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How to be confident if ventilating will improve damp issues in cold & even wet weather

The headlines

It is a common belief that if windows are opened when it is wet outside it will make damp and humidity issues worse indoors.

It sounds counterintuitive, but this is actually untrue most of the time, ESPECIALLY in cold weather, and even in cold wet weather.

The tools below will give you confidence that when you open a window or trickle vents, you are improving your damp issues and NOT making them worse.

They use simple information such as outside weather information from wherever you live.

Humidity is a understandably a pretty mysterious subject to most people and is widely misunderstood.

The tools I describe below are:

  • Two simple (utterly brilliant!) free web tools.

  • A physical product - a smart humidity meter


A little bit about humidity "witchcraft" first

When air warms up its % humidity figure and 'dampness' drop.

This means that when air comes in through a window on a cool day, the % humidity figure of that air actually falls once it has warmed up to the indoor temperature.

The photo below has an example:

In this case the weather outside is 10C with 90% humidity and the indoor temperature is 18C.

As the air heads indoors it's humidity will fall to 55% once it has warmed up to the room temperature indoors.


In the above example it means if the air indoors was at 70% humidity before the window was opened it will start to fall.

The air inside will actually gradually become dryer.

In simple terms:

  • Cold air doesn't hold much water in it, and the colder it is the less it can hold less.

  • When raining the rain is actually falling 'through' the air, and not 'in' the air. Relative humidity meters measure the amount of water held in the air as a gas, not a liquid.

This was some feedback from someone after reading this article:

Try these tools for yourself if you are doubtful. Just follow the web links in the sections below, punch in the numbers below and see what the tools predict.


The free web tools

These give you clear information for your situation and are quick and easy to use.

Each tool works in slightly different ways. One might suit you better than the other.

Sadly there are no mobile phone apps that do this, (but it would not be hard to produce an app to do this if someone out there has the skills and interest!).

If you use these tools, you'll hopefully soon find that in cooler weather in particular (even in cold damp weather with high outdoor humidity), that opening windows will help reduce high levels of humidity in winter rather than increase it.

In winter, there are very few circumstances when ventilating a home will make indoor humidity or damp issues worse.

Ventilating is really important to for getting rid of a wide range of pollution generated from day to day activities indoors.


Tool 1 - 'Humidity calculator' web tool

This tells you what the relative humidity % will be for air coming in once it has warmed up.

The screenshots below show you how to use it, and what it tells you.

You just need to punch three things into the boxes on screen:

  1. The humidity outside. This is easy to find from weather forecasts on mobile phone apps or dedicated humidity apps.

  2. The temperature outside. Again easy to find from the same apps.

  3. The temperature inside. You really need some sort of thermometer for this.

In the example I've used below:

  • The outside temperature is 10C

  • The outside humidity is 85%

  • The temperature in the home is 18C

This example shows that if you open windows, the air coming in will be 52% once it has warmed up to 18C.

So, if for example your indoor humidity is 70%, it will fall when you open a open a window or trickle vent.

It may not fall rapidly and it may not fall as low as 52%. But it WILL definitely fall. How quickly it falls will depend on how much ventilation you have provided. It will also depend on whether you are currently generating much moist air indoors, such as drying laundry.

A windy day will mean it your indoor humidity will drop more quickly.

It seems too good to be true, and magic, but trust me this is all real.

Outdoor humidity and temperature vary a lot day to day. If you use this tool do try a few different figures for different times of day to get an overall feel.

Don't worry if it goes up and down a lot during the day. Outside conditions change a lot during the day and you'll find that most of the time is sensible to ventilate.


Tool 2 - 'Humidity Forecast' web tool

This next tool gives much more information.

It gives you a graph that predicts what your indoor humidity could be for the hours and days ahead if you ventilate. It might not suit everyone though.

This just needs you punch in your postcode and indoor temperature.

I've included a screenshot below.

The orange line shows the outdoor humidity in the days ahead. As you can see on the scale on the left, it is around 90% but is currently forecast to fall in a few days time.

The blue line shows what the humidity of that air if that air was allowed into the home.

On these days it is forecast peak at just over 60% and the lowest would be around 25%.

As you can see the blue 'indoor humidity' line is very low a lot of the time.

In this case, I would expect that over the days ahead the indoor humidity will settle somewhere between the peaks and troughs of the blue line. So, in the example below the indoor air should end up somewhere around 45-50%.

Again it is really important not to worry about peaks.

The ideal range is 40%-60% indoors. Some people with certain health conditions need a humidity that is a little different.

I've also had a go at explaining this tool in this three minute video.


Physical product - a pretty clever humidity meter

Inexpensive relative humidity meters are becoming a very familiar sight these days. They typically cost under £5. Unfortunately though, most of them have no idea what is going on outside.

I've found a jolly clever humidity meter that has an external sensor. It automatically works out for you it is sensible to ventilate based on the temperature and humidity outside.

You get a clear simple symbol on an indoor display showing whether you are best opening or closing windows or vents.

They are sold on Amazon here currently at £32 but there are other places also selling the same product. Just Google 'BRESSER 7007402'.

A YouTube review of it can be found here. You don't need sound as for some weird reason it is in Russian with English subtitles!


A simple experiment

To give you confidence that I'm not a looney, do have a play

  • Pick a time when your indoor humidity is high.

  • Use one of the web tools above to work out if it makes sense to ventilate in the next few hours.

  • Put your humidity meter in the middle of the room.

  • Shut the door.

  • Let the meter and room settle for a couple of hours.

  • Open the window a half an inch or inch and make a note of the meter reading.

  • See what the humidity meter says a few hours later.

Assuming the temperature in the room hasn't changed much during the test, the humidity % figure should fall.


Indoor air pollution - I do need to mention...

Off topic a little, but ventilation is the only way to get rid of indoor pollution. This is actually a serious issue that I'll be talking more about in other blogs shortly.


The geeky zone

Why 40-60% is generally ideal for indoor humidity

The risk of issues from bacteria, viruses and moulds gradually increase if the relative humidity is outside this range. It doesn't mean that going outside these levels is harmful for short periods.

This is all specific to indoor situations. None of this is relevant to being outside or humidity levels outdoors.

This chart below is very useful as a guide for indoor situations. It can be found on this web site with some further very useful information on this specific subject.

Chart source Energy Vanguard.

All electricity costs assume energy unit costs as of Oct 2022 - 34p/kWh


Quick links to the other main articles on this subject.

Other supporting articles on damp:


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If you have any other suggestions for additions or changes to site content do please let me know. While I have made every effort to ensure that the information contained on this website is correct, I cannot take responsibility for errors or omissions.

All content on the site should be treated as information and not advice. You should take professional advice where appropriate to different site articles.


Mark Thompson

Get Energy Savvy - simple practical home energy efficiency information

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Jan 03

Very helpful ,thanks mate


Sep 07, 2023

This was a very helpful article, thank you so much!

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