What damp and mould actually do to a household, and how we find out
what is causing yours.
Almost everybody who calls us about mould apologises for it first. They should not have to
— and the government guidance on this says so in stronger terms than we would dare to.
We are absolutely clear that it is totally unreasonable to blame damp and mould in the
home on ‘lifestyle choices’. It is unavoidable that everyday tasks, such as
cooking, bathing, washing and drying laundry will contribute to the production of indoor
moisture.
UK Health Security AgencyUnderstanding and addressing the health risks of damp and
mould in the home, 2023
10–15litres
of water go into the air of a four-person home every day, just from
living in it — showering, cooking, drying washing, breathing.
0.65litres
from two adults asleep for eight hours, in one room, with the door shut. Nobody is going
to stop doing that.
10–12hours
a child spends in that room a night — which by four in the morning is the coldest,
stillest, dampest place in the house.
None of it is optional
The only question is whether that water has somewhere to go. In a house that has been
draught-proofed, double-glazed and had its chimneys blocked up, usually it has not —
so it finds the coldest surface in the room and sits on it. That is why the mould is always
in the same places: behind the wardrobe on the outside wall, in the window reveal, in the
corner where the ceiling meets the gable.
It is a moisture problem with a mechanical cause, and it is why we measure before we
recommend anything.
Children get the worst of it
UKHSA names them among those at greater risk, describing
children and young people whose organs are still developing and are therefore more
likely to suffer from physical conditions such as respiratory problems.
And it is where they are that concentrates it. Ten to twelve hours a night, in one room,
with the door shut, in the coldest and dampest air in the house — which is exactly
where the mould grows, and exactly when they are breathing hardest against it.
What the evidence says
The occupants of damp or mouldy buildings, both houses and public buildings, are at
increased risk of respiratory symptoms, respiratory infections and exacerbation of
asthma.
World Health Organization · Guidelines for Indoor Air
Quality: Dampness and Mould, 2009
Some people who are exposed to damp and mould might experience poor mental health as
a result of living in a home with damp and mould.
UK Health Security Agency · Understanding and addressing
the health risks of damp and mould in the home, 2023
House dust mites — one of the commonest asthma triggers in this country —
need humid air to thrive, and their numbers fall as indoor humidity is brought down.
Ventilation is the lever that moves it.
Why the humidity reading matters more than the mould you can see
In November 2022, a coroner concluded that two-year-old Awaab Ishak had died from a severe
respiratory condition caused by prolonged exposure to mould in his home. It is the reason
Awaab’s Law exists, and the reason damp and mould stopped being treated as a decorating
job.
We are not doctors, and this page is not medical advice. The part that is ours is the
mechanical part, and it is the part that fixes it:
moisture that has somewhere to go does not condense on your walls, and mould with no
water does not grow. Everything else on this page is about working out what
“somewhere to go” means for your particular home.
Sources. World Health Organization, Guidelines for Indoor Air Quality:
Dampness and Mould (2009). UK Health Security Agency and the housing department,
Understanding and addressing the health risks of damp and mould in the home (first
published September 2023). Moisture generation figures from BS 5250 and BRE guidance.
Quotations are reproduced as published.
Find the Cause, Not Just the Mould
Mould is a symptom. Painting over it, or fitting a bigger fan without knowing why the moisture
is there, tends to move the problem rather than solve it. Here is how we work out the cause.
01
We measure, before we recommend
Relative humidity and temperature in each affected room, surface temperatures on the walls
where mould is showing, and the airflow your existing extract fans are actually achieving —
which is very often a fraction of what they are rated for.
02
We find where the moisture is coming from
Showering, cooking and drying washing indoors put several litres of water into the air every
day. We work out how much your home is generating, where it is settling, and whether the
cause is condensation, a cold bridge, penetrating damp or rising damp — because the fix is
different for each.
03
We check the cold surfaces
Mould grows where warm, damp air meets a cold surface — behind wardrobes on external walls,
at loft and joist junctions, in window reveals. Finding those cold spots explains why the
mould is exactly where it is, and often why it keeps coming back in the same place.
04
You get a written report and a costed option
What we found, what is causing it, and what would fix it — with the ventilation option sized
properly for your property and the Part F position set out clearly. If a system is not the
right answer for your home, we will tell you that too.
