How to manage Radon in a Retrofit

Let’s talk about your house. It’s probably your biggest investment. It’s your sanctuary. It’s also, I’m sorry to say, a bit of a disobedient, money-wasting idiot.

It leaks heat like a sieve made of other, smaller sieves. You pay a fortune for gas or oil, that heat rises, and it flies right out the attic, which you probably haven’t looked at in years. Your house is basically a machine for turning your bank account into slightly warmer air for the local pigeons. It’s also, by doing this, helping to gently boil the planet. You’re not a bad person, but your house is making you look bad.

So, you decide to be a Good, Smart Person. You hear about retrofitting. You see the ads. You learn about grants. You’re going to fix this! You’re going to seal that sucker up. You’ll get new windows, pump the walls, and, most importantly, you’ll roll out so much insulation in that attic it’ll look like a cloudy day in a sheep-shearing factory.

You’re a hero. You’re saving money. You’re saving the planet. You’re cosy. You’ve won at homeownership.

And then you hear a whisper. A whisper on the internet. A whisper from a nervous-looking neighbour. A whisper that says by sealing your house, you’ve… trapped something. Something invisible. Something radioactive. Something that was always there, but now, thanks to your brilliant, money-saving plan, it’s building up. And up. And up.

You’ve, uh, accidentally built a highly-efficient, gas-fired… radon trap. And you’re in it.

This is the “Retrofit Paradox,” and it’s one of the most important and least-discussed topics in our national rush to upgrade our homes. The core, terrifying question is this: Does making my house energy-efficient also make it cancerous?

The short answer is: it absolutely can.

The long answer is: it absolutely doesn’t have to, and a properly retrofitted house is actually safer than your old leaky one.

To understand why, we need to go on a journey. A journey deep under your house, into the mind of a physicist, and into the very specific, and slightly terrifying, geology of Ireland. Grab a torch. Don’t breathe too deeply.

Chapter 1: Meet Radon, the Earth’s Invisible, Radioactive Fart

Before we talk about houses, we need to meet our villain. His name is Radon.

Radon is a gas. You can’t see it, smell it, or taste it. It is, however, radioactive. It comes from the natural, extremely slow decay of uranium in the rocks and soil pretty much everywhere. The Earth is, in very slow motion, radioactive. And as the uranium in the ground under your feet has spent the last billion years breaking down, it creates other things, and one of the last things it breaks down into, before it becomes stable old lead, is radon gas.

If you’re outside, this is not a problem. The radon seeps out of the ground, whispers “boo!”, and is immediately scattered to the winds, diluted into the atmosphere to completely harmless concentrations. Good one, radon.

The problem is when it seeps out of the ground… and into your house.

Your house acts like a big, dumb container. The radon seeps in through tiny cracks in your foundation, gaps around pipes, and all the little imperfections where your house meets the dirt. And once it’s in, it’s in. It continues to decay, releasing tiny radioactive particles. You breathe these in, they lodge in your lungs, and like tiny, subatomic bullets, they fire off radiation, damaging your lung tissue. Over time, this damage can lead to cancer.

This is not a small, hypothetical “everything gives you cancer” risk. Radon is the second leading cause of lung cancer in the world, right after smoking. In Ireland, the Environmental Protection Agency (EPA) estimates that radon is linked to around 350 new lung cancer cases every single year.

And here’s the kicker: Ireland is special. In a bad way. Because of our specific geology, we have a massive radon problem. The world average for indoor radon is about 39 Bq/m³ (that’s “Becquerels per cubic metre,” the unit for measuring this stuff). The UK average is 20. The Irish national average? Somewhere between 77 and 89 Bq/m³. We are, geologically speaking, one of the most radon-prone countries in the world.

The national “Reference Level” – the “oh crap, you must fix this” level – is 200 Bq/m³. Data from the EPA shows that in counties like Sligo, one in four homes tested is over this limit. In Kerry, one house was measured with a level of 49,000 Bq/m³. That is not a typo. That is 245 times the national action level.

So, this gas is bad. It’s everywhere. And it’s been seeping into your house this entire time. Which, weirdly, brings us to why your old, leaky house was kind of helping.

Chapter 2: Your Old House and its “Accidental Breathing System”

Your old, un-retrofitted house is a thermal disaster. It’s full of draughts, “uncontrolled ventilation,” and gaps. The single biggest gap, as we discussed, is almost certainly the attic. It’s unsealed, uninsulated, and all your expensive warm air is screaming out of it 24/7.

This process has a name: The Stack Effect.

It’s simple physics. Warm air is more buoyant than cold air. So, the warm air in your house rises. It hits the ceiling, finds all the little gaps, and escapes out the top, primarily through your leaky attic. It’s a giant, invisible chimney.

But as all that air leaves the top of your house, it has to be replaced, right? The house can’t just become a vacuum. So, it sucks in replacement air from the path of least resistance. And what’s the path of least resistance? Not always your closed windows. It’s the thousands of tiny, unsealed cracks in your foundation and the gaps around your pipes.

