Last updated: August 2026
Home ventilation is the upgrade nobody gets excited about - until the condensation arrives. Ireland's push to insulate and seal draughty homes has a side effect the grants leaflets rarely mention: a well-sealed house traps moisture as well as heat. Every shower, every dinner, every night's breathing puts litres of water vapour into your air, and if it has no controlled way out, it finds the coldest surface and becomes condensation, damp and eventually mould. This guide explains how home ventilation actually works, why insulating without ventilating backfires, the four main systems and their costs, and the cheap fixes that solve most problems.
⚡ Quick Summary
🌿 The golden rule: insulation keeps heat in, ventilation lets moisture out - a home upgrade needs both or it grows mould.
🌿 A family of four releases roughly 10 litres of water vapour a day into the air through cooking, washing and breathing.
🌿 The cheap fixes first: unblocked wall vents, working extractor fans in kitchen and bathroom, trickle vents open, lids on pots.
🌿 The four systems: natural (wall vents), extract fans, demand-controlled ventilation (DCV) and heat-recovery ventilation (MVHR).
🌿 SEAI cares too: grant-funded insulation works include ventilation checks - sealed homes must still breathe by design.
🌿 Target humidity: roughly 40-60% - a €10 humidity meter tells you where you stand.
💨 Why Sealed Homes Need Designed Ventilation
Old Irish houses ventilated themselves - through rattling windows, gappy floors and chimneys. Wasteful, but the moisture left with the heat. Modern retrofitting closes those accidental escape routes: attic insulation, wall insulation, new windows and draught-proofing can transform a leaky house into a sealed one. The heat stays - and so does the water vapour.
When warm, moist air touches a cold surface - single glazing, an uninsulated corner, a thermal bridge - it condenses. Persistent condensation feeds black mould, which damages plaster, furnishings and, more importantly, respiratory health. The fix is never "stop insulating" - it is ventilating by design instead of by accident.
🔍 The Four Ventilation Systems, Plainly
🌿 Natural ventilation (wall vents): the round vents in your walls. Free to run, no moving parts - but uncontrolled: they over-ventilate on windy days and under-ventilate on still ones. Rule one: never block them, however draughty they feel.
🌿 Extract fans: kitchen and bathroom fans that dump moist air outside at the source. Every home should have working ones - a humidity-triggered bathroom fan costs €150-€300 fitted and runs itself.
🌿 Demand-controlled ventilation (DCV): smart inlet vents and extract units that open and close based on humidity - ventilating hard when you shower, easing off overnight. A popular whole-house retrofit at roughly €1,500-€3,000, and a sensible partner for a deep insulation job.
🌿 Heat recovery ventilation (MVHR): the premium option - continuously extracts stale moist air and supplies fresh filtered air, passing them through a heat exchanger that keeps most of the warmth. Superb in airtight homes, but a serious retrofit (ducting throughout) at roughly €4,000-€8,000+. Standard in new builds and deep retrofits; overkill for a moderately sealed semi.
All costs are indicative - system size, ducting runs and house layout move them substantially.
🫧 Condensation and Mould: The Practical Playbook
🌿 Measure first: a cheap hygrometer in the bedroom and kitchen. Sustained readings above ~60-65% mean the moisture is winning.
🌿 Attack moisture at source: lids on pots, bathroom door closed and fan running during and 15 minutes after showers, no drying clothes on radiators in sealed rooms (a dehumidifier or vented space beats the radiator).
🌿 Let rooms breathe: furniture a few centimetres off external walls, wardrobes not jammed against cold corners, trickle vents open even in winter.
🌿 Fix the cold surfaces: condensation targets the coldest spot - insulating the wall or upgrading the glazing removes the landing site. This is where ventilation and insulation work together, not against each other.
🌿 Mould already there? Clean it properly, then fix the cause - mould cleaned without changing moisture or surfaces always returns.
🏠 Ventilation and the Grants System
SEAI-funded insulation works include ventilation assessment for good reason: the State does not want to fund 500,000 sealed, mouldy homes. If you are planning grant-backed retrofit works, your contractor should address ventilation as part of the job - ask how, specifically, before signing. There is currently no standalone SEAI grant for ventilation systems; DCV or MVHR is usually funded as part of a One Stop Shop package or paid directly (the Home Energy Upgrade Loan can cover it within a wider grant-backed project).
⚠️ Common Mistakes to Avoid
🌿 Blocking wall vents to stop draughts: the single most common - and most damaging - DIY "fix" in Irish homes. Cold draught now, black mould later.
🌿 Insulating with no ventilation plan: if your retrofit quote never mentions ventilation, that is a red flag about the contractor, not a saving.
🌿 Buying a dehumidifier instead of fixing the cause: useful tactically (drying clothes), but running one permanently is paying electricity to do a vent's job.
🌿 Turning off MVHR/DCV to save power: these systems sip electricity; the damp they prevent costs far more than they use.
🌿 Ignoring the bathroom fan: if it can't hold a sheet of toilet paper against the grille, it is not extracting - replace it.
❓ Frequently Asked Questions
🔹 Why does my house get condensation after insulating?
Insulating and sealing removed the accidental ventilation the house relied on, so moisture that used to leak out now stays in. The fix is designed ventilation - not removing the insulation.
🔹 Should wall vents be open in winter?
Yes. They exist precisely for winter, when windows stay shut and moisture peaks. If the draught is uncomfortable, fit a humidity-controlled (DCV) vent rather than blocking it.
🔹 What is the best ventilation system for an Irish home?
For most retrofitted homes, working extract fans plus DCV hits the sweet spot of cost and control. MVHR is the right answer for very airtight, deep-retrofit homes.
🔹 How much does a ventilation system cost?
Indicatively: humidity-controlled extract fan €150-€300, whole-house DCV €1,500-€3,000, retrofit MVHR €4,000-€8,000+. Get itemised quotes - layouts move prices a lot.
🔹 Is there an SEAI grant for ventilation?
Not as a standalone measure. Ventilation is assessed and addressed within grant-funded insulation and One Stop Shop retrofits.
🔹 What humidity should my house be?
Roughly 40-60% relative humidity. Below 40% feels dry and scratchy; sustained readings above 60-65% invite condensation and mould.
📚 Related Guides
🌿 Attic Insulation
🌿 Wall Insulation
🌿 BER Rating
🌿 BER Cert Cost
👉 All guides and calculators
Climate Jargon Buster is an independent resource providing plain-English information on climate, energy and sustainability topics for an Irish audience. System costs are indicative and vary by home and installer; grant scheme details are set by the SEAI and correct as of August 2026. For official guidance, visit seai.ie. This site is not affiliated with SEAI.