Why Is It So Hot Upstairs But Cold Downstairs?

Is your two-story Colorado house hot upstairs but cold downstairs? This is also hard on your air conditioning system as you constantly adjust the thermostat to maintain a cool temperature upstairs. Whether your home is a 1980s colonial in Arvada, a split-level in Lakewood, or a two-story in Aurora or Englewood, this hot-and-cold temperature imbalance is a common complaint. Explore the multiple reasons why your upstairs is so hot and what can be done to fix it. 

Key Takeaways

• Heat naturally rises, leaving the second floor in your Colorado home warmer than the rest of the house.
• Attics in older Denver-area homes trap massive amounts of solar heat during afternoon temperature spikes.
• Poor duct design, leaky ductwork, and missing return air vents are leading causes of temperature imbalance.
• Solutions range from simple thermostat adjustments to ductless mini splits and zoned HVAC upgrades.

Why Is Your Upstairs So Hot?

The temperature gap between your bottom and top floors is a combination of overlapping factors, and many of them are more noticeable in older Colorado homes. 

Heat rises

Hot air rises because warm air is less dense than cool air, so it tends to migrate upward. By the time your air conditioner cools the lower level, that heat has already made its way toward the ceiling and upper floors. This is why the upstairs always gets so hot. Adjusting the thermostat constantly may temporarily resolve the issue, but it makes your AC work harder and never truly fixes it. 

Attic heat gain

Most of Colorado’s regions, especially Denver, are at an elevation of 5,000 feet or higher. High altitude means a thinner atmosphere, making UV radiation hit harder. Once heat gets trapped in your attic, it eventually radiates down into your ceiling’s insulation and onto your second floor. Older homes from the 1970s and ’80s in communities like Englewood and Aurora frequently have inadequate attic insulation by today’s standards, making this problem significantly worse.

Thermostat location

Most thermostats are mounted on the main floor. Once it reaches the desired temperature on the main floor, it stops cooling, leaving your top floor many degrees higher. For a thermostat to function properly, it should never be placed in direct sunlight. 

Duct design & airflow imbalance

Duct systems need to be properly balanced for a two-story home, and for many older Colorado homes, duct design wasn’t done properly. Supply ducts on the second floor are usually smaller in diameter and not properly sized to ensure proper airflow. Ultimately, the second-floor supply ducts produce less cool air.

Lack of return air vents upstairs

Return air vents pull hot air from your home back into your air conditioner so that it can be cooled.On top of an improperly designed duct system, older Colorado homes usually have fewer or sometimes no return air vents upstairs. Without adequate returns upstairs, hot air gets trapped on the second floor with nowhere to go.

Leaky or poorly insulated ductwork

Ductwork can become damaged and full of microscopic holes over time. If it’s poorly insulated, it loses a significant amount of conditioned air before even reaching the return vent. Instead, it bleeds into your hot attic, rather than the second floor. Typical duct systems can lose 20 to 30% of airflow through leaks alone.

Oversized or aging AC system

Any AC installation must ensure proper sizing. An oversized system will cool the space too quickly, telling the thermostat the desired temperature has been reached before the air even makes it to the upper floor. Similarly, an aging system that’s lost efficiency may simply not have the capacity to push enough cooled air to the upper level. 

Sun exposure on second-floor windows

Second-floor windows that face the sunlight take the full brunt of the heat. Single-pane or older double-pane windows, common in pre-1990s Colorado homes, offer minimal heat resistance and essentially bake your upper-floor rooms and hallways. 

How To Fix Hot Upstairs Cold Downstairs

How do you fix the common hot upstairs, cold downstairs issue? The most effective long-term solutions involve upgrading the AC system and how it distributes air throughout your home. 

Zoned HVAC systems

A zoned HVAC system divides your home into separate temperature-controlled areas, each guided by its own thermostat. For a two-story Front Range home, this is the most impactful upgrade available, especially with an ENERGY STAR-rated system that provides high efficiency and lower energy bills. 

Smart thermostat

Pairing your air conditioner with a smart thermostat is a great way to learn when you’re losing conditioned air and precool your home before those big temperature peaks. These thermostats can be controlled on your phone via WiFi. Smart thermostats also learn your cooling patterns and show your energy efficiency breakdowns.

Ductless mini-split for the second floor

If your home’s ductwork is the problem, installing a ductless mini-split upstairs can be a game-changer. Like central systems, ductless mini splits have two main components: an outdoor compressor/condenser and an indoor air-handling unit. The difference is that mini-splits deliver conditioned air directly to the space without relying on ducts. They’re highly efficient, allow precise room-by-room temperature control, and are less invasive to install than a full ductwork installation.

There are also some quick fixes a homeowner can do:

• Adjust vents: Partially close supply vents on the lower floor to push more airflow toward the upper level.
• Run the thermostat fan: Set your thermostat fan to “ON” instead of “AUTO” to keep air circulating even when the AC isn’t actively cooling.
• Add ceiling fans: Fans help break up hot air layers and create a cooling effect in occupied rooms.
• Upgrade attic insulation: Adding blown-in insulation is one of the highest-ROI improvements you can make in a pre-1990s Denver home.
• Seal duct leaks: Have a professional perform duct leakage tests and seal the problem areas.
• Add a return air duct: If certain rooms always feel stuffy or warmer than the rest of the house, a missing or undersized return air duct is often the culprit. Adding one helps the system pull air back efficiently, balancing pressure and improving airflow throughout the home.

When To Call a Professional

Some quick fixes are easy to DIY, but the more involved upgrades require a professional. If you’re experiencing any of these issues, rely on the certified technicians at Sanders & Johnson:

• Your upstairs is always 5°F higher than your downstairs, even after basic adjustments. 
• Your AC is 15 to 20 years old and needs replacing.
• You’re considering replacing your HVAC system with a zoned HVAC system, a ductless mini split, or a smart thermostat, and want to know which option is best.
• Your system needs an AC maintenance and tune-up.

Learn why it is necessary to service your AC every year in Denver, CO.

Fix Your Hot and Cold Imbalance With Sanders & Johnson

Don’t spend another Colorado summer sweating when you’re upstairs. Let the experts at Sanders & Johnson educate you on your home comfort options. With our 49 years of experience serving Colorado homes, we offer a range of air conditioning and heating services, including professional, guaranteed installation. Contact us today to schedule a service.