5 Things You Need to Know About Damp, Mould and Heating Efficiency

At the start of 2025, we set out to get a better understanding of the root causes of damp and mould in rented homes.

With the help of Tenancy Deposit Scheme and over 100 private landlords and estate agents, we dug deep into how heating, ventilation, insulation, and daily habits all play a part.

The result? A clear, actionable list of the five key areas to focus on when tackling damp, mould, ventilation and heating issues. Here's what we found:

1. Heating: It’s Not Just About the Temperature

• 18°C is a good rule of thumb for avoiding condensation, but not all homes can manage this with heating alone. Poor insulation can make this target hard to reach—or too expensive to maintain.

• We calculated the ideal temperature and humidity needed in each home to avoid condensation and considered whether retrofitting was more cost-effective than increasing heat.

• Heater size and placement matters. For example, a 1kW heater in a room losing 1.4kW of heat per hour just won’t cut it.

• Cold hallways are a hidden culprit. Warm, moist air from a bathroom can hit a cold hallway and condense quickly—creating prime mould conditions.

• Heating costs add up. Raising a poorly insulated home from 16°C to 18°C could cost an extra £40/month. Affordability matters.

• Sharp temperature spikes for short periods often indicate fuel poverty or poor control over heating systems. Identifying this allowed us to recommend better heater setups or additional support for residents.

2. Moisture: We All Breathe, Cook, and Shower

• People generate a surprising amount of moisture. Even leaving a window slightly open—just 1cm—can help keep humidity levels in check.

• That said, sometimes we're producing more moisture than the home can remove—leading to condensation.

• We identified key points in the day when air needed to be purged—and for how long—to avoid build-up.

• Outdoor moisture affects indoor conditions. Inside humidity levels often mirror what’s happening outside. Recognising this helped us identify when behaviour or mechanical solutions were needed.

3. Ventilation & Fans: Set It and Forget It? Not Quite

• Many extractor fans are set incorrectly. Often tied to a light switch and activating only at 70% RH, they run for too short a time and shut off before doing their job.

• We worked out how much moisture needed to be removed and converted that into optimal RH settings and overrun times for each fan—much more effective than guessing.

• Does opening a window help? Yes—if done right. On still, humid days it’s less effective, but even leaving a window slightly open helps. A 1cm gap can maintain balance, while opening a window wide for 10 minutes can level out a room fast.

• Timing is everything: opening windows before high-moisture activities like showers or drying laundry, helps prevent condensation and damage.

4. Insulation: Not All Walls Are Created Equal

• The majority of walls we tested would have remained damp-free if the room stayed at 18°C and 55% RH - even Victorian homes

• We calculate the heat losses and cold bridge effect to help determine whether existing walls could perform well enough—or if insulation upgrades were truly needed.

• There’s widespread scepticism about EPC ratings, despite many high-cost decisions relying on them. By calculating the exact conditions required to keep mould at bay—and the cost savings of retrofitting—we gave landlords solid data to make confident, cost-effective decisions.

5. Behaviour: Small Habits, Big Impact

While the technical side of heating and ventilation is crucial, resident habits play a big role too. From knowing when to leave a window slightly open, to how long to run a fan, or managing heating schedules—these behaviours often tip the balance between a healthy home and one where damp and mould take hold.

We took the data we gathered and turned it into everyday language that was easy to follow—tailored to the specific home and the person living in it. That made it easier for residents to take action, understand why it mattered, and stay engaged in the long term.

Final Thoughts

Damp and mould are rarely caused by just one issue. It's usually a combination of poor heating, inadequate ventilation, insulation gaps, and daily habits. The good news? With the right knowledge—and a bit of planning—most of these issues can be tackled affordably and effectively.

If you're a landlord, estate agent, or just someone looking to make your home healthier, focusing on these five areas is a great place to start.

Talk to us to find out how we can help you dampsmart.co/contact

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