Move your house from C3 to higher (might be the windows)

There is a specific type of conversation that happens in kitchens all across Ireland every winter. It usually involves someone wearing a thick jumper, holding a mug of tea, and staring suspiciously at a window. The radiator is on. The thermostat says 21°C. But the person is cold.

“It can’t be the windows,” they say, tapping the glass. “They’re double glazed. Look. Two panes. There’s a gap. We’re sorted.”

This is the Great Glass Lie.

We tend to think of home improvements as binary. You either have a thing, or you don’t. Do you have an extension? Yes. Do you have a heat pump? No. Do you have double glazing? Yes.

But when it comes to the energy performance of your home, this binary thinking is a trap. It’s the reason why hundreds of thousands of Irish homes—particularly those built in the “Pre-Tiger” boom of the 1990s—are stuck in a sort of thermal limbo. They aren’t old enough to be crumbling ruins, but they aren’t new enough to be warm. They are the “C3” generation. And the biggest culprit holding them back is the very thing their owners think is saving them: their windows.

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To understand why, we need to go on a deep dive into the invisible world of thermodynamics, molecular diffusion, and a very boring spreadsheet column known as “UValueWindow”.

The Glazing Paradox

If you were to look at a thermal image of a street in a Dublin suburb, the 1990s houses would stand out. They wouldn’t be glowing red all over like the single-glazed cottages of the 1950s. Instead, they would look… confused.

The walls might be okay (maybe they had some home energy upgrades pumped into the cavities). The roof might have a bit of yellow fluff in it. But the windows would be leaking heat like a sieve.

The problem is that “Double Glazing” is not a technical specification. It is a description of a shape. It just means “two sheets of glass with a space in between.” That’s it. It says nothing about what the glass is made of, what is in the space, or what holds the sheets together.

In the early 1990s, when Ireland started building houses at a rate that would eventually make the Central Statistics Office graphs look like vertical lines, the technology of glazing was primitive. The windows installed in 1993 look almost identical to the windows installed in 2023. But thermally speaking, they are different species.

A modern window is a high-tech sandwich of noble gases, microscopic metal coatings, and thermal breaks. A 1990s window is essentially two sheets of glass held apart by a cold metal bar, filled with 30-year-old air. The difference in performance is roughly 300%.

This is why, when a BER assessor comes to your house and plugs your details into the software, the result is often a disappointing C3. The computer isn’t being mean. It’s just looking at the physics.

The Detective Story: Column K

In the world of Irish energy ratings, there is a legendary database. It is the SEAI National BER Research Tool. This is where the energy soul of every rated home in the country lives, stored as a row of data.

If you download this dataset (warning: it is enormous) and scroll to the right, past the floor area and the wall type, you will find a variable often labelled “UValueWindow” or similar, residing in the vicinity of Column K.

For a modern A-rated home, the number in this column is usually 1.2 or lower. This is the “U-value,” which measures how easily heat passes through a material. A lower number is better. A U-value of 0 would be a perfect insulator (impossible). A U-value of 5.8 is a single pane of glass (terrible).

For the vast majority of Irish homes built between 1990 and 2005, the number in Column K is 2.8.

This number, 2.8 W/m²K, is the “default” value for a standard double-glazed unit with a 12mm air gap and a metal spacer bar. It is a number that screams “mediocrity.” It means that for every square metre of glass, for every degree of temperature difference between inside and outside, 2.8 Watts of heat are marching out of your house.

You might think, “Well, my windows are in good nick, surely they are better than the default?”

Almost certainly not. In fact, they are likely worse. To understand why, we need to zoom in. Way in.

The Anatomy of a Thermal Disaster

Let’s take a cross-section of a standard 1995 window and see what’s actually going on inside. There are three main components that turn this “double glazing” into a liability.

1. The Metal Bridge

In the 90s, the “spacer bar” (the bit that separates the two panes of glass) was made of aluminium. Aluminium is a fantastic material for making aeroplanes and cans. It is a terrible material for insulation. It is a highly efficient conductor of heat.

This aluminium strip runs around the entire perimeter of the window. In winter, it acts as a thermal highway, carrying heat from the inner pane directly to the outer pane, bypassing the air gap entirely. This is why you often see a strip of condensation around the edge of old windows. That is the “cold bridge” in action.

2. The Invisible Coat (That Isn’t There)

Modern glass is coated with a microscopically thin layer of metal oxide, known as a “Low-E” (Low Emissivity) coating. This coating is like a bouncer for heat. It lets sunlight (short-wave radiation) come in, but when heat tries to leave (long-wave radiation from your radiator or your body), the coating reflects it back into the room.

Most 1990s glass is just… glass. It is transparent to heat. Without a Low-E coating, the glass absorbs heat from your room and happily radiates it out to the garden. It offers almost no resistance to radiative heat loss.

3. The Gas That Left the Building

This is the big one. The “insulation” in a double-glazed unit is supposed to be the gas trapped between the panes. In modern windows, this is Argon. Argon is a noble gas. It is heavy, lazy, and slow. Because it is denser than air, it doesn’t move around much, which makes it hard for heat to travel through it.

See this explanation of noble gases and thermal insulation for a deeper look at why Argon is the king of the cavity.

