ᐅ How does a house store heat when it is hot outside?

Hello,

I registered on this forum to get an answer to why an apartment sometimes never really cools down during hot periods.

I live in a 50 sqm (540 sq ft) attic apartment. It is located directly under the entire roof, both the north and south sides. My apartment is mainly one 40 sqm (430 sq ft) large room with sloping ceilings on both sides and a peak height of 3.80 meters (12.5 feet). The rest consists of a hallway and a bathroom. The total volume is approximately 100 cubic meters (3,530 cubic feet). My problem is that even when I ventilate and the apartment becomes cooler, the heat quickly returns. I already use a strong fan to move the hot air that accumulates near the roof away.

I am curious where such a building stores the heat and then releases it (into my apartment)? Is the warm air trapped in the roof tiles or in the masonry? Or is it still within the attic apartment itself?

The heat is stored in the mass of the roof and masonry. Therefore, ventilation only provides short-term relief: the air has significantly less mass compared to the roof and walls and can only absorb and carry away a small portion of the stored heat. A fan is therefore not helpful in this case.

The heat mainly comes from solar radiation, which warms the mass of the roof and masonry. Effective solutions are shading and an air conditioning system.

First of all, thank you for the very clear explanation.

Fortunately, I have had an energy-efficient air conditioner since last year. Although it is only sized for about half of the living space by volume, it lowers the room temperature by almost three degrees and creates a comfortable climate (I don’t sweat), so it works well when it’s running. I just have to turn it off occasionally because of the noise when I’m on the phone. I work from home (small consulting business).

I’ve also thought that the heat is stored somewhere in the walls (and roof) and that you can’t simply "push out" the warm air with a fan.

A non-scientific way to put it: We build houses designed to keep heat from escaping in winter. These stubborn things follow the same principle in summer as well.

If the air conditioning system is slightly oversized, it can run at a lower setting = quieter.

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I would be interested to know where such a house stores heat and then releases it into my apartment?

Everything inside the insulated building envelope stores heat. Walls, floors, furniture—everything. The more massive the material, the more heat it stores. As mentioned before, air with low mass stores and transfers very little heat. On the other hand, a high-mass building takes longer to warm up.
The only effective measures are consistent shading, active cooling, and minimizing internal heat sources. If heat penetrates through the roof insulation after a few hours of sunlight, there is little you can do except actively cool the space.