Why does my new build feel stuffy or have poor air quality?

What this usually means

Modern new builds are constructed to be airtight, because uncontrolled air leakage wastes energy; the building regulations and the energy calculations assume a tight envelope. But a tight envelope removes the accidental ventilation that older, leaky homes relied on, so a new build must have purpose-provided ventilation — continuous mechanical extract, or a mechanical ventilation with heat recovery (MVHR) system — to supply fresh air and remove the moisture and pollutants a household generates. The airtightness and the ventilation are two halves of one design: tight envelope, controlled ventilation. When the ventilation half does not work, the airtight home has no way to breathe, and it feels stuffy and becomes humid.

The usual reason the ventilation does not work is that it was never properly commissioned. An MVHR or extract system is frequently installed and switched on but never set up to deliver the design airflow to and from each room, so it moves far too little air; or it is out of balance, has its boost functions disabled, or the ducts are poorly run. The occupant is then often told to 'open the windows' — which defeats the airtight, energy-efficient design and still does not provide reliable ventilation. The stuffiness, humidity, lingering cooking smells, and any condensation or mould are the symptoms of a ventilation system that is present but not performing.

Because this is a defect rather than an inherent feature, it should be measured and put right rather than tolerated. Measuring the actual airflow at each terminal reveals whether the system delivers the designed ventilation; checking the commissioning records (or their absence) and the installation shows why it does not. Where the home is in warranty, documenting that the ventilation does not achieve its design flow rates is evidence the developer should act on, because the home does not meet the ventilation standard it was built to. The fix is to commission or correct the system so it delivers fresh air and removes moisture as intended. An assessment establishes the shortfall, provides the evidence, and defines the remedy, so the airtight home gets the working ventilation it depends on.

Common causes

Signs and symptoms

A dwelling's indoor air quality and humidity depend on the rate at which fresh air is supplied and stale, moist air removed. In a leaky older home, infiltration through the envelope provided a large, if uncontrolled, share of this exchange; deliberately airtight modern construction suppresses that infiltration to save energy, so the required air exchange must be delivered by the designed ventilation system. If that system fails to achieve its design flow rates, the air-change rate falls below what is needed to dilute pollutants and remove moisture, and the indoor humidity, carbon dioxide and contaminant levels rise — experienced as stuffiness, condensation and poor air quality. The airtightness is not the fault; the unmet ventilation is.

Ventilation systems deliver their performance only when commissioned and correctly installed. An MVHR or continuous extract system is designed for specific supply and extract rates per room, balanced across the dwelling; commissioning measures and adjusts the terminals and fan to achieve these. Uncommissioned systems run at arbitrary, usually deficient, flows; imbalance, disabled boost, clogged filters, and crushed or poorly routed ducts all reduce the delivered air. The consequence in an airtight home is direct and predictable: insufficient ventilation and the resulting humidity and pollutant build-up, because there is no compensating infiltration. Advising occupants to open windows substitutes an uncontrolled, intermittent, energy-wasting exchange for the designed one and does not reliably meet the ventilation requirement.

Diagnosis is therefore a measurement of the ventilation against its design, and it doubles as evidence. Airflow measurement at each supply and extract terminal establishes whether the system meets the design rates and balance; inspection of the installation and commissioning documentation explains any shortfall; and humidity and air-quality monitoring confirm the effect. Where the home is within warranty, a documented failure to achieve the design ventilation demonstrates that the dwelling does not meet the standard it was built to — the basis for the developer to commission or correct the system. The remedy follows from the measurement: commissioning, re-balancing, duct correction or installation rectification, after which the airtight home receives the controlled ventilation that makes it healthy and efficient. This measured approach both fixes the home and supports a claim, in contrast to living with stuffiness or masking it with open windows.

How to fix a stuffy new build

Measure the ventilation against its design, document the shortfall as a defect, and get the system commissioned or corrected so the airtight home receives the fresh air it depends on.

1. 01

   Measure the airflow

   Check the actual supply and extract rates at each terminal against the design and balance.

2. 02

   Check commissioning and installation

   Establish whether the system was set up and whether the ducts and unit are sound.

3. 03

   Confirm the air quality

   Monitor the humidity and air quality to confirm the ventilation shortfall's effect.

4. 04

   Document the defect

   Record that the ventilation does not meet its design rates as evidence for the developer.

5. 05

   Commission or correct the system

   Set the system to its design flow rates and balance, or rectify the installation.

6. 06

   Verify the result

   Confirm the home no longer feels stuffy and the humidity and air quality have improved.

How to prevent it coming back

• Ensure the ventilation is commissioned with records when handed over.
• Do not rely on opening windows to compensate for failed ventilation.
• Maintain the system's filters and ducts.
• Treat a stuffy airtight home as a ventilation defect to fix.