Calculate meaningful internal humidity readings
This short blog is to give a basic guide as to how to calculate and monitor internal humidity in your own home. It is not aimed at being a scientific research paper, just the basics to record and interpret data in your old house.
I see many posts on social media about people recording relative humidity and temperature in their own homes and looking for advice on whether the readings are acceptable. I then see many replies offering advice which do not really appear to understand the basics.
What you need to know
Use a humidity recorder that records Absolute Humidity and record dew point / surface temperature of your walls.
If you do not have a humidity recorder that records absolute humidity, use a free online calculator to convert temp and relative humidity to absolute humidity such as’; https://humiditycalculator.vaisala.com
Internal absolute humidity should not be significantly raised above external levels.
Raised internal absolute humidity causes raised dew points , which can create conditions where high internal moisture load can condensate on cooler surfaces.
If high absolute humidity exists, reducing humidity can usually be achieved by resolving any external defects and providing proper ventilation and heating.
What not to do
The problem with recording relative humidity alone
Relative humidity is linked to temperature. As such, if we use the example below we can see that;
External and Bedroom 1 Relative Humidity is around 70%.
The bathroom Relative Humidity is LOWER at 57.8 %, but because of the higher internal temperature the absolute humidity (total moisture in the air) is actually higher.
This results in the bathroom walls dew point being at the same temperature as outside, thus there is a high chance when external conditions get cooler (at night) condensation could occur.
Using ABSOLUTE HUMIDITY
Absolute humidity and dew point are far more useful tools than temperature and relative humidity for basic householder calculations.
We will work through an example to see how we might recording some usable data at your own home only using temperature, relative humidity and surface temperature.
Step 1 – Get a pen and paper and record the temp / relative humidity externally and in a few rooms internally. Write them down in a table like the example above and below.
Step 2 – Calculate the ABSOLUTE HUMIDITY and DEW POINT using a humidity calculator and enter the results into the table; Vaisala have a free one; https://humiditycalculator.vaisala.com
Step 3 – (Optional) record surface temperature to see problem areas for condensation formation.
Write the results in a table
Use an online calculator to calculate the Absolute Humidity
Record surface temprature
The first thing to remember is these readings are always a snapshot in time. External conditions change and internal conditions change.
The recorded data table should demonstrate quite quickly and clearly if there are any specific areas where absolute humidity is too high, or the dew point is too close to the external wall temperature.
In this example we record the humidity in the Heritage Consulting Office. We can see that the absolute humidity is raised by 2/m3 which is elevated. However, the dew point is quite high at 14.6 degrees in comparison to the external temperature.
However, if we look at surface wall temperature (15.2 degrees) we can see that this is only marginally above the dew point. As such we have a “potential” issue in this location.
For the purposes of this article, we will pretend the humidity was raised slightly and the revised readings are in the table below. If these conditions existed we can clearly see that the dew point is significantly above the wall temperature of 15.2 degrees and there is a high moisture load of about 40% greater internally than externally. CONDENSATION ALERT !
Modified results for desktop study
Tools of the trade
We use Vaisala or Testo thermal hygrometers which record absolute humidity. We use FLIR thermal imaging cameras for recording surface temperature. However these units are very expensive and probably not economical for the home owner, We need these expensive units as we use them commercially (and need them to last a long time) and for expert witness work (so need demonstrable reliability).
However, I have tested some cheap home hygrometers and surface temperature recorders and found them quite good enough to record the data required for a home owner.