Ventilation, condensation and mould after insulation

The essentials

Include ventilation in the work from the start

Insulation reduces heat loss; ventilation brings in fresh air and removes stale, moist air. Both belong in the upgrade plan.
Ask the contractor to assess existing ventilation and explain in writing what must be kept, repaired or added before insulation starts.
Condensation and mould need investigation. A leaking roof, pipe or wall needs repair; a fan alone cannot fix water entering the building.
Keep required air supplies clear. Room ventilation, airflow around roof or floor timbers, and air for fuel-burning appliances serve different purposes.

What changes when you insulate?

Insulation slows heat escaping through walls, floors and roofs. Airtightness limits unwanted air leakage through gaps. They are different, but an insulation project often includes sealing gaps or replacing windows. That can remove draughts the house previously relied on for air exchange.

Ventilation deliberately brings fresh air in and takes stale air out. It removes moisture from cooking, showers and laundry, along with other indoor pollutants. A draught through a cracked window frame is an unreliable substitute.

Insulation can make internal surfaces warmer, reducing the risk of condensation: water forming when moist air meets a cold surface. But the upgrade must also deal with moisture and air supply. More insulation is not, by itself, a remedy for every damp problem.

If condensation or mould is already present

Arrange an inspection before agreeing to cover affected areas. Record where and when the problem appears, take dated photos, and tell the surveyor about leaks, heating difficulties and previous repairs. A patch that worsens after rain is useful evidence, not a diagnosis.

Ask a building surveyor or other suitably qualified building professional to check for water entering through the roof or walls, plumbing leaks, cold surfaces and inadequate ventilation. More than one cause can be present. A thermal bridge is a place where heat escapes more easily, such as an uninsulated junction around a window; its colder surface can encourage condensation.

Agree repairs and drying before insulation conceals the affected construction. Do not accept painting over mould as the complete repair. If it returns after an upgrade, contact the contractor with your records and ask for the cause to be investigated, including ventilation performance and insulation junctions.

The UK Government’s household mould guidance explains why removing visible mould alone does not resolve its moisture source. We use its health and building guidance here, not its English tenancy rules. Arrange prompt professional help for extensive or recurring mould, or suspected hidden damage. If you are concerned about symptoms, speak to a healthcare professional while arranging the building work. If renting, report the problem in writing to your landlord or housing provider and keep a copy.

What should the ventilation survey cover?

For SEAI-funded upgrades, the Sustainable Energy Authority of Ireland (SEAI) sets out ventilation assessment requirements in its Domestic Technical Standards, version 4.0, July 2026. Ask the contractor to explain how these apply to your proposed work.

Walk through the house together and ask for a written plan covering:

Fresh air in: which wall or window vents supply occupied rooms, whether they work, and whether insulation or new windows will obstruct or replace them.
Moist air out: how kitchens, bathrooms, shower rooms and utility rooms extract air to outdoors. A cooker hood that only filters and recirculates room air does not remove moisture outside.
Air between rooms: how it reaches extract points when internal doors are closed. The designer must coordinate this with fire safety; do not cut or prop open fire doors yourself.
Existing problems: damp, mould, blocked openings, noisy fans and equipment that is switched off or broken.
The finished house: all planned insulation, window and draught-sealing work, including later phases. Ask whether an airtightness test is needed to establish the design; this measures air leakage under controlled pressure.
Safe installation and access: duct (air pipe) routes, outside inlet and outlet covers, electrical work, controls and access for servicing. Include roof, floor and fuel-appliance air supplies separately.

Have the quote identify who designs and installs the system, what repairs must happen first, and who pays for them. Compare the complete installed price, including VAT, testing, finishing and handover, rather than the price of a fan alone. Our contractor guide explains how to compare written scopes.

Which system could suit the house?

There is no single system to prescribe from the age of the house or the insulation thickness. The designer needs the house layout, planned airtightness and a workable installation route. Ask them to explain the choice in everyday terms.

Background vents with intermittent extract fans

Fresh air enters through designed wall or window openings. Fans in rooms that produce moisture run when needed and discharge outdoors. Openable windows also allow purge ventilation, a larger, short burst of air exchange.

This can suit some existing homes, provided the complete system is adequate after the work. Opening a window occasionally is not a replacement for that assessment. Keep required vents clear and operate fans and their timers as instructed.

Continuous mechanical extract, including demand control

Fans continuously take air out of moisture-producing rooms. Fresh air enters through designed inlets and moves through the home. Demand-controlled ventilation (DCV) varies extraction with the room’s needs, using sensors or controls. It still needs a designed fresh-air route.

