Understanding Condensation and Vapour Pressure

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. 

We Are Approved & Accredited By

Gold RGB

Our customer reviews

  • “We recently had the pleasure of working with the team from Trace. We found all of the office staff, managers and contractors a pleasure to deal with. Friendly, helpful and knowledgeable team. Very highly recommended !”

    Finalfix Ltd Builders in Cheshire

  • “I couldn't recommend Trace Remedial enough. I was extremely impressed with all of the advise received and delighted with their speedy and professional service. I have even sought some advise months after my initial business with Trace Remedial and they were more than happy to help again. Fantastic!”

    Lolos V R

  • “Newton Waterproofing Systems would highly recommend the services of Trace Basements & Trace Remedial Building Services. We have a fantastic working relationship with Trace. As Newton Specialist Basement Contractors, they are reliable, knowledgeable and skilled in their field.”

    Newton Waterproofing Systems

  • "I can highly recommend Trace. I rang to get a damp survey booked in on a property we were looking to buy. Lewis was very helpful over a couple of phone calls, even ringing back on a day off! Offered advice and help even though we didn't end up going through with the survey. Great service, will use again."

    Nick Renshaw

  • “Just had Trace in to deal with a fairly significant damp problem in our house. The work was completed to a very high standard in the timescale agreed before commencement. Really good communication throughout and the tradesmen were really professional. Left the house as tidy as possible every day and I’m made up with the outcome. I wouldn’t hesitate to recommend to family and friends.”

    Kealey Clarke

About Trace
Basements & Remedial Systems

Have our experts quote on your Waterproofing, Damp Proofing or Timber Treatment job.