Insulated plasterboard and condensation.

[John D v2.0]

Condensation risk is only relevant to timber framed structures, not plasterboard on a brick wall.

You mean to say, in practice you've found condensation risk is potentially high on timber frame (which includes standard cold roofs) whereas with plasterboard on a brick wall it's usually low so not worth calculating.

 

[hard-work]

Have to disagree here, Woods.
Two years ago on a refurbishment, a client applied insulated plasterboard (polystyrene-type, and with no vcl) to a vaulted roof. There was 25mm Kingspan between the rafters - giving 50mm airgap - and the insulated plasterboard below. There was plenty of draft in the airgap as it was an old house. During the first cold snap, brown condensation stains appeared down the ceiling and it had to be taken down and re-done. On cold, still, Winter's nights there is often little air movement, and the moisture condensed on the cold side of the insulation. In the OP's case, it is likely there will be little ventilation of the gap, and being a bathroom, the risk is higher.

If a vent ridge tile was fitted there would not have been a problem. It looks like the air gap was open only at the bottom not the top, so there was no through air to take away any moisture laden air. My situation is very different. Also, the plasterboard in the bathroom will have tiles floor to ceiling.

BTW, good debate.

 

[^woody^]

You mean to say, in practice you've found condensation risk is potentially high on timber frame (which includes standard cold roofs) whereas with plasterboard on a brick wall it's usually low so not worth calculating.

No.

Condensation is only relevant to timber frame because of the associated risk of mould and rot.

The risk of condensation occurring is not the same as the risks following the occurrence of condensation.

 

[mikeey84]

. Any moisture that might evaporate out at the rear, will be very little as to be insignificant - it will be just normal air and humidity.

Just to clarify, it won't be normal humidity. When we refer to humidity readings, we refer to relative humidity, as in relative to temperature. Air with the same amount of water vapour will have a higher humidity at a lower temperature, and therefore a higher risk of condensation.

 

[John D v2.0]

No.

Condensation is only relevant to timber frame because of the associated risk of mould and rot.

The risk of condensation occurring is not the same as the risks following the occurrence of condensation.

Earlier you said there won't be condensation because the bricks are porous, now you're saying it doesn't matter if there is because the bricks won't be damaged? But if that, the cavity where the build up will occur needs to be drained to outside, not just soak in inside the wall construction above the dpc.
To be honest we probably are coming at the same thing from opposite directions and probably agree in in reality!

 

[John D v2.0]

Just to clarify, it won't be normal humidity. When we refer to humidity readings, we refer to relative humidity, as in relative to temperature. Air with the same amount of water vapour will have a higher humidity at a lower temperature, and therefore a higher risk of condensation.

Normal humidity is absolute humidity? Usually the humidity sensors you buy would read relative humidity.
Relative humidity is relative to the capacity of the air, not to the temperature. The dew point is the temperature the air needs cooling to in order to reach 100% relative humidity.

 

[mikeey84]

Normal humidity is absolute humidity? Usually the humidity sensors you buy would read relative humidity.
Relative humidity is relative to the capacity of the air, not to the temperature. The dew point is the temperature the air needs cooling to in order to reach 100% relative humidity.

The capacity of air to hold moisture is directly related to temperature (and pressure) so whilst you are correct, the capacity is a function of its temperature. Woody seems to be saying, and apologies if this is misinterpreted, that the humidity of the air between the wall and the insulation would be the same as in the room. This is unlikely as it's more likely to be colder between the wall and insulation and therefore the relative humidity (which is much more relevant to the real world) would be much higher.

 

[^woody^]

Earlier you said there won't be condensation because the bricks are porous, now you're saying it doesn't matter if there is because the bricks won't be damaged?

No. Earlier I said

How does moisture (ie condensation) form on the surface of a porous material?

and

You seem to be applying the bog standard "air condensing on a cold piece of glass in a room" scenario to completely different situation.

and

Just normal air, going through a normal cycle of humidty changes, and passing through materials.

You seem to be thinking of condensation like that which occurs on the windows on a cold night.

Air might condense on (in to) the wall but there wont be condensation. The moisture will either remain within the air, diffuse into the wall or evaporate back out as part of a normal cycle.

Bear in mind that we are talking about a very small quantity of air and even smaller amount of moisture. Its not as if moisture is going to keep going from the bathroom and filling up the cavity with more and more water. There will only ever be a tiny bit of moisture going around in a cycle.

 

[John D v2.0]

Yes I see what you're saying! I think we're actually in agreement in essence! Thanks for the discussion

 

[garyo]

I'm confused... why do people batten out walls with a vapour barrier and an air gap if the walls are porous and can breathe? Is it that the battens are the only practical way of fixing the panels, and then that creates the risk of decay which necessitates the vapour barrier? So for a straight solid wall you could just dot-and-dab a PIR backed panel to the wall and not bother with gaps or barriers at all?