisel and a hammer. Low spots will be filled by the first mortar layer.

3. Use a Pre-Assembled Glass Block Window

Laying glass block is much like laying brick. There is not a lot of room for error and since glass block is transparent, mistakes will really show. Consider using a pre-assembled glass block window instead. First, once you are sure the window will fit, place some shims on the window sill at least 3 inches away from the corners. Then you center the window on the shims and fill the bottom gap in with some mortar. Pre-assembled windows are heavy, so have a second person on hand to help you lift it. Next put shims in the side gaps and check to make sure they match the bottom gap. When that is done apply mortar to the sides, removing the shims as you go. Finally, with a caulk backer rod and a silicone seal fill in the top of the window. All that is left is to clean up any excess mortar and silicone.

4. Use a Mortar-Less Glass Block Window Kit

Mortar is messy and takes a lot of time to dry. There are mortar-less glass block window kits on the market and when finished it looks just as good as a glass block window installed using mortar. First you screw mounting channels into the window opening. Then you do a test run with the glass blocks to make sure they fit. When you are ready, you start placing the blocks in the opening and use plastic connectors to hold them together. You continue until the opening is filled. The final step is to seal everything with silicone.

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