Data loggers

These instructions are for the ORIA Wireless Thermometer Hygrometer currently costing £17.49 for two on Amazon (we recommending using two or more dataloggers to differentiate types of damp and to track down the source).

  1. Place one on or near the main damp patch, ideally on the coldest part of the wall.
  2. Place another in the nearest source of vapour often a kitchen or bathroom, ideally on an internal wall out of the way, such as above a door frame.
  • ORIA also sell dataloggers with displays (they are larger and slightly heavier, so harder to place by damp patches or under floors) £13 Brifit Wireless Thermometer Hygrometer, Bluetooth 5.0 with data display and 35M blue tooth range.
  • Smart Hygrometers, with 50M blue tooth range are useful for measuring sub-floor humidity, that is under timber flooring.
  • Landlords should consider using at least one Govee WIFI connected hygrometer. These costs about £42 through Amazon, the benefit is remote monitoring through WIFI (make sure you have WIFI). So the tenant can see the humidity and temperature and the landlord can monitor ambient conditions, and agree changes accordingly, for example proving improved ventilation or requesting the tenant increases nighttime temperature. After all it takes teamwork between tenant and landlord to maintain a property.

Dataloggers for identifying the root cause of damp

Dataloggers are instruments that monitor and log data collected from the environment, such inside a building. They can form part of Internet of Everything (IoE). The datalogger that interests us here is the hygrometer datalogger. It collects relative humidity and temperature, typically logging every 10 minutes.

For the past 5 years we’ve been using ever more data to pursue the root cause of damp. Data-loggers are an important part of that mix.

We ask clients to install one or more dataloggers for some time before the onsite damp survey begins. This helps understand when and where water vapour is being produced, ventilated, heated, insulated and therefore which areas are suffering from low temperature, condensation and mould. They help differentiate rainwater and mains water leaks from ambient vapour.

Setting up your datalogger the first time.

This applies to the Smart Hygrometer (which costs about £16 for two, is light weight, low profile (easy to stick near damp wall) and has a 100 day battery life). However, most dataloggers are designed with similar set-up and functionality.

Get “sensor blue” from app store. 
Turn bluetooth “on”.
Tip, start device(s) together on the hour.

If you find any of the above stages difficult, please at least pull out the small piece of plastic to initiate the loggers, and place them as below, we can complete the process during the survey.

  1. Place one on or near the main damp patch, ideally on the coldest part of the wall.
  2. Place another in the nearest source of vapour often a kitchen or bathroom, ideally on an internal wall out of the way, such as above a door frame.

Dew point formula

During and after the survey we will help with the next part, processing and understanding the data.

Alternatively download data to an excel or other spreadsheet, to find the Dew point the formula is

Dew point is a proxy for vapour pressure, i.e. amount of vapour in the air (note atmospheric pressure affects the dew point for the same vapour pressure, but if the devices are at the same atmospheric pressure, then vapour pressure correlates directly with dew points). Therefore you can compare the dew point of one device to another to work out where vapour is being produced and when it is being removed.

Screenshot from an excel spread sheet. Note you may have to remove the % and ° symbols. Also, occasionally the datalogger exports % as 0 – 1, in which case remove the two instances of “/100” from the formula. Note you can find the mould point by using “/85” instead of “/100” see, with daily forecasts.

Ex. 1 condensation

Sheet shot of complex datalogger graph

In this interesting example, four dataloggers were used in a mouldy top floor modern flat.  I don’t normally make the changes myself for insurance reasons, but in this case I foolishly provided the details of a general builder, who made a mess of the vents and was unwilling or unable to make the necessary improvements.

The issue was the ducting from two extractor fans were combined before exiting the building (as a top floor flat is was difficult to drill out an additional hole). Unfortunately back-flow drafts were not included in the original set-up, so the kitchen was venting into the bathroom and vice versa. On 5th January I installed two back-flow shutters and improved distribution of the loft insulation

From the graph, looking at the blue bathroom lines you can see a drop in %RH from nearby 100%RH to an average below 80%RH. The bedroom has similarly dropped. It’s looking considerably better. I have asked for replacement of the thermostat, that I found was unreliable, so as to reduce the temperature fluctuations. Otherwise the issues will disappear over time.

Top floor flat suffering from mould and condensation.

The problem was the extractor fan from the kitchen was venting out into the bathroom and vice-versa.

The data-loggers helped identify the problem, and provide a good comparable of before and after intervention.

