Bedroom Air Quality: CO2 Buildup, Dampness, and Practical Fixes for Healthier Winter Sleep

Original publisher: The Conversation

A winter study of airtight Irish homes shows that in more than four out of five bedrooms the air remains unsafe due to high CO2 levels for the entire night, even though occupants judge the air as good. The author explains how living-room behavior as a thermostat fails in bedrooms after sleep, letting CO2 and moisture accumulate and potentially foster mould. The piece also discusses health risks highlighted by Awaab Ishak’s case and offers practical fixes that balance warmth and fresh air.

• In most bedrooms, CO2 stays above safe thresholds all night in winter.
• People tend to keep windows shut in winter, enabling moisture buildup and mould risk.
• Trickle vents are often shut after energy upgrades, reducing background ventilation.
• Practical fixes include keeping vents open, ajar doors, Lüften rituals, and using dehumidifiers with humidity targets around 60%.

Overview

The article reports on a winter study of around two hundred highly energy‑efficient but airtight Irish social homes. The core finding is stark: in more than four out of five bedrooms, the concentration of carbon dioxide (CO2) in the indoor air remained above what is considered a safe threshold for the entire night. The author notes that occupants’ perception of air quality often contradicts this measured reality, with more than 90% of people rating their bedroom air as good even when CO2 levels are high. The piece explains that the problem is not the room itself but the occupants’ behavior: awake in the living areas, people regulate temperature and ventilation like a thermostat with hands. Once asleep, this automatic correction stops in bedrooms, allowing CO2 and moisture to accumulate and create conditions that favour mould growth.

CO2 as a proxy for stale air

CO2 is used as a proxy indicator for stale or poorly ventilated air. Levels are measured in parts per million (ppm). European standards set thresholds for living spaces to be 1,350 ppm above outdoor levels, and bedrooms to 950 ppm above outdoor levels. Outdoor fresh air CO2 typically ranges from 420 to 450 ppm, providing a baseline for indoor measurements. The article highlights that high CO2 levels correlate with insufficient airing and increased humidity, which can lead to damp and mould problems.

What the data show

Between 2022 and 2025, the author continuously monitored temperature, humidity, and CO2 using sensors in roughly 200 Irish rooms in energy‑efficient but airtight social housing. The bedrooms spent about two hours per night in winter above unhealthy levels, while the adjacent living rooms were generally fine. The same rooms were fine in summer, underscoring the influence of seasonal ventilation behavior. The likely reason is that people shut windows during cold weather to stay warm and open them in milder seasons, creating a seasonal swing in air quality.

Health implications and social context

Moisture is a byproduct of respiration. Two sleeping adults emit about a pint of water per night as they breathe, which, if trapped, can condense on cold surfaces such as windows or walls behind furniture. Cold, damp surfaces in warm, moist rooms create ideal conditions for mould growth. This is more concerning for small children whose lungs are still developing. The article cites a historic dampness incident in Rochdale that killed a two‑year‑old child in 2020, leading to Awaab Ishak’s law in England which obligates social landlords to address damp and mould within fixed deadlines. While airing a room (opening a window) clears stale air and moisture quickly, it also expels heat, illustrating the trade‑off between fresh air and warmth.

Ventilation, mould, and the energy‑efficiency paradox

The piece draws a parallel to sick building syndrome, observed after energy shocks when offices were sealed off to save fuel. In modern homes, energy efficiency can inadvertently amplify indoor air quality problems if ventilation is neglected. A 2024 Irish study of retrofitted homes found that bedroom CO2 levels were higher after work than before, with roughly half of these homes showing visible mould, highlighting a potential unintended consequence of energy upgrades unless ventilation is preserved or improved.

What to do: practical steps

The author offers actionable strategies to reduce CO2 and damp without substantially sacrificing warmth. If an open window costs heat, maintain trickle vents and keep doors ajar to provide moisture a path to escape. A German practice of Lüften has gained popularity on TikTok as “house burping”: venting for ten to fifteen minutes after waking to purge stale air and moisture quickly while minimizing heat loss. When extremely cold, dehumidifiers are a sensible option; typical units cost between £90 and £300, draw around 250 watts, and run at roughly 6–9 pence per hour. Most dehumidifiers include a humidistat set to a target around 60% humidity. This helps the machine operate efficiently and maintain comfortable conditions. While a proper ventilation system remains the best solution, dehumidifiers and humidity control can meaningfully alter what is breathed at night. An indoor air quality monitor is recommended for definitive proof of air quality issues, as real‑world data often prompts changes in ventilation or dehumidification habits.

Summary and implications for policy and everyday living

The findings suggest a need to rebalance energy efficiency with adequate ventilation in housing policy and home design. Trickling vents and user behavior play crucial roles in nocturnal air quality, and regulatory frameworks should consider how to ensure these ventilation paths remain open or effectively replaced by other measures that maintain air exchange without unduly increasing energy costs. The article concludes with a practical toolkit for households: monitor air quality, keep vents open, embrace Lüften after waking, and use dehumidifiers with humidity targets while maintaining a healthy indoor environment as part of a broader strategy to prevent damp, mould, and its health risks.