Mike Halmarack <mikehalmarack@gmail.com> wrote:

In a discussion about who has the responsibility regarding the damp
and mould in our flats, the discussion seems to come down to two possibilities. It's either a structural problem, which the management
company is responsible for, or a condensation problem caused by the
flat occupier.

The management company refuse to consider the situation further unless
all rooms within each flat are heated at all times in cold weather,
even those which are unoccupied, and powered extractors are fitted to
each bathroom and kitchen.

That's sensible. You don't have to heat it to a lot, eg a room thermostat
set to 13C might be sufficient. If you have a cold unventilated room with humans living in it it's a recipe for damp. Also daily variations can
trap damp (similar to how dew forms in the morning).

The first rule of damp control is to stop it coming in in the first place
(fix leaks, bad roofs, guttering, etc). Don't dry clothes inside (or if you do, get a dehumidifier).

The second rule is to expel moist air if you do generate it - that's
extractor fans for steamy bathrooms and kitchens (less important for
standalone toilets where fans are mostly for smells, although they do have
a big bowl of standing water), and opening windows to exchange the air. The German way is to open all the windows for 5 minutes a day to get a through draught to blow out the stale damp air - you don't lose very much heat that way.

The third rule is to use heating to drive it out.

One aspect of the situation that I can't quite fathom is that if warm
air holds more moisture than cold air, when this air meets a cold
surface like an external wall or a window, wouldn't that produce more condensation than the less saturated air would in an unheated room?

There are roughly two types of construction. The old way houses were built
is to 'breathe'. They rely on being warmer inside than outside (a coal fire originally, central heating now). The walls are permeable and so that
pushes out damp through the walls where it gets blown away by the wind.
Even quite recent construction can be breatheable (plaster+block+rockwool cavity fill+bricks).

Modern construction uses vapour barriers. You put an impermeable barrier on the damper side (which would be inside, during the winter anyway, unless you have driving rain - or live in Florida where it's hot and humid outside) to stop the moisture going into the wall in the first place. Then the wall
stays dry. A common way this is done is a foil-backed plasterboard,
insulation board or vinyl paint on the inside.

If you mix those two techniques you can get problems where the water goes
part way through the wall and then meets an impermeable barrier, where you
get condensation inside the wall and thus damp and potentially rot. Unless things have been retrofitted then it's less likely this has happened, but
watch for finishes eg non-porous external paint.

If you are getting condensation inside from normal breathing (ie not showers etc), and the heating is keeping the room at a comfortable temperature, then the walls are insufficiently insulated. But you can't jump to that
conclusion without checking heating and ruling out sources of moisture first.

Theo

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On 27/03/2025 10:17, Mike Halmarack wrote:

In a discussion about who has the responsibility regarding the damp
and mould in our flats, the discussion seems to come down to two possibilities. It's either a structural problem, which the management
company is responsible for, or a condensation problem caused by the
flat occupier.

The management company refuse to consider the situation further unless
all rooms within each flat are heated at all times in cold weather,
even those which are unoccupied, and powered extractors are fitted to
each bathroom and kitchen.

One aspect of the situation that I can't quite fathom is that if warm
air holds more moisture than cold air, when this air meets a cold
surface like an external wall or a window, wouldn't that produce more condensation than the less saturated air would in an unheated room?

Warm air has the *capacity* to hold more moisture before reaching
saturation, however the temperature only sets the upper limit before saturation, not the actual amount of water in the air. You can have warm
dry air and warm wet air.

The logic goes that if the room is heated, then the inner wall surfaces
will be at a higher temperature, and are thus less likely to be below
the dew point, so the water held in air in contact with those surfaces
stays in the air and does not condense on the surface.

--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk | \=================================================================/

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On 27 Mar 2025 at 11:43:02 GMT, Theo wrote:

Mike Halmarack <mikehalmarack@gmail.com> wrote:

In a discussion about who has the responsibility regarding the damp
and mould in our flats, the discussion seems to come down to two
possibilities. It's either a structural problem, which the management
company is responsible for, or a condensation problem caused by the
flat occupier.

