How to Prevent Damp & Mould in Existing Houses – An Expert’s Guide

Find out how to protect your property’s structure from the damage caused by damp and mould, with practical tips and insight from Build It’s period property expert, Alan Tierney

Alan Tierney

Damp and mould can pose serious problems in your home. If you’re lucky, they’ll only lead to the discoloration and deterioration of internal surface finishes. But at the other end of the scale, because wet walls conduct heat out of a building more than dry walls, damp can have a negative impact on the thermal efficiency of your house – creating a cold and uncomfortable environment that’s expensive to warm.

The presence of excess water in building fabric tends to promote rot. The highest risk is in timber elements that are subject to attack from wood boring insects and fungal decay. This can affect everything from floors, windows and doors to roofs and structural beams. Over time, this can become very serious indeed.

Mould growth inside homes is potentially the most serious of all. The spores of some species of mould are harmful to our health, particularly in terms of causing or aggravating respiratory illnesses and allergies.

Here, I’m exploring some common causes of damp and mould, and looking into the different ways you can prevent further damp-related issues when renovating a house, including:

Damp vs mould – what’s the difference?
The common causes of damp in existing homes
Why damp & mould can occur in modern homes
My top tips for preventing further damage
Damp & mould FAQs answered

What’s the difference between damp and mould?

Damp is caused by water penetrating the building fabric and becoming trapped there. Usually, this is from external sources but can also be from leaking plumbing or heating.
Mould grows where there is excessive condensation within the building. Generally, the source of this is internally generated moisture from bathing, cooking, washing and (to a surprising extent) occupants breathing.

Preventing Damp & Mould in Houses – What Do You Need to Do?

Penetrating damp is quickly deteriorating the surface and finishes of this home’s walls

Together, damp and mould can create an internal environment with high levels of humidity, leading to condensation on any cold surfaces. Poor ventilation is the main factor in promoting condensation-driven mould growth. It often goes hand in hand with damp, which tends to cool wall surfaces, providing large areas for condensation to occur.

Understanding theses causes is essential to preventing and dealing the root causes of these problems. Too much damp treatment work only seeks to treat the symptoms, such as stained internal finishes, rather than identifying and fixing the underlying issues.

What are the most common causes of damp & what do I need to look out for?

One of the biggest mistakes when dealing with damp in period properties is to focus on the kind of damp that’s present. The only effective way to deal with damp is to identify where the water is coming from and the mechanisms involved. Often this can be a complex issue, and it may even be the result of a combination of factors – hence why remedies sold as catch-all solutions may not be up to the task.

Older houses (generally built before 1918) are very different from modern homes. They comprise permeable materials that readily absorb moisture, which then evaporates when conditions are favourable. Over time, almost all will have been adjusted, usually more than once. It’s common for alterations to introduce impermeable materials, such as cement render or masonry paint that prevent evaporation. These interventions can cause serious dampness.

The most common sources and causes of damp in old buildings are:

Gutters & downpipes

Inadequate, poorly maintained or leaking gutters can concentrate large amounts of rainwater in one part of a wall. Even an apparently insignificant drip from a leaking joint will yield a surprisingly large volume of water, risking damp in the adjacent wall.

This poorly detailed porch is deflecting rainwater into the wall

Similarly, extreme rainfall can overwhelm gutters, pouring large amounts of water into the wall below. Climate change is increasing the frequency of such events, so gutters that used to be adequate are often not any more. In addition, pinholes in downpipes can spray a powerful jet into the wall behind.

Cement renders

Using impervious materials or finishes on the outside of a wall will disrupt its breathable performance as they will prevent evaporation. To make matters worse, a cement render is rigid and inflexible; while a period home’s underlying building fabric, because of its softer construction materials, is subject to a higher degree of natural movement. The inevitable result is that the render will crack; and these fissures will let in driving rain that becomes trapped, making the wall progressively damper.

Modern floors

Replacing a breathable solid floor, such as brick or quarry tiles, with an impermeable option (ie solid concrete with a damp-proof membrane) will prevent rising ground water from evaporating through the structure as it should. When the water hits the barrier, it is forced sideways into the base of the wall, which then becomes damp.

Defective brick pointing

If there are gaps in the pointing of the external face of a brick wall, driving rain will penetrate into the structural fabric but be unable to escape, making it damp. In this case, you’ll need to repoint the brickwork.