The cost of solving the problem was very low, maybe £10 plus 1 hours work, it saved about £1,200 (plus annual maintenance) from an unnecessary PIV (Positive Input Ventilation). PIV are untargeted, expensive to install and run and often suffer from short-circuiting.

Ex. 2 mould

In this second mouldy property the client solved most of the mould problem with a dehumidifier before we surveyed the property – note the improvement from late November. We further improved the property with no need for additional equipment.

This image has an empty alt attribute; its file name is datalogger-dehumdifier.jpg
This mouldy property was solved in part by installing a dehumidifier, in late November

Understanding data

Occupiers will know their habits, changes and movements. Dataloggers can help the own, landlord and damp expert identify the main issues. 
Key elements to consider and look for as:

Surface %RH

  • Relative humidity (%RH) is a function of both temperature and vapour pressure, so where the data logger is placed can make a huge difference to it’s temperature and therefore relative humidity, but little difference to the dew point.
  • Rough every drop of 1°C increases % relative humidity (%RH) by about 5%RH.
  • To compensate for the temperature difference between the data logger and cold surface:
    1. Measure the temperature of the coldest part of the damp wall and compare it to the temperature on the data logger.
    2. The 365 version of the excel file includes a calculation of the surface relative humidity on the interface tab.
    3. Use a laser thermometer or infra-red lens, with accurate temperature sensor to identify the coldest part of a wall.
  • For ease, the graph assumes a constant temperature difference – it would be more accurate if the logger was placed on the coldest part of the wall, although it is hard to find and difficult to measure the very coldest surface.

Dew point differential

Dew point is a proxy for vapour pressure, or quantity of vapour in the air. Comparing the vapour pressure between two data logger can identify where vapour is generated or removed (through ventilation or condensation / dehumidification).

External data

  • The dew point and temperature within a similar nearby location is fairly constant across the area, such as London.
  • Comparing internal dew point to external dew point helps determine the benefit of ventilation.
  • Comparing temperature helps determine heat loss, such as through poor insulation is a factor.
  • We don’t normally show external relative humidity as it is not relevant and can distort the perceived benefits of ventilation, which should be through comparing internal to external dew points).


  • Ideally there should be a fairly constant temperature across a property and over the 24 hour cycle. This is hard to achieve.
  • Some people like to zone their house so as to conserve heat, or keep wine chilled. In this case you should increase ventilation out of the source of vapour (bathroom, kitchen, laundry) and in the cold zone, or used dehumidifiers, constantly piped out.

Relative humidity

  • If the surface relative humidity is at or close to 100%, then water is almost certainly condensing on or in the wall.
  • The dew line is where a wall is cold enough for condensation to take place within the building building material – known as interstitial condensation.
  • If the surface is above 85%RH for 6 or more hours then it is likely to become mouldy. Mould is omnipresent, however, spore will be far greater in properties where mould is growing. Therefore it is wise to keep washing away mould with bleach or anti-mould products and reduce relative humidity by:
    1. Extracting vapour at source (bathroom, kitchen, laundry).
    2. Keep doors closed in vapour generating rooms.
    3. Keep balanced heating.
    4. Improve installation and anti-mould products.
    5. Use dehumidifiers, regularly decanting water.
    6. Consider making the bathroom scrifical y keeping it colder, and wash away mould regularly.

How to use

  1. The simplest way to use the data loggers is to open a .csv file, copy and paste the data into the data sheet, making sure that the start times line up.
  2. We can’t support the use of file. Tip, use YouTube to better understand excel.
  3. Do share concerns, issues and tips amongst each other in the comments below.

Known issues

These analytical excel files will keep evolving, issues are time consuming to resolve, so sometimes are left in a slightly user-unfriendly state – sorry.

  1. When opening the data logger file:
    Excel may say: “Excel found a problem with one or more formula references in this worksheet.”
    Why: Because a formula runs from the start and finish data.
    Solution: Go to interface and select start and finish date.
  2. Importing data is easy, but occasionally a file is corrupted.
    Why: The date column is out of line and Excel recognises the temp (temperature), humidity (%RH) and time as text. It needs a quick find and replace (Command H – Mac / Control H – Windows).
    Solution: Find and replace % with % and : with : and remove ° by replacing with nothing.
  3. Make sure the data is align.
    Why: A datalogger may have been started at a different time.
    Solution: Cut and paste appropriately – note an offset is automatically set up for external data.
  4. Do post other issues in comments, we can’t help with excel, but contributors may be able to help.
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