Three problems that look identical and aren't

Six questions that separate the three

Work down the left column and answer honestly. You are looking for a pattern across all six rows rather than a single match — any one of these problems can borrow one feature from another, but none of them can borrow all six.

One caution before you start. Two of these three can be present in the same house at the same time, and often are, because a wall that is genuinely damp low down and a room that runs cold and humid overnight are not mutually exclusive. The table tells you which one to spend money on first.

Condensation: about 70% of the mould, and almost none of the invoice

Air holds water, and how much it can hold depends on temperature. When a surface is colder than the dew point of the air touching it, water comes out of the air onto that surface. That is not a theory about your house, it is the definition of dew point, and it is why the bathroom mirror fogs.

In a house, the coldest surfaces are external corners, uninsulated masonry, window reveals, and any wall with a wardrobe or a lounge pushed hard against it — because furniture blocks both the warmth and the air movement. Floor level is also colder than head height. That is exactly why condensation so often presents as damp along the bottom of a wall, with blistered paint, bulging plasterboard, decaying skirting boards and black speckling, which is precisely the description in every rising damp advertisement you have ever seen.

The distinguishing feature is that condensation tracks people, not ground. It is worse in the rooms that are shut up, worse in winter and overnight, worse after a shower without extraction, worse where washing is dried indoors, and worse where an unflued gas heater is running, because unflued combustion puts its water straight into the room. Open the house up on a dry mid-morning and the pattern changes. Rising damp does not care whether you dried the washing inside.

There is a version of this that happens inside the masonry rather than on it. As a wall warms, it exhales air from its pores; as it cools, it inhales. If the incoming air is humid and the masonry drops below dew point, water condenses within the pores. Where hygroscopic salts are present in the wall, that inhaled moisture is captured and retained. This is how a wall with a perfectly good damp course and no liquid water source whatsoever can read wet on any instrument, stay wet, and continue to decay. Any coating that stops the wall breathing shortens its life.

South East Queensland loads the dice here. High ambient relative humidity suppresses evaporation from wall faces, which lowers the rate of salt decay but raises the height of rise, and makes hygroscopic salt effects and condensation far more prominent in the differential diagnosis than they are in Adelaide. Queensland copy that reads like Adelaide copy is describing somebody else's climate.

On health, one number and no adjectives. Persistently damp surfaces support mould growth, and the accepted control threshold is keeping surface relative humidity below about 80 per cent over month-long periods. That is the target. Everything past that number in this market is sales copy.

The test you can run this week

The definitive instrument for condensation is a data logger recording air temperature, relative humidity and wall surface temperature over at least a week including cold nights — a fortnight to a month is more realistic, and seasonal behaviour may need a full year. You do not need to hire one to get a usable answer. You need two readings and a table.

What you are testing is a single proposition: is the wall surface colder than the dew point of the air in the room. If it repeatedly is, water is condensing on it, regardless of what any moisture meter says.

1. Get two numbers, not oneA cheap hygrometer that reads room air temperature and relative humidity, and an infrared thermometer to read the surface temperature of the wall. Both are inexpensive. The single number a damp salesperson shows you on a pin meter is not one of them.
2. Read at the worst timeFirst thing in the morning, in the affected room, before opening any windows or doors. Overnight is when surfaces are coldest and when the room air has had all night to load up with moisture from breathing, cooking and drying.
3. Read the damp spot and a reference spotTake the surface temperature at the damp patch and again on an internal wall in the same room, away from the corner. The difference between the two is the cold-bridge effect that is doing the damage.
4. Look up the dew pointUse the table below. Find your room air temperature down the left and your relative humidity across the top. If the wall surface temperature is at or below the figure in that box, water is condensing on that wall. This is arithmetic, not an opinion.
5. Repeat for at least a weekOne morning proves nothing. Seven to fourteen consecutive mornings, including the coldest nights available, gives you a pattern. Write the numbers down with the date — undated readings are worthless, in this test and in a contractor's report.
6. Then change exactly one thingRun the extract fan for twenty minutes after every shower. Stop drying washing indoors. Pull the wardrobe 50 mm off the external wall. Open the house up mid-morning when the outside air is drier than the inside air. Change one variable at a time and keep reading. If the numbers move, you have your answer for the price of a hygrometer.
7. Know what would not changeGenuine rising damp is indifferent to all of the above. If the band along the base of the wall is still there in February, at the same height, with the same level upper edge and the same salt bloom, after a fortnight of aggressive ventilation, condensation is not what you are looking at.