Raised ground

The external ground level should always be lower than the internal floor. If not, water contained in the soil will penetrate the wall above the internal floor level, causing it to become damp. If there is a suspended timber floor this can provoke severe decay, particularly to the joist ends. The situation is made very much worse if there is hard landscaping (such as driveway materials) right up to the wall, especially if it does not have a good fall (incline) down and away from the house.

Inadequate ventilation

Modern living creates a large amount of water vapour from sources such as cooking, washing, bathing and occupants breathing. If there is not sufficient ventilation to remove this airborne moisture, serious condensation problems can result, especially if walls are already cold, damp or both. Effective extract ventilation in kitchens and bathrooms is crucial for preventing this.

Why do damp and mould-related issues occur in newer, modern homes?

Modern houses incorporate impermeable materials, multiple vapour barriers and damp-proof courses to control moisture penetration. They strongly rely on the integrity of these barriers to prevent issues arising.

For many early to mid-20th century houses, the materials used have not stood the test of time and have degraded, allowing water penetration. Changes to and the installation of new services frequently lead to the perforation of the protective barriers. These buildings were mostly constructed with uninsulated cavity walls.

Black mould growing in condensation on cold, damp walls

In recent years, there has been an extensive programme of retrofitting cavity wall insulation that has had some serious unintended consequences. Because the cavity wall doesn’t have a physical moisture barrier, filling it with insulation material can result in damp penetrating the inside. This can be quite extreme in more exposed locations (eg the western British Isles).

More recent houses should have effective barriers to moisture and be resistant to damp penetration. There is still a risk of later work penetrating these barriers, so due care is essential. Unfortunately, the biggest problem experienced relates to poor construction. There are numerous examples of moisture barriers not being correctly installed or even not installed at all.

It’s not uncommon for newly fitted membranes to be perforated by the next person on site, compromising performance before the house is even finished. For economy, roofs often have minimal eaves and no overhang at the verge at all. This causes runoff to saturate the outer leaf of the wall; if there is any flaw in the moisture barrier this will penetrate through the wall.

How can you prevent damp & mould from growing? Here are my top tips:

Upgrading gutters & downpipes

Unsurprisingly, the principal source of water penetrating buildings is rainfall. The best means of protection is rainwater goods (gutters and downpipes). The most important factor in preventing damp in buildings is to ensure that these goods are functioning effectively.

You will probably be alarmed to discover that, in more than 20 years of inspecting buildings, I have never found the rainwater goods to be performing as well as they should and I’ve never investigated damp in a building where failing rainwater goods were not a contributing factor.

So, the message is clear: sort out the gutters, keep them clear, clean and well maintained. There are many things to look out for. Leaking joints (even seemingly small drips) can concentrate a surprisingly large amount of water in a small area. Damaged or degraded gutters and downpipes can be cracked, deformed or broken – cast iron and PVC are the most susceptible.

Featuring a milled finish, this aluminium rainwater system from Yeoman Rainguard retains its silvery hue over time, for a modern look

Gutters are often simply too small for the job, either due to poor initial design or increased rainfall as a result of climate change. Downpipes often don’t lead to effective drainage to take the collected water away from the building – sometimes they simply empty at the base of walls.

Looking at alterations

The next thing to consider is past alterations that interfere with the intended performance of the building. For older properties the priority will be assessing the introduction of any impervious materials that are trapping water – cement is always the prime culprit. If the material is causing problems it should be removed, provided it can be done without causing harm. If reinstatement is needed this should be done using vapour permeable materials.

Any building that has been extended or had roof details altered runs the risk of those new details or junctions concentrating rainwater runoff in vulnerable spots. The best way to check for any issues is to go out in heavy rain and see how the water flows.

Addressing poorly-installed insulation

For a number of reasons, any insulation that has been retrofitted has potential risks. For old homes, this is mostly the potential to block evaporation pathways. I’ve already mentioned the risks to exposed cavity walls. External wall insulation must be fitted to a very high standard with extreme attention paid to details that ensure water cannot get in behind the insulation. Spray foam products can cause such serious condensation problems that a recent report found houses where it had been fitted to be effectively unmortgageable.

Improving the house’s ventilation

After ensuring that your building fabric is dry, effective ventilation is key to preventing internal condensation and mould growth. Older buildings will have plenty of air filtration – usually too much! Here are some tips for keeping a modernised home well-ventilated:

Trickle vents are background ventilators integrated into window and door frames to allow a constant airflow to an internal space. While it may be tempting to block trickle vents to stop draughts, it’s essential that you don’t

Extract ventilation is usually most effectively provided by high volume extractor fans in bathrooms and cooker hoods in the kitchen. They should be fitted with an overrun timer set to keep the fan going for at least 30 minutes after the last use. Like trickle vents, people are often tempted not to run extractor fans to save electricity, but the energy cost is trivial compared with the benefits they convey.

