Vapour pressure in buildings

The following article talks about vapour pressure in buildings large and small. Last time we talked about condensation and relative humidity, how this is expressed as a proportion of how much moisture air can hold at a given temperature, and how 'dew point' temperature, is the exact temperature where vapour condenses back into a liquid (100% RH). Today let's think about vapour pressure. This is actual quantity of moisture in the air.

Simplistically therefore, high vapour pressure = wet air, low vapour pressure = dry air. The thing to understand about differential vapour pressures, is that they will seek to equalise. What does this mean? Example: Two sealed rooms with a wall and door separating them. One has a shower in it which has been running and it's steamy/humid, i.e. high vapour pressure. The other room has a dehumidifier running and it's very dry / low vapour pressure. When the door is opened between the rooms, what happens?

Does each level of vapour stay exactly the same in each room? No. They will seek to equalise until the level of vapour is the same in each room. This is called 'vapour pressure equalisation' or 'vapour drive'. Sounds complicated, but you probably already inadvertently understand this. What do you do before you jump in the shower of a morning? Do you open a window? The air outside is cooler than inside, (therefore can hold less moisture), and is typically lower vapour pressure than it is in an occupied property, especially in a bathroom with a shower running. Therefore what you're doing when you open a window pre-shower, is allowing for steam (vapour) to drive / be lost to the exterior through vapour pressure equalisation.

Why do people say that to solve condensation issues, people need to open their windows? Vapour drive! Vapour pressure will always be higher inside an occupied property Pics: 1. Rates of vapour production in an occupied property. 2. Graph shows information collected over time using data-loggers, with one placed inside an occupied property, and one outside.

It illustrates the difference in internal (green line) versus external (red line) vapour pressure over time, note that internally it is always higher because of internal vapour production and also that the interior tracks the exterior to a degree, i.e. internal vapour pressure is influenced by external vapour pressure, because we do not live in sealed boxes. IMG_8029 vapour pressure To find out more about waterproof systems click here. 

It’s that time of year… and it’s probably worth having a basic understanding of condensation and this and how it can be dealt with. The issue can effect your entire house as well as your basement waterproofing system. Water can be solid (ice), a liquid, or a gas (vapour). Condensation is the process whereby it changes from a gas to a liquid. We all know that warm air can hold more moisture right?  Well this is true, but there are different ways in which we can consider HOW MUCH moisture there is within the air, this being: relative humidity (RH) and vapour pressure. RH does not actually tell us how much physical moisture there is in the air.  What it tells us is how much moisture is in the air RELATIVE to the total quantity of moisture that the air can hold at a given temperature. It is expressed as a proportion, i.e. a percentage. If we have 50% RH, that air is holding half of the total moisture that it could hold, at a particular temperature. Remember – it’s just a proportion, not a physical quantity. If we don’t change the amount of moisture in the air, but heat it up or cool it down, what we’re doing is influencing what the air can hold, nothing else, but as a result of this the RH proportion would change. As simplistically as I can make it and using the example above, i.e. air with 50% RH, if we heat up the air the RH might drop to 25%.  If we cool down that air, the RH might increase to 75%. Remember, we’ve not changed how much moisture is in the air, we’ve just influenced how much moisture the air can hold, by changing its temperature. So… condensation occurs when you reach 100% RH, i.e. the air is cooled to a point whereby it cannot hold the moisture, and this condenses, i.e. changes from vapour (a gas) into its liquid state. This is why water condenses on cold surfaces, such as windows, window reveals, poorly insulated substrates etc. etc., and it is why homes need to be heated/insulated in order to prevent issues of condensation. The temperature at which you reach 100% RH is called the ‘dew point’ temperature, and when assessing condensation one of the things to do is to work out what that (dew point) temperature is and then compare surface temperatures to it.  The closer those surfaces are to dew point, the higher the risk.  Don’t worry, there are easy ways to do this with the right equipment, but I’ll come back to this. There is more to it in respect of considering the influence of vapour pressure, and how issues of condensation can be controlled other than heating and insulating, i.e. what we can do in respect of ventilation and vapour production, but RH is enough for one day and a post on vapour pressure can follow. Photos: 1. Condensation forming on 'cold bridges' where dot & dab adhesive was used to fix plasterboard to all wall, providing localised patches of cooler surface temperature. This was just after skimming so high vapour levels from that skim drying down and releasing it.  2. Condensation on a concrete soffit, in tunnel linking a house to a separate swimming pool pavilion where we remedied a number of issues.

dot and dab condensation

dot and dab condensation

Condensation on soffit

Condensation on soffit



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