You want light in a dark hallway, a landing or a bathroom, but you do not want an open doorway. Then a glass block wall is exactly the answer: it lets light through and keeps sight and sound out.
Short and concrete: you build a glass block wall as a free-standing, non-load-bearing panel inside an opening. You fix a U-channel all the way round (40-80-40-3 mm, for example) with a felt strip inside it, lay the blocks in stiff cement mortar with joints of at least 10 mm and no more than 30 mm, bed reinforcement bar (around 6 mm) into the joints at no more than 65 cm apart, and leave an expansion joint at the sides and the top so the wall can move. Work no more than about 1 metre of height a day, and never lay in temperatures below 5 degrees Celsius.
The standard block is 19 x 19 x 8 cm. With a 10 mm joint the module size is 20 cm, so reckon on 25 blocks per m²
and roughly 30 kg of mortar per m² at a 10 mm joint thickness.
Sounds manageable? It is. But there are a few traps that people walk straight into: letting the wall carry load, leaving no expansion joint, building too high too fast, and thinking you can just cut a block down to size. Below I go through the lot.
Can I build a glass block wall anywhere?
No, and that is the most important rule of the lot. Glass blocks are non-load-bearing. They must not take any structural load. They always stand free inside an opening, with room all around them.
In practice that means:
- You must not rest floor joists, a roof deck or masonry above on top of them.
- Want to replace a load-bearing wall with glass blocks? Then a proper lintel or steel beam has to go over the opening first, carrying the load out to the sides. That calculation is a job for a structural engineer, not for a rule of thumb from a blog.
- There should always be an open expansion joint above the wall, so the structure above can deflect without pressing down on your glass.
Beyond that, a non-load-bearing internal partition inside your own home usually needs no planning permission, as long as you change nothing structural and nothing on the outside of the building. Put a glass block panel into an external wall or take out a load-bearing wall, and you are into Building Regulations territory and will normally need approval and a building control notice. Check it with your local authority before you start. It costs you half an hour and saves you an enforcement letter.
Worth knowing for a bathroom too: glass blocks do not absorb moisture, so as a partition or shower wall they are excellent. But they do not replace ventilation. A bathroom still has to meet the ventilation requirements; a glass wall with no vent or extractor only makes a damp problem worse.
Mortar or glue? The two methods compared
This is the question most articles skip, even though it decides how the whole job goes. There are two ways.
| Laying in mortar | Glue system with plastic spacer strips | |
|---|---|---|
| Look | Classic, wide joint (10 mm), grey or white | Crisp, narrow joint, very even |
| Difficulty | Moderate, needs a bricklayer’s feel | Easier, close to foolproof |
| Speed | Slower, about 1 m per day at most | Faster, but still curing time per metre |
| Reinforcement | Needed, steel bar in the joints | Built into the strip system |
| Suitable outdoors | Yes, with a waterproofing additive | Only if the manufacturer allows it |
| Strength | High, proper masonry | Ample for internal walls |
| Correction possible | Yes, while the mortar is still wet | Barely, the glue grabs quickly |
| Material cost | Lower | Higher (the strip kit) |
My advice: if this is your first time and it is an internal wall up to about 2 metres high, take the glue system. You get even joints automatically and the chance of the whole wall coming out skew is far smaller. Working outside, building something bigger, or after that classic laid-in-mortar look? Then mortar.
The rest of this article is mainly about laying in mortar, because that is the method with the most detail in it. I cover the glue system separately further down.
Which size of glass block should you choose?
The most common block is 190 x 190 x 80 mm. But there is more choice than people think.
| Size (mm) | Module size with 10 mm joint | Blocks per m² (approx.) | Typical use |
|---|---|---|---|
| 115 x 115 x 80 | 125 x 125 mm | 64 | Small panels, fine pattern |
| 190 x 90 x 80 | 200 x 100 mm | 50 | Half block, edges and courses |
| 190 x 190 x 80 | 200 x 200 mm | 25 | Standard, by far the most used |
| 190 x 190 x 100 | 200 x 200 mm | 25 | Heavier, better insulation and sound |
| 240 x 240 x 80 | 250 x 250 mm | 16 | Large panel, fewer joints |
| 300 x 300 x 100 | 310 x 310 mm | approx. 10 | Large façade panels |
Besides the size, you also choose:
- Clear (you can see through reasonably well, but distorted), cloud or raindrop (light through, view blocked), or coloured.
