On Monday, November 5, 2012 9:48:27 AM UTC, stuart noble wrote:
On 05/11/2012 08:23, polygonum wrote:
On 05/11/2012 08:10, harry wrote:
On Nov 4, 12:17 pm, stuart noble wrote:


Where do you get the idea that indoor RH is normally higher than
outdoor? Do you live in a cave?

Showers kitchen and bath rooms of course. When the warm damp air
encounters a cold surface, it condenses.

"Normally" seem to imply an ongoing, more-or-less permanent state of
affairs. Our kitchen and bathroom might transiently have RH towards the
high end - but very rapidly reduce - we do have opening windows! Hence
ours are not running in condensation, are not coated in black mould, etc.

Well exactly. What's so complicated about that?

Nothing. It seems to confuse increased RH with condensation & mould. Condensation only happens where RH locally reaches 100%. Mould only happens where condensation happens so much the wall is too damp over time.


You get rid of condensation either by ventilation or by raising the air
temperature so that it can carry more moisture (i.e. it dries) or, shock
horror, a combination of both. Yes it hurts to have the windows open and
the boiler on but it's not for long.

Most of the time such meaures aren't being taken, and water vapour is being put into the air all the time a house is occupied. Heating enables it to carry more water vapour, but obviously that alone wouldnt keep it dry for long, there is then ventilation which replaces interior air with greater vapour content with outdoor air with lower water vapour content.

In walls without a VB, you also have higher water vapour content on one side than the other, so the vapour slowly migrates through the walls from inside to outside.

Its basic physics.