An insulated home behaves differently

If you have added loft insulation, cavity wall insulation or internal wall insulation in recent years, you have almost certainly made your home warmer and cheaper to run. You have also, perhaps without realising it, changed the way air moves through the building. Older UK homes were draughty by design, or at least by accident. Chimneys, floorboards, sash windows and unsealed joists all leaked air, and that leakage carried moisture out with it.

Seal those routes and the moisture has nowhere to go. A typical household produces somewhere between 10 and 15 litres of water vapour every day, from showering, cooking, boiling kettles, drying laundry and simply breathing. In a leaky house that vapour drifts out through the fabric. In a well-insulated, airtight house it condenses on the first cold surface it finds. Ventilation is not an optional extra for a retrofit; it is part of the insulation package.

What trapped moisture actually does

Damp, stale air is not just an unpleasant smell. It causes real damage to both the building and the people inside it.

• Surface condensation and mould on windows, external wall corners, ceilings and behind furniture, especially in bedrooms where occupants breathe out moisture overnight.
• Interstitial condensation inside the wall or roof build-up, where moisture condenses out of sight. Wet insulation performs far worse than dry insulation, so your carefully calculated U-values quietly stop delivering.
• Fabric decay in timber joists, rafters, lintels and fixings, which can be expensive and disruptive to put right.
• Health effects including dust mites, aggravated asthma, eczema and respiratory infections, particularly for children and older occupants.

The sweet spot for indoor relative humidity is roughly 40 to 60 per cent, and closer to 40 to 55 per cent if you want to keep mould at bay. A cheap hygrometer in a bedroom and a living room will tell you more about your home than any rule of thumb.

Your ventilation options, in plain terms

Ventilation in a UK home is usually split into background ventilation (a slow, continuous trickle) and intermittent extract (a boost when you cook or shower). The main systems you will come across are:

• Trickle vents and window vents providing background air paths. Passive, cheap, and only effective if they are actually left open.
• Intermittent extract fans in kitchens and bathrooms, ideally humidity or boost triggered rather than just switched with the light.
• Decentralised mechanical extract ventilation (dMEV) — continuous, low-power extract units in each wet room. Straightforward to retrofit and a genuine improvement over a tired old fan.
• Centralised mechanical extract ventilation (MEV) — one unit ducting from several wet rooms, running continuously with a boost setting.
• Positive input ventilation (PIV) — pushes filtered air in from the loft, gently pressurising the home and pushing damp air out through the fabric.
• Mechanical ventilation with heat recovery (MVHR) — extracts from wet rooms and supplies warmed fresh air to living spaces, recovering typically 70 to 90 per cent of the heat that would otherwise be lost.

Making MVHR work in a retrofit

MVHR is the gold standard for an airtight, well-insulated home, but retrofitting it needs thought. Duct routes are the biggest constraint: rigid ducting performs better than flexible, and any duct passing through a cold loft must be insulated to prevent condensation inside the pipe. Units are often sited in a loft, a utility cupboard or a bulkhead above a landing.

Two details matter more than the specification sheet. First, commissioning: the system must be balanced so that supply and extract airflows match the design figures, and this should be measured, not guessed. Second, filtration and maintenance: dirty filters strangle airflow and undo the benefit.

If MVHR simply will not fit, a well-installed dMEV system in the bathrooms and kitchen, combined with properly sized trickle vents elsewhere, is a sensible and far cheaper compromise. It will not recover heat, but it will keep moisture under control.

Everyday habits that do the heavy lifting

Whatever system you install, occupant behaviour determines how well it works.

• Run bathroom fans during and for at least 15 to 20 minutes after a shower, and kitchen extract while cooking.
• Dry laundry outside or in a vented tumble dryer. If you must dry indoors, do it in a room with extract running and the door closed.
• Keep saucepan lids on and avoid leaving water simmering unnecessarily.
• Leave trickle vents open, and never block them with curtains or furniture.
• Keep internal doors open, or trim them so there is a gap underneath, to let air travel from dry rooms to wet ones.

Checking it is working

Ventilation is easy to install badly and easy to forget. Ask for airflow measurements at commissioning, keep the paperwork, and check filters every six months. If you are planning a deep retrofit, an airtightness test before and after will show you exactly how much the building has tightened and how much mechanical ventilation it now needs.

Insulate well, seal carefully, and ventilate deliberately. Do all three and you get a home that is warm, dry, healthy and genuinely efficient. Skip the third and the first two will quietly unravel.