Condensation will be at the same rate on the uninsulated portion of Scenario B as it is on the whole wall in Scenario A. (i.e. g/m²s not total rate). If the condensation is occurring faster than the available water vapour is replaced so that the humidity is reduced, then condensation will decline at a faster rate in scenario A than scenario B. So scenario B will look wetter (i.e. greater g/m²) but condense less total water than scenario A. But if the humidity stays constant then the g/m² will be equal.

I was wondering if there was an effect of the insulated wall in Scenario B make the uninsulated wall warmer the closer it is to the insulation, but then I realised the opposite takes place too - the insulated wall is colder as it gets closer to the uninsulated portion.

Posted By: djhCondensation will be at the same rate on the uninsulated portion of Scenario B as it is on the whole wall in Scenario A. (i.e. g/m²s not total rate). If the condensation is occurring faster than the available water vapour is replaced so that the humidity is reduced, then condensation will decline at a faster rate in scenario A than scenario B. So scenario B will look wetter (i.e. greater g/m²) but condense less total water than scenario A. But if the humidity stays constant then the g/m² will be equal.

In the situation where the water vapour is not being replaced - why would scenario B condense less total water than scenario A? What's happened to the difference - doesn't it all have to condense somewhere?

Posted By: djh

Posted By: lineweightIn the situation where the water vapour is not being replaced - why would scenario B condense less total water than scenario A? What's happened to the difference - doesn't it all have to condense somewhere?

Sorry, I wasn't meaning in the limit as all the water condenses (assuming conditions allowed that) but at any given time after the start of the experiment.

Ok.

I guess part of the reason behind my question is to understand to what extent doing a partial-insulation job can increase risks compared to a no-insulation job.

This is aside from what willinaberdeen rightly says above, about such renovations commonly being accompanied by changes in airtightness and hence opportunities for water vapour to disperse to the outside.

Let's say there is a bathroom, in which, prior to being partly insulated, there tends to be some condensation on the inner face of the wall when someone has a shower. That wall becomes somewhat wet, but it then gradually dries out when the shower is turned off, and an extract fan is turned on, or a window opened.

Compare this with the same bathroom (and occupant behaviour) where half, or three quarters, of the wall has been insulated but an insulated portion remains. Does this mean that that section of wall is going to collect a similar amount of condensed vapour, but concentrated into a smaller area, which might, say, result in water running down the face and pooling somewhere, where it is going to take longer to dry out than the pre-insulation scenario.

Based on your explanation - I think there is potentially an increased risk, but not in fact during the period the shower is running and there's a constant supply of vapour, because the portion of wall we're looking at is not behaving any differently in either scenario. However, after the shower is finished, and there is no longer a supply of 'new' vapour, then there is potentially a longer period of time during which the vapour is condensing onto the wall surface, and because the rate per m2 is the same, that means that there will indeed be a larger quantity collected by the portion of wall we are looking at.

Is that right?

The other question is whether the type of surface is relevant... for example, if it's a surface which water runs off easily, does this mean that there will be a higher rate of condensation?

Posted By: Cliff PopeIf part of the wall is insulated then by definition it will be warmer than an uninsulated part, so condensation will be higher on the colder section.

The temperature of the uninsulated part is unchanged (modulo edge effects) so the condensation rate on it is unchanged. The insulated part is warmer, so the condensation rate on it is decreased. Similar to what you say but the detail is important. It is the bahaviour of the newly insulated part that changes, not the behaviour of the unchanged, uninsulated part.

Posted By: Cliff PopeIf the air is undisturbed then the still air near the insulated section will be happy to stay carrying its moisture, and only the cooled air adjacent to the uninsulated section will condense.

But if you move the air (eg by an extractor fan? !) then this will give all humid air the chance to move over the cooler surface and deposit its moisture too?

It's true that this effect occurs (i.e. still air acts as a slight vapour retarder) and that we've been ignoring it up until now. I don't think it has much practical effect. The air movement caused as a person moves around or opens a door is enough to stir the air in the room in our house at least. To be fair we do have MVHR so the air is never completely still anyway.

