Why Is My Room Hotter Than Outside?

Most people would assume that their entire house gets colder at night when there’s no direct sunlight. However, that’s not always the case.

It’s entirely possible that you’ll find one or more rooms hotter at night, even though the outside temperature is much lower. So, is your room in the house hotter than the outside?

When your room is hotter than the outside, it’s possible that the room has poor ventilation. Proper ventilation allows hot air to exit while cool and fresh air enters the room. South-facing rooms also heat up from more sunlight, while upstairs rooms will experience the Stack Effect as heat rises through the building. Lastly, some house-building materials also retain and give off more heat even at night.

As you continue reading this article, you’re going to learn about each of those reasons why your room at home is hotter than the outside. On top of that, you’ll even learn what you can do to reduce the effects of those reasons and keep your room cool well into the night!

Poor Ventilation

Whenever you find that the temperature in a room is higher than that of the outside, the first thing you should always troubleshoot is your ventilation system. There’s a very high chance that your ventilation system is working poorly, leading to insufficient or even blocked airflow.

You see, one of the most crucial aspects of comfort inside the home is continual airflow throughout the house and to the outside. There are two main reasons for this:

  • Hot, Stuffy Air Needs To Escape: Firstly, good ventilation will ensure that hot air has somewhere to go. For instance, hot air will rise into the ceiling or attic and flow out of the house into the surrounding environment.
  • Cold, Fresh Air Needs To Circulate: As the hot air leaves, cold and fresh air must also be able to flow throughout the entire house. That way, there isn’t a significant difference between indoor and outdoor temperatures.

In some cases, this might mean allowing air to flow into the home from the outside. However, the same is also true if cold air is being generated by your household’s air conditioning system. Excellent ventilation is crucial for the AC’s cold air to reach your bedroom as hot air is expelled out of the house.

Related: How Long Does an AC Take to Cool an Apartment?

However, poor ventilation will have the opposite effect as it hinders the exchange of air. Hot air will not be able to escape your room, which keeps the indoor temperature high. Plus, cooler, fresh air will not be able to enter the room and lower the temperature either.

What you can do about it: Suppose you’ve confirmed that the ventilation is poor in your room. If that’s the case, then here are a few things you need to do:

  • Clean your vents: Thoroughly clean your vents from any dust or dirt buildup. If the blockage is too far in the ducts to reach, call your local HVAC technician to do it for you.
  • Ensure that there is no furniture or any other objects blocking any vents in your room.
  • If possible, open your windows to allow the indoor and outdoor air to exchange, thereby balancing the temperatures.

Excessive Sunlight 

Some bedrooms in a home are predisposed to getting much hotter than others. Bedrooms with windows facing the South side of the house, in particular, will experience much more sunlight than any other side.

As a result, the sunlight will heat up the floor and everything else in the room much more than anywhere else in the house.

That is known as ‘solar gain’ or ‘passive heat gain’. Even though it happens in the daytime, its effects can last well into the night when it’s cold outside yet the room remains hot.

When the sun goes down and temperatures outside drop, the heat that was absorbed by your floors, walls, and furniture start to radiate which will keep the room warm for much longer at night.

What you can do about it: When dealing with solar heat gain, the most effective solution is to prevent it from happening at all. Thankfully, there are plenty of approaches you can take to improve your windows, walls, and doors:

  • Windows: Most of the heat gain will come through the windows thanks to direct sunlight. The easiest way to reduce or prevent this is by keeping your blinds or curtains closed during the day. When there’s less direct sunlight penetrating into the room, there will also be less heat.

Still, sunlight can pass through some curtain materials and still heat up the room. That’s why another effective solution is to use blackout curtains. These are multi-layered curtains that prevent sunlight and heat from penetrating through.

As a long-term solution, you could get your bedroom windows tinted to reflect some of that direct sunlight and prevent heat gain.

  • Walls: Heat is also absorbed and retained by your South-facing bedroom walls. The general rule of thumb for this type of problem is to paint the walls a lighter color, as that will help to reflect some of the solar heat.

However, you can also find specially-designed exterior wall paints that are extremely effective at preventing solar heat gain. These paints do cost more than standard exterior paints, but they’re well worth the investment.

  • Doors: Doors also experience solar heat gain, albeit to a lesser extent. Much like walls, you can paint any South-facing doors with light-coloured or heat-reflecting paints. 

Besides that, weatherproofing doors to keep them airtight will also help that bedroom retain as much cool air as possible.

Don’t forget about pet doors! Doggy or kitty doors also need to be sealed and covered, as transparent ones can let sunlight in while also letting cool air escape,

Room Position

The position of your bedroom within the home can also affect how hot it gets at night. That is particularly true for bedrooms that are located on the top floor of the home, which suffer from what’s known as the Stack Effect.

Remember: warm air inside your home is less dense than cold air. Therefore, warm air will always rise, typically going to your ceiling or attic before it’s vented out of the roof.

If your bedroom is located on the top floor, that means all of the heat will travel towards you on its way out of the house. When that happens, it will create negative pressure inside your home, which pulls cold air in from the lower level that helps the hot air rise further through your room.

What you can do about it:

To reduce the Stack Effect that’s keeping your room hotter than the outside, you’ll need to prevent incoming air from entering the home in the first place. That means insulating any areas on the lower floor where air might leak into the home. 

Then, when the incoming air is reduced on the lower floor, there will be less of a Stack Effect involving hot air going through your room on the way to the attic.

Essentially, you’ll be cutting the problem off at the source.

There is also one other solution, although it’s probably less practical for most people. You could repurpose the room and move somewhere else in the house. For instance, you could turn that bedroom into a home gym while using a cooler room as your bedroom.

House Structure Retains Too Much Heat

Depending on the materials used to build your home, it’s also possible that your house is retaining too much heat even at night when it’s cooler outside.

You see, certain building materials like brick and concrete have higher ‘thermal mass’. Simply put, that means these materials can store much more heat and take a very long time to cool down.

So, during the day, the direct sunlight will heat the structure of your home to a high temperature. Once the sun goes down, the structure will remain relatively hot compared to the cooler temperature in the surrounding environment.

Related: How Long Should It Take To Cool A House?

In simpler terms, the structure of your home suddenly becomes a source of heat at night, rather than being on the receiving end of the heat. It can take several hours for that brick or concrete material to cool down completely.

During that time, the heat can make your room feel hotter than it is outside.

What you can do about it:

This is yet another situation where prevention is the most effective solution. When it comes to thermal mass, as explained above, the way to prevent it is with high-quality insulation.

With professionally installed insulation, your home will be able to keep itself free from hot air during the day when the sun is shining at its brightest. At the same time, the insulation will also ensure that the cool air inside your room will stay there for much longer as well.

As a result, the structure around your room (and the rest of the house) will not absorb too much heat during the day. Plus, what little it does absorb will be quickly released back into the environment at night.