When we’re talking about condensation it makes logical sense to equate those water droplets with the amount of moisture in the air at any given time, or the relative humidity. However when we’re talking about building science and interstitial condensation (condensation in between two sides e.g. a wall cavity), what we actually need to talk about is the dew point. So what’s the difference?
Relative humidity is the amount of moisture 
present in the air relative to the current maximum moisture capacity of that air and it varies depending on the air temperature.
Dew point is the temperature at which the relative humidity in the air will turn into water droplets.
Exercise
Take a look at your preferred weather forecasting website (I personally like Weatherzone) today and navigate to your town or city.
- What is today’s air temperature?
- What is today’s relative humidity?
Below I have provided a Dew Point Calculator that I came across thanks to the esteemed and hilarious Allison A. Bailes III, PhD. Enter both of these numbers into this tool and it will give you the dew point.
So today, if the air finds any surface that is at the dew point temperature or below, water vapor from the air will condense and form liquid water droplets.
If you have a steel framed home for example, 
due to the high levels of conductance of steel, if this frame is not thermally separated from the outside conditions, there is a high risk that, during winter, the night time temperature of the steel will drop below the dew point on a regular basis and condensation will form on the surface, which carries with it a huge mould and corrosion risk for your wall and roof systems.
The same goes for thermally unbroken aluminium window frames and single pane glass. If you live in a house like mine, you will see condensation on the inside of your windows regularly during winter as cold air can hold less water vapour than warm air. This condensation means the glass has dropped below air temperature and reached the dew point where the moisture vapour in the air becomes liquid.
Have a play with the dew point calculator and see for yourself how the 3 factors of air temperature, relative humidity and dew point influence each other.
If you would like to reduce your risk of condensation appearing inside the walls and roof of either a project that you are building or designing for someone (builder/architect) or in your own house (owner), click the chat now button!
Jessica Allen
Climasure