However, 1990s windows were typically filled with just dry air. Air is a much better conductor of heat than Argon (about 34% better, which is bad for you). But even if they were filled with Argon, it wouldn’t matter anymore. Because of a phenomenon called “The Pumping Cycle.”

The Physics of Decay

A window is not a static object. It is a pressure vessel that breathes. Every time the sun comes out, the gas inside the unit heats up and expands. This pushes the glass panes outward, putting stress on the seals. When the sun goes in, the gas cools and contracts, sucking the panes inward.

Over 25 years, a window goes through about 9,000 of these cycles. Eventually, the seals get tired. They develop microscopic cracks. This is where Fick’s laws of diffusion come into play. Even without a visible hole, gas molecules will naturally migrate from an area of high concentration (inside the unit) to low concentration (outside).

The Argon (if there was any) leaves. But worse, atmospheric air gets in. And atmospheric air brings a passenger: water vapour.

Once moisture gets into the unit, the game is over. The desiccant (a drying agent inside the spacer bar) soaks it up for a while, but eventually, it gets full. Then, you get condensation inside the glass. The unit has “blown.” But long before you see the fog, the thermal performance has collapsed.

The Comfort Crisis: Why You Feel Cold

Let’s go back to our freezing homeowner with the mug of tea. Why do they feel cold if the air temperature is 21°C?

Human thermal comfort is not just about air temperature. It is also about “Radiant Temperature.” Our bodies are constantly radiating heat to the surfaces around us. If we are standing next to a warm wall, we feel warm. If we are standing next to a cold surface, we radiate our body heat towards it rapidly. We feel this heat loss as a chill.

This is a phenomenon known as “Radiant Asymmetry.” A detailed study on Window Performance for Human Thermal Comfort explains that when a surface in a room is significantly colder than the others, it creates a discomfort that no amount of air heating can fix.

The inner surface of a modern, efficient window might be 17°C on a freezing night. The inner surface of an old 1990s double-glazed unit might be 12°C or lower. That 12°C surface acts like a “heat vampire,” sucking the warmth right out of your skin from across the room.

This creates a draft effect. The air touching the cold glass cools down, becomes heavy, and drops to the floor. This creates a circulating current of cold air that swirls around your ankles. You think there’s a gap in the frame, but there isn’t. The draft is being created by the glass.

The Whole-Home Strategy (Or: Don’t just fix the holes, fix the bucket)

So, you have these C3 Liability windows. You need to replace them. But wait.

If you just replace the windows and do nothing else, you are making a classic error. You are putting a brand new door on a tent. Energy efficiency is about the “Thermal Envelope”—the entire barrier between you and the weather.

I recently read a great guide on why 1990s houses are so cold, and it makes a crucial point: these houses fail on multiple fronts. The windows are bad, yes. But the walls are often leaky, and the attic insulation is usually a sad, thin layer of fibreglass that has been compressed by Christmas decorations since 1998.

If you upgrade your windows to triple-glazed fortresses but leave your attic uninsulated, the heat will simply change direction and leave through the ceiling. In fact, because the house is now more airtight, you might even increase the risk of condensation on those cold ceiling spots.

This is why the wisest approach is the “Fabric First” approach. You fix the shell. You do the attic insulation to stop the heat rising out. You pump the walls. And then you replace the windows.

It’s only after you have secured the envelope that you should look at the “fancy” stuff. I often see people rushing to install renewable tech on leaky houses. It’s like buying a jet engine for a glider that has holes in the wings. It might look cool, but it won’t fly well.

For example, if you are considering Solar Panels Dublin based companies offer, they will often ask about your BER. Why? Because generating cheap electricity to heat a house that loses heat instantly is an exercise in futility. It’s better to reduce the demand first (windows, insulation) and then meet that reduced demand with solar.

The Solution: Replacing vs. Refurbishing

Can you just change the glass? Sometimes. If your PVC frames are in good condition, you can sometimes retrofit new, modern “Low-E” Argon-filled units into the old frames. This is cheaper and less disruptive.

However, you still have the old frames, which might not have the complex multi-chamber insulation of modern ones. And you might still have drafty hinges.

The “Gold Standard” is full replacement. New frames, new glass. This allows you to address the airtightness around the window (where the frame meets the wall) and eliminate those cold drafts forever.

Is it expensive? Yes. But there is help. The SEAI Grants have evolved to focus heavily on this “Whole Home” approach. While individual window grants are trickier to get on their own, they are a huge part of the “One Stop Shop” deep retrofit grants.

Conclusion: The End of the C3 Era

The C3 rating is a badge of honour for a house that has survived 30 years of Irish weather (which is no joke), but it is time to retire it. The “Double Glazing” of the 1990s was a necessary step in our evolution, like dial-up internet or the VHS tape. It did its job. But its job is done.

We now know too much about physics, comfort, and carbon to keep living in these transparent fridges. We know that the gas has left. We know the coatings are missing. We know that the cold spacer bar is a bridge to nowhere.

So, the next time you are standing in your kitchen, freezing, looking at your “good” double glazing, remember: it’s not you. It’s the physics. And physics always wins.

But you can win too. You just need to stop trusting the glass.

Would you like to stop heating the garden and start heating your home? Check out our guide to solar panels in Dublin to see how you can power your newly efficient home.