Glass Block Window in an Exterior Wall
Retrieved from Get Up! And DIY (
Introduction [ edit this section ]

Here is the finished glass block window seen from the outside of the house.
Here is the finished glass block window seen from the outside of the house.
Our upstairs bathroom had a regular window in the shower area.  Here's a list of all the reasons why this isn't a great idea:

  • Bad for privacy (can't have blinds in a shower).
  • It wouldn't pass inspection these days because it lacked tempered glass.  This is against code in showers because if the glass breaks (elbow hits it or something) it would create big shards of glass.
  • Being an old standard window, the frame was wood which would likely be moisture problems in the future
  • Old windows are not the best for insulation so it would result in a cold, drafty shower

We decided the best idea was to replace it with glass block for added privacy and safety.  This project describes how to install a 4 block by 6 block window using the ProVantage glass block system.  This system is developed by Pittsburgh Corning and is available at some hardware stores like Home Depot.  You can also get it online at various sites.  

Glass block windows are traditionally done in mortar like a brick wall.  We decided to go with the ProVantage system instead of a mortar window for several reasons:

  • Mortar creates really thick seams between the blocks.  The ProVantage system comes with spacers which makes the seam as small as possible and looks a lot better.
  • I have heard that it's difficult to do a mortar glass block system.  Although it doesn't seem like it would be that difficult, I decided one less thing to learn is better.  Using the provantage system is still time consuming, but the end result is great.
  • The instructions are pretty straight forward for installing the glass block system.  This meant less trial and error and less time researching how to get the job done.

Price : We were quoted $900 to install the glass block window (using mortar).  Doing the window ourselves cost about $250 and the provantage system is an easier alternative to learning all the mortar techniques.

Tools and Supplies
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Supplies needed:

  • Safety glasses
  • Glass blocks (we used the "iceberg" pattern for maximum privacy)
  • 1 inch screws (to attach channels to the frame)
  • Lumber planks for making the window frame.  1x8 planks are adequate.  We used pressure treated lumber for added weather protection.
  • Grout sealant
  • White paint (just a small amount is needed for covering screw heads and any other noticable objects the channel)
  • latex or rubber gloves (for working with sealant and grout)
  • 1 or 2 cans of expanding foam (for insulating the gaps around the window frame)
  • 5-gallon bucket (for cleaning glass blocks)
  • Big sponge (for cleaning glass blocks)
  • Paper towels (for cleaning glass blocks)

ProVantage System components:

  • Channels (enough to fit around the perimeter of the glass blocks)
  • Vertical spacers (one for each vertical seam between glass blocks)
  • Horizontal spacers (we had 6 rows so we needed 5 horizontal spacers.  If your window is wider you may need additional spacers as they only come in one length)
  • Grout
  • Anchors
  • Sealant

Tools needed:

  • Drill
  • Level
  • Rubber float (for spreading the grout)
  • Table saw or circular saw (used to cut down the frame lumber to the right size)
  • Miter saw (useful for cutting the frame to size)
  • Hack saw or Jig Saw (to cut the channels and horizontal spacers)
  • Reciprocating saw (sawzall) (for cutting the old window frame out)
  • Utility knife (for cutting sealant out of the seams after curing)
  • (optional) Tweezers (useful for picking out sealant from the seams)
  • (optional) Two suction cups with handles.  We found ours in the tile section of the hardware store.  These are especially useful when installing the final two glass blocks at the top.
  • (optional) Soft rubber mallet.  This can help to get the blocks into position.  Don't hit them too hard!
  • (optional) Mixing paddle for a drill.  This is used for mixing the grout - way better than mixing by hand.  I found mine in the hardware store around the drywall tools.
Removing the Old Window
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Our old window used the big iron counterweights in the wall to help balance the weight of the window.  These days windows are made with lighter materials and a tighter fit so they support themselves by friction alone.  The ropes connected to the counterweights often get painted over which makes it more difficult for them to operate effectively.  Also they need a big space in the wall to move up and down - this creates a big air gap in the exterior wall which should really be filled with insulation for energy efficiency.