Above: This Rega MVHR unit has been fitted in the property’s utility room. The insulated ducks reduce the risk of condensation

In some of today’s highly-airtight homes, a sophisticated ventilation system is required to maintain both infiltration and extraction with minimal heat loss. These are usually mechanical ventilation and heat recovery (MVHR) systems, though there are many options available.

Damp & mould FAQs

How do I know whether the issue is rising damp, penetrating damp or condensation?

Rising damp will rise up from the ground, through the walls. The issue will likely be fairly obvious, with dark, damp patches either outside on the external facade, or inside above the skirting board. If internal, you may see peeling paint or wallpaper towards the bottom of the surface. The root cause of rising damp is usually a failed damp proof course (DPC).

Penetrating damp is usually caused by external defects such as damaged brickwork, or render, or blocked gutters and downpipes. Internally, it will appear as damp circles or patches on the wall, or dry flaking paint or plaster.

Condensation will be fairly obvious, but if not dealt with can cause issues. It’s triggered when moisture from cooking, bathing, drying washing etc is combined with poor ventilation and cold surfaces, preventing evaporation. You can deal with this by opening windows, turning on extractor fans and turning on the heating regularly.

How can making energy-efficiency upgrades backfire and potentially cause condensation/the development of mould?

Elements such as your windows, walls, floors and roof, cannot always be upgraded independently. These features have junctions with each other, and by insulating one thermal element you make another relatively cooler. By doing this, you’re creating the perfect environment for condensation and mould growth.

A retrofit strategy must also go beyond insulation. The lead technical author of the Publicly Available Specification on retrofit (PAS 2035) has a saying, adapted from protests made ahead of the US Declaration of Independence: “no insulation without ventilation!” The process of improving the thermal performance of walls, roofs windows etc reduces natural ventilation (air leakage). Many houses rely on this to provide adequate fresh air to breathe. So, it’s essential to investigate this whenever you’re retrofitting insulation, and to improve ventilation wherever necessary.

Answered by Nigel Griffiths

Nigel Griffiths is director of the Sustainable Traditional Buildings Alliance and a consultant for energy efficiency policy and evaluation. He specialises in sustainable construction, renewable energy and building performance. He is also the author of the Haynes Eco House Manual.

How can I tell whether mould is growing behind surfaces inside my home (for example, behind wallpaper)?

Mould can grow behind your interior surface finishes, such as paint and wallpaper, and go unnoticed as it thrives in dark environments. It’ll likely manifest in dark, shadow-like patches and may give off a damp, musty smell, or feel soft to touch. You may see peeling wallpaper, too, due to the adhesive wearing off. To be sure, you can also use a moisture meter or detector to identify moisture levels in the walls.

Can I access funding to help combat and get rid of damp and mould inside my home?

Yes, you can access funding to make improvements to your home and combat the growth of mould and damp. The Warm Homes Grant is a government-funded scheme, delivered by local authorities, which provides homeowners with grants to make upgrades such as insulation or switching to a heat pump. Eligibility is usually based on a combination of income and the home’s current energy performance certificate (EPC).

If you’re undertaking a wider retrofit project and your plans include making energy efficient upgrades, however, it’s worth exploring green mortgage options. “These are loans that reward borrowers for investing in energy-saving measures by providing lower interest rates, cashback offers or more generous borrowing terms,” says Build It’s project management expert, Mark Stevenson. “Qualifying improvements might include insulation upgrades, heating improvements, installing photovoltaic and solar panels, fitting triple glazed windows, or a range of other renewable energy technologies.” Most high street lenders offer this kind of renovation funding. “They’ll usually ask to see some kind of retrofit plan, alongside an estimated project timeline,” says Mark.

Answered by Mark Stevenson

Mark Stevenson has worked as a construction professional for over 35 years and is a serial self builder. He runs project management consultancy Elsworth Projects and, as chair of NaCSBA, advocates for the interests of self builders and contributes to the success of the emerging custom build industry.

Alan Tierney

Alan Tierney is a period property and conservation specialist. He ran a historic building consultancy, offering hands-on advice to the owners of heritage homes, but is now focused on renovating Old Barnstaple House.