Ventilation, CO2, humidity, and temperature

Our house has been built in the 50s and uses “natural ventilation”. Essentially this means there is poor isolation and air can escape/get in through random holes.

Ventilation is important, so that’s great. However it also means cold air can easily get in and hence we need a lot more gas to heat the house. Furthermore it is hard to control the indoor climate, and as a result we have a fairly high humidity (70%) inside. In the basement, which we use for storage, we actually noticed blankets to be wet and food

We’re going to isolate our cavity walls and replace single-glass windows with new double-glass windows, which should poorly affect the natural ventilation.

The house does have a ventilator in the kitchen and bathroom upstairs. There is no controlled entrance of fresh air, except for opening windows (and random places due to current poor isolation).

We first bought a few cheap hygrometers which confirmed the high humidity. We next purchased the Netatmo Weather Station (temperature, humidity, CO2) and added 3 additional indoor modules to try and better understand the current house climate and see how we can improve things. Since it has an outdoor module, it provides better perspective on the humidity inside compared to outside.


Without proper ventilation the CO2 will quickly rise as soon as there is people in a room. With the bedroom window closed, indeed CO2 started spiking during the night. It dropped again when we opened a window and were no longer in the room.

To ensure sufficient oxygen, we simply need to ensure fresh air can get in (and bad air out).


To be able regulate temperature inside the house, you want to keep heat out during summer and cold during winter. The house is heated using gas, which is effective but expensive. We currently have no system to cool the house, except for a portable air conditioner. The house remains fairly cool downstairs, since due to the orientation not a lot of sun comes in. The attic becomes very hot.

The insulation is supposed to keep cold air out. Ventilation during winter however will allow cold air to get in. It felt contradictory to “seal the house” first and next add ventilation. However, the coldness also enters via the windows, walls, etcetera (e.g. not only via airflow).


Humidity should be between 40% and 60%. It has been 70%+ whenever I checked.

Humidity (and CO2) increases from our breathing, cooking, aquarium, cat water fountain, etcetera. Using ventilation you want to get the moist air out. If the outside humidity is even higher though, how can you get it reduced?

Warm air can contain more water, so relative humidity drops if you heat the air. Therefore in winter, humidity tends to be (too) low inside.



The basement has a different dynamic. It’s relatively cold, so when warm air coming in from upstairs or outside cools down, the humidity will increase. For now, I’ve put in a dehumidifier. If the Netatmo station notices humidity exceeding 60% it will activate the powerplug using IFTTT. If it drops below 55%, it will shutdown the dehumidifier again. I’ll work on further fine-tuning this to minimize energy usage. It naturally remains cold so no need to bother about the temperature. The gas heating is in the basement, so I added a Nest Protect to monitor for CO (not to be confused with CO2). Since we don’t spend a lot of time in the basement, CO2 is not really of concern.

Living room

To be added.


To be added

Bed rooms

To be added.

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