  1. Start by removing the window sections from the inside.  Cut the rope attached to the counterweights so you can grab those later.  There should be an access panel on the side of the window frame.  It might be hard to find if it has been painted over several times.  If you cant get at them easy just leave them for now because the whole frame is coming out anyways. 
  2. Unscrew the storm window on the outside.  Since we were on the second level it was easier for us to just pull it inside rather than try to carry it down the ladder.
  3. Insert the blade of a sawzall in the seams between the window frame and the rough opening.  Run the blade around the sides to remove any nails or screws holding it in place.  It should be easy to pull out at this point.
  4. It's a good idea at this point to attach a tarp or plastic sheet to the outside.  Since it was winter when we did this we wanted the plastic sheet there to prevent the cold from getting in, but even in the summer it's a good protective measure in case a surprise rain storm appears.
Creating the Frame
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The glass blocks need a frame which will perfectly fit the end product.  This will get inserted into the rough opening of the wall and your glass blocks will go inside.  We used pressure treated lumber for added protection against moisture.

The booklet which comes with the ProVantage system tells you the exact width and height of your frame according to how many blocks you're using.  Create your frame according to this size.  For our purpose we had 8 inch blocks, 4 blocks wide and 6 blocks tall.  This amounted to a width of 32 and 1/4 inches and height of 48 inches.

The bottom piece of the frame is called the sill plate.  This needs to be cut in a special way to allow rain to fall off correctly.  The outer edge extends out past the window opening.  The extra material is also wider than the opening and this creates a lip which wraps around the outer edge of the window frame.  There is also a drip channel cut into the bottom close to the outer edge.

Dry fit the pieces in the rough opening before putting it all together.  Once you're satisfied that it will all fit, screw the frame together and insert it into the opening.  Add shims to make it snug.  Check the top and sides with a level and adjust the shims as necessary.  Attach the frame to the rough opening using 3 inch screws or whatever will reach to the studs in the rough opening.  If you're using pressure treated lumber make sure you use exterior grade screws to protect them against the chemicals in the wood.  They are usually galvanized or have some other protective plating or coating.

Add insulation where needed.  Specifically, use the cans of expanding spray foam to fill the gaps between the frame and the rough opening.  When we pulled the lath away from one of the walls we found a big space where there wasn't much insulation at all so we added some batting in this area.

I used a weather wrap around the edges of the frame to further protect against weather.  The one I got was called "Protecto Wrap" but there are others.  It's basically rubbery tar paper with one sticky side to apply to the surface.  Since we were installing this window in a wet environment (the shower) we also used the wrap around the inner edges to prevent against water coming from the shower area.  Maybe this was overkill to wrap it up like a christmas present, but it's a cheap and easy step to provide a little extra protection.  
Note : Make sure the wrap isn't too cold when you apply it or it won't adhere and latch on properly.  We used a work lamp to heat up the strips before applying them.

Adding the channel
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There are two ways to install the ProVantage glass block system. 

  • Use channels around the outside perimeter of the blocks.  These channels are held in place against the frame with screws.  They form an inner frame which holds everything in place. 
  • Use anchors on all the horizontal spacers.  These anchors are little L shaped brackets that get attached to the frame with screws.  If you don't like the white border around outside from the channels you may want to just use the anchor system.  If you put interior trim around the perimeter to hide the channels then this might not be an issue.

We decided channels were good because the cement board and tile going over the wall can cover up all of this.  In the end we actually decided to use a combination of both methods just for added support.  We put the channels around the outside then used anchors to hold the horizontal spacers.  This isn't necessary, but the anchors are cheap and we decided this would provide the best possible support in case someone leans up against it in the shower.

Using a jig saw or hack saw, cut the channels to the right size according to your opening.  If you want the cleanest looking corners you can make 45 degree cuts on the corners, but we decided to just cut them at right angles and but them up against each other because after we apply cement board and tile around the edges it will hide the seam anyways.

When you're ready to put the channels in place, apply a bead of sealant along both edges of the channel.  This will glue them in place and create a weather tight seal against any possible drafts.  Screw them in place with some small screws (we used one inch long screws).  You need to drill two holes at the top and bottom of the channels to get the edges properly attached.

For the top channel you need to cut it in half lengthwise after sizing it to the space.  The instructions say you can cut it with a utility knife, but I instead used a jig saw with a fine, thin blade because it seemed easier.  One half is screwed in place on the back side (the side facing the outdoors).  Keep the other half of the track handy.  It will be added after you finish placing the glass blocks.