Your house, driven by the heat escaping from its own leaky roof, is actively sucking radon gas out of the ground and into your living room. It is a giant, poorly-designed, radon-powered vacuum cleaner.

Now, in this old, leaky system, this was a high-turnover situation. The house was sucking in radon, yes, but it was also so leaky that all that air (radon and all) was just flying out the top again, along with all your money. It was a constant flow: in from the bottom, out from the top. The radon concentration might have been high, or it might have been acceptably low, diluted by the sheer volume of air passing through. You were paying a fortune to live in a draughty, inefficient sieve, but you might have been accidentally diluting your radon problem.

And then you, the Good, Smart Person, decide to fix it.

Chapter 3: The Fix. The Trap. The 730% “Oh Crap” Moment.

You call the retrofitters. You decide to finally get that attic insulation sorted. They come in, they roll out hundreds of millimetres of beautiful, fluffy insulation. They seal up all the gaps. They put a nice insulated hatch on your attic door. They’ve put a lid on your chimney. The Stack Effect is dead. Your heat stays in. You are a genius. Your heating bill plummets. Your cosy-smugness is off the charts.

But the radon? The radon didn’t get the memo.

It’s still seeping in from the ground. The source hasn’t changed. But now, it has nowhere to go. The giant “out” vent at the top of your house is sealed. You have, in effect, taken your leaky sieve and turned it into a Tupperware box. You’ve trapped it. The concentration of the gas, which is still seeping in 24/7, begins to build. And build. And build.

This isn’t a theory. This is documented, measured, terrifying fact. A 2023 study by researchers from the University of Galway, funded by the EPA, modelled this exact scenario. They found that a standard energy retrofit, if it didn’t account for ventilation, had the “potential to more than double” radon levels. One model predicted an increase of up to 107%.

But real-world data is even scarier. An EPA-funded project called UNVEIL surveyed 142 real Irish homes that had undergone energy-efficiency retrofits. They measured the radon levels before the work, and then again after the work. The results were… wild. The post-retrofit radon levels ranged from a 90% reduction (a ratio of 0.1) to a 730% increase (a ratio of 7.3).

Think about that. The exact same process (retrofitting) made one house drastically safer and turned another one into a carcinogenic hotbox. The difference wasn’t luck. It was planning. It was about whether they just “trapped” the house, or whether they followed the one, true mantra of building science…

But before we get to the solution, I have to tell you, it gets worse.

Chapter 4: How You Weaponised Your Kitchen Fan

You thought you killed the “suck.” You stopped the Stack Effect. But you didn’t. You just created a new, more powerful, and more idiotic radon-sucking monster: your kitchen extractor fan.

In your old, leaky sieve-house, when you turned on your kitchen fan to get rid of the smell of burnt toast, it was no big deal. The fan sucked the air from the kitchen and blew it outside. To replace that air, it just pulled in more air from the thousand other draughts and leaks all over the house. Easy.

Now, try that in your new, beautifully-sealed, air-tight Tupperware-box-house.

You turn on the fan. It tries to suck 300 cubic feet of air per minute out of your kitchen. But the house is sealed. There are no easy draughts to pull replacement air from. The fan, which is a powerful motor, creates a massive and immediate pressure drop. Your whole house becomes a giant vacuum. It must get replacement air from somewhere.

And where is the path of least resistance? Where is the one part of your house that isn’t sealed with membranes and insulation?

The ground.

Your kitchen fan, your bathroom fan, even your clothes dryer, have all been transformed into on-demand, high-power radon-sucking machines. Every time you turn them on, you are actively hoovering that radioactive gas from the soil, through your foundation, and into your home.

This “pressure inversion” risk is how you get those catastrophic 730% spikes. You haven’t just trapped the radon. You’ve installed a pump for it.

…So, we’re all doomed? We either freeze to death in a leaky, wasteful house or we die of cancer in a warm one?

No. Thank god. There is a solution. And it’s not even complicated.

Chapter 5: “Build Tight, Ventilate Right”

This is the mantra. Say it three times. “Build Tight, Ventilate Right.”

The problem isn’t “building tight.” Building tight is good. Building tight is smart. You want an airtight house. You just don’t want to live in a sealed Tupperware box, breathing in your own fumes and the Earth’s radioactive farts.

The “leaky house” myth—the idea that “a house needs to breathe”—is one of the most dangerous myths in construction. As building scientist Joseph Lstiburek famously said, people who say “I want my house to breathe” are really saying, “I want to rely on the mistakes that were made by the plumber and the electrician to provide me with fresh air.”

You don’t want random, “accidental” air pulled from your damp foundations or your dusty wall cavities. You want fresh air, from a known source, at a known rate.

This is the “Ventilate Right” part. And it’s not an optional extra. It’s the law.

The problem is that too many people (and, frankly, too many contractors) see a retrofit as only being about Part L of the Building Regulations (Conservation of Fuel and Energy). They completely forget about Part F (Ventilation). But these two parts are a team. They were updated together. The official government Technical Guidance Document F (Ventilation) was amended specifically to support the new airtightness measures in Part L.