- Standard or insulating. There are blocks with an improved fill that get down to a U-value of around 1.1 to 1.4 W/m²K; an ordinary glass block wall sits nearer 2.2 W/m²K. For an internal wall that makes little difference. For an external wall it is a big difference, and even then a good window solution is often still better.
- Fire resisting. There are special fire resisting versions classified at 30, 60 or 90 minutes. An ordinary glass block wall does not achieve that classification automatically, so do not assume it does when you are dealing with a fire compartment wall.
- End blocks and corner blocks. If you want a free-standing end (a half-height wall projecting into the room, for instance), you need end blocks with a rounded finished face, or you cap the end with a stainless steel profile.
On sound: manufacturers quote airborne sound insulation for glass block walls in the order of 40 to 49 dB, depending on the type and thickness. That is fine for separating a hallway from a living room, but it is not a studio wall.
Important: buy all your blocks in one go, and ideally from a single batch. Colour and clarity can vary slightly between batches, and in a continuous panel you spot it straight away.
Materials and tools
Materials:
- Glass blocks (plus 5 to 10 per cent spare for breakages and mistakes)
- Purpose-made glass block mortar, or your own stiff cement mortar mixed at roughly 1 part cement to 3 parts sand
- For external walls: a waterproofing additive for the mortar
- Reinforcement bar, around 6 mm; indoors plain steel is fine, outdoors it has to be hot-dip galvanised
- Cold-rolled U-channel all round, for example 40-80-40-3 or 50-100-50-3 mm (matched to the block thickness)
- Felt strip or backer rod to go inside the channel
- 10 mm spacers
- Neutral-cure silicone (in bathrooms: a mould-resistant sanitary silicone, also neutral cure)
- Stainless steel screws and wall plugs
- Jointing compound or pointing mortar
Tools:
- Spirit level (long, at least 100 cm) and ideally a cross line laser
- Trowel, pointing trowel, mixing tub and a paddle mixer for the drill
- Rubber mallet
- Sponge and a bucket of clean water
- Drill, with hammer action if your plugs need it
- Tape measure, pencil, carpenter’s pencil, spirit level or laser
- Gloves and safety glasses
What you do not need but often see: a masonry saw. Do not go there.
Why you do not cut glass blocks
This is the mistake that goes wrong most often. A glass block is a hollow block: two half glass shells fused together with an air cavity between them.
Cut one through and:
- you lose the cavity, and with it the insulation and sound values,
- you are left with a razor-sharp, unfinished glass edge,
- and it often cracks anyway from the internal stress.
So solve it with your setting out. Work your panel out in modules of 20 cm (for 19 x 19 blocks with a 10 mm joint) and take up the leftover with the opening, the U-channel and the expansion joint. If it still will not come out, use half blocks of 190 x 90 mm, or split the leftover dimension evenly between top and bottom so it looks right.
A quick worked example. Your opening is 1.64 m wide. 1.64 m divided by 0.20 m is 8.2 blocks. So you go 8 blocks wide, which gives you 8 x 200 mm = 1600 mm of masonry, less the final joint. That leaves you roughly 4 to 5 cm, which you split left and right across the U-channel and the expansion joint. Perfect.
Step by step: building a glass block wall in mortar
Step 1: Measure and set out the wall
Measure the opening at every height. Old houses are rarely square. Work out the number of blocks across and up in module sizes, and mark the bottom course on the floor with a pencil or a line laser.
Check that your base is flat, clean, dry and solid. Loose surfaces, old paint and dust have to go. If the floor is not level, level it first; you will never pull a skew first course straight again.
Step 2: Fix the U-channel
Screw the cold-rolled U-channel round the opening: bottom, left, right and top. Use stainless steel screws and plugs that suit your substrate.