Approved Document F sets a minimum extract rate for every wet room, and a minimum ventilation
rate for the dwelling as a whole \u2014 and nothing at all for a living room or a
conservatory, which is a thing worth knowing rather than a gap. Both have to be met. This works out what those minimums are
for your property, and flags where the whole-dwelling figure is the one that governs.
Work out my minimum ratesTakes about a minute
1 How is the property ventilated?
2 Which rooms does it have?
Wet rooms have an extract rate. Dry rooms \u2014 living room, dining room, conservatory \u2014 do not, and adding them shows what they need instead.
3 The dwelling as a whole
All storeys added together.
Indicative only — not a compliance certificate. These are the minimum rates from
Approved Document F (2021, Volume 1: Dwellings) for England. A minimum is a floor, not a target:
duct length, bends, terminal type and filter condition all reduce what a fan actually delivers,
and a system that meets the figure on paper regularly fails it on site. Commissioning with a
calibrated anemometer is the only way to prove a rate has been achieved. Wales, Scotland and
Northern Ireland have their own regulations.
For landlords, letting agents and estate agents
Damp and mould — and the evidence that you dealt with it
A black bathroom ceiling is not really a decorating problem. It is a disrepair claim, a
renegotiated sale, a void month, or a tenant who has stopped answering the phone — and
by the time it is any of those, the question is no longer whether you fixed it. It is
whether you can show when you were told, what you found, what you did and when you
did it.
That is a record-keeping problem as much as a ventilation problem. Most contractors hand you
a paper invoice and nothing else. We do both halves, and the record builds itself.
If you are the landlord
The Homes (Fitness for Human Habitation) Act lets a tenant take you to court directly, and
damp and mould is a category 1 hazard. Repainting it is not a defence — the
cause has to be dealt with, and you need to be able to prove it was.
You get a report naming the cause and a commissioning certificate proving the fix works.
If you are the letting agent
You are the one taking the call, and you are carrying it with the landlord and the tenant
at the same time. What you need is one contractor who goes out, reports back in writing,
and gives you something you can forward the same afternoon.
You get a dated record per property, in an account you can pull it out of at any time.
If you manage a block
Communal extract that has not been touched since it was fitted, flats with no working
ventilation, and a Section 20 to get through before anything can happen. A specified
job with a measured outcome is a great deal easier to consult leaseholders on than
“we will put some fans in”.
You get a survey of the whole block, a specification you can consult on, and figures at the
end that show what was actually delivered.
If you are the estate agent
A survey that flags condensation or mould takes the momentum out of a sale and puts a
number in the buyer’s head. Nine times in ten it is a ventilation shortfall, not
rising damp — and that is a fixable, provable, quotable thing.
You get a fast survey, a written cause and a fix that closes the point out before exchange.
Surveyors and damp specialists: if you diagnose but do not install, send it
over. We do not survey for damp causes in competition with you — we take the ventilation
half, do it properly, and hand your client back a measured figure with your name still on the
diagnosis.
What that looks like on a job
Step one
Go out and measure it
Not a look and an opinion. A damp and mould survey with the readings written down as they
are taken: relative humidity, surface temperatures at the cold spots, moisture readings,
and the airflow the existing extract is actually achieving.
Why it matters
A measured airflow is the difference between “there is a fan in there” and
knowing whether it moves the air the room needs. Most of the ones we test do not.
Step two
Put the cause in writing
A written report naming the cause, not the symptom — a ventilation shortfall
against the Approved Document F minimum, a cold bridge, a defect, or an occupancy load
the fabric cannot carry.
Why it matters
“Tenant lifestyle” is the answer that ends up in front of a judge. A figure
against a standard is the answer that ends the argument.
Step three
Fix it and prove it
Extract upgrades, continuous mechanical extract, positive input, or full heat recovery
where the property warrants it — then commissioning with a calibrated anemometer and
a certificate of the rate measured at each valve.
Why it matters
A system that meets the figure on paper regularly fails on site. The certificate is what
stops the same property coming back next winter as a claim.
And the paperwork lands with it
RAMS and method statementIssued from our own system for every job, signed online.
Survey reportThe cause, the readings behind it, and what it needs.
Commissioning certificateThe rate measured at each valve, against the Part F figure.
Dated photographsBefore, during and after, timestamped as they were taken.
Quotes and invoicesApproved online, downloadable years later.
