Why your home feels like a tropical rainforest during a heatwave

The secret behind that stuffy feeling at home has everything to do with the physics of our indoor climate. Many people therefore wonder whether the temperature affects the humidity in the home. The answer is, of course, a resounding ‘yes’, and it explains exactly why your home can feel so uncomfortable in summer. There is a direct link between the temperature of the air and the amount of moisture it can hold. Warm air expands and acts like a sponge. So, the warmer the air, the more water vapour it can hold, and therefore the more humid the air becomes.When it’s sweltering outside, and certainly when a thunderstorm is brewing during a heatwave, the outside air is saturated with moisture. As soon as that warm, humid air finds its way into your home, the air inside also becomes saturated. Because the temperature indoors is often just a little lower than outside, or simply because the air isn’t circulating, the relative humidity indoors rises rapidly. The result is that typical, oppressive heat in which sweat can no longer evaporate from your skin. Your home no longer cools down, but instead traps the heat and moisture.

At AquaConsult, during the hot summer months, we’re often asked: “When is the humidity in the house actually too high?” In winter, we tend to associate damp with condensation on the windows, but in summer the signs are more subtle. To determine whether your indoor climate is still healthy during a heatwave, we look at the percentages. In a healthy Flemish home, the ideal relative humidity is between 40% and 60%. Within these limits, the air feels comfortable, your respiratory system functions optimally and mould doesn’t stand a chance. But during a heatwave, this figure indoors can easily rise to 70% or higher. And that brings us back to a frequently asked question: “Is 70% humidity too high?”

• Less than 40%: In that case, the humidity in your home is too low. This can cause dry eyes and irritated airways. However, this is more common in winter.
• Between the 40% and 60%: The ideal percentage for a healthy home. This ensures you enjoy optimal living comfort, healthy breathing and no risk of damage to your home and your health.
• 70% or higher: This humidity level is far too high for your home. What’s more, it makes the air feel sticky and causes extreme stuffiness indoors. It is also the ideal breeding ground for fungi.

So a humidity level of 70% indoors is certainly too high. But what does this actually mean for your comfort? And why does it feel so stifling?

The last straw

During a heatwave, something unusual happens to the humidity indoors.

• The sponge becomes saturated: The air outside is simply warmer, which means it is full of moisture due to summer evaporation. And during a heatwave, this air also seeps into the house, leading to higher humidity.
• The air cools down indoors, but the damp remains: As you’ve already taken all the necessary measures, it’s a few degrees cooler inside than outside. But when this warm, humid air cools down indoors, it shrinks, as it were. The sponge gets smaller, but the amount of moisture remains the same.
• The last straw: Because the cooler indoor air can hold less moisture as it contracts, the relative humidity shoots up immediately to 70% or 80%. The air is then supersaturated.

So why does this feel so ‘tropical’ and stifling?

This is the point at which your body starts to protest. Normally, the human body cools itself down by sweating. The normal, dry ambient air absorbs the tiny droplets of sweat, causing your skin to cool down.

But if the relative humidity in your home exceeds 70%, the air is already so saturated with water vapour that there is no room left for your sweat. The sweat can no longer evaporate and remains on your skin. Your body can no longer release its heat, making you feel stifled and as if there’s not a breath of wind to be found.

The signs: How can you tell if the humidity is too high during a heatwave?

As you don’t have the heating on in summer and the windows are often open, you don’t tend to see condensation on the windows. However, there are other, very clear signs that indicate the humidity in your home is too high.

• Damp sheets and furniture: You get into bed and the bedding feels clammy or almost damp, as if it hasn’t dried properly.
• A musty smell: When you walk into a room, especially the bedroom or the cellar, there’s a typical, musty ‘cellar smell’.
• Skin that sticks: Even if you’re sitting still with the fan on, your skin won’t dry out and you’ll still be left with a shiny, sticky film.
• Wood that swells: Kitchen cupboards, interior doors or wooden floors may suddenly start to stick or become harder to close because the wood absorbs excess moisture from the air.
• Unexplained patches of mould: Small, dark spots suddenly appear behind cupboards or in the corners of the bathroom and bedroom. This is because mould thrives in these warm, tropical conditions.

Do you notice any of these signs in your home? If so, there’s a good chance that the moisture balance in your home has been seriously disrupted.

When you start to break out in a sweat and the walls of your home begin to radiate heat, most Belgians’ first instinct is, quite naturally, to seek relief through natural ventilation. We open everything up.

Although this feels very intuitive, natural ventilation during a heatwave is, unfortunately, the biggest pitfall for your indoor climate.

1. Leave windows and doors wide open in the evening and at night

• The idea: It’s getting cooler outside, so if I open everything up now, the heat will blow out of the house.
• The pitfall: Yes, the outside air feels cooler on your skin in the evening and around midnight, but that same night-time air is often extremely saturated with moisture in the summer. By leaving your windows wide open for hours on end, you create a draught that literally lets litres of invisible water vapour into your bedrooms and living rooms. As soon as you close the windows again in the morning, the moisture becomes trapped. You are then literally bringing the heat into your home yourself.

2. Leave windows slightly ajar or in the ‘tilt’ position during the day

• The idea: If I let in a breath of fresh air, at least it’ll still be possible to breathe inside.
• The pitfall: Leaving a window tilted open during a heatwave is an open invitation to the hot, humid air from outside. The hot, humid outside air flows in slowly but continuously. Because it is slightly cooler inside thanks to your sun blinds, the incoming air cools down, contracts and causes the relative humidity in your home to rise immediately to between 70% and 80%. In doing so, you create exactly the greenhouse effect you are trying to avoid.

3. Relying on built-in window ventilation grilles

• The idea: My windows have ventilation grilles, so natural air circulation takes care of it.
• The pitfall: Window vents operate on the basis of wind pressure and temperature differences. On a windless, sweltering summer’s day, there is hardly any pressure difference between indoors and outdoors. As a result, the vents do absolutely nothing, or they passively allow the warm, humid air to flow in without any active extraction to counteract it. There is no control whatsoever.

4. Creating cross-ventilation

• The idea: If I open the front door and the back window at the same time, I effectively drive the warm air out.
• The pitfall: Although a breeze indoors feels wonderfully cooling on your sticky skin at that moment, you’re filling the structure of your home with damp air. As soon as the doors close again, the airflow stops and the oppressive humidity is trapped in your furniture, walls and fabrics.

With natural ventilation, you are completely at the mercy of the whims of nature. You cannot filter the airflow, you cannot regulate it and, most importantly, you cannot remove the moisture from the air. You are bringing the tropical climate from outside straight into your own home. Anyone who wants to keep their home truly comfortable and dry during a heatwave needs to move away from natural ventilation and opt for control with our solution.