Lay felt strip or backer rod into the channel at the sides and the top. That felt is not decoration. It makes sure the wall is not locked in place there but can move. That is your expansion joint.
At the bottom the wall does sit fast in the mortar, because that is what it stands on.
No U-channel, or it will not fit? Then the alternative is a rebate at least 40 mm deep in the surrounding structure, with a wall tie into the side wall every two courses.
Step 3: Mix the mortar
Mix the mortar according to the bag. The right consistency is damp and stiff: squeeze a ball and it holds its shape, but it does not slump. Too wet and your blocks sink, too dry and the mortar will not bond.
Do not mix more than you can use in about two hours. Rule of thumb: around 30 kg of mortar per m² at a 10 mm joint thickness.
Do not lay below 5 degrees Celsius. Below that cement does not cure properly and you end up with a wall that never reaches its strength.
Step 4: Lay the first course
Lay a bed of mortar about 10 mm thick into the bottom channel. Set the first block into it, tap it down to height with the rubber mallet and check it with the spirit level in two directions.
Put spacers between the blocks. They do two jobs: they hold the joint at exactly 10 mm, and they stop the weight of the block squeezing the mortar out underneath.
Check after every block. Really: after every block. A millimetre out on course 1 is a centimetre out on course 10.
Step 5: Bed in the reinforcement
Reinforcement is not optional, it is the reason your wall stays up.
- Bed horizontal reinforcement bar (around 6 mm) into the bed joints. Space it as evenly as you can in the spanning direction, with no more than 65 cm between two bars. With a block height of 19 cm plus the joint, that works out at roughly every second or third course; better too often than too rarely.
- Put vertical reinforcement into the perpend joints as well, in practice every two blocks.
- Do not bend the bar tightly round the corner; run it on into the U-channel or the rebate.
- Outdoors: hot-dip galvanised steel. Indoors, ordinary reinforcement bar is fine.
Important: the reinforcement has to sit fully within the mortar, not up against the glass. Steel touching the glass creates stress points, and that means a risk of cracking.
Step 6: Build up course by course
Lay a fresh bed of mortar, set the next course, tap the blocks down to height, level across, spacers in between.
Stick to the build rate. A fresh glass block wall is top-heavy, and the mortar at the bottom is carrying nothing yet. Rule of thumb: no more than about 1 metre of height, or around 7 courses, per day, then leave it 24 hours to dry before you carry on.
Push on regardless and the bottom half slowly settles out of true, and you only see it when it is too late.
Step 7: Take the spacers out
Good quality spacers have a snap-off outer section. You take that out once the mortar has gone off but has not gone rock hard. The inner part can stay; it disappears under the joint.
Forget this and your spacer sits in your finished joint, and you will see it forever.
Step 8: Point the joints
Fill the joints completely with jointing compound and strike them smooth with a pointing trowel or a jointer. Work from the top down.
Clean the glass immediately with a damp sponge and clean water. Wait until tomorrow and the cement haze is stuck fast, and you will only get it off with an acidic cement haze remover. That stuff attacks the joint itself, so you want to avoid it.
Some suppliers advise leaving the final pointing of a mortar-laid wall for about two weeks, so the wall can cure and settle first. Always follow your own supplier’s instructions on that.
Step 9: Finish the junctions
Seal the side and top junctions with a neutral-cure silicone. Acetoxy silicone attacks the lime in the mortar and lets go after a year.
In a bathroom or shower: mould-resistant sanitary silicone, also neutral cure.
Important: that silicone joint is an elastic seal, not a glued connection. Fill the joint with backer rod first so the silicone only bonds on two sides and can stretch. Bond it on three faces and it tears at the first bit of movement.
If you have a free-standing end, finish it with end blocks or a stainless steel cap strip. Never leave a bare cut edge.
And what about the glue system?
With a glue kit you work with plastic cross and strip spacers that hold the joint at exactly the right size.
- Make sure the substrate is clean, flat and sound. Glue does not bond to dust, old paint or loose plaster.
- Fix the perimeter profiles with adhesive sealant.
