Why Your House Is Heating Up, Even When It’s Cool Outside

Don’t you love it when the temperatures drop at night, and you can open the windows?  

Fresh, instead of recycled air, and cool breezes wafting through the windows. Except . . .  why is your house heating up, even though it’s cool outside?

What’s up with that?

Your home builds up radiant heat throughout the day, thanks to the sun. And stores it. In the attic, the walls, even your furniture. Your appliances are also a source of heat. Despite being cooler outside, if the cool air has no way to circulate through the house, your indoor temps stay warm.

Keep reading. I’ll cover some more reasons for a house that’s heating up and share some tips to help you cool it down.

Why is Your House Heating Up?

There are several factors at play when it comes to indoor heat.

You and Your Appliances Generate Heat

First, there’s a lot of heat inside. Natural heat—you, your family, and your pets—and heat generated by appliances and electronics. Now couple that with the fact that we insulate our homes to keep the heat in and the cold out.

electric oven
Electric ovens releases 3,000 watts energy.

Those two facts alone will give rise to your indoor temps. Even when you open the windows on a cool day or night.

Here are some examples of the typical heat sources in a home and the approximate watts of energy they release.

What’s Generating Heat in Your Home

ObjectAmount of Energy(heat) in Watts
A resting human body100 watts
Someone exercising or exerting themselves200 to 300 watts
A television100 watts
A computer100 watts
An LED bulb20 watts
Refrigerator70 watts
Clothes dryerBetween 1,800 and 5,000 watts
Washing machine100 watts
Electric oven3,000 watts

That could be more than 8,000 watts of energy being released in your home. And this is assuming you only have a single TV or computer running.

Released energy = heat.

And your insulation is doing its job and stopping heat transfer.

Heat Buildup

The sun rises in the morning and hits your roof. It comes through your windows and hits your walls, your floors, your furniture. When objects are in the path of radiant heat waves, the energy from them is absorbed and they heat up.

window with plants
Objects that is getting sunlight will have heat buildup.

While that is going on, all the things mentioned above are going on too. So as the day progresses objects in your home absorb more heat. And as long as the outside temps remain hot, they hold on to that heat.

Your sofa, your mattress. They have become a heat source.

And remember, your insulation is doing its job and stopping heat transfer. But there’s another layer to this too. Literally.

Thermal Mass

The exterior construction of your home plays a part as well. Because of something called thermal mass. Portland Cement puts it better than I ever could. And remember, brick is made from concrete, which is made from cement.

“Thermal mass is a property that enables building materials to absorb, store, and later release significant amounts of heat… These materials absorb energy slowly and hold it for much longer periods of time…”

stacked bricks for construction.
Bricks has more thermal mass than timber and aluminum.

So a brick home has more thermal mass than a timber or aluminum siding home. Which is why brick homes are the norm in northern climates. But of course, depending on how far north you are, you’re getting hot summers too. So for several months out of the year, that brick is working against you.

As it begins to cool off outside, the heat that’s been absorbed all day begins to release. Keeping it warm inside.

Thermal Inertia

Yes, more science. Sorry.

Thermal inertia is tied to thermal mass in that the density of an item dictates its inertia. Huh? Let me explain.

To put it as simply as possible, thermal inertia refers to how long it takes an object to gain or lose heat in comparison with its environment. It depends on its density(mass) and conductivity (heat’s ability to travel through it).

An object with high thermal inertia will take a long time to reach the heat of its surroundings, while an object with low thermal inertia will do it quickly.

So some of the contents of your home may take longer to absorb heat than others, and some might take longer than others to release heat.

Here’s an example. If you took a piece of cotton cloth and a steel ruler of the same size and put them in the freezer, one will get colder faster than the other. Now take them out and test how long it takes for each to warm up.

Set them both on the counter beside each other at room temperature. The piece of cotton will warm up much faster than the ruler since the ruler has a lot more thermal mass and a lot more thermal inertia.

So all the upholstery fabric in your home will gain and lose heat quickly. But not necessarily all the foam padding below the fabric. All the ultra-modern metals in your home will gain and release heat slowly. So when you open your windows to let in cooler air, the amount of time it will take your warm interior air to reach equilibrium with the cool air will depend on the contents of your home.

Is all that science making your headache? Yeah, me too.

So now that you understand—maybe—why your house is staying warm or maybe even feels like it’s heating up when it’s cooler outside, let’s talk about what you can do.

What You Can Do to Cool Your House Down

Short of sitting still all day and unplugging all electronics and appliances, what can you do to cool down your house? There are both long and short-term alternatives. A blend of both should do the trick, but the long term can be very long term, so be aware of that.

Long Term Solutions

The sun beating down on your home is part of the problem. It hits the roof, and depending on where you live, you’ll likely have a lot of insulation in the attic. Which is good.

The problem is many of us live in temperate areas. Meaning part of the year is hot and part of the year is cold. But houses are built for one climate or another. And until we can have Transformer type homes that just morph into the most appropriate type of dwelling as the seasons change, we need to compromise.

So try to protect your home from the heat of the sum. If your property allows for it, plant high, fast-growing trees to give your house some shade.

Here’s a list of trees that will grow to anywhere from 15 to 40 feet in 3 years.

  • Thuja Green Giant Cedars
  • Leyland Cyprus
  • Red Maple
  • Weeping Willow
  • Poplar
  • Royal Empress

The next remedy that you might have to wait for is a new roof. When shopping for shingles, choose a light color. That will reduce heat gain in your home.

Quick Solutions

You want to find some equilibrium between the indoor and outdoor air. You need to mix the air up.

As you’ve already discovered, simply opening your windows doesn’t mean the air outside will magically enter them. It has to be pulled or pushed in. If it’s a breezy day and the wind is blowing in just the right direction, it will come in your window.

And then sit there.

If there is no wind blowing in the right direction, you need to pull cool air in by using a window fan and directing the air in. But you also need to draw the hot air out, not just mix it with the new cooler air.

To draw air out, there needs to be a window open at the other end of the house. In that window, put another fan but this time you want it blowing air out. You want to exhaust the hot air it’s pulling through your home. This is called cross-ventilation.


We’ve discussed why your home is heating up. It’s due to both exterior and interior forces. And in many cases, there isn’t a lot you can do about either.

The sun will shine down, and your body will give off heat. So will many other things in your home. All of this leads to an escalating heat gain throughout the day.

Your solutions are to provide some shade to block the sun outside or create cross-ventilation inside to pull in cool air and push out hot air.

Hopefully, this helps, and you’ll be able to sleep in a cooler house tonight.

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