Keep in mind that everything on the channel is visible through the blocks.  This is not as much of a problem with this frosty "iceberg" pattern we used, but you can still make out a dark object like a screw head if you're looking through the glass block.  The channels are white to give the best light reflection so the screw heads can be seen as a dark spot on the otherwise seamless channels.  For this reason it's a great idea to apply some white paint to the screw heads and any other material that is not white.  If you are butting the channels up against eachtoher this will create an opening at the edges where you can see the wood frame behind.  Apply some white paint here as well.  It doesn't have to be perfect - just make sure it looks white.  Keep the paint handy for later to cover the metal anchor brackets (if you intend on using them).

Placing the Blocks
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  1. Dry fit the bottom row to make sure everything fits properly.  The blocks at the far right and left get inserted into the adjacent side channel first then pushed down into position.  Once you are satisfied with the dry fit take the blocks out.

  2. For the first row of blocks run a bead of sealant along both edges of the bottom channel. Sealant is not needed on the side facing the side channel.
  3. Apply a bead of sealant along side glass block where the spacer will be applied. 
    Useful Notes:
    • Always apply sealant to the glass blocks instead of the spacers.  This prevents you from applying it too close to the edge which will result in sealant squeezing out into the seam.
    • On the glass blocks, only apply sealant behind the raised edge.  This helps prevent it from squeezing out into the seam.
    • Don't apply too much sealant or it will squeeze out of the edges and show up in the seams between the blocks.  Some will likely get squeezed out, but try to avoid it since it will need to be removed later.  Don't try to wipe it away - it's much cleaner to cut it out with a utility knife after it cures.  If you smear it all over the seam there will be nowhere for the grout to adhere to later on.  
  4. Position a vertical spacer against the glass block.
  5. Get another block and apply a bead of sealant along the inner edges on one side and slide it against the spacer.  
  6. Continue this process until the last block.  On the last block don't apply any sealant.  Just slide it into position into the channel.  The surrounding elements provide all the necessary support.  The last block is on alternating sides for each row to gain strength on all sides.
  7. Check the blocks with a level at this point.  I had no problem with level - the spacers and track system pretty much automatically makes everything level.
  8. Cut a horizontal spacer to the size of the opening.  It should be long enough to fit inside the side channels.
  9. If you are going to use the metal anchors for added strength, insert these into the ends of the horizontal spacers.  Dry fit the spacer on top of the blocks and drill holes where the screws will be inserted.  Be careful to brush away any pieces of plastic or wood resulting from drilling the holes.
  10. Apply a bead of sealant to the top of all of the glass blocks of the first row (behind the raised edge).
  11. Apply the horizontal spacer onto the top of the first row and push it in place.
  12. Start laying blocks for second row but this time start from left side (assuming you started from the right side for the first row).  
  13. This time for each of the inner blocks you will apply sealant to both the bottom and the side of the block (the side facing the vertical spacer which was just inserted).  On the final block add sealant only to the bottom of the block and slide it down through the channel.
  14. Keep laying blocks using this procedure until you get to the final row.

  15. On the final row first apply a bead sealant along the top inside edge of the half channel.  The blocks will press up against this.  Slide one block against the right channel and one block against the left channel.  Insert the remaining blocks until you get to the last one.  On the final block do not apply any sealant to the final vertical space.  The spacer will fit into this space completely dry.  It seems strange but it will get sealed up by grout later on.  To help position this last block attach a suction cup onto the two adjacent blocks.  Slide the vertical spacer into the last seam.  Using the suction cup handles, push the two blocks against this spacer so the gap is the same width as everywhere else and aligned with the gaps below.
  16. Apply a bead of sealant to the frame above the top row of glass blocks.  This is where the other half of the channel will fit.
  17. Apply a bead of sealant to the inner edge of the remaining half channel.
  18. Push this half channel into position.  If it doesn't fit tight then use some tape to keep it in position until the sealant cures.
  19. Wait 24 hours for the sealant to cure.
Glass blocks all in place.
Glass blocks all in place.
Glass blocks in daylight.
Glass blocks in daylight.


