How Cool Should My House Be If It’s 90F Outside (Or More)?

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 90°F outside (striking a balance between 72°F, 75°F, and 78°F thermostat setting).

72°F indoor temperature is most often cited as the most appropriate as far as our comfortability levels are concerned. However, when outdoor temperatures are higher (90°F or more), it makes sense to adjust the thermostat settings accordingly.

What temperature to set the thermostat when it’s 90°F outdoors? Why not just set it to 72°F?

As we will explain further on, the humidity levels play a key role when deciding how cool should your house be if it’s 90°F  outside. Based on that, we will outline what AC thermostat setting makes sense in 90-degree heat. Example: We experience 75°F AC thermostat setting as:

  • 72°F at 20% indoor humidity levels.
  • 77°F  at 70% indoor humidity levels.
what temperature to set the ac thermostat if its 90 degrees outside
At very low humidity levels (between 0% and 20%), you can set the AC thermostat to 78°F. It will feel like 74°F, despite the 90°F outdoor temperatures. At high humidity levels (80% to 100%), 78°F will feel like 85°F; at high humidity, AC thermostat settings between 72°F and 75°F make much more sense.

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 90°F 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 7°-10°F 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 90°F:

  • 90°F outdoor temperature and 72°F indoor temperature give us an 18°F temperature gap. Example of the cooling bill: $200/month.
  • 90°F outdoor temperature and 75°F indoor temperature give us a 15°F temperature gap. Example of the cooling bill: $181/month (9% savings compared to 72°F).
  • 90°F outdoor temperature and 78°F indoor temperature give us a 12°F temperature gap. Example of the cooling bill: $162/month. (18% savings compared to 72°F).

As you can see, you save about 18% on cooling costs if you set the AC setting to 78°F instead of 72°F. That can save you up to $100 per cooling month.

Let’s see when it makes sense to increase the thermostat higher than 72°F if the outdoor temperatures reach 90°F:

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 (72°F), 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 72°F. If the outdoor temperatures reach 90°F, however, you have to ask yourself how comfortable you are at 75°F or even 78°F 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:

how cool should my house be if its 90 degrees outside depends on humidity levels

Let’s say it’s 90 degrees outside and you are thinking about setting the AC thermostat to 75°F. Here is how we will perceive this AC setting at different indoor humidity levels:

  • At 90°F outside temperature and 10% indoor humidity levels, we will perceive the 75°F AC thermostat setting as 70°F.
  • At 90°F outside temperature and 20% indoor humidity levels, we will perceive the 75°F AC thermostat setting as 72°F.
  • At 90°F outside temperature and 30% indoor humidity levels, we will perceive the 75°F AC thermostat setting as 73°F.
  • At 90°F outside temperature and 40% indoor humidity levels, we will perceive the 75°F AC thermostat setting as 74°F.
  • At 90°F outside temperature and 50% indoor humidity levels, we will perceive the 75°F AC thermostat setting as 75°F.
  • At 90°F outside temperature and 60% indoor humidity levels, we will perceive the 75°F AC thermostat setting as 76°F.
  • At 90°F outside temperature and 70% indoor humidity levels, we will perceive the 75°F AC thermostat setting as 77°F.
  • At 90°F outside temperature and 80% indoor humidity levels, we will perceive the 75°F AC thermostat setting as 78°F.
  • At 90°F outside temperature and 90% indoor humidity levels, we will perceive the 75°F AC thermostat setting as 79°F.
  • At 90°F outside temperature and 100% indoor humidity levels, we will perceive the 75°F AC thermostat setting as 80°F.

In general, we should strive to achieve perceived indoor temperatures between 72°F and 76°F even at 90°F outdoor temperature.

This means that we can set the thermostat temperature to 75°F 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 75°F. We will have to lower it to 74°F, 73°F, or even 72°F.

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 90°F Heat Depending On Humidity Levels

Here we strive for 72°F  to 76°F perceived indoor temperature at ninety degrees Fahrenheit outdoor temperature. Depending on the humidity levels, here is how cool should your house be:

  • 72°F thermostat setting if humidity levels are very high (between 80% and 100% humidity).
  • 73°F thermostat setting if humidity levels are high (between 60% and 80% humidity).
  • 74°F thermostat setting if humidity levels are higher-than-average (between 40% and 80% humidity).
  • 75°F thermostat setting if humidity levels are average (between 20% and 60% humidity).
  • 76°F thermostat setting if humidity levels are below-average (between 15% and 50% humidity).
  • 77°F thermostat setting if humidity levels are low (between 10% and 40% humidity).
  • 78°F 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 90°F and indoor humidity levels are 40%, you can set the thermostat to 74°F, 75°F, 76°F, or 77°F.

Basically, in 90°F heat, avoid setting the AC thermostat temperature too low if you have low humidity (Example: Setting the thermostat to 72°F 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 78°F in 80% humidity will be too hot).

You can check similar calculations and recommendations of how cool your house should be if it’s 100°F outside here (heatwaves).

We hope that this illustrates well how cool your house should be if it’s 90°F 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.

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