Fix your Chimney and upgrade your D rated home

If you live in an Irish housing estate built between 1980 and 1995, I have some news for you. You don’t live in a house.

You live in a wind tunnel with a roof.

Specifically, you live in a “Victim.” This is a standard semi-detached, 3-bed, concrete-block box. It’s the architectural equivalent of a ham sandwich—functional, ubiquitous, and completely uninspiring. But the Victim has a secret. It has a hole in it.

A really, really big hole.

And this hole is the reason why, no matter how much you spend on kerosene or how many jumpers you wear, your Building Energy Rating (BER) is stuck in the D-band purgatory. Today, we are going to look at the math behind this hole, meet the “Judgy Robot” that calculates your energy rating, and discover how a €30 piece of plastic might just save you thousands.

The Physics of “The Suck”

Let’s go back to 1985. Hair was big, music was synthesized, and in Ireland, we were building houses with a very specific feature: The Open Fireplace.

The open fireplace was the social media of the 80s. It was where you gathered to stare at something changing rapidly and feel warm inside. But there was a flaw in the design. To get the smoke out, builders installed a 200mm clay flue straight up through the middle of the house.

Here is the problem. Hot air rises. You know this. It’s why attics are hot and basements are creepy. But in your house, that 200mm flue acts like a massive straw. Even when the fire is not lit, the warm air from your radiators floats over to the fireplace, looks up, sees the sky, and thinks, “Freedom!”

This is called the Stack Effect. As the warm air shoots up the chimney, it creates a vacuum downstairs. Physics hates a vacuum, so it pulls cold air in from outside to replace the warm air you just lost. It sucks this cold air through the floorboards, the window seals, and the letterbox.

According to the laws of physics (and Wikipedia), your chimney is basically an engine that converts money into cold drafts.

Meet The Villain: The Judgy Robot (DEAP)

In Ireland, we don’t just look at a house and say, “Yeah, that looks warm.” We use a piece of software called DEAP (Dwelling Energy Assessment Procedure). DEAP is a Judgy Robot. It doesn’t care about your feelings. It only cares about data.

When a BER assessor comes to your house, they measure everything and feed it into the Judgy Robot. And there is one specific column in the spreadsheet that the Robot is obsessed with: Column AI: NoOfChimneys.

If you have an open fire, the assessor puts a “1” in this column. And the Robot loses its mind.

The Robot has been programmed with a “Rule of Thumb” from the Building Research Establishment (BRE). It assumes that a single open chimney sucks out 40 cubic meters of air per hour. Every hour. 24 hours a day. Forever.

To put that in perspective, 40 cubic meters is roughly the volume of a shipping container. Imagine a shipping container full of warm air leaving your house every single hour. That is what the Robot thinks is happening. And according to the BRE’s research, the Robot might actually be underestimating the problem.

The D1-to-C3 Loophole

Here is where things get interesting. Most 1980s “Victim” houses are sitting right on the edge of a decent rating. They are usually a D1 or a D2. They are tantalizingly close to a C3.

You might think the solution is to spend €20,000 on upgrading your 90s house with fancy windows or a new boiler. But there is a cheaper way. A “Cheat Code,” if you will.

It turns out that the DEAP software has a second-order effect—a “Double Dividend”—that most people miss. It’s hidden in something called Appendix S.

See, the Robot assumes your house is leaky because it’s old. It assigns a “Default Air Permeability” score. If you have an open chimney, the Robot assumes your house is basically a sieve. It gives you a terrible permeability score.

But!

If you permanently seal that chimney, two things happen:

1. The Direct Benefit: You stop the 40m³/hr air loss penalty. The Robot stops deducting points for the hole in the roof.
2. The Appendix S Bonus: This is the magic part. Because you have sealed the biggest hole in the house, the Robot’s logic circuit switches tracks. It re-classifies your home’s “Draught Proofing Percentage” to 100%. Suddenly, it assumes the rest of your house is tighter too. The default air permeability score drops.

It’s a multiplier effect. By doing one small thing, you get credit for two big things. This single change is often enough to jump a house from a D1 (sad face) to a C3 (happy face). And you didn’t even touch the walls.

The Context: Fabric First, Gadgets Second

We often get excited about the shiny stuff. We want the heat pumps and the Tesla Powerwalls. And sure, solar is great. But trying to heat a house with an open chimney is like trying to fill a colander with water. You don’t need a bigger tap; you need to plug the holes.

This is why home energy upgrades should always follow a “Fabric First” approach. Deal with the structure before you add the tech. It makes zero sense to install a €15,000 heat pump if all that expensive heat is being sucked up the flue by the Laws of Thermodynamics.

Think of it like this: If you were going to space, you wouldn’t ask, “What kind of heater should I put in my suit?” You’d ask, “Is the suit airtight?”

For many Dublin homes, a massive intervention like external wall insulation Dublin residents often ask about is a fantastic long-term goal. It wraps the house in a tea cosy and solves the wall heat loss problem forever. But if you have limited funds right now, sealing the chimney is the “low hanging fruit” that stops the immediate bleeding.

The Solution: The Balloon vs. The Cap

So, how do you fix it? You have two options: The Band-Aid and The Surgery.

1. The Chimney Balloon (The Band-Aid)

This is exactly what it sounds like. You buy a thick plastic balloon, shove it up the chimney, and inflate it. It blocks the air flow. It costs about €30.

Pros: Cheap. DIY-friendly. Stops the draught immediately.

Cons: The Robot doesn’t trust it. Because you can just pull it out, it’s not considered “permanent.” So while your house feels warmer, your official BER rating won’t change unless you do a blower-door test to prove it works.

2. The Permanent Seal (The Surgery)

This involves capping the chimney at the top and blocking it at the bottom with a vent. This is the gold standard.

Pros: The Robot loves it. You get the full BER bonus. The hole is gone forever.

Cons: You can never have a fire again. (Sorry, romantic 80s nostalgia).

A Serious Note on Safety (Please Don’t Skip This)

Before you run off and stuff a pillow up your flue, we need to talk about death. Specifically, Carbon Monoxide.

That “Hole in the Roof” was actually doing one good thing: it was ventilating your house. Old houses don’t have fancy mechanical ventilation systems; they rely on being leaky to keep the air fresh. If you seal the chimney, you make the house airtight.

If you have a gas cooker, an oil boiler, or a stove in another room, and you block the chimney without adding new wall vents, you could be creating a gas chamber. Carbon Monoxide is silent and deadly. If you seal a chimney, you must ensure you have adequate wall vents in the room. This isn’t bureaucracy; it’s biology. We need oxygen to live.

Also, wind matters. Data from Met Éireann shows that Ireland is a windy little rock. The windier it is, the stronger the stack effect becomes. On a stormy night in November, that open chimney isn’t just a hole; it’s a jet engine sucking heat out of your living room.

The Bottom Line

The Irish housing stock is getting older. Census data from the CSO confirms that we aren’t building new homes fast enough to replace the old ones. That means we have to fix the ones we have.

We are seeing progress. The SEAI recently reported over 38,000 upgrades in the first three quarters of 2024. But for every deep retrofit, there are ten houses that just need a few smart tweaks.

If you are stuck in a D-rated freezer, stop looking at the walls and look at the roof. The “Hole” is the enemy. Kill the hole, appease the Robot, and save your money for something better—like a holiday to somewhere that isn’t a wind tunnel.

Once you’ve plugged the leaks, you’re ready for the next step in your whole-home strategy. Check out our attic insulation solutions and start reducing your energy bills, now.