- Clip or set the glue strips between the blocks. Apply the adhesive only at the contact points, not across the whole face. Full glue joints are not stronger, they just squeeze out and give you dirty edges.
- Here too, build about 1 metre at a time and then let it cure.
- Point or seal it, depending on the system.
- Finish the exposed ends with end blocks or stainless steel cap strips.
Upside: crisp, narrow joints and far less chance of a skew wall. Downside: you are tied to the manufacturer’s system, corrections are barely possible, and not every system is approved for outdoor use. Read the product sheet.
Glass blocks outdoors: the extra rules
Outside, the requirements are stricter, and for good reason: the wall stands in the rain, the frost and the sun.
- Hot-dip galvanised reinforcement. Plain steel rusts and forces the joint open. That is not a question of if, but of when.
- Waterproofing additive in the mortar. Ordinary masonry mortar soaks up water, and that water freezes.
- Never lay below 5 degrees. And preferably not in full sun either, because then the mortar dries out too fast and cures badly.
- Expansion is sacred. Glass expands when it warms up; the linear expansion coefficient of glass is around 8.5 · 10⁻⁶ per Kelvin. Small number, but across a wide panel warming up by 40 degrees it adds up. Trap the wall and it cracks.
- Break large panels up. Manufacturers specify expansion joints for panels of about 6 metres maximum. Go bigger than that and split the panel with an expansion joint or an intermediate mullion.
- Radiators and heat sources at least 25 cm away. Heating one part of the wall locally creates stress and cracking.
- Walking on it horizontally? That is a different job. Glass pavement blocks in a floor or roof are allowed up to a maximum permissible load in the order of 5 kN/m², but that really is a structural application. Have it calculated, do not do it by feel.
What does a glass block wall cost?
The figures are a sensible range and can vary considerably by supplier and type. Always run the numbers yourself with current prices.
| Item | Guide price | Notes |
|---|---|---|
| Glass block 19x19x8, clear or cloud | approx. £2.50 to £6 each | 25 per m² |
| Glass material per m² | approx. £65 to £150 | Coloured and insulating sit higher |
| Glass block mortar | approx. 30 kg per m² | Bagged, a few tens of pounds per m² |
| Reinforcement bar | A few pounds per m² | Galvanised costs more |
| U-channel | Per linear metre | Depends on thickness and material |
| Spacers, silicone, felt | Small money | Do not skimp here |
| Glue system (alternative) | Dearer than mortar | Per kit, per m² |
| Total doing it yourself | roughly £100 to £215 per m² | Heavily dependent on the block you pick |
| Having it installed | Labour on top | Get at least two quotes |
Compare that honestly. A wall of 1.6 x 2.0 metres is 3.2 m², so around 80 blocks and roughly £340 to £680 in materials. That is more than a stud wall, but you get light, sound damping and a wall that does not mind moisture in return.
The 10 most common mistakes
- Letting the wall carry load. Glass blocks are non-load-bearing. Full stop.
- No expansion joint at the top and sides. Then the structure presses down on it and the glass or the joint cracks.
- Building too high too fast. More than about a metre a day and your bottom courses settle out of true.
- Forgetting the reinforcement, or spacing it too far apart. Keep to 65 cm maximum.
- Reinforcement against the glass. It has to sit fully in the mortar.
- Mortar that is too wet. Blocks sink and the joints come out uneven.
- Letting cement haze dry on. Sponging off straight away with clean water saves you hours of fiddling.
- Using acetoxy silicone. It attacks the lime and lets go. Always neutral cure.
- Cutting blocks. Loss of insulation, sharp edges and a risk of cracking. Set out in module sizes.
- Laying in frost or below 5 degrees. The mortar never reaches its strength.
And a bonus mistake: buying your blocks from several different suppliers because the last five were on offer somewhere. You will see the colour difference, guaranteed.
Maintenance and cleaning
Good news: a glass block wall is nearly maintenance free.
- Clean it with warm water, a splash of general purpose cleaner and a microfibre cloth. Squeegee it dry to avoid limescale streaks.
- Do not use scouring powder or wire wool on the glass. Scratches never come out again.