You cannot legally or safely do Part L without also doing Part F. Doing so is building a car with an engine and no exhaust pipe. It’ll run, for a little while, and then it will kill you.

“Ventilating Right” can mean a few things. It can mean new, smart wall vents. It can mean a system called Positive Input Ventilation (PIV). But in a truly airtight, high-performance home, it almost always means installing a Mechanical Heat Recovery Ventilation (MHRV) system. This is the gold standard. It’s a “smart” breathing system for your house. It constantly extracts the stale, humid, radon-laden air from your bathrooms and kitchen, and at the same time it brings in fresh, filtered air from outside. And—this is the “recovery” bit—it uses a heat exchanger to transfer over 90% of the heat from the outgoing stale air to the incoming fresh air.

You get fresh air, but you don’t lose your heat. You’ve solved the paradox.

This is what that Galway study also found. When “appropriate ventilation measures were implemented during the retrofit process, radon levels could be reduced below the initial levels.” This is how you get the 90% reduction, instead of the 730% increase.

Chapter 6: The Real Hero: A Tiny Straw Under Your House

Okay. So you’re building tight, and you’re ventilating right. You’ve got your MHRV system. You’re a building-science champion.

But what if you test your house, and the radon is still high?

This can happen. If you’re one of those unlucky people who built your house on a geological radon-volcano, even a good ventilation system might not be enough to dilute the sheer volume of gas seeping in.

In this case, you don’t need dilution. You need a cure. You need to stop the radon from ever getting in.

Meet the Active Radon Sump.

This is the single most effective, brilliant, and surprisingly simple solution to a radon problem. If your house is the Tupperware box, and the radon is the problem, the sump is a tiny straw that sucks the radon out from under the box before it can ever get inside.

A contractor drills a small hole in your foundation, creates a small cavity (the size of a bucket) in the hardcore beneath, and inserts a pipe. This pipe runs up the outside of your house to a special, quiet, low-energy fan that runs 24/7. This fan constantly sucks the radon-laden air from the soil and vents it harmlessly above your roofline. It’s called “sub-slab depressurisation,” and it’s so effective it typically reduces radon levels by 90% or more. It solves the problem at the source.

In fact, Irish building regulations already require all new homes built in “High Radon Areas” to have a “standby” sump (the pipe, with no fan) installed from day one, just in case it needs to be activated.

Chapter 7: Your Simple, 4-Step, Don’t-Die-From-Radon Plan

Alright, that was a lot. You’ve gone from smug retrofit-hero to terrified Tupperware-dweller and back again. Let’s boil this all down to a simple, actionable plan. If you are thinking about any kind of home energy upgrade, this is your new bible.

Step 1: TEST. (The “Before” Photo)

Before you call a single contractor. Before you get a single quote. Test your home for radon. This is non-negotiable. You cannot know the impact of your retrofit if you don’t have a baseline. You must get a 3-month test from an EPA-registered service. It’s simple, it’s cheap (around €60), and it’s the most important data you will ever have about your home. You can also check the EPA’s map to see if you live in a High Radon Area, which makes this step even more critical. If your level is already high (over 200 Bq/m³), you need to fix that first.

Step 2: RETROFIT (The Smart Way).

This is where you plan your home energy upgrades. But you are no longer just a “heat loss” warrior. You are a “whole-home health” champion. When you talk to a contractor, your first question isn’t “How much insulation?” Your first question is “How will this retrofit comply with both Part L and Part F of the building regulations?” If they look at you blankly, they are not the right contractor. You are upgrading your home’s thermal envelope (its skin) and its ventilation system (its lungs) at the same time. The good news is that SEAI grants exist to help you do this properly, as part of a whole-home system.

Step 3: TEST AGAIN. (The “After” Photo)

The work is done. Your house is sealed. Your new ventilation system is humming away. You’re cosy. Are you safe? You don’t know until you test. Get another 3-month radon test. This is your report card. This is the only way to know if your plan worked. Did your number go down? Amazing! You’re a hero. Pop the champagne.

Step 4: SUMP (If Needed).

Did your number go up? Or is it still stubbornly above 200 Bq/m³? Don’t panic. You just live on a radon-y bit of earth. This isn’t a failure; it’s just new data. You now call an EPA-registered radon remediation contractor and get an Active Radon Sump installed. This will fix it. Then, you test one last time to confirm. Problem solved. For good.

Conclusion: Don’t Be Scared, Be Smart

Retrofitting your home is one of the best things you can do. It saves you money, makes you more comfortable, and is critical for the environment. The “Retrofit Paradox” isn’t a reason not to do it. It’s a reason to do it properly.

That 142-home study is the perfect summary. The same action led to a 730% increase in one home and a 90% decrease in another. The difference was knowledge and planning. One person just built a trap. The other built a system.

Don’t just build a trap. Test your home. Plan for ventilation. Be a smart homeowner, not a suffocating one. The takeaway is simple: test, plan, and ventilate, because a smart retrofit means upgrading your home’s thermal envelope and its lungs at the same time.   Upgrading your attic insulation is a great place to start.