Another damp and mould question

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.

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.

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

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

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

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

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.

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.

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..

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

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.

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

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.

Hence my 'rules'.

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

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:

Theo

Great read, very helpful and educational. Thanks a lot for that.