- Old cement haze? A cement haze remover works, but protect the joints first and rinse thoroughly afterwards.
- Check the silicone joint every few years. Loose or discoloured? Cut it out, clean up and reseal with neutral-cure silicone.
- If you see a hairline crack in a joint, work out where it is coming from first. Often it is an expansion joint that cannot move freely, and in that case patching it up alone does you no favours.
Alternatives to glass blocks
Sometimes a glass block wall is not the best answer. Fair is fair.
| Alternative | When it is better | Downside |
|---|---|---|
| Channel glass (U-profile glass) | Large tall panels, crisp modern look | Dearer, installation by a specialist |
| Fixed glass screen in a steel frame | You genuinely want to see through | No privacy, far more expensive |
| Polycarbonate multiwall sheet | Light, cheap, quick | Yellows, scratches, looks less classy |
| Stud wall with a borrowed light | Cheapest way to share light | Less light, less character |
| Internal door with glass | The existing wall stays put | Light only when it is open, or with a glazed door |
Glass blocks are at their best in wet rooms, in a hallway or landing with no daylight, and anywhere you want light without being overlooked. Just want as much daylight as possible and privacy is not an issue? Then a window or a glazed door is almost always cheaper and more effective.
Frequently asked questions
How many glass blocks do I need per m²?
With the standard 19 x 19 x 8 cm block and a 10 mm joint, the module size is 20 x 20 cm, so 25 blocks per m². With 24 x 24 cm blocks it is around 16, with 11.5 x 11.5 cm blocks around 64. Always allow 5 to 10 per cent extra for breakages.
How wide should the joint between glass blocks be?
At least 10 mm and no more than 30 mm, according to the manufacturers’ installation instructions. In practice almost everyone works with 10 mm, because that is what the standard spacers are made for and it gives you tidy 20 cm module sizes to set out with.
Can I make a glass block wall load-bearing?
No. Glass blocks must not be structurally loaded and have to stand free inside an opening. If you want to replace a load-bearing wall, a lintel or steel beam goes over the opening first to take the load, and underneath that you set the blocks free with an expansion joint. Have that structure calculated by a structural engineer.
Can you cut glass blocks or trim them to size?
Do not. It is a hollow block with an air cavity; cutting it through ruins the insulation and sound values, gives you sharp edges and can still lead to cracking. Set your panel out in module sizes and use half blocks (190 x 90 mm), or take up the leftover dimension with the U-channel and the expansion joint.
Do glass blocks insulate well?
Only moderately, compared with modern glazing. An ordinary glass block wall sits at a U-value of around 2.2 W/m²K; special insulating versions get to about 1.1 to 1.4 W/m²K. For comparison, a good modern double glazed unit is well under 1.2. For an internal wall it makes no difference. For an external wall it is a compromise you should make with your eyes open.
How long does it take to build a glass block wall?
Reckon on about 1 metre of height a day at most, with 24 hours of drying time in between. So a wall 2 metres high is a two to three day job, spread out, plus the day for pointing and finishing. It is not a Saturday afternoon job, and that is exactly where people come unstuck.
Can a glass block wall go in the shower?
Yes. Glass does not absorb moisture, so as a shower wall it is a sound choice. Do use a mould-resistant, neutral-cure sanitary silicone at the junctions, make sure the junction with the shower floor is watertight, and make sure the bathroom stays properly ventilated.
To finish
A glass block wall is one of the few jobs where the result changes a room the moment it is done. A dark hallway becomes light. A bathroom gets daylight without being overlooked. And it is work you can do yourself perfectly well with a bit of patience.
Remember the three things that separate a wall that stands for twenty years from a wall that cracks after two: set it free-standing with an expansion joint, reinforce it at 65 cm maximum spacing, and build no more than a metre of height a day.
The rest is measuring, levelling and working steadily on.
Want to be sure your dimensions work before you order 80 glass blocks? At Fred’s DIY Plans you will find worked-out building plans with a material list and full dimensions, so you do not discover halfway through that your panel does not quite come out.
Have a look at the plans at Fred’s DIY Plans