The management company refuse to consider the situation further unless
all rooms within each flat are heated at all times in cold weather,
even those which are unoccupied, and powered extractors are fitted to
each bathroom and kitchen.

That's sensible. You don't have to heat it to a lot, eg a room thermostat set to 13C might be sufficient. If you have a cold unventilated room with humans living in it it's a recipe for damp. Also daily variations can
trap damp (similar to how dew forms in the morning).

The first rule of damp control is to stop it coming in in the first place (fix leaks, bad roofs, guttering, etc). Don't dry clothes inside (or if you do, get a dehumidifier).

Wouldn't the first rule be to avoid cold bridging during construction or retrofit?

The second rule is to expel moist air if you do generate it - that's extractor fans for steamy bathrooms and kitchens (less important for standalone toilets where fans are mostly for smells, although they do have
a big bowl of standing water), and opening windows to exchange the air. The German way is to open all the windows for 5 minutes a day to get a through draught to blow out the stale damp air - you don't lose very much heat that way.

'Stossluften' - open morning and evening, depending upon conditions. Others
air duvets and pillows first thing - all helps.

The third rule is to use heating to drive it out.

One aspect of the situation that I can't quite fathom is that if warm
air holds more moisture than cold air, when this air meets a cold
surface like an external wall or a window, wouldn't that produce more
condensation than the less saturated air would in an unheated room?

Probably, yes. The thing to do is design out those cold 'bridging' (from outside to in) surfaces. But if the room is barely heated you're on a hiding
to nothing - any external wall becomes a huge cold surface, and moisture in
the air will condense on the coldest part once the surface hits dew point.

There are roughly two types of construction. The old way houses were built is to 'breathe'. They rely on being warmer inside than outside (a coal fire originally, central heating now). The walls are permeable and so that
pushes out damp through the walls where it gets blown away by the wind.
Even quite recent construction can be breatheable (plaster+block+rockwool cavity fill+bricks).

Older solid brick homes did indeed rely on porosity. The problem comes when they're maintained and/or retrofitted without regard to this need to wick moisture.

Two main aspects to this - 1) sealing up drafts, and 2) creating vapour barriers of various degrees, such as non-lime mortar, acrylic and other
limited porosity paints and decorative finishes, right through to foil backed plasterboard. Even using plain plasterboard is potentially a problem - can't imagine it wicks much moisture.

Modern construction uses vapour barriers. You put an impermeable barrier on the damper side (which would be inside, during the winter anyway, unless you have driving rain - or live in Florida where it's hot and humid outside) to stop the moisture going into the wall in the first place. Then the wall stays dry. A common way this is done is a foil-backed plasterboard, insulation board or vinyl paint on the inside.

Which only works to a point - modern non-MHVR homes still need to be ventilated. That internal moisture has to go somewhere. Which does set in
chain a viscous loop of heating and ventilating that warm air. Completely counterintuitive for many households, especially those without much money..

If you mix those two techniques you can get problems where the water goes part way through the wall and then meets an impermeable barrier, where you get condensation inside the wall and thus damp and potentially rot. Unless things have been retrofitted then it's less likely this has happened, but watch for finishes eg non-porous external paint.

Joist ends in particular. Bouncy ground floor floors a possible sign.

If you are getting condensation inside from normal breathing (ie not showers etc), and the heating is keeping the room at a comfortable temperature, then the walls are insufficiently insulated. But you can't jump to that conclusion without checking heating and ruling out sources of moisture first.

Yes, agreed. I'd add that persistant damp in a particular part of the room could well be pointing to a building weakness. But as mentioned above, all buildings succumb if there's a perfect storm of high moisture and little heat.