Efflorescence is a symptom, not a diagnosis

The white stuff on your bricks is salt. Water carried it into the masonry, the water evaporated at the surface, and the salt could not follow it into the air, so it crystallised there in the open. That is efflorescence. It is not a health hazard and on its own it is cosmetic. The question worth asking is never what it is. The question is whether it keeps coming back.

On new brickwork it is normal. Portland cement contains soluble sodium and potassium compounds, and construction water mobilises them to the evaporating face. The bloom appears in the first year or two, washes off with rain and progressively diminishes. Left alone, it fades. Dry-brush it if it bothers you.

Bloom that returns season after season is different. Recurring efflorescence means water is still arriving and still evaporating at that point, which means there is a live source to find. Cleaning it without finding the source guarantees it comes back, which is why the cleaning aisle of the hardware store is not, in the end, a treatment.

There is a version of this that does the actual damage, and it is invisible. When evaporation is faster, the drying front sits inside the wall rather than on it, and salt crystallises within the pores. That is sub-florescence, or crypto-florescence, and the crystallisation pressure is enough to disrupt the strongest masonry. It is what makes bricks fret and lose their fired outer skin, sandstone shed its hardened surface, and mortar turn to sand. So the white bloom you can see is the harmless one, the one you cannot see is the destructive one, and a wall with no visible salt at all can be in worse condition than a wall covered in it — rain simply washes the evidence away.

Two hard rules. Never acid-clean efflorescence: acid cleaning introduces chloride into the masonry, chloride does not come out, and a cosmetic problem becomes a permanent one. And where salt analysis returns more than about 0.5 per cent by weight, desalination belongs in the scope of works, not in the optional extras.

When it really is the first one

Rising damp is real and it does happen here, and pretending otherwise would be as dishonest as the over-selling. The signature set is specific: a continuous band along the base of the wall rather than isolated patches, a fairly level upper edge typically between 1.0 and 1.5 metres, present through the year rather than after cold nights, correlating with external ground level and with where the damp course sits, salt bloom and mortar deterioration within that band, and the mortar going before the bricks.

Even with all of that present, nobody should be quoting. The evidence for rising damp is a vertical profile of drilled samples showing moisture content decreasing with height, with hygroscopic moisture content measured separately at 75 per cent relative humidity so that free capillary water can be separated from moisture the salt is holding out of the air. That single subtraction is what distinguishes the two most expensive outcomes in this field from each other.

Which brings up the fourth possibility that belongs on a page about lookalikes: the wall where the source is already gone. High hygroscopic moisture content, negligible free water, a meter that screams, bloom that returns after every repaint, damp on humid days and dry on dry days. The leak was fixed in 2019. The salt stayed, and the salt is hygroscopic — common salt goes wet at around 75 per cent relative humidity, sodium nitrate in the low-to-mid 70s, calcium nitrate at 47 to 55 per cent, magnesium chloride around 33 per cent and calcium chloride at 29 to 32 per cent. Below about 33 per cent relative humidity is genuinely rare in an occupied Australian building, so a wall carrying chlorides is permanently in the wet phase. It will read damp on any instrument, forever, with no liquid source at all. That wall needs salt removed and replastering in lime, not a damp course. It is the most expensive misdiagnosis in the book, and it is the reason the hygroscopic moisture test is not optional.

The fourth answer: a leak with an alibi

The three-way comparison is a useful frame, but there is a whole fourth family — water arriving from a specific defect — and two of them impersonate rising damp so well that they are routinely treated as it.

Failed wet-area waterproofing presents as damp at the base of the wall in the room next to the bathroom, not in the bathroom itself. Blistered paint and lifting skirting boards in the hallway, drummy or loose wall tiles, damp that has no relationship to the ground outside. It is remedied by removing and reinstating the membrane to AS 3740:2021, which is licensed waterproofing work in Queensland and runs $3,000 to $15,000 per wet area. A regrout and reseal usually buys a year.

Slab edge dampness is the other. Persistent damp on the exposed slab or footing edge, efflorescence and drummy render below the damp course, external paint blistering below the damp course, damp carpet edges, pungent odours, tile bond failure near external walls and rust staining on metal near the slab edge. It very often follows landscaping added after construction that buried the edge. It is a drainage, membrane-termination and concrete-quality problem, and injecting brickwork does not address it.

