Modern homes are built to be airtight — and that's exactly why ventilation matters more than ever. As insulation standards have risen and building envelopes have become more sealed, the moisture produced by everyday living has fewer ways to escape. Get the ventilation wrong and that moisture ends up where you least want it: inside the wall.

Why airtight building makes ventilation critical

Tighter, better-insulated buildings lose less heat — but they also trap more water vapour. Cooking, washing, drying clothes and simply breathing all add moisture to the air. In a draughty older house, that moisture leaked away through gaps. In a modern airtight build, it has to be managed deliberately, or it condenses on the coldest surfaces and inside cavities.

That's why current guidance combines an airtight envelope with controlled ventilation rather than relying on accidental draughts. In practice that means two jobs working together: keeping the wall cavities dry, and giving the rooms a steady supply of fresh background air. The Active Trickle Vent handles the rooms — moving fresh air through living spaces without the heat loss of an open window — while the Active Weep Vent keeps the cavity itself draining and breathing.

What good ventilation controls

• Moisture — preventing condensation, damp patches and the mould that follows.
• Air quality — clearing stale air, cooking smells and indoor pollutants.
• Building fabric — keeping walls, insulation and timbers dry so they last.
• Energy performance — because dry walls insulate far better than damp ones.

That last point is the one most people underestimate. Insulation only works because it traps still air, and air barely conducts heat. Water conducts it around 25 times more readily — roughly 0.6 W/(m·K) versus 0.024 W/(m·K) (thermal conductivity reference data) — so as soon as damp replaces that trapped air, heat flows straight through. Worse, the water tracks between insulation boards and bridges across wall ties, mortar and cavity trays, creating a continuous thermal bridge from the warm inner leaf to the cold outer skin (Premier Guarantee).

The numbers are stark. In controlled tests, mineral-wool insulation at just 5% moisture content lost enough performance for its thermal conductivity to rise by around 90% — nearly doubling the heat it lets through (Hayrullin et al., Transportation Research Procedia, 2022). For masonry, every 1% rise in moisture content increases thermal conductivity by roughly 5%, and saturated brick or stone can conduct up to about twice as much heat (Ismaiel et al., Journal of Building Physics, 2022). Keeping cavities dry is one of the cheapest ways to protect a home's energy efficiency.

Where cavity wall ventilation fits in

Whole-house and mechanical systems handle the air inside your rooms, but the cavity in your external walls needs to breathe too. Weep vents let moisture drain and air move through the outer leaf. UK guidance (NHBC Standards Chapter 6 and Approved Document C) sets out weep vents at a maximum of 900mm centres, tightening to around 450mm with at least two per opening above windows and doors — we cover this in full in our NHBC cavity wall ventilation guide.

The catch with a standard weep vent is that it can block — mortar dropped into the cavity during the build is the classic culprit — and once blocked, it can't easily be cleared or inspected.

Why choose Active Vent

The Active Weep Vent tackles exactly that problem. Its removable insert means the vent can be opened up to flush out any dropped mortar and to inspect that the cavity is still clear — something a fixed weep vent simply doesn't allow. It's a small feature that makes a big difference to whether a cavity actually stays dry over the life of the building.

Installation in brief

1. Assess the property and identify where moisture is most likely to collect.
2. Follow the installation guide and specified spacing.
3. Keep the cavity clean, and use the removable insert to clear any debris.
4. Check airflow and drainage before final detailing.

For the full range and specifications, visit the shop or get in touch for advice tailored to your project.

Sources

• Hayrullin, A. et al. (2022), Transportation Research Procedia, Vol. 63 — insulation thermal conductivity under dry and moist conditions
• Ismaiel, M. et al. (2022), Journal of Building Physics — thermal resistance of masonry walls: a literature review
• Premier Guarantee — thermal bridging: what it is and how to prevent it
• NHBC Standards, Chapter 6 — Superstructure (excluding roofs)
• Approved Document C — Site preparation and resistance to contaminants and moisture (Building Regulations)
• Thermal conductivity of water vs air — reference data