Ask about normal operation and the boost setting for extra extraction, bedroom noise, electricity use and cleaning. A continuous system should not have to be switched off to make the home tolerable: report excessive noise or draughts so the installer can investigate.

Mechanical ventilation with heat recovery

MVHR supplies fresh air and extracts stale air through ducts. A heat exchanger transfers some heat from the outgoing air to the incoming air. It does not replace the home’s heating system.

For both DCV and MVHR, have the designer specify compatible kitchen extraction. SEAI’s current standard rules out a separate externally vented cooker hood with these systems; the designed room extraction must remove cooking moisture.

MVHR needs suitable airtightness, space for equipment and ductwork, and maintenance access. Ask what building work is needed, how filters are reached and what running and servicing costs to expect. Very airtight homes need mechanical ventilation, but that does not mean every insulated home needs MVHR.

Keep these air supplies distinct

Roof and floor timbers: ventilation of a cold attic, where insulation is at ceiling level and the roof space is unheated, protects the roof structure. A suspended timber floor is supported above a space underneath; ventilation of that space protects the floor timbers. It does not provide the ventilation needed in bedrooms or living rooms. Insulation must preserve the required air paths; bathroom extract air must discharge outdoors, not into the attic. See the attic and floor guides for those separate checks.

House section: the ceiling and suspended floor separate three spaces. Blue arrows show their separate air routes; roof-space and underfloor ventilation do not ventilate the rooms. Room supply and extraction are illustrative, not a system design. Fuel-burning appliances need their own air-supply assessment. Keep required openings clear. Schematic, not a construction detail.

Fuel-burning appliances: some stoves, fires and other appliances take combustion air from the room. Their permanent air supply must remain open. Tell the ventilation designer and the qualified appliance installer about each appliance before changing vents, adding extraction or sealing draughts. Ask them to check compatibility and whether tests for fumes spilling into the room are needed. A room vent or fan must not be altered without considering this supply.

Radon: this radioactive gas can enter from the ground. You cannot judge its level from the absence of condensation or from a ventilation brochure. The Environmental Protection Agency’s testing page explains how to order a three-month home test through a registered service. Discuss testing before work and retesting afterwards: the EPA specifically identifies insulation and window changes as reasons to reconsider a previous low result. If a result is high, follow its remediation and retesting advice; do not assume a new fan has solved it.

What to receive at handover

Ask for a demonstration in your own home, not just the product leaflet. You should know which openings stay clear, what runs continuously, how boost works and how to recognise a fault.

Keep these with the insulation records:

The agreed ventilation design and equipment details.
Commissioning records: the installer’s tests and adjustments showing the system delivers its intended airflows.
Independent validation where required: a separate qualified validator checks the installed system against the design and applicable requirements. Agree who arranges this and supplies the certificate; commissioning and validation are different tasks.
Instructions for controls, filters, cleaning and servicing, with warranty and defect contacts.

For SEAI-funded mechanical systems, check the commissioning and independent validation requirements in sections 4 and 6.7 of the current standards with your provider before signing. Ask for the relevant records when the work is completed.

Follow the system’s maintenance instructions and keep vents unobstructed. The Department of Housing’s homeowner leaflet is a useful companion. If condensation persists, record it and arrange a return inspection.

For a wider upgrade, use our which insulation first guide to coordinate repairs, ventilation and later heating work. The costs and grants checker compares insulation options; a home survey is still needed to design the ventilation and confirm its cost.

Common questions

Does insulation cause mould?

Insulation can warm cold surfaces and reduce condensation risk. But associated draught sealing or new windows can reduce air leakage, so the home still needs planned ventilation. Existing leaks, cold junctions and inadequate extraction can also contribute. Ask for an investigation if mould appears or returns after work.

Can I close the vents to keep the heat in?

Keep the ventilation openings required by your system clear and use them as instructed. Never block a permanent air supply for a fuel-burning appliance. If a vent causes an uncomfortable draught, ask the installer to check the design and operation rather than covering it.

Is opening a window enough after insulation?

Opening windows can clear a short burst of moisture or pollutants, but it does not establish that the home's everyday ventilation is adequate. Ask the contractor to assess the whole system. Very airtight homes need mechanical ventilation, chosen and designed for the house.

Will a dehumidifier solve the problem?

A dehumidifier can remove moisture from the air, but it does not supply fresh air or repair a leak. It may help while the cause is addressed. Persistent condensation or mould still needs the building, heating and ventilation checked together.

Work out your own numbers

Grant total: add up what each SEAI route pays for your upgrades.
Which grant route: five questions to find where to apply first.

Sources and review

Sources were last checked on 30 October 2026. The review checks published guidance; ask a contractor to survey and design the work for your home.