Simple guide to relative humidity (RH)
Relative humidity (RH) is the proportion of water vapour the air can carry at a given temperature – it is expressed as a percentage. “The warmer the air the more water it can hold. “
(Click on the chart to see a larger version)
1000grams of air at 50C can hold about 5 grams of invisible water vapour.
1000grams of air at 250C can hold about 20 grams of water (4 times more)
Tracing the numbers on the psychrometric chart please following this argument :-
1 |
Air at 00C, carrying 90% of its capacity in water vapour (90% RH) enters the home and is heated up to 200C. |
2 |
At 200C the RH reduces to only 23% but it soon takes on the water from the damp air inside the house |
3 |
Going up the chart the RH of the air rises to that which is in the home - 70% |
4 |
On a cold surface, as at position 4, about 140C, this air becomes 100% humid and the water condenses out forming water droplets – CONDENSATION. |
If air is slowly introduced into the dwelling the relative humidity gradually falls to below 60% and the damp problem goes away. This is because any air coming from outside, soaks up the moisture, and leaves the building with lots of water vapour.
The house dries and the living conditions improve immediately.
To control damp you need more heat or more air (or a combination of both – a cold house is harder to keep dry) – air costs very little to heat and air from a roof void is not only dry but often warmed by the sun.
Positive pressure ventilation works by pumping in “dry” air from the roof space.*
*Dutch research by professor Prof. Mollier
The relationship between RH (relative humidity), temperature and energy necessary to heat 1m3 of air.
At 20˚C and a RH of 80% 50 K joule is needed
At 20˚C and a RH of 55% 40K joule is needed
A saving of 10K Joule which is 20%.
In practice this does not lead to a 20% saving on your total energy because it will contain more than only energy consumption for heating. It proves however that your heating bill will go down when humidity levels in the house drop. (Source vochtfree.nl)
From around March to Sept warm air from the roof enhances the heating within the house – free heating from the sun
So - in most homes only 20% of all the heating costs actually heats up the air – 80% heats up the walls, ceiling and floors. So only a very small percentage of the heating cost is lost when air is introduced and this is balanced by being able to turn down the thermostat and still feel warmer!
Heat recovery ventilation can reduce RH by removing “wet air” and pumping dry air in from outside at the same time. These units are particularly specified where there is no roof void, where the damp generation is focused as in the bathroom or kitchen, or when dwelling is small. There is no solar gain but about 80% of the heat is extracted and returned to the home making them very efficient.
A humidity controlled fan will automatically switch on when the humidity level is too high. This is a cheap and effective way of combating mild cases of condensation.