Then the cheap ones, which are worth ruling out before anything else. A blocked or leaking gutter, downpipe or rainwater head produces damp that is worst high on the wall and traceable upward, with the salt appearing wherever the water finally evaporates, which may be well away from where it entered — $400 to $6,000 to put right. An air-conditioning condensate line discharging at the base of a wall. An irrigation dripper 100 mm from the brickwork. A concealed supply or sewer leak, which shows up as damp with no relationship to weather or season and a water meter that creeps with every tap shut.

Causation is established by exclusion testing, not by walking around with a meter. Isolate the water meter and watch it. Flood-test the shower. Run the downpipes and hose the ground to check the falls. Dye or CCTV the waste lines. Put a borescope into the cavity to look for mortar droppings on the wall ties and blocked weepholes. Dig a small pit and find out whether the damp course exists and whether the ground is sitting over it.

Why the cheapest answer gets sold as the dearest one

This section names no company. It describes a structure, and the structure explains most of what goes wrong here.

The instrument comes first. A two-pin electrical resistance moisture meter measures conductivity between two pins and is calibrated for timber. Soluble salts are electrolytes, so on masonry the reading responds to salt as much as to water, and a dry but salty wall reads as high as or higher than a genuinely wet clean one. It is common on salt-loaded walls to get readings above 100 per cent moisture content, which is a physically impossible figure leaving no room for the masonry itself. The Australian heritage guide, jointly published by three state governments, states that moisture meters should never be used as the sole basis for diagnosing a damp problem, and never on their own to prove a wall is unacceptably damp. That instrument is nonetheless the entire evidence base behind a great many five-figure quotes.

Then the structure. The diagnosis is usually performed, at no charge, by the business that sells the cure. RICS guidance holds that where a surveyor's recommendations may include sales, the conflict must be clearly noted so the client understands the survey is separate from the quotation. A free inspection is not free — it is a customer acquisition cost, recovered in the works, by a party with a direct interest in the finding.

Then the weather. Inspect after heavy rain and every wall in the street reads worse. Declare success after a dry spell and nothing has been demonstrated, because a long dry period can produce an incorrect assessment that the damp has been controlled. Undated readings prove nothing in either direction, which is why dates and conditions belong on every measurement.

And then the guarantee. A chemical damp course stops water rising through the treated zone. It does not remove the salt already in the wall above it, and it has no effect whatsoever on condensation — which, on the published figures, is where about 70 per cent of the mould came from in the first place.

We should be explicit about our own position, since we sell one of the treatments discussed on this page. That is exactly why the diagnosis should be separated from the quotation, why the evidence a diagnosis rests on should be published rather than asserted, and why the most useful thing a report can contain is the sentence that says you do not need us.

What would change these answers

No comparison table survives contact with every building. These are the observations that should make you re-open the question, whichever answer you had arrived at.

• The damp band has no level upper edge. Whatever the meter says, that is not the signature of capillary rise.
• The worst of it is above about 1.5 metres. Look up — gutters, flashings, a rainwater head, a balcony above.
• It appeared within one to three years of landscaping, paving, a new path or a renovation. Suspect a bridged damp course first; the time lag between creating the problem and seeing it is measured in years.
• The building is less than two years old. Construction moisture, with readings that decline steadily across successive tests, and a bloom that washes off and does not return.
• Drilled samples show negligible free water but high hygroscopic moisture content. The source has already gone. Salt is the remaining problem, and a damp course would be treating nothing.
• It is worse in humid summer weather and there is a clear 150 mm or more of bare brick above the ground. That combination points at hygroscopic salt rather than a live source.
• The wall is wetter above the damp course than below it. That is the diagnostic signature of lateral damp from higher ground on the other side, and chemical injection cannot resist a hydrostatic head.
• It only shows up in the room adjoining a bathroom, shower or laundry. Flood-test the wet area before anyone drills brickwork.
• It shows up on a slab-on-ground floor rather than in masonry. There is nothing to inject and no course to cut; the correct measurement is an in-situ relative humidity probe to ASTM F2170 at 40 per cent of slab depth.

Who fixes which one, including when it isn't us

Nine likely answers, and the honest allocation of each. Two rows on this table are our work. The other seven are not, and where the evidence points at them, that is what we will say and who we will point you to.

• Chemical damp-proof course to external brickwork. Holes drilled roughly every 120 mm along a mortar joint, 10 to 15 mm in diameter, to within about 30 mm of the far face, with silane or siloxane cream or fluid introduced at low pressure or by gravity feed. It forms a continuous water-repellent zone that capillary moisture cannot climb past, without blocking the pores, so the wall still breathes and dries. It does nothing about salt already above it, and where a wall needs treating from both faces the internal side is outside our scope.
• Hydropoxy negative treatment to internal concrete slabs. Grind back, prepare the surface, and apply an epoxy moisture barrier to the top of the slab before new flooring goes down. It manages moisture at the flooring interface. It is not a damp-proof course, and it is not a retrofitted under-slab membrane — a membrane cannot be installed beneath a slab that has already been poured.

