A-rated house too hot? Why new Irish homes overheat and what actually works

Published: 22 July 2026 8 minute read

If your new build turns into an oven every time Ireland gets a warm week, you're not imagining it and your builder didn't botch the insulation. The Irish Times reported this month that one in ten Irish homes is at risk of overheating, and the homes most affected are the newest and best-rated ones. This guide explains why it happens and what genuinely works, in the order you should spend money.

Why the best-insulated homes are the hottest

An A-rated home is designed to do one thing exceptionally well: hold onto heat. Airtight construction, deep insulation, high-performance glazing. Through an Irish winter that's exactly what you want.

In summer the same physics runs against you:

• Solar gain comes in through the glass. Modern builds have big south and west-facing windows for daylight and passive winter heating. In June and July, those same windows are heat collectors. A large south-facing window can admit more heat in an afternoon than a radiator puts out.
• The insulation won't let it back out. In an older draughty house, heat leaks away overnight. In an airtight A-rated home it has nowhere to go. Each warm day stacks on the last, which is why the house feels worse on day four of a warm spell than day one.
• Internal gains add up. Cooking, appliances, showers, computers, and people all release heat into a sealed envelope. A home office with two monitors adds a constant trickle of warmth that the fabric of the house faithfully stores.
• MVHR ventilates, it doesn't cool. Mechanical ventilation systems exchange air while recovering heat, which is the last thing you want in July. Many have a summer bypass mode; plenty of homeowners don't know it exists.
• Attic conversions and dormers get it worst. They sit directly under the roof, absorb heat all day, and are usually bedrooms, exactly where overheating hurts most at 11pm.

None of this is a defect. It's a design trade-off that Irish building standards made in favour of winter performance, in a climate that used to make summer cooling irrelevant. That assumption is now out of date, and if you want the detail on where the market is heading, our cost guide covers what fixing it costs.

What doesn't work (and why you've already noticed)

Fans

Fans move air; they don't cool it. At moderate temperatures moving air helps sweat evaporate, which feels cooler. Once the room itself is at 27 or 28 degrees, a fan is stirring warm air around a sealed box. Retailers sold out of fans during this summer's heatwave, and most of those fans are now doing very little in very warm rooms.

Opening windows during the day

If it's hotter outside than inside, daytime ventilation imports heat. The old advice holds: keep windows and blinds closed through the heat of the day, open everything after dark.

Turning off the MVHR

Tempting, wrong. You lose ventilation and keep the heat. Check for a summer bypass mode instead; it skips the heat-recovery core and brings in cooler evening air.

What works, in order of cost

1. Stop the sun at the glass (free to a few hundred euro)

Heat you keep out is heat you never have to remove. Close blinds and curtains on south and west windows before the sun reaches them, not after the room is hot. If you're serious: external shading, awnings or external blinds, stops heat before it passes the glass and is several times more effective than anything internal. Standard practice in Austria and Germany, almost unknown in Ireland, and genuinely worth considering on a badly affected elevation.

2. Cut internal gains (free)

Cook outside or cook cold meals during warm spells, run the dishwasher and washing machine at night, switch off idle equipment. Small individually, meaningful together in a sealed house.

3. Night purge (free)

After dark, open windows on opposite sides of the house to pull cool air through, and leave internal doors open. In an airtight house this is the main way stored heat escapes. Secure vents and restrictors help if open windows overnight worry you.

4. A portable AC unit for one room (€280 to €700)

The first option that actively removes heat. A vented portable unit will hold a bedroom at a livable temperature. Honest limitations: they're the loudest option, the exhaust hose must go out a window, and they cost more to run than an installed unit. Our portable AC guide covers the ones worth buying.

5. An installed split system (€2,000 to €3,500 per room)

The fix that makes the problem go away. A split unit actively removes heat, dehumidifies, runs quietly enough to sleep under, and doubles as an efficient heater in spring and autumn. For an attic conversion or a couple of bedrooms, a single split or twin-head multi-split is a one-day install. Sizing matters more than brand; our BTU sizing guide covers how to get it right, and the cost guide covers what a fair quote looks like.

If you're building right now

Plan cooling before first fix. That means shading on the south and west glazing at design stage, and if you're adding AC, getting the installer coordinated with your electrician and plumber so pipework is hidden and unit positions are right. It is dramatically cheaper than retrofitting a finished house, and summer comfort will be a selling point of your home, not a flaw.

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