In the summer, outside temperatures can easily reach 90 degrees or more. In these higher temperatures, it makes sense to adjust the AC thermostat set temperature. We are going to look into how cool should your house be if it’s 90ยฐF outside (striking a balance between 72ยฐF, 75ยฐF, and 78ยฐF thermostat setting).
72ยฐF indoor temperature is most often cited as the most appropriate as far as our comfortability levels are concerned. However, when outdoor temperatures are higher (90ยฐF or more), it makes sense to adjust the thermostat settings accordingly.
What temperature to set the thermostat when it’s 90ยฐF outdoors? Why not just set it to 72ยฐF?
As we will explain further on, the humidity levels play a key role when deciding how cool should your house be if it’s 90ยฐF outside. Based on that, we will outline what AC thermostat setting makes sense in 90-degree heat. Example: We experience 75ยฐF AC thermostat setting as:
- 72ยฐF at 20% indoor humidity levels.
- 77ยฐF at 70% indoor humidity levels.
We all know that setting the AC temperature to a lower temperature will increase the cooling costs. According to the US Department Of Energy, “The smaller the difference between the indoor and outdoor temperatures, the lower your overall cooling bill will be.”
If we are experiencing 90ยฐF outdoor temperatures, this gap between the outdoor temperature and indoor AC thermostat set temperature is going to be quite big (increasing the cooling bill). The DOE also states that “You can save as much as 10% a year on heating and cooling by simply turning your thermostat back 7ยฐ-10ยฐF for 8 hours a day from its normal setting.” That’s about 1% savings per 1 degree in 8 hours and 3% per 1 degree in 24 hours.
Let’s look at this temperature gap when outdoor temperatures reach 90ยฐF:
- 90ยฐF outdoor temperature and 72ยฐF indoor temperature give us an 18ยฐF temperature gap. Example of the cooling bill: $200/month.
- 90ยฐF outdoor temperature and 75ยฐF indoor temperature give us a 15ยฐF temperature gap. Example of the cooling bill: $181/month (9% savings compared to 72ยฐF).
- 90ยฐF outdoor temperature and 78ยฐF indoor temperature give us a 12ยฐF temperature gap. Example of the cooling bill: $162/month. (18% savings compared to 72ยฐF).
As you can see, you save about 18% on cooling costs if you set the AC setting to 78ยฐF instead of 72ยฐF. That can save you up to $100 per cooling month.
Let’s see when it makes sense to increase the thermostat higher than 72ยฐF if the outdoor temperatures reach 90ยฐF:
What To Set AC Thermostat When In 90 Degree Heat?
Namely, when we are setting the AC thermostat temperature in 90-degree weather, we are trying to find a balance between:
- Cooling costs + overworking AC. At lower AC temperatures (72ยฐF), our cooling costs are going to be higher. On top of that, our air conditioning system will have to work extra hard (at 90-degree outdoor temperature, we need the AC to lower the indoor temperature considerably compared to the outdoor temp). The increased AC output can shorten the lifespan of our air conditioning systems.
- Comfortability levels. Most of us feel comfortable at 72ยฐF. If the outdoor temperatures reach 90ยฐF, however, you have to ask yourself how comfortable you are at 75ยฐF or even 78ยฐF thermostat setting.
Finding the right balance between cooling costs and comfortability levels in 90-degree heat depends, quite interestingly, on:
- Our personal preferences. Some people can handle higher indoor temperatures at 90-degree outdoor temperatures better than others.
- Humidity levels. This is the most important factor. Humans experience hot and humid temperatures as higher than set on the AC thermostat and we experience hot and dry temperatures as lower than set on the AC.
When deciding what to set the AC thermostat at 90-degree outdoor temperatures, it is essential that we take the indoor humidity levels into account. Here is a chart that illustrates how we perceive indoor temperature levels at 90-degree outdoor temperatures:
Let’s say it’s 90 degrees outside and you are thinking about setting the AC thermostat to 75ยฐF. Here is how we will perceive this AC setting at different indoor humidity levels:
- At 90ยฐF outside temperature and 10% indoor humidity levels, we will perceive the 75ยฐF AC thermostat setting as 70ยฐF.