What it costs

Questions we actually get asked

Does efflorescence mean I have damp?

It means water has been in the masonry and evaporated at that spot, which is not the same thing as a problem. On new brickwork it is normal, comes from soluble compounds in Portland cement mobilised by construction water, and fades over the first year or two. What matters is recurrence: bloom that returns season after season means water is still arriving and still evaporating there, and something is supplying it. Efflorescence is a symptom that tells you to go looking. On its own it is not a diagnosis and it is certainly not a reason to inject a wall.

Can condensation cause efflorescence?

Indirectly, yes. Salts already present in masonry are mobilised by any water moving through it, and that includes moisture condensing inside the pores. As a wall warms it exhales air from its pores and as it cools it inhales; if the incoming air is humid and the masonry drops below dew point, water condenses within the wall, and where hygroscopic salts are present that moisture is captured and held. That is how a wall with a sound damp course and no liquid source can keep blooming. The classic recurring bloom low on external brickwork is more often a liquid source, but the two mechanisms genuinely overlap, which is another reason the salt content and the free water content have to be measured separately.

It's black speckles, not white powder. What does that tell me?

That you are almost certainly in the condensation family rather than the salt family. Black spot mould is a biological growth that establishes on surfaces which stay humid — the common culprits are Cladosporium and Aspergillus — and it appears on cold external corners, window reveals, behind furniture and in wardrobes on external walls. Salt does not go black. The accepted control threshold is keeping surface relative humidity below about 80 per cent over month-long periods, and the levers that move it are ventilation, extraction ducted outside, heating pattern, insulating cold spots and reducing the moisture being put into the air.

Is it rising damp or a leak?

Timing and traceability separate them. A leak correlates with an event — rainfall, a shower being used, a tap running, a sprinkler cycle — and it can be traced back to a defect. Rising damp correlates with ground level and stays broadly the same through the year. The way you settle it is exclusion testing rather than surface readings: isolate the water meter and watch it with all fixtures off, flood-test the shower, run the downpipes and hose the ground to check falls, dye or CCTV the waste lines, and borescope the cavity. Negative results are only as good as the duration of the test, which is why a fifteen-minute inspection excludes very little.

Why does the same wall feel damp on humid days and dry on dry days?

Because the wall contains hygroscopic salt and is responding to the air, not to the ground. Chlorides and nitrates pull water vapour straight out of the atmosphere: above a critical humidity they dissolve, below it they crystallise. Common salt goes wet around 75 per cent relative humidity, calcium nitrate at 47 to 55 per cent, magnesium chloride around 33 per cent and calcium chloride at 29 to 32 per cent — and mixtures, which is what real walls contain, go wet at a lower humidity than either constituent alone. A wall carrying chlorides is effectively in the wet phase permanently. The treatment for that is salt removal and replastering, not another damp course.

If I run a dehumidifier and it improves, does that prove it was condensation?

It is strong evidence, and it is a fair experiment provided you keep dated readings and change one variable at a time. Two cautions. First, drying is limited by how fast water can move through a wall to the surface, not only by how fast you strip it out of the air, so beyond a point extra dehumidification achieves little. Second, on a salt-loaded wall, increasing the drying rate increases the rate of salt crystallisation inside the masonry and therefore the rate of decay — the heritage guide notes that adding heating or air-conditioning increases drying and so increases the associated decay. Steady moderate ventilation beats aggressive forced drying, and a dehumidifier is a diagnostic aid rather than a repair.

I've been told I need a damp course. What should the quote contain before I sign anything?

Ask for eleven things and see how many are there. Whether anyone drilled the wall and took masonry samples, which BS 6576 requires. A vertical moisture profile at multiple heights. Hygroscopic moisture content reported separately from total moisture content. Salt type or percentage. External ground and paving levels measured against the existing damp course. A subfloor and cavity inspection. Gutters, downpipes and plumbing assessed and excluded. Drill hole spacing and depth stated. Fluid volume per linear metre stated. Whether desalination is in the scope. A drying period specified before replastering, and the guarantee's exclusion list attached. If nobody drilled the wall, nobody diagnosed anything, and the document you are holding is a sales proposal rather than a report.