Best Practice for Retrofitting Glass Block

May 5, 2012 9:16 AM

Opening in concrete block wall doesn't evenly divide by glass block sizes. Now what?

Construction-knowledgeable mefites, I have a home built in the 40s with a 39.5 x 44.5 opening in one of my basement rooms. This is close but not quite for glass block. What's best practice here?

Additional info: Planning on using Pittsburgh Corning loose glass block (local!) that I'll mortar together. Not trying to make an architectural statement here. Trying to deal with a broken pane on the old window.

Piggyback question: do I need to put a vent in the window? I'm leaning towards not putting a vent in.

posted by bfranklin to home & garden (12 answers total)

Pittsburgh Corning's site says that the glass block is 8x8x4 - can you clarify if that's the actual dimension or the nominal dimension? Because a typical concrete masonry unit (CMU) will be listed as 8x8x16, but will actually measure 7 5/8" x 7 5/8" x 15 5/8" - the 3/8" is made up by the mortar joint. If the glass block follows the same convention (and it would be incredibly stupid for them not to, since they're specifically meant to work with standard CMU sizes), then you might be okay on at least the 39.5 dimension (you may be able to eat up a 1/2" in mortar thickness if you need to). For the 44.5, maybe they have glass half-blocks?

Actually, if you look here (pdf file), the actual size is 1/4" less than the size stated (the nominal size). There are some styles of glass block that have 4x8 face sizes, so if your complete window ends up being five 8" units wide x five 8" units and one 4" unit high, you may be OK.
posted by LionIndex at 9:29 AM on May 5, 2012

Basically, for your 39.5 dimension you're looking at 5 blocks with 1/8" mortar joints between them and the existing opening.

The other way, you'll have 5 whole blocks, one half block, and slightly larger than 1/4" mortar joints.

You may want to check with Pittsburgh Corning to see if the 1/8" joints are acceptable for them. I would guess that they would be if the 39.5 dimension is horizontal since it's pretty common to do a curved wall with glass units by altering the thickness of the joints, but it would be better to hear it from them.
posted by LionIndex at 9:44 AM on May 5, 2012 [1 favorite]

You might want to check if having a vent is required by code. I think they're ugly, but I would probably install one if I were re-blocking a window. Also, I'm assuming this isn't in a bedroom or if so there's another egress window?

If the blocks+mortar don't come to a nice round number, can you use those flat, thin cinderblocks to make up the difference?
posted by MonsieurBon at 9:47 AM on May 5, 2012

You can always frame it in to the size you need as well. The opening will be smaller, but you can make it the exact size you need.

They also have glass block vents available.
posted by Vaike at 9:53 AM on May 5, 2012

If the broken pane is the reason for switching to glass block, why not fix the old window instead?
posted by jon1270 at 9:58 AM on May 5, 2012

You may run afoul of fire code requirements for basement egress.
Repairing the broken pane is probably the safer option.
posted by the Real Dan at 12:05 PM on May 5, 2012

LionIndex: Yes, those sizes are nominal.

Monsieur/Real Dan: Have another window and a door for egress. Safe under code. The basement exits at grade at the rear of the house.

jon1720: Original 1940s steel frame window with 0 R value. It's overdue for an upgrade.
posted by bfranklin at 12:26 PM on May 5, 2012

I just used some bricks to edge around the window. I used grey bricks the same thickness as the glass blocks and it worked fine.
posted by gjc at 1:46 PM on May 5, 2012

Original 1940s steel frame window with 0 R value. It's overdue for an upgrade.

Glass block has a low R-value. It's better than single pane glass but that's about it. If efficiency is one of your targets there are modern double pane windows (with argon fill, low-E coatings, etc) that handily beat glass block.
posted by 6550 at 2:26 PM on May 5, 2012

Yes, those sizes are nominal.

Yeah, I figured that out. The dimensions I gave on the mortar joints and stuff are based on actual sizes.

Have another window and a door for egress. Safe under code. The basement exits at grade at the rear of the house.

You need a window (or door) that can function as an exit directly to the exterior, not a door to the interior of the house and a window. You do need ventilation to the room, but if you've already got another operable window in the same room you may be OK.
posted by LionIndex at 2:40 PM on May 5, 2012

Yes, the second window functions as egress directly to my rear patio. I am comfortable with the requirements of fire code in my area and am confident that converting this window to block will not present an issue.

6550: Pittsburgh Corning manufactures high efficiency block.
posted by bfranklin at 3:01 PM on May 5, 2012

The R-value of their most efficient glass blocks is well under that of the cheapest double pane window available at Lowe's. I love the look of glass black walls and windows and I truly wish it was more efficient, but the conclusion I came to is that it simply is not a good choice in any climate with a significant heating requirement.
posted by 6550 at 3:23 PM on May 5, 2012