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You need to determine the proper measurements in order to allow the space required for the blocks, mortar joints, expansion foam and the finish trim. If you need to change the size of your rough opening, check the inside of the wall for electrical wires before making any modifications and make sure you don't cut through load-bearing studs. In this project, you'll notice that 4 inch-wide glass blocks were added in the middle of the 8-inch glass blocks in order not to resize the original window opening (shown in the image below). They also complement the vertical side windows.
Using a circular saw, cut 2 x 4's to the proper size and place them in the opening for the side windows; secure them to the wall with 2-1/2" wood screws. Use the lines that you drew on the sheathing to make sure the blocking is level and plumb as you put each piece in.
You may have to build up the frame around the main window to the proper dimensions to accommodate the glass block so that the grout lines on the center window match the smaller side windows. Add 1x 4 or 2x4 blocking to the top or the bottom to get the grout joints to line up on all of the windows.
Take the measurements of the rough frame openings for the side windows. Using a circular saw and a table saw, cut the interior finish trim pieces out of 1 x 6 pine. Slide it into the opening of the 2 x 4 frame, making sure it is flush to the exterior and interior walls and that it fits snugly and is level and plumb. Nail it in place using 2-1/2" finish nails. Repeat the same process for the main window.
After all of the finish trim is in place, prime the bottom sill. Priming the sill will keep the wood from absorbing the moisture of the mortar when you begin setting the glass block. You can prime the jambs and the top trim piece as well, but it's not critical as expansion foam will be used to fill the gaps between the glass block and the sides and top.
Mix up a batch of mortar in a wheelbarrow. Glass blocks don't absorb moisture like regular bricks, so the mortar needs to be dryer than normal, about the consistency of putty. Place a bed of glass block mortar on the window sill and insert spacers into the grout (Image 1). Set the blocks on one at a time. Do not butt the blocks up to the sides of the frame because you need a small amount of room for the expansion foam up the sides and across the top. As each block goes in, mud one side of the block with mortar and set it beside the previous block. Press the block into the mortar until it rests on the spacers (Image 2).The plastic spacers are not absolutely necessary, but they'll make it easier to maintain even mortar joints on each row and column.
After the first row is installed, insert the expansion foam between the block and the pine trim. Let it dry completely, then cut the foam off at the top of the block.
Place mortar on the top of the first row of blocks. Bend a metal panel anchor at a 90-degree angle and press it into the mortar. Place the panel anchors every 3 rows, beginning with the top of the first row. Attach it to the side of the trim with 1 1/2-inch screws.
Begin the second row of blocks, inserting the spacers and setting the blocks in place just as before. Add mud to one side of the block set it snugly against the previous one. Continue this process until you have only one empty row left at the top; then place the expansion foam all the way up the sides and insert the panel anchors where needed.
Once you get to the top, you'll need an additional person to help on the outside of the window. Start the top row with one block at the end and then go to the opposite end and the set the blocks coming back toward the center. You need to meet in the middle for the final block to go in properly. Insert expansion foam along the top of the blocks.
The side windows go in exactly the same as the center window. It's a good idea to have someone help you with the smaller windows because you can't reach around as easily as with the larger window. After the blocks are in, break off the spacer tabs and tuck and point the joints and corners using a tucking tool or even a small spoon. Smooth out the joints to an even depth.
On the exterior, you need to prime the window frame before you caulk the perimeter. Be sure to use glass block sealant caulk. This type of caulking will seal all of the cracks between the glass and frame and allow for expansion and contraction for this type of glass block window. Use the same caulk on the interior.
You may need to remove some siding in order to see the edges of the window. Cut the siding 2-1/2 inches from the window frame (Image 1). Install brick molding on the edges of the window, then nail the "J Channel" (used for vinyl siding) along the side of the brick molding (Image 2). For other types of siding than vinyl siding used in this project, this process will vary slightly depending on the material.
When installing vinyl siding, leave the nail heads up just slightly so the siding has a little play in it. This will keep it from buckling as it expands and contracts with the weather. Nail and set the vinyl starting from the bottom and working up (Image 1). Use a siding removal tool to get under the edge of the siding and slide it back into place (Image 2). Make cuts along the narrow edges with a pair of snips and make sure the siding is level before nailing it. For wood or cementious siding, the process is similar but will require different tools. Make sure the siding is level to the other siding and gaps between pieces of siding are minimized.
Caulk and fill the nail holes and paint the trim. On the inside wall, paint the inside edge of the pine trim. Trim out the windows with 2-1/4" inch case molding. Install the molding with 2-1/2" inch finish nails, leaving a 16th of an inch reveal. Once the molding is up, fill in the nail holes, caulk along the edges and corners and give the trim a final coat of paint.