The 2nd Law of Thermodynamics
The 2nd law of thermodynamics tells us that heat, energy and pressure all move from high concentration to low concentration. This is important to understand because when its hot outside, like it has been over this past week in Fresno, that heat outside wants to attack the cooler air indoors. If your home has a large number of air leaks to the outside through poor seals around doors, penetrations in walls and leaks around windows, then that heat has an expressway into your home.
That 2nd law of thermodynamics also tells us that the greater the difference between two temperatures the faster the change. So when it was 109 degrees outside and you kept your home at 80 degrees the heat will move towards the indoors at a faster pace than if it was 85 degrees and 80 degrees inside.
This is why it is so important to make sure you’ve got a proper seal around your doors and any penetration or “hole” to the outside. One quick and easy way to find out if you’ve got a proper door seal is to take a piece of paper and close it in a door, then try to pull out that piece of paper. You should feel some resistance, if you don’t then you don’t have a good seal.