- At 90ยฐF outside temperature and 20% indoor humidity levels, we will perceive the 75ยฐF AC thermostat setting as 72ยฐF.
- At 90ยฐF outside temperature and 30% indoor humidity levels, we will perceive the 75ยฐF AC thermostat setting as 73ยฐF.
- At 90ยฐF outside temperature and 40% indoor humidity levels, we will perceive the 75ยฐF AC thermostat setting as 74ยฐF.
- At 90ยฐF outside temperature and 50% indoor humidity levels, we will perceive the 75ยฐF AC thermostat setting as 75ยฐF.
- At 90ยฐF outside temperature and 60% indoor humidity levels, we will perceive the 75ยฐF AC thermostat setting as 76ยฐF.
- At 90ยฐF outside temperature and 70% indoor humidity levels, we will perceive the 75ยฐF AC thermostat setting as 77ยฐF.
- At 90ยฐF outside temperature and 80% indoor humidity levels, we will perceive the 75ยฐF AC thermostat setting as 78ยฐF.
- At 90ยฐF outside temperature and 90% indoor humidity levels, we will perceive the 75ยฐF AC thermostat setting as 79ยฐF.
- At 90ยฐF outside temperature and 100% indoor humidity levels, we will perceive the 75ยฐF AC thermostat setting as 80ยฐF.
In general, we should strive to achieve perceived indoor temperatures between 72ยฐF and 76ยฐF even at 90ยฐF outdoor temperature.
This means that we can set the thermostat temperature to 75ยฐF in 90-degree heat if the indoor humidity levels are 20%, 30%, 40%, 50%, or 60%. If we have 70% humidity levels or higher in 90-degree heat, most of us will not feel comfortable setting the AC thermostat to 75ยฐF. We will have to lower it to 74ยฐF, 73ยฐF, or even 72ยฐF.
Here is the general outline of how cool your house should be if it’s 90 degrees outside (based on humidity levels):
How Cool Should A House Be In 90ยฐF Heat Depending On Humidity Levels
Here we strive for 72ยฐF to 76ยฐF perceived indoor temperature at ninety degrees Fahrenheit outdoor temperature. Depending on the humidity levels, here is how cool should your house be:
- 72ยฐF thermostat setting if humidity levels are very high (between 80% and 100% humidity).
- 73ยฐF thermostat setting if humidity levels are high (between 60% and 80% humidity).
- 74ยฐF thermostat setting if humidity levels are higher-than-average (between 40% and 80% humidity).
- 75ยฐF thermostat setting if humidity levels are average (between 20% and 60% humidity).
- 76ยฐF thermostat setting if humidity levels are below-average (between 15% and 50% humidity).
- 77ยฐF thermostat setting if humidity levels are low (between 10% and 40% humidity).
- 78ยฐF thermostat setting if humidity levels are very low – dry air (between 0% and 20% humidity).
You can use this general outline to adequately adjust the AC thermostat setting at 90-degree outdoor temperature. Example: If the outdoor temperature is 90ยฐF and indoor humidity levels are 40%, you can set the thermostat to 74ยฐF, 75ยฐF, 76ยฐF, or 77ยฐF.
Basically, in 90ยฐF heat, avoid setting the AC thermostat temperature too low if you have low humidity (Example: Setting the thermostat to 72ยฐF in 20% humidity levels will be too cold) and setting the AC thermostat temperature too high if you have high humidity (Example: Setting the thermostat to 78ยฐF in 80% humidity will be too hot).
You can check similar calculations and recommendations of how cool your house should be if it’s 100ยฐF outside here (heatwaves).
We hope that this illustrates well how cool your house should be if it’s 90ยฐF outside. Following these general directions will enable you to save on cooling costs as well as have a comfortable temperature in your house, despite the 90-degree heat.