Grout the Seams
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Grout is used on the seams between the blocks to add strength to the system.  The system also allows you to use sealant on the seams instead of grout, but they don't recommend using sealant in wet environments.  Since ours is a wet environment (the shower) we used the grout.  Personally I think it looks better with grout anyways.  

You apparently need to use the grout that is supplied with the system.  That's what we did, but I imagine you could also use standard grout with a latex admixture mixed in.  There wouldn't be much price benefit though.  The standard tub of grout covered our 24 block window with some to spare.

  1. Before worrying about the grout, clean up the seams on the window.  Use a utility knife to slice out any visible pieces of sealant that squeezed into the seams.  A tweezer is helpful for pulling out small pieces.

  2. Take the bag of grout out of the container and split it roughly in half.  Half will be applied to the inside and half to the outside.  The instructions state that the whole bag needs about 4 cups of water mixed in for proper consistency.  Since we split the grout in half we added 2 cups for each half.
  3. Add the half bag of grout into the supplied container
  4. Add 2 cups of water.
  5. Mix it up with a paddle attached to a drill.  If you don't have a drill paddle then a stick will work.
  6. The grout should be thick and fairly smooth like sandy mashed potatoes.  If it's too coarse add a bit more water and keep stirring until it gets to the right consistency.
  7. Wait 10 to 15 minutes for it to set up a little bit.

  8. Apply the grout into the seams of the window using a float.  The standard method is to push against the seams at a 45 degree angle using the float.  I found that the glass blocks are too slippery for this method - the float pushes against one block and pulls the grout slightly away from the other block.  I just kept pressing it at different angles until it was adequately filled.  Do the best you can to get it pushed into the seams and fill them up.  
  9. Go over the surface of the blocks using a damp sponge to wipe off the excess grout.  Frequently clean out the sponge in a bucket of water so it doesn't have too much grout stuck in it.  This will make the final clean up easier.
  10. Let the grout set for about 15 minutes
  11. Wipe everything down again with a damp sponge using clean water to frequently rinse out the sponge.  
  12. Let the grout set for at least 4 hours.
  13. Clean up any haze on the glass blocks using some paper towels.  Alternate between damp and dry paper towels for the best result.  The damp the paper towel helps clean off the grout residue while the dry one can be used to do a final cleaning of the surfaces.
  14. Apply grout sealer to the grout lines.  It helps to use a paper towel to wipe the glass surfaces down as you apply the sealant.  This prevents it from drying onto the glass.  Apply at least two coats of sealant.

  15. Let the grout cure for 72 hours.
  16. Let the grout cure for 72 hours then apply a thin bead of sealant around the outer perimeter where the channel meets the glass block.
    Let the grout cure for 72 hours then apply a thin bead of sealant around the outer perimeter where the channel meets the glass block.
    Apply a thin bead of the silicone sealant to all the perimeter joints where the glass blocks meet the channels.  This is the same sealant used to glue the glass blocks together.
Exterior Trim
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When you removed the window you likely also had to remove some trim around the outside.  This trim hides the seams between the rough opening and the window frame.  Unless you think you can salvage the old trim you'll need to replace it.  Since the window is probably not the same size you'll likely need new ones.  This is easier than creating the frame.

Before the exterior trim is added.
Before the exterior trim is added.
After the exterior trim is added.
After the exterior trim is added.
  1. Before creating the trim, caulk all the underlying seams with silicone sealant to prevent any moisture from entering the wall under the siding.
  2. Measure the opening of the frame.  
  3. Windows are all a bit different.  In my case it's an old window on a stucco wall.  It had fairly wide trim pieces.  Get some appropriate size lumber planks to fit over the exposed surface.  I used the same 1x8 lumber which was used for the window frame.  
  4. The side trim should be long enough to reach the top edge of the window.  The top trim piece should be as wide as the window plus the width of the two side trims.
  5. To make things easier you can paint the trim before adding them to the outside of the window.  This way you don't have to stand on a ladder trying to paint outside.  Since it was winter at the time it helped to paint these inside the house to let them dry properly before bringing them outside to apply them to the window.
  6. Tack the trim in place with some finish nails.  6d size nails are adequate for this.
  7. Caulk all the seams using a good silicone sealant.  If your trim is white like mine it's easy - just get white sealant.  If you can't find a sealant of the correct color to match your trim you might have to paint over the sealant after it cures.