When it is hot outside, you expect the inside of your house to be cooler.
But it is puzzling – how can the indoors be hotter than the outdoor when you have a roof over your head?
Why is it hotter in the house, even after the sun goes down?
Worse, the house is still hot with the AC on, and you have trouble falling and staying asleep.
How do you cool the house in this situation?
How Hot Is Too Hot Inside a House?
Maintaining a comfortable indoor air temperature is crucial for sleep, relaxation, and daily tasks.
The optimal indoor temperature varies with the seasons and humidity levels.
Is 80-degrees too hot for a house?
For the summer months, an indoor air temperature between 75 to 80 degrees Fahrenheit is ideal:
- 75 degrees when the humidity is 50% or higher.
- 80 degrees when the humidity is lower.
When the temperature inches towards the 90-degree Fahrenheit mark, you need to understand why the house gets so hot and implement concrete solutions for the many summers ahead.
Why Is Your House Always Hotter than the Outside?
The condition of each house differs.
Some homes have inadequate design or a deteriorated structure; others may suffer heat issues due to lifestyle choices.
1. Dark-Colored and Cluttered Interior
The root cause of a warm house is the net energy gain over the amount it releases when the sun shines and warms the air.
Your experience of a higher indoor temperature than the outdoor is the symptom of the building releasing the heat it absorbs – plus trapped heat.
This occurrence is why you experience high temperatures in your home, and Reasons #2 to #7 (below) are causes that lead to the net energy gain.
The floor, wall, ceiling, and furniture absorb the warm air’s energy when the air temperature inside the house increases.
Dark-colored objects, especially black, absorb more light energy (and thus heat) than lighter-colored ones.
As air warms and cools quicker than solid matters, this heat absorption continues throughout the day and builds in the furniture and building parts.
When the outdoor temperature falls in the evening, the indoor air temperature decreases.
However, the drop in indoor air temperature creates a state where the energy retained in the furniture and building is comparatively higher, prompting energy release in the opposite direction.
This effect is typically more pronounced in cluttered homes.
2. Insufficient or Absence of Roof Insulation
The roof is the single largest surface of a house’s external envelope.
When the sun beats down, the roof absorbs the most heat as it faces the rays directly for longer.
Heat gain through the roof peaks between 11 am to 3 pm, and the air temperature in the roof space or attic rises rapidly.
A roof without appropriate insulation lets the heat transfer downwards to the floors below it.
You will feel the air warming, and the inside can get hotter than the outside when poor ventilation traps the heat indoors.
The lack of roofing insulation or a damaged one is the primary reason a house becomes warm when the outdoor temperature increases.
3. Undesirable House Facing
A south-facing house in the northern hemisphere (US, UK, and Canada) enjoys the winter sun, but highly-glazed homes experience massive heat gain in summer.
The cardinal direction your house faces significantly impacts its overall heat gain.
If the majority of your house’s windows and glass doors suffer from exposure to direct sunlight, the affected rooms and the furniture in them absorb the sun’s energy and can warm considerably.
A lack of solar screens – natural or architectural – exacerbates the problem.
Determine your house’s facing and ascertain if it has undesirably large window openings that require shielding.
4. Poor Thermal Insulation in the Walls
Some homes stay cool when the weather gets hot; others can cool when the air-conditioning system goes to work.
These houses have excellent thermal insulation and will equally stay warm when the heater switches on during winter.
Heat penetrates the walls readily when no insulation material stops or slows down the energy transfer.
Different wall materials carry varying R-values (a material’s thermal resistance).
Walls with a higher R-value may perform better at keeping the house cool. Still, the lack of insulation undermines the house’s overall performance keeping the heat out.
If an uninsulated wall faces the scorching sun, the energy it absorbs is massive, and the room stands no chance of maintaining a cool interior.
5. Heat Gain Through the Windows
Warm air from outside the house can enter through the glazing in your home when there is:
- Air leaks – the weatherstrips around the frame can deteriorate and fall apart as they age. Repeated use over many years and the window’s weight can mean misaligned frames that cannot close tightly.
- Single pane – this window design does not have the air gap a double-pane window possesses to insulate against incoming heat.
Older houses typically suffer from these conditions and perform inadequately in keeping the heat in or out.
Windows with poor weatherstripping are not a significant issue until they receive direct sun rays and as the outdoor air becomes hotter under sun exposure.
6. Inadequate Cross-Ventilation
Hot air enters the house and warms the interior.
