ᐅ Strong increase in indoor air humidity during summer

Hello everyone,

In 2019, my wife and I purchased a semi-detached house built in 1925, which we completely renovated. Heating, plumbing, electrical system, flooring, new windows with double-glazed PVC frames, cavity wall insulation with hydrophobic loose-fill (Lammers insulation technology from Farven), roof insulation with 180mm (7 inches) Rockwool between rafters along with a Precit moisture-variable vapor retarder foil with an SD value of 0.2 to 50. External wall insulation of a 13 sqm (140 sq ft) extension with 100mm (4 inches) Neopor covered by a diffusion-open render. The roof of the extension is insulated with 200mm (8 inches) XPS (roof terrace) plus a polymer weldable membrane.

After two winters and two summers, the following recurring pattern emerges:

In winter, relative humidity ranges between 35% and 50%, depending on how many humidifiers we use.
In summer, the relative humidity rises to 50% to 80% in all rooms.

We regularly ventilate by cross-ventilation early in the morning and late in the evening throughout the entire house, but this has no significant effect. Currently, we use an electric dehumidifier, sometimes on the ground floor, sometimes on the upper floor, which extracts about 6 liters (6.3 quarts) of water from the air within 24 hours. This lowers the humidity to around 50% to 55%. However, this is not a permanent solution.

I would be very grateful for helpful, technically sound explanations regarding the cause and possible approaches to address this issue.

Best regards to all

Michael Dellas

Quite normal in summer. Warm air can hold much more moisture than cold air. Also, right now there is rain or humid warm weather almost all over Germany. Where should the moisture go then? Ventilation doesn’t help much in this situation. It is simply the way it is.

That’s quite high. But in our house, we also have around 60% humidity everywhere, and 65% in the basement. It also smells slightly musty down there, so the dehumidifier is running and I empty it every two days.

I think this year it’s due to the weather, either extremely wet or humid.

Purely in terms of humidity, ventilation can sometimes be counterproductive. We used to live in a natural stone house in a valley with a stream. Even in the middle of summer, the house always stayed cool. In unfavorable weather conditions and with an open stairwell, moisture would condense on the cold terrazzo stairs. It looked as if someone had just mopped without wringing out the cloth. This clearly shows how much moisture warm air can carry and then release as it cools down.

Mike Dellas schrieb:

In winter, relative humidity is between 35 and 50% depending on how many humidifiers we use.

This correlates with the outdoor humidity.

Mike Dellas schrieb:

In summer, the relative humidity rises to 50 to 80% in all rooms.

This also matches the humidity outdoors.

Mike Dellas schrieb:

We regularly ventilate early in the morning and late in the evening by opening windows throughout the house, but this has no significant effect.

That’s to be expected if the outdoor air is humid.

Mike Dellas schrieb:

Currently, we use an electric dehumidifier alternately in the ground floor and the upper floor, which extracts 6 liters (6.3 quarts) of water from the air in 24 hours.
The humidity then drops to 50 to 55%. However, this is not intended as a permanent solution.

A permanent solution is to turn off the dehumidifier and accept that it will be more humid in the summer than in winter. THAT IS NORMAL!!!

Mike Dellas schrieb:

I would appreciate helpful, professionally based comments on the causes and possible approaches.

You have received those. Possible approach: build a completely airtight house, keep windows always closed, and supply fresh air using mechanical ventilation. Only this way can you control the indoor humidity as desired. Alternatively, run an air conditioning system in summer that significantly dehumidifies the air.

No chance at the moment with this weather. It has been raining here continuously for the past 2 months, not just feeling like it. Dehumidifier on and that’s it.