Why Your Top-Floor Units Overheat Every Summer (and How to Fix It)
Every summer it is the same set of phone calls. The tenants on the top floor say their units are unbearable by mid-afternoon, the AC runs nonstop, and the bills climb anyway. Meanwhile the lower units feel fine. If you own or manage a multifamily building in Chicagoland, you already know which apartments are going to complain first when a heat advisory hits.
The frustrating part is that the usual fixes (a bigger AC unit, new window units, telling people to keep the blinds closed) rarely make the problem go away. That is because the real cause is not the cooling equipment. It is the building itself.
It is the stack effect, not the air conditioner
Warm air rises. In a multi-story building, that simple fact turns into a real problem. Hot air collects at the top of the building and pushes its way out through gaps in the attic, the roofline, and around can lights, plumbing chases, and bathroom fans. As that air escapes up high, it pulls hot outdoor air in down low to replace it.
Engineers call this the stack effect, and it runs all day during a Chicago summer. The top floor takes the worst of it: heat from the floors below rises into those units, the sun bakes the roof directly above them, and a poorly sealed attic lets even more hot air pour in. No thermostat setting fixes a building that is breathing hot air in and out all day.
Why a bigger AC will not solve it
When the top floor cooks, the instinct is to throw more cooling at it. The catch is that an oversized or harder-working AC is just fighting the leak instead of closing it. The cool air you pay for keeps slipping out through the same gaps, so the equipment runs longer, wears out faster, and your energy bill goes up without the comfort complaints going down.
You are paying to cool air that the building keeps pushing outside. Sealing and insulating the top of the building does the opposite: it keeps the cooled air where your tenants are and lets a normal-sized system actually keep up.
The fix is air sealing first, then insulation
The work that actually solves top-floor overheating happens in two steps, in this order.
First, air sealing. Before any insulation goes in, the leaks at the top of the building get sealed: the gaps around light fixtures, plumbing and electrical penetrations, the attic hatch, and the joints where the ceiling meets the walls. This is the step most contractors skip, and it is the one that actually slows the stack effect. Adding insulation over an unsealed attic just lets the air keep moving through it.
Second, insulation. With the building sealed, we add a deep, even layer across the attic floor (we typically blow in about 14 inches of cellulose) so the heat radiating off the roof stops pouring down into the top-floor units.
To prove it worked, we run a blower-door test before and after, so you can see the change in the building's air leakage instead of taking our word for it. Tighter building, cooler top floor, less runtime on the cooling system.
For most Chicagoland multifamily buildings, this is often covered
Here is the part most owners do not realize. Illinois utility rebate programs through ComEd, Nicor Gas, Peoples Gas, and North Shore Gas can help fund insulation, air sealing, and weatherization for qualifying multifamily buildings, generally those with three or more units. For some income-eligible properties, the out-of-pocket cost can be very low.
Programs vary by building, by utility territory, and by the funding available each year, and the money is first-come, first-served. That makes it worth checking your building's eligibility before the season's funding runs out. We handle the rebate paperwork as part of the project, so you are not the one chasing forms. (For a closer look at how owners use these programs, see How Multi-Family Property Owners Can Use Energy Efficiency Rebates.)
Start with a free building assessment
The only way to know what your building actually needs, and what it qualifies for, is to look at it. Building Energy Experts has spent more than 15 years sealing and insulating multifamily buildings across Chicagoland. We are BPI certified, and we were named ComEd's Service Provider of the Year in both 2025 and 2026. We have done this work for more than 500 building owners.
If your top-floor tenants are already dreading the next heat wave, now is the time to get ahead of it.
Or call 888-623-3769 and we will tell you exactly what is making those units overheat and what it will take to fix it.
