Damp and mould survey: finding the cause, measuring and giving advice

In a damp survey, you find out why a home suffers from damp or mould. Then you set out a plan of action. You look at the building and at how the home is used.

Measuring the damp is one part of it. If it is mainly about mould, it is also called a mould survey.

The problem is growing. In the WoonOnderzoek Nederland 2024 (the Dutch national housing survey), 20% of households say they suffer from damp and mould. In 2021, that was 15%.

Among tenants of a housing corporation, it rose from 24% to 29%. In private rental, it went from 23% to 31%.

Where is the mould? Of the households with mould, 59% have it in the bathroom. For 35% it is in the bedroom, for 23% in the living room.

Who does this work? Site supervisors and specialists of housing corporations, contractors and expert advisers, and independent damp and mould experts.

Often there is not one cause, but a combination. Sometimes the groundwater level also plays a role. That is why you go through all possible causes.

Take care with sustainability upgrades too. Upgrading a home without attention to ventilation can cause new damp problems. That also applies if new thermal bridges arise in the process.

These are the causes you check:

• Condensation: a lot of moisture from cooking, showering and drying laundry, with too little ventilation or heating.
• Thermal bridges and interstitial condensation.
• Rising damp and groundwater pressure.
• Penetrating damp and leaks.
• Too few provisions for ventilation.
• Construction moisture.

The Aedes framework “Aanpak van vochtproblemen in woningen” (tackling damp problems in homes) names a fixed set of measuring tools. With them, you support what you see.

The framework also gives a decision tree per type of problem, from condensation and thermal bridges to construction moisture.

Note: the framework still refers to the old Bouwbesluit 2012 (the former Dutch building regulations). For today's requirements, look in the Bbl (Besluit bouwwerken leefomgeving, the current Dutch building regulations).

These are the measuring tools the framework names:

• A handheld meter for temperature and humidity.
• A moisture meter for materials.
• Meters for temperature and relative humidity over a longer period.
• A CO2 meter to assess the ventilation.
• A thermal camera (thermography) to find thermal bridges.
• An endoscope.

The Besluit bouwwerken leefomgeving (Bbl) sets minimum ventilation capacities. Different requirements apply to new builds than to an existing home.

The Bbl has applied since 1 January 2024. The Bouwbesluit 2012 lapsed at that time. What NEN 1087 and NEN 8087 cover is explained on the NEN 1087 page (in Dutch).

For new builds, you determine the capacity according to NEN 1087 (the Dutch standard for ventilation in new buildings). The Bbl then requires 0.9 dm³/s per m² of habitable area (verblijfsgebied) and 0.7 dm³/s per m² of habitable room (verblijfsruimte). Each habitable area and each habitable room gets at least 7 dm³/s (art. 4.122).

In new builds, a room with a cooking appliance requires 21 dm³/s. A toilet requires 7 dm³/s and a bathroom 14 dm³/s. For the bathroom, IPLO (the Dutch government's information point on building rules) states the aim: preventing too much water vapour and mould.

For purge ventilation (spuiventilatie) in new builds, 6 dm³/s per m² of habitable area and 3 dm³/s per m² of habitable room apply (art. 4.131).

For an existing home, you determine the capacity according to NEN 8087 (its counterpart for existing buildings). These are the requirements:

• Habitable room: at least 0.7 dm³/s per m², and at least 7 dm³/s (art. 3.67).
• Toilet: 7 dm³/s. Bathroom: 14 dm³/s.
• Room with a cooking appliance: 21 dm³/s (art. 3.67). At least 21 dm³/s goes directly outside (art. 3.71).
• Purge ventilation: 3 dm³/s per m² (art. 3.73).

NEN 2778 is called Vochtwering in gebouwen (moisture control in buildings). The standard gives methods to determine four properties of a construction. These are watertightness, rain resistance, water absorption and the temperature factor of the inner surface.

On nen.nl, NEN 2778:2015 is listed as final (definitief). With a free account, you can view the standard through NEN Connect.

