How to Prevent Mould in Rental Properties Between Tenancies

Why empty properties are at higher risk of mould

It might seem counterintuitive - no cooking, no showers, no people - but empty properties are often at higher risk of mould than occupied ones. With no heating running and no natural air movement from daily life, cold damp air sits in the rooms and moisture condenses on cold walls and in corners.

Any existing mould that was present at the end of the tenancy but went unreported or was hidden behind furniture can spread rapidly in an unheated, unventilated property. By the time the new tenant moves in, what was a patch in a corner can cover an entire wall.

The void period is therefore not just a cleaning opportunity - it is your window to inspect properly, deal with any issues found, and set the property up so it does not develop mould problems during the next tenancy.

Step 1: Investigate before you clean

The instinct is to get a cleaner in straight away, but a damp inspection should come first. Cleaning mould without finding the source just means it comes back - and if the new tenancy is already in progress when it returns, you have a complaint on your hands.

• Use a damp meter on external walls, particularly north-facing walls, walls below windows and external corners. Any reading above around 20% on a plaster surface warrants closer attention.
• Check under every sink and around all bath, shower and toilet fittings for slow drips or softening timber.
• Inspect the window reveals and sill returns - these cold junctions are where condensation mould often starts.
• Look in the loft or roof space for signs of condensation or missing insulation.
• Check externally: gutters, downpipes, flashings around chimneys and dormers, and any cracked pointing on external walls. These are the routes for penetrating damp.

Step 2: Treat any mould you find properly

If there is visible mould, treat it before the new tenancy begins. Wiping it with bleach or painting over it is not treatment - it is concealment, and it does not protect you legally.

• Use a proper biocide mould treatment, not a diluted household bleach solution. Biocides kill the mould at root level; bleach removes the colour but leaves the structure intact and the spores ready to regrow.
• Allow the treated surface to dry fully - this can take days in a cold, still property. Use dehumidifiers and ventilation to speed up drying.
• If the wall surface has been damaged or if mould has penetrated into plaster, the affected area will need cutting back and replastering before decoration.
• Anti-mould paint is a useful final layer but it is not a substitute for treating the cause. Paint over a damp wall and you will have peeling paint and mould again within weeks.

Step 3: Fix the underlying moisture source

Every mould problem has a moisture source. Until that is fixed, the mould will return regardless of what you apply to the surface. During the void, you have the access to find and fix it without disrupting a tenant.

Leaks and plumbing

Replace any failing bath or shower sealant, check waste pipes under sinks and baths, and have the roof and flashings inspected if there is any sign of water ingress through the ceiling or in the loft. A slow drip behind a panel or under a bath can go unnoticed for a whole tenancy and cause significant timber rot and mould.

Ventilation

Test every extractor fan in the property. Put your hand or a tissue near the grille with the fan running - if there is little suction, the fan needs replacing. An extractor fan that does not extract is useless against bathroom and kitchen moisture.

If the property has had persistent condensation problems across multiple tenancies, the underlying issue is usually that the building cannot shift moisture fast enough. Options to consider include trickle vents on windows (which allow low-level ventilation even when windows are closed), upgrading extractor fans to higher-flow or humidistat-controlled models, or installing a positive input ventilation (PIV) system, which gently pushes fresh, slightly warmed air into the property and displaces the damp air.

Cold spots and insulation

Cold spots on internal wall surfaces - often external corners, below windows and in north-facing rooms - are where warm moist air loses heat and deposits moisture. Improving insulation reduces the temperature difference between the wall and the air, which means less condensation. This might mean insulating a cold wall internally, upgrading loft insulation, or improving window seals and draught-proofing.

Step 4: Manage the void period itself

How you manage the empty weeks between tenancies makes a big difference to whether the property arrives dry for the new tenant.

• Set the heating to a background minimum - around 14-15°C is enough to keep the fabric of the building above dew point and prevent condensation settling. Turning the heating off entirely in winter is a false economy; you risk mould, frozen pipes and a cold, unwelcoming property for your new tenants.
• Leave internal doors open so air can circulate through the whole property rather than sitting in individual rooms.
• On dry, mild days, open windows briefly to flush out damp air. Avoid doing this on cold, foggy or wet days when it will introduce more moisture than it removes.
• If the void runs longer than two or three weeks, use a dehumidifier set on continuous to manage humidity levels, particularly in winter.
• Do a second damp check in the week before the new tenancy starts. Conditions change, particularly if the void has crossed a period of heavy rain or cold weather.

A mould prevention checklist for the void period

Use this list as your standard process between every tenancy:

• Clear and inspect gutters and downpipes.
• Inspect roof, flashings and external pointing.
• Check all window and door seals for gaps or failures.
• Run a damp meter on all external walls and under-window areas.
• Check under sinks and around all plumbing fittings.
• Test all extractor fans and replace any that are not working properly.
• Treat any visible mould with a proper biocide, not bleach.
• Fix any identified source - leaks, seal failures, ventilation gaps.
• Set heating to a minimum background level throughout the void.
• Ventilate on dry days; use a dehumidifier on wet or cold days.
• Do a final damp check one week before the new tenancy begins.
• Brief new tenants on the importance of ventilating the property - particularly in bathrooms and kitchens - and ask them to report condensation or damp early rather than leave it.

What to tell new tenants

A short note to new tenants on how to manage moisture in the property goes a long way. Explain where the extractor fans are and how to use them, remind them to keep the heating at a reasonable base level in winter rather than letting rooms get very cold and then very hot, and ask them to report any sign of condensation or damp early. A problem spotted in week two is a quick fix; the same problem ignored for six months becomes a dispute.

You cannot control how a tenant lives, but giving them the right information at the start sets clear expectations and reduces the chance of a mould complaint later in the tenancy.

We treat the source, not just the surface - and we back every mould removal job with a 12-month guarantee. If you are preparing a property between tenancies, send a photo on WhatsApp and we can give you a fast quote for any treatment or remedial work needed.

If you need a hand, Simpled Services can help. Call us on 020 4571 7367, message us on WhatsApp at the same number (020 4571 7367), or email hello@simpledservices.co.uk and we will take it from there.