The Underlying Problem Both Systems Solve
UK homes are generating more moisture than ever. We shower more, dry laundry indoors more, and cook more — and because we have been sealing our homes for energy efficiency, that moisture has nowhere to go. The result is condensation on windows and cold walls, which leads to black mould growth, musty odours, and in serious cases, respiratory health problems particularly in children and the elderly.
Both Positive Input Ventilation (PIV) and Mechanical Ventilation with Heat Recovery (MVHR) address this problem by introducing controlled fresh air. They differ fundamentally in how they do it, what they cost, and how effective they are in practice.
How Positive Input Ventilation Works
PIV is the simpler of the two systems. A single fan unit is installed in the loft (for houses) or in a ceiling void (for flats). It draws air from the loft space or outside, gently filters it, and pushes it into the property through a ceiling diffuser — usually in the hall. This positive pressure pushes stale, moist air out through natural gaps around windows, doors, and vents.
PIV units are inexpensive (£400-£900 supplied and installed), easy to retrofit in any property, and require minimal disruption. Modern PIV units like the Nuaire Drimaster Eco include gentle heating to avoid cold draughts. Running costs are low: typically 10-30W, costing £15-£40 per year in electricity. PIV works well in moderately draughty properties — it relies on natural gaps for exhaust, so in a very airtight new-build, it is less effective.
How MVHR Works
MVHR is a whole-house ventilation system with two airflows: supply air from outside to habitable rooms, and extract air from wet rooms (bathrooms, kitchen, utility). Both airflows pass through a heat exchanger unit, which transfers 70-90% of the heat from the extracted air to the incoming fresh air. This means you get fresh air without losing your heating.
MVHR is significantly more complex and expensive. The unit itself costs £1,500-£3,000, and the full installation with ducting and commissioning costs £3,000-£8,000 for a typical house. MVHR works best in new builds or whole-house retrofits where ductwork can be routed cleanly. In an existing home with low ceilings and limited ceiling voids, MVHR ducting becomes a significant construction project.
Performance Comparison: When PIV Is Good Enough
For a typical semi-detached or terraced house in East Anglia with moderate condensation issues, PIV is almost always good enough — and dramatically cheaper. It will reliably reduce condensation on windows, reduce surface mould, and improve background air quality for around £500-£900 installed. If you are retrofitting ventilation as part of an insulation or heat pump upgrade and the property is not exceptionally airtight, PIV is the pragmatic choice.
MVHR becomes the right choice when: you are building or whole-house retrofitting to Passivhaus or high-insulation standards where the building is highly airtight, you want to genuinely recover the heat from extract air and reduce heating demand, or you need whole-house fresh air supply as part of a premium build specification.
For East Anglia Properties: Our Recommendation
The majority of retrofit properties we survey in Cambridgeshire, Norfolk, and Suffolk benefit most from PIV. It is reliable, low maintenance (annual filter check), and solves the condensation problem effectively. We install Nuaire Drimaster units, which are the market-leading PIV product for loft installations.
For new-build projects and whole-house retrofits with significant insulation upgrades — particularly EWI or internal wall insulation that raises airtightness significantly — we specify MVHR as part of the design. In these projects, the ventilation strategy is designed at the same time as the insulation specification, ensuring the ductwork is integrated properly. Contact us to discuss which approach is right for your property and retrofit goals.
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