NPR 2652 from 2008 gives examples of construction details for homes. When built correctly, they are watertight and rain-resistant, tested according to NEN 2778.

For new builds, the Bbl also sets a requirement for the floor above the crawl space. It lets through at most 20 × 10⁻⁶ m³ of air per m² per second, measured according to NEN 2690.

The Bbl refers to NEN 2778 for habitable rooms, toilets and bathrooms. These are the requirements:

• Watertight to the outside, such as roof and facade: art. 3.64 (existing buildings) and 4.118 (new builds)
• Watertight towards the crawl space: the same articles
• Existing buildings: water absorption in the bathroom up to 1 m above the floor (art. 3.65)
• New builds: water absorption in toilet and bathroom up to 1.2 m above the floor (art. 4.120)
• New builds: a temperature factor of at least 0.65 for a home (art. 4.119)

Disputes can arise about the cause: is it the building or the way the home is lived in? Mould also sometimes comes back after earlier measures.

From 2027 to 2029, the central government is setting aside €2 million a year for damp and mould experts. In that plan, landlords pay 75% of a survey. The government pays 25%, through municipalities. It expects about 4,600 surveys a year.

The expert writes an independent report with conclusions and recommendations. The Aedes framework also recommends such a specialist for recurring reports and complex damage. That also applies to a court case or a case at the Huurcommissie (the Dutch Rent Tribunal).

In a defects procedure, the Huurcommissie carries out its own building investigation in the home. The tenant must first report the defect to the landlord. For serious damp that the tenant did not cause, the Huurcommissie can temporarily reduce the rent.

According to the Nationale Prestatieafspraken (the national performance agreements on housing), these are the cases where an independent expert is called in:

• The cause is unclear: is it the building or the way the occupants use it?
• The corporation and the tenant cannot resolve it together.
• The mould comes back, even after earlier measures.

A good report is drawn up independently and gives conclusions and recommendations. It distinguishes between the building or the installations and the use of the home. The Aedes framework assigns the former to the corporation and the use to the tenant.

The location and the size of the patches also matter. They determine whether it is a serious defect for the Huurcommissie.

For rising or penetrating damp in a habitable room, the threshold is generally 0.25 m². In other spaces, such as a storeroom, basement, hallway or toilet, that is from 1 m².

Mould from condensation on the ceiling or walls of a habitable room also counts from 0.25 m². That applies if a technical shortcoming is the cause. A room that can no longer be used because of damp is also a serious defect.

This belongs in a good damp report:

• All causes investigated, with the cause or causes you found.
• The measurements, with the room and the location of each measurement.
• For each room, the location and the area of the patches.
• Where relevant: the ventilation set against the Bbl requirements.
• Measures for each cause you found.
• A check a few weeks after the measures.

The damp and mould survey is listed in Keursnel as coming soon. You cannot choose it yet. There is no date, and which features will come has not been decided.

You already come across damp in the building inspection. For this, the template has the chapters “22 Optrekkend vocht” (rising damp) and “14 Ventilatie” (ventilation). If further investigation is needed, you can give the rating “nader onderzoek” (further investigation).

There are also rows for moisture measurements: in the bathroom, at the base of the walls and at the chimney. There, you choose the outcome of your measurement. Keursnel does not test against NEN 2778. For chapter 14, the template states that the ventilation was assessed visually, without measuring the amount of air.

The NHG annex (NHG is the Dutch National Mortgage Guarantee) has its own code for this subject: B.5 Ventilatie/vocht (ventilation/damp). Line items with the rating “nader onderzoek” appear in the NHG annex under the remarks.

With a paid plan, the report uses your firm's house style. Do you carry out damp surveys and would you like to do so in Keursnel later? Let us know via the contact page.

You can already use this in the building inspection:

• Photos with arrow, circle, frame, numbered marker and measuring line. The original photo never changes.
• The time the photo was taken, from the file itself (Exif).
• Built to work without internet: changes and photos go to the server later.
• Team and roles: main admin, manager and team member.
• A report with a checksum, which no longer changes after delivery.
• A secret share link per report version, which you can revoke.