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Air Quality · The problem library

What kind of damp do I actually have, and how do I tell the difference?

Recently reviewed·Reviewed June 2026, next review December 2026

Damp comes in three forms and each has a different remedy. Condensation forms when warm humid air meets cold surfaces. Penetrating damp enters through a failure in the external fabric. Rising damp travels up from ground level through capillary action in walls that lack a damp-proof course. The first step is correctly identifying which one is in front of you.

Editorial confidence
High confidence. Multiple regulator, government or academic sources agree.
Evidence status
Recently reviewed · reviewed June 2026 · next review December 2026
Monitoring
Every six months, plus whenever BRE damp guidance or Approved Document C is updated.
Evidence drawn fromBuilding standards: ADF, ADLProfessional bodies: EST

You may not need to read the rest of this page

You probably already have enough to act if:

  • There is visible mould or a persistent musty smell
  • Damp appears in a recognisable pattern (high corners, low tide-line, or external-wall patches)
  • Indoor humidity sits above 60% for weeks at a time
Open the damp diagnosis guide

This is probably the wrong page for you if:

  • Water is actively entering from a burst pipe or flood
  • The issue is summer overheating rather than moisture

Indoor relative humidity guideline

40-60%

Source: Building Regulations; Approved Document F · Approved Document F treats sustained relative humidity above 60% as the threshold at which mould risk becomes significant.

Who this advice is for

Applies to

  • UK houses showing damp, mould or repeated condensation
  • Solid-walled Victorian, Edwardian and interwar properties
  • Properties with blocked sub-floor vents, defective gutters or failed pointing

Not intended for

  • Flood damage and burst-pipe water ingress (different remedies)
  • New-build snagging within developer warranty
  • Commercial premises

What it usually looks like

These are the symptoms readers describe most often. None of them alone is diagnostic, but together they build a picture.

  • Black spotting on cold walls
  • Window condensation in winter
  • Musty smell
  • Damp tide mark at low level
  • Crumbling plaster behind skirting

Most common in: Victorian terrace · Edwardian semi-detached

Before you buy anything

Watch the house respond as you scroll.

These checks are listed in the order we would work through them. The illustration on the left changes with each one, so you can see what each check is actually addressing before deciding whether it is worth doing.

Warm wet air settles on the coldest surface and stays there. The window streams; mould follows.

01

Look at where the damp appears

Damp in upper corners is almost always condensation. Damp in patches on external walls suggests penetrating damp. Damp at low level around the base of walls points to rising damp.

02

Inspect external walls and roof for water ingress

Blocked gutters, cracked render and missing pointing cause many cases that are misdiagnosed as condensation. Fixing the leak makes everything else cheaper.

03

Measure humidity in the affected rooms for a week

Persistently above sixty per cent points to a ventilation problem rather than an envelope problem. The data prevents you spending on the wrong fix.

04

Get an independent damp survey if the cause is unclear

Many damp companies sell the treatments they recommend. An independent surveyor without skin in the game pays for themselves quickly.

05

Treat the cause before treating the symptom

Anti-mould paint over an unresolved damp problem fails within months. The cost saving comes from doing the underlying work once.

Before you spend money

The cheapest answer is usually the one nobody sold you.

  1. 01

    Photograph the affected area at the worst time of year

    Damp patterns name their own cause: high tide-marks point to rising damp, ceiling stains to roof or plumbing, cold-wall patches to condensation. Diagnosis precedes spending.

    Free
  2. 02

    Check gutters, downpipes and ground level around the wall outside

    Most external damp in UK houses is rainwater in the wrong place: a blocked gutter, a cracked hopper, paving above the damp-proof course. Clearing it costs nothing and often resolves the problem.

    Free
  3. 03

    Log indoor humidity for a fortnight before commissioning a damp survey

    Many 'damp' diagnoses by commission-paid surveyors are actually condensation. The number on the hygrometer protects you from buying a chemical injection you do not need.

    Free
  4. 04

    Get a second opinion from an independent damp surveyor (not one who sells the cure)

    The single biggest source of damp overspend is commissioned diagnosis. £200 on an independent assessment can save thousands.

    Free

What we recommend, and why

Every recommendation explains why it comes first, the evidence behind it, how confident we are, and when it would not apply.

01

Diagnose the type of damp before treating anything

Why this comes first

Condensation, penetrating damp and rising damp look superficially similar but need entirely different remedies. Treating one as another is the most common reason damp problems recur within months of an expensive intervention.

Evidence

EST damp guidance and BRE digests both treat correct diagnosis as the determining step; pattern and location of damp identify the cause far more reliably than chemical tests.

EST · ADF

Confidence

High confidence. Multiple independent sources agree on the direction and the order.

Exceptions
  • Cases where two damp types co-exist and need sequenced treatment
Next step
See the damp diagnosis guide
02

Use an independent surveyor when the cause is unclear

Why this comes first

Many damp-proofing companies sell the treatments they recommend. An independent surveyor without commission interest is almost always cheaper than the unnecessary work they prevent.

Evidence

EST and RICS guidance both highlight the conflict of interest in supplier-led damp assessments and recommend independent inspection for any significant remediation.

EST

Confidence

High confidence.

Exceptions
  • Clear-cut cases of single-cause condensation in modern dwellings
Next step
Find an independent surveyor

The trade-offs

Every choice gives you something and asks for something back.

  • Fix external defects first (gutters, pointing, ground level, flashings)

    You get

    The most common cause resolved at the lowest cost and the fewest unintended consequences.

    You give up

    A few hundred to a few thousand pounds and the realisation that this is not the glamorous answer.

  • Ventilation and humidity control (extract fans, PIV, behaviour change)

    You get

    The right answer when the issue is condensation rather than ingress, and a much wider safety margin year-round.

    You give up

    Several hundred to a thousand-plus pounds and a willingness to live with a fan running quietly in the background.

  • Chemical damp-proof injection or full re-plaster

    You get

    A confident-sounding answer and a contractor's guarantee.

    You give up

    Often four-figure sums for a treatment whose evidence base is weak and which can mask the real defect underneath.

Real homes that faced this

Publicly documented UK projects that ran into the same problem and reported what happened.

We did not run these projects. Each links back to the original publisher so you can read the figures in context and judge for yourself what carries over to your home.

How we'd approach this

Name the cause before buying the cure.

If we had damp in our own house we would refuse to commission anything from anyone who also sold the cure. The single biggest reason UK homeowners overspend on damp is that the diagnosis arrived in the same envelope as the quote. An independent surveyor, paid by us and not by the outcome, is the cheapest insurance we know.

We would walk the outside of the wall before the inside. Gutters, downpipes, pointing, ground level, flashings: in most older terraces, the cause of an internal stain lives within three metres of it on the outside. Two hours with a ladder beats two thousand pounds of chemical injection.

And we would be cautious about anyone selling a single explanation. Rising damp is real but rare; condensation is common and misdiagnosed; bridging defects sit somewhere in between. The honest answer is usually a combination, and the proportionate response addresses each in proportion to its share of the problem.

Evidence behind this page

Every recommendation on this page is traceable to its source. Click a publication to read the original.

Products that may help

Only consider these once the checks above have been ruled out. A product fitted into the wrong cause is rarely satisfying.

House Summary

The cheapest answer to damp, mould or condensation is usually the one that addresses the cause rather than the symptom. The list above is in the order we would work through it, because the checks at the top tend to rule out the most expensive mistakes further down.

Related decisions
Next Best Step

Ventilation; what your home actually needs

Take the next step