How Do I Check If My Internal Walls Have Moisture Damage?

Moisture damage inside walls is one of the most common — and costly — building problems in New Zealand homes. You can do a solid initial check yourself using your eyes, nose, and a few basic tools, but a licensed building inspector will give you the definitive picture if you find anything concerning.

Why Moisture Gets Into Internal Walls

NZ homes face moisture from several directions: weather-tightness failures in the cladding or roof, condensation from poor ventilation, plumbing leaks behind linings, and rising damp from the subfloor. Older homes — particularly those built between the 1990s and early 2000s with monolithic cladding — are especially prone to weather-tightness issues. Understanding the source matters, because the fix is very different depending on where the water is coming from.

Step 1 — Start With a Visual Inspection

Walk through every room in good natural light and look for:

• Staining or discolouration on walls or ceilings — yellow, brown, or rust-coloured marks often indicate water tracking down from above
• Paint bubbling, peeling, or flaking — moisture trapped behind paint breaks the bond with the substrate
• Efflorescence — white chalky deposits on plaster or masonry walls, caused by salts left behind as water evaporates
• Visible mould or mildew — black, green, or grey patches, often in corners, around windows, or near skirting boards
• Warped or swollen skirting boards and architraves — timber swells when it absorbs moisture
• Cracks in plasterboard or plaster — can allow moisture in, or indicate movement caused by wet framing

Check areas that are statistically higher risk: around windows and sliding doors, behind showers and baths, under sinks, near the base of exterior walls, and in rooms with poor ventilation like bathrooms and laundries.

Step 2 — Use Your Nose

A musty or earthy smell — even without visible mould — is a strong indicator of moisture damage or mould growth hidden inside a wall cavity. Trust your nose, especially in rooms that have been closed up for a while.

Step 3 — Touch and Tap Test

Gently press on suspect areas of plasterboard. Soft, spongy, or crumbly plasterboard has absorbed water and lost its structural integrity. Tap the wall lightly with your knuckle — a dull, hollow sound in an area that should be solid can indicate the lining has separated from the framing behind it due to moisture damage.

Step 4 — Use a Moisture Meter

A pin-type or non-invasive moisture meter is the most useful DIY tool for this job. You can hire or buy one from Bunnings or Mitre 10 for a reasonable cost.

• Pin-type meters insert small pins into the surface and give a direct reading — useful on timber skirting boards, window frames, and exposed framing
• Non-invasive (capacitance) meters read moisture through the surface without penetrating it — handy for scanning plasterboard without causing damage

As a general guide, timber framing above 18–20% moisture content is considered elevated and at risk of mould growth or decay. Always check the manufacturer's guidance for your specific meter and material type.

Step 5 — Check the Exterior Too

Internal moisture damage usually has an external cause. Walk around the outside of your home and look for:

• Cracked, missing, or poorly sealed cladding joints and flashings
• Blocked or overflowing gutters and downpipes
• Ground level sitting too close to the bottom of the cladding (less than 150 mm clearance is a common issue)
• Vegetation growing against the wall

The MBIE Building Performance website has excellent guidance on NZ Building Code clause E2 (External Moisture), which sets out how buildings must be protected from weather-tightness failures.

Healthy Homes and Rental Properties

If you're a landlord, be aware that the Healthy Homes Standards require rental properties to be free from moisture ingress and drainage issues. Tenancy Services has full details on your obligations.

When to Call a Professional

DIY checks are a great starting point, but call a licensed building inspector if:

• Your moisture meter is showing consistently high readings
• You can see or smell mould but can't find the source
• You suspect a weather-tightness issue with monolithic or EIFS cladding
• The home was built between approximately 1990 and 2005 (higher leaky-building risk)
• You're buying or selling a property
• Damage appears extensive or structural framing may be affected

A building inspector can use thermal imaging cameras and professional-grade moisture meters to map moisture without opening walls unnecessarily. If plumbing is suspected as the source, that investigation must be carried out by a licensed plumber — plumbing work is restricted under the Plumbers, Gasfitters and Drainlayers Board rules.

For mould remediation involving large areas, consider engaging a specialist — WorkSafe NZ has guidance on health risks associated with mould exposure.

You can find qualified local building inspectors and tradespeople on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance — Building Code clause E2 (External Moisture)
• BRANZ — Moisture and weathertightness resources
• Tenancy Services — Healthy Homes Standards
• WorkSafe NZ — Mould in the workplace and home
• GIB — Plasterboard installation and moisture guidance
• Plumbers, Gasfitters and Drainlayers Board — Licensed tradespeople