Why Your Windows Are Streaming With Condensation (And What Actually Fixes It)

You know the drill. You wake up, go downstairs, and the kitchen windows are running with water. The bedroom windows are fogged. There’s a puddle on the windowsill. Maybe there’s already a dark patch of mould forming in the corner.

You wipe it down. You open the window for ten minutes. You close it because it’s February and it’s freezing. By tomorrow morning, it’s all back again.

If this sounds like your house, you’re not alone. Condensation is the single most common damp problem in Irish homes — and most of the advice you’ll find about it is either incomplete or flat-out wrong.

Let’s fix that.

The Quick Version

The problem: Your home produces too much moisture, and there’s nowhere for it to go. When that moist air hits cold surfaces — windows, walls, corners — it condenses into water.

What doesn’t work: Wiping windows, leaving trickle vents open (you’ll freeze), running the heating higher (makes it worse), or hoping it’ll sort itself out.

What actually works: Removing the moisture at source through proper ventilation — ideally with heat recovery, so you’re not pumping your expensive heat out the window.

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Why Do Windows Get Condensation?

Here’s the science in plain English.

Air holds moisture. Warm air holds more moisture than cold air. When warm, moist air touches a cold surface, it can’t hold the moisture anymore — so it dumps it as water droplets. That’s condensation.

Your windows are usually the coldest surfaces in the room (even double-glazed ones), so that’s where the moisture shows up first. But make no mistake — if it’s on your windows, it’s on your walls too. You just can’t see it yet.

Where does all this moisture come from?

You’d be surprised how much moisture an average Irish family pumps into the air every single day:

• Breathing and sleeping — up to 1 pint per person per night
• Cooking — up to 3 litres per day
• Showering — up to 1.5 litres per shower
• Drying clothes indoors — up to 5 litres per wash load
• Boiling the kettle — every cup of tea adds moisture

A family of four in a three-bedroom semi can easily produce 10 to 15 litres of moisture per day. That moisture has to go somewhere. If your home isn’t ventilated properly, it stays in the air until it finds a cold surface.

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Why Modern Irish Homes Are Worse

Here’s what nobody tells you: newer homes get more condensation than older ones.

Older Irish houses were draughty. Badly insulated. Gaps around doors and windows. All that cold air wasn’t pleasant, but it did one useful thing — it carried moisture out.

Modern homes (and retrofitted ones) are built tight. Better insulation, sealed windows, draught-proofing. Brilliant for energy bills. Terrible for moisture.

You’ve essentially sealed the moisture inside.

The government’s push for better energy ratings (BER) and the SEAI retrofit schemes have made thousands of Irish homes warmer and more efficient. But they’ve also created a ventilation crisis that’s only now becoming obvious — in the form of condensation, damp, and mould.

The irony: The better insulated your home is, the more likely you are to have a condensation problem — unless you also address ventilation.

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What Doesn’t Actually Fix Condensation

Let’s go through the list of “solutions” you’ve probably already tried:

1. Wiping the windows every morning

You’re treating the symptom, not the cause. The moisture is still in your air. You’ll be wiping those windows every day for as long as you live in that house.

2. Opening windows

This works — briefly. You’re letting the moist air out and dry air in. But you’re also letting all your heat out. In an Irish winter, you’re paying to heat your home and then literally throwing that heat out the window. And as soon as you close them again, the moisture starts building up immediately.

3. Trickle vents

Trickle vents are those small flaps at the top of your windows. They provide some ventilation, but with no heat recovery, no filtration, and they create draughts. Most people in Ireland close them because they’re cold and noisy, especially on windy days. Closed trickle vents are doing precisely nothing for your condensation. We’ve written more about this in our post on why trickle vents aren’t real ventilation.

4. Turning up the heating

Warmer air holds more moisture, which means turning up the heat can temporarily reduce condensation on windows — but it doesn’t remove any moisture from your home. As soon as the heating cycles off at night, all that moisture-laden warm air cools down and dumps water on every cold surface it can find. You’ve just made the problem worse and spent more on heating.

5. Dehumidifiers

Dehumidifiers do pull moisture from the air. They work. But they’re expensive to run (a typical unit costs €150+ per year in electricity), they’re noisy, they need emptying, and they don’t bring in any fresh air. You’re still breathing stale, CO2-heavy air — you’re just breathing slightly drier stale air.

6. Anti-condensation paint and sprays

These products create a slightly warmer surface that delays condensation forming. They don’t address the moisture in the air at all. At best, they’re a temporary cosmetic fix. At worst, they hide the problem while mould grows behind the paint.

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What Actually Stops Condensation

The answer is straightforward: you need to remove the moisture from your air before it has a chance to condense.

That means ventilation. Specifically, continuous ventilation — not opening a window for ten minutes when you remember. The moisture is being produced 24 hours a day (you breathe all night), so the ventilation needs to run 24 hours a day too.

But here’s the catch. If you ventilate by opening windows or using extract-only fans, you’re throwing away your heat. In an Irish winter, that’s like burning money.