--
Cheers, Rob, Sheffield UK

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Mike Halmarack <mikehalmarack@gmail.com> wrote:

Thanks. That's taken me from trying to understand one complex subject
(for me) to yet another level.

Look Joe Lstiburek's 'perfect wall' on Youtube. He mostly gives talks to US-based audiences, but here's a talk to a New Zealand audience whose
climate is a bit more like ours:
https://www.youtube.com/watch?v=IaozbiujnWM

The main point is that buildings have layers designed to control water, air, vapour and thermal transfer from outside to inside or vice versa. The arrangement of these layers is key to having a healthy building and
comfortable occupants. How they are arranged in any particular building,
and especially the detailing at interfaces (eg windows), constrains how a building will perform.

Theo

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RJH <patchmoney@gmx.com> wrote:

On 27 Mar 2025 at 11:43:02 GMT, Theo wrote:

Mike Halmarack <mikehalmarack@gmail.com> wrote:

In a discussion about who has the responsibility regarding the damp
and mould in our flats, the discussion seems to come down to two
possibilities. It's either a structural problem, which the management
company is responsible for, or a condensation problem caused by the
flat occupier.

The management company refuse to consider the situation further unless
all rooms within each flat are heated at all times in cold weather,
even those which are unoccupied, and powered extractors are fitted to
each bathroom and kitchen.

That's sensible. You don't have to heat it to a lot, eg a room thermostat set to 13C might be sufficient. If you have a cold unventilated room with humans living in it it's a recipe for damp. Also daily variations can
trap damp (similar to how dew forms in the morning).

The first rule of damp control is to stop it coming in in the first place (fix leaks, bad roofs, guttering, etc). Don't dry clothes inside (or if you
do, get a dehumidifier).

Wouldn't the first rule be to avoid cold bridging during construction or retrofit?

That's the zeroth rule ;-) These are all things the occupant should do while living there. 'Buy a different house' is a bit trickier.

One aspect of the situation that I can't quite fathom is that if warm
air holds more moisture than cold air, when this air meets a cold
surface like an external wall or a window, wouldn't that produce more
condensation than the less saturated air would in an unheated room?

Probably, yes. The thing to do is design out those cold 'bridging' (from outside to in) surfaces. But if the room is barely heated you're on a hiding to nothing - any external wall becomes a huge cold surface, and moisture in the air will condense on the coldest part once the surface hits dew point.

Indeed. Heat keeps more moisture in the air and prevents it condensing. If you make the house colder (or better sealed) then it's more important the
house is built right. When there was a big coal fire burning all day long
it didn't matter too much about how things were constructed because there
was lots of leeway. When you turn down the heat and rely more on insulation you're putting much heavier requirements on the building being built
perfectly, and that's when all the defects show up.

Modern construction uses vapour barriers. You put an impermeable barrier on
the damper side (which would be inside, during the winter anyway, unless you
have driving rain - or live in Florida where it's hot and humid outside) to stop the moisture going into the wall in the first place. Then the wall stays dry. A common way this is done is a foil-backed plasterboard, insulation board or vinyl paint on the inside.

Which only works to a point - modern non-MHVR homes still need to be ventilated. That internal moisture has to go somewhere. Which does set in chain a viscous loop of heating and ventilating that warm air. Completely counterintuitive for many households, especially those without much money..

Hence my 'rules'.

If you are getting condensation inside from normal breathing (ie not showers
etc), and the heating is keeping the room at a comfortable temperature, then
the walls are insufficiently insulated. But you can't jump to that conclusion without checking heating and ruling out sources of moisture first.

Yes, agreed. I'd add that persistant damp in a particular part of the room could well be pointing to a building weakness. But as mentioned above, all buildings succumb if there's a perfect storm of high moisture and little heat.

It's worth keeping a log of exactly what happens when. eg if you can see visible condensation, does it happen after a bath, after a rainstorm, in the morning after sleeping? It helps in tracking down sources of moisture.

Theo

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