But heat trapped inside your home causes it to become hotter than outside.
So, which is it?
Should you close or open the doors and windows when the day is at its hottest?
Sun-facing windows need shielding with heavy drapery to block direct heat gain. Still, as the outdoor air temperature gradually warms and the indoor air follows, you want the heat to escape and not continue to warm the interior.
Natural cross-ventilation utilizes the difference in atmospheric pressures to generate airflow from one end of the house to the other.
Houses with an elongated floor plan benefit the most from this effect.
7. Heat-Generating Chores Indoors
House chores mostly take place during the day – when temperatures are highest.
The sun is the most significant heat radiator and energy contributor to a warm house, but heat-generating appliances compound the problem.
Household appliances that release heat include:
- Washing machine – cold and warm washes release heat; the latter more.
- Clothes dryer – most dryers operate between 125 (low) to 135 (high) degrees Fahrenheit. Heat can escape through leaky vents.
- Oven – requires airflow for operation, so it is normal to feel heat leaking around the oven door.
- Dishwasher – runs between 130 to 150 degrees Fahrenheit. Small air fans pump hot air out of the dishwasher through exhaust vents.
Why Is the House Still Hot with the AC On?
Air cooling systems are handy to keep the house cool, but air leaks through the doors and windows, and poor insulation in the walls, ceilings, and roof compromise their performance.
Your house remains hot even with working air-conditioning because:
- The cool air escapes through gaps and openings in the building envelope – cold air is denser and therefore has a higher pressure, so air flows from cold (inside) to hot (outside).
- Inadequate wall and ceiling insulation – cold and warm airs are energy, and energy transmits both ways through poorly-insulated building structures.
- High indoor humidity – air with higher humidity traps more heat, causing the AC to work extra hard.
Why Does the House Stay So Hot at Night When It’s Cooler Outside?
You try to tolerate the heat when the day is warm, but when night falls, you look forward to a cooler home so you can enjoy the remaining hours before going to bed.
Frustratingly, that is not the case.
When the house has poor insulation, and warm air enters the room throughout the day, the walls, ceilings, and furniture absorb the heat.
The structures and furniture release stored heat when the ambient temperature falls, causing the house to stay hot at night.
The effect of a hot indoor space when it is cooler outside is common in houses with inadequate insulation, poor ventilation, and clutter.
Tips on Cooling the House
A warm house that gets hotter when it is cool outside can benefit from various means of shielding it from the sun’s heat – some are inexpensive; others cost more but pay off long-term.
Take advantage of inexpensive manners to cool your house:
- Plant high on sun-facing sides – rooms with direct sun exposure get warm quickly. Grow plants tall enough to shield the room from the sun at an angle. Alternatively, grow vines to cover the external walls for a low-maintenance year-round cover.
- Utilize cross-ventilation – choose and open at least two windows, one at each end of the house where they do not get direct sun rays, to prevent heat buildup inside. Alternatively, install intake and exhaust fans at similar locations to induce airflow.
- Lighten the interior – choose lighter-colored furniture and paint colors to reduce heat gain when it is hot and heat release when the day cools.
- Shut the door to sun-facing rooms – when you cannot insulate the whole house, you want to compartmentalize it to reduce heat transmission between rooms. This approach is less desirable, but it is inexpensive, and it works.
- Ventilate the attic – if your house has an attic, ventilating the space ensures the heat can escape as it travels upwards.
- Extricate hot air for a cooler bedroom – contrary to a house that can cross-ventilate, a bedroom typically has windows only on one side. Method #12 in this post explains the details.
- Reschedule heat-generating chores – if you can, carry them out when it is cooler in the evening.
- Opt for LED lights – incandescent lights generate more heat. Switch on fewer lights when possible.
Installations and mechanical systems can be more effective, depending on your house’s room layout:
- Install an attic fan – draws out hot air where it matters most.
- Install thermal blinds – sun-facing windows can benefit from this protection. Open the blinds and windows when sunlight no longer hits them directly.
- Apply window insulation films – an inexpensive solution to control heat, light, and UV transfer.
- Install solar screens – architectural solutions that effectively block the sun and heat radiation without obstructing the view.
- Install awnings – if the windows or patio door face the sun but you still need them opened, a retractable awning lets you adjust how much shade you get.
- Insulate the house – can be expensive depending on the size of your home. But you can apply insulation selectively where heat gain is the most severe.