This is because heated air is thinner and less dense than cooler air.
Third floor hot because heat rises.
First why is the second floor hot.
Heat from any source naturally rises.
As the weather warms up indoor temperatures are starting to rise as well.
So your upstairs has a natural tendency to be warmer than lower levels.
Two story homes often require more cool airflow to the second floor during the summertime and more warm airflow on the first floor in the wintertime.
Ductless mini splits are ideal for large homes where you only want to heat or cool the rooms you use the most.
These small heating and cooling systems focus on one room so you can control the temperature independently in each room where a unit is installed.
Although true this is far from the main culprit of an uncomfortable upstairs and there is plenty that can be done to even the temperatures in your 2 story home.
Don t wait until summer to correct this issue.
Your third floor is hotter than the rest of the house in the summer because the heat gets trapped there.
The most common rationale for a warmer second floor is that heat rises.
Your roof absorbs a ton of heat from the sun unless you have a lot of shady tree cover.
Unfortunately for many owners of two story homes varying heat loss or heat gain is a common problem.
You have warmth rising from the first floor plus the sun beating down on the roof and those two factors work together to make the third floor the hottest place in the house.