Posted By: lineweightI thought the tendency was for vapour pressure to equalise within a volume of air, regardless of whether the air itself is actually moving as such.

Yes, indeed, but diffusion is a much less powerful force than convection.

Posted By: lineweight
Let's say there is a bathroom, in which, prior to being partly insulated, there tends to be some condensation on the inner face of the wall when someone has a shower. That wall becomes somewhat wet, but it then gradually dries out when the shower is turned off, and an extract fan is turned on, or a window opened.

Compare this with the same bathroom (and occupant behaviour) where half, or three quarters, of the wall has been insulated but an insulated portion remains. Does this mean that that section of wall is going to collect a similar amount of condensed vapour, but concentrated into a smaller area, which might, say, result in water running down the face and pooling somewhere, where it is going to take longer to dry out than the pre-insulation scenario.

Based on your explanation - I think there is potentially an increased risk, but not in fact during the period the shower is running and there's a constant supply of vapour, because the portion of wall we're looking at is not behaving any differently in either scenario. However, after the shower is finished, and there is no longer a supply of 'new' vapour, then there is potentially a longer period of time during which the vapour is condensing onto the wall surface, and because the rate per m2 is the same, that means that there will indeed be a larger quantity collected by the portion of wall we are looking at.

Is that right?

The other question is whether the type of surface is relevant... for example, if it's a surface which water runs off easily, does this mean that there will be a higher rate of condensation?

^^

I'm quoting myself from further up the thread - because I'm interested whether folk agree or disagree with my third-last paragraph.

Posted By: tonyThe way I see it is that if air near the coldest point reaches dew point and you add one more molecule of moisture to the air in the room then one molecule of water will condense on the cold surface no matter where you add the moisture. This is almost instant and something like drying washing, yes the washing dries but some water already in the air ends up as condensation at the cold place.

It does not need to convect or diffuse to get there you could think of it being transported there a bit like in Star Trek if you like, or like adding a building block to a row of blocks laid across a table - adding one causes one to fall off the others side. i.e. is is not the same molecule that condenses out as the one that was added to cause the problem.

You're willing to ignore the whole of science then?

Tony said that water molecules are interchangeable, the one that was added last to the room doesn't have to be the same one that condenses first. Also that molecular diffusion works by all the molecules moving a short distance down the gradient on average, not by one of them having to move all of the way while the rest of them all stand still.

That's 100% correct, according to classical science. Quantum mechanics offers a more subtle description but is not substantially different. Which bit did you disagree with?

Lineweight, I think your 3rd-last para is matching with what happened to my window panes in the 3rd post of the thread?

Posted By: WillInAberdeen

Lineweight, I think your 3rd-last para is matching with what happened to my window panes in the 3rd post of the thread?

Yes indeed - but as you said, there is also the factor of increased airtightness - do you think you would still be seeing additional condensation in those locations, had you done the insulation but somehow maintained the same level of ventilation (ie. as previously by leakage) to the space?

A few aspects to consider:

- increased risk of surface condensation and mould growth to uninsulated areas (always insulate your opening reveals!).

- potential risk for your neighbor if improving your own insulation levels reduces surface temperatures on their side of a party wall.

- increased level of air tightness - making intentional ventilation / humidity control much more important to get right and relevant to the overall issue.

- but, also an Increased overall air temperature in the dwelling from the same level of heat inputs.

Provided the internal relative humidity levels are controlled to about 65% Rh or below for most of the time and internal surface temperatures kept above 15°C for most of the time, then risks can be kept fairly low.

I wouldn't suggest insulating just half a wall for example, but staged interventions might be okay if any condensation issues that were to arise were monitored and managed.

Of course condition of external walls and drainage / rainwater goods etc. is also all highly relevant.

It sounds silly. Or rather it sounds like something designed to cope with perceived legalities. Why not insulate the whole lot so there is no part prone to condensation? What's the whole story?