Awaab’s Law does not apply to you yet
Worth saying plainly, because plenty of people will imply otherwise. Awaab’s Law is
named after Awaab Ishak, who died in 2020 after prolonged exposure to mould in his
family’s flat. It came in through the Social Housing (Regulation) Act 2023, and
the first phase of it took effect for social landlords on 27 October 2025
— fixed legal timescales to investigate a reported damp or mould hazard, report the
findings in writing and start the work.
For privately rented homes it is not in force. The Renters’ Rights
Act 2025 gives the government the power to extend it, and the published roadmap puts
that in a later phase, subject to consultation. No commencement date has been set.
Anyone telling you there is one is guessing.
It is worth knowing now, because the record it will ask you for is the record you should be
keeping anyway — and the properties that will struggle are the ones with no
before-and-after figures on file. Everything above already produces that record. Nothing
needs to change when the timescales land.
Not legal advice. Awaab’s Law currently applies to social housing; the
extension to privately rented homes under the Renters’ Rights Act follows its own
timetable, and the regulations are amended from time to time. Check the current position, or
take advice, for what applies to your properties today. What we can tell you precisely is what
a property needs under Approved Document F, and prove by measurement that it is achieving
it.
Included with every job
The evidence trail builds itself
We built our own engineer’s app, and every client gets the portal side of it. It is
not a brochure — it is where the job actually lives, and the readings land in it as
the engineer records them on site.
The reason that matters when you manage property is simple: the record you would have to
produce is the same record we have to keep anyway to do the job properly. So you get it as
a by-product, dated, rather than reconstructing it from an invoice eighteen months later
when somebody asks.
No chasing.
Every property, every job and every stage in one list. You stop spending Friday afternoon
ringing round to find out what happened at Flat 3.
Documents where you can find them.
Reports, quotes, invoices, commissioning certificates and RAMS, downloadable any time
— including three years later when a claim, an inspection or a sale needs them.
Damp cases with the readings behind them.
Not a verdict, the numbers. Humidity, surface temperature and measured airflow, with the
dates they were taken on.
Approve a quote in a click.
And sign RAMS and variations online, so a job is not held up waiting for a wet signature
to come back in the post.
One thread per job.
Message the engineer against the job itself, so the history is attached to the property
rather than buried in somebody’s inbox.
Where the money goes.
Fixing the cause once costs less than repainting the same ceiling every spring, and a
measured commissioning figure is what stops the same property coming back as a claim. The
portal is what lets you prove which of those you bought.
A look inside
This is what you would be working in
One property or two hundred — tell us what you have and where, and we will tell you
what it needs.
In most cases, yes. Building Regulations Part F sets minimum ventilation requirements whenever you're altering a wet room or kitchen, replacing windows, or changing the layout of extract points. The exact requirement depends on the room type, extract rate, and how the space is used, so it's always worth having a survey before work starts rather than after.
MVHR (Mechanical Ventilation with Heat Recovery) continuously extracts stale, humid air from wet rooms and supplies filtered fresh air to living spaces, recovering most of the heat in the process. It's especially valuable in well-insulated or airtight homes where natural background ventilation has been reduced. Whether you need it depends on your property's airtightness, layout, and how it's currently ventilated.
Common warning signs include condensation on windows, a musty smell in enclosed spaces, and dark spotting around cold-bridge areas such as loft/joist junctions, ceiling breaks, and the wall behind wardrobes pushed against external walls. Our humidity simulator further up this page gives an illustrative sense of risk, but a site survey is the only way to get an accurate picture.
As a general guide, filters should be checked roughly every 6 months and replaced or cleaned as needed, though this varies with usage, local air quality, and the unit itself. Skipping servicing reduces airflow efficiency and can let dust and moisture build up in the ductwork.
We cover most of South East London and the Bromley borough — Beckenham, Bromley, West Wickham, Chislehurst, Dulwich, Penge, Sydenham, Forest Hill, Catford, Crystal Palace, Petts Wood, Orpington, Blackheath, Lee, Eltham and the areas around them. If you're just outside, get in touch anyway: we travel further for larger installations and commercial fit-outs.
Not sure where to start?
Send us a message with what you have noticed at home. We will tell you honestly whether you need a survey, a system, or neither.
Most trade sites show you a row of logos. These are the actual certificates, both sat in
August 2026. Tap either one to read it, or download the PDF.
Company no. 12092552Registered in England & Wales. Trading as Cavalli Ventilation since July 2019, and in ventilation since 2003.
Airflow measured on every installA calibrated anemometer at the grille, recorded and handed over. The only figure that counts is the one measured in your room.