The solution: Heat Recovery Ventilation

A heat recovery ventilation (HRV) system extracts the stale, moist air from your rooms and pulls in fresh air from outside — but passes the two air streams through a heat exchanger first. The outgoing air gives its warmth to the incoming air, so you get fresh, dry air without losing your heat.

Modern units like Optim Vent recover up to 97% of the heat. That means the fresh air entering your home is almost the same temperature as the air leaving it. No draughts. No cold spots. No condensation.

And because the unit runs continuously — at whisper-quiet levels under 33dB — the moisture never gets a chance to build up. Your windows stay dry. Your walls stay dry. The mould never starts.

What about cost?

This is where people expect the catch. But a single-room ERV unit like Optim Vent uses just 7.8W of power. That’s less than a phone charger. Running 24/7, it costs about €10 per year in electricity.

Compare that to:

• A dehumidifier: €150+ per year
• Higher heating bills from opening windows: €200–400 per year (estimated)
• Mould treatment and repainting: €300+ per room

The maths speaks for itself.

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How to Tell If Your Problem Is Condensation (Not Structural Damp)

Not all damp is condensation. Here’s how to tell:

Sign	Condensation Damp	Structural Damp
Location	Windows, cold walls, corners, behind furniture	Ground level, chimney breasts, around gutters
Timing	Worse in winter mornings	Year-round, worse after rain
Pattern	Streaming windows, mould in corners and ceilings	Tide marks on walls, damp patches after heavy rain
Smell	Musty, especially in closed rooms	Earthy, like wet soil
Touch	Walls feel cold and clammy	Walls feel wet, plaster may be crumbling

If your damp matches the condensation column, it’s almost certainly a ventilation problem — and ventilation is the fix.

If it matches the structural column, get a surveyor. You may have rising damp, penetrating damp, or a leak. No amount of ventilation will fix a structural problem.

The good news? The vast majority of damp complaints in Irish homes turn out to be condensation damp. It’s the most common type and the most fixable.

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A Room-by-Room Breakdown

Condensation doesn’t hit every room equally. Here’s why:

Bedrooms

The worst offenders. You spend 8 hours breathing out moisture into a sealed room. Windows stream every morning. Mould appears on window reveals and behind wardrobes pushed against external walls.

Fix: Continuous ventilation that runs overnight. A wall-mounted ERV in the bedroom keeps the air fresh and dry while you sleep — silently, with no draught.

Kitchen

Cooking, boiling kettles, dishwashers — kitchens produce enormous amounts of moisture. Extractor fans help but only run when you remember to turn them on, and they throw heat out with the steam.

Fix: ERV ventilation combined with using your cooker hood when cooking. The ERV handles the background moisture; the cooker hood handles the peaks.

Bathroom

Showers can dump 1.5 litres of moisture into a small room in minutes. If you don’t have an extractor fan, this moisture spreads through your house when you open the door.

Fix: A good extractor fan for peak shower moisture, plus continuous ERV ventilation to handle the lingering humidity.

Living room

Less obvious, but with multiple people breathing, possibly drying clothes on a radiator, and the room sealed up in the evening — condensation builds overnight.

Fix: Continuous background ventilation. Stop drying clothes on radiators (use a heated airer in a ventilated room if you can’t use a dryer).

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Taking Action: What to Do This Week

You don’t need to wait for a full ventilation system to start improving things. Here’s a practical order of action:

1. Stop drying clothes on radiators. This is the single biggest source of avoidable moisture in most Irish homes. Use a tumble dryer, a heated airer, or dry outside when you can.

2. Use your extractor fans. Run them while cooking and showering, and leave them on for 15 minutes after.

3. Open trickle vents — if you have them and the weather allows it. Some ventilation is better than none.

4. Don’t push furniture against external walls. Leave a gap so air can circulate and moisture doesn’t get trapped.

5. Consider a heat recovery ventilation unit. For a permanent fix that doesn’t cost you heat, an ERV like Optim Vent removes moisture continuously while recovering up to 97% of your heat. One unit per room, installed in a couple of hours, and it runs for about €10 a year.

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Frequently Asked Questions

Can I claim an SEAI grant for ventilation? Mechanical ventilation systems may be eligible for SEAI grants as part of a home energy upgrade. Check our guide to SEAI grants for ventilation for the latest figures.

How quickly does an ERV fix condensation? Most homeowners notice a dramatic improvement within the first week. Windows that used to stream every morning stay dry. Within a few weeks, damp patches begin to dry out and mould stops spreading.

Will an ERV make my house cold? No. With 97% heat recovery, the incoming air is pre-warmed to near room temperature. There’s no draught and no cold spot — it’s the opposite of opening a window.

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Condensation isn’t something you should just live with. It’s a sign that your home’s air is too moist — and that has consequences for your health, your home’s structure, and your heating bills. The good news: it’s fixable.

If you’d like to find out how Optim Vent can solve your condensation problem, get in touch for a free home assessment.