There are a few reasons why your house may heat up at night, even when the temperature outside is dropping:
Radiative Cooling: During the day, the sun's rays heat up the objects and surfaces inside and outside your house. At night, these objects and surfaces radiate the stored heat back into the surrounding environment, including your house. This radiative cooling can contribute to the heating of your house at night, even if the air temperature outside is colder.
Thermal Mass: Some materials, such as concrete or brick, have high thermal mass. These materials absorb and store heat during the day and release it slowly over time. So, even as the outdoor air temperature drops, the thermal mass of your house continues to release the stored heat, leading to a warmer interior.
Infiltration: If there are gaps or leaks in your house's insulation or windows, colder air from the outside can infiltrate into your house, displacing the warmer air inside. This can make your house feel colder overall. However, in some cases, the cold air entering the house can mix with warmer air, leading to a slight rise in temperature.
Internal Heat Sources: The activities inside your house, such as cooking, using electronic devices, or even body heat from occupants, can generate heat. If these heat sources are present at night, they can contribute to the warming of your house.
It's important to note that the specific factors affecting the temperature change in your house may vary based on various factors like insulation, ventilation, orientation, and local climate. Conducting a thorough evaluation of your house's insulation, air leaks, and thermal properties can help identify the specific reasons for the heating of your house at night.