Why Two-Flats, Three-Flats, Four-Flats — and Even Multi-Story Homes, Condos, and Townhomes — Freeze in Winter and Bake in Summer
Starting with What You’re Really Looking For: Solutions
If you live in a Chicago two-flat, three-flat, four-flat, townhome, condo, or multi-story single-family home and your first-floor or garden unit/ basement is freezing every winter, you’re not imagining it. You’re also not alone. Thousands of Chicago buildings built before modern insulation and air-sealing standards were implemented waste energy year after year, and as a result, they never feel comfortable from floor to floor.
You can crank the thermostat, run extra heaters, or wrap yourself in blankets—but until you interrupt the stack effect, you’ll keep fighting a losing battle.
Here are the three most important solutions that actually work, whether during a full renovation or in a no-renovation situation:
1. Seal and insulate the roofline (spray foam or dense-pack cellulose).
2. Air-seal and insulate the first-floor or basement ceiling (spray foam or dense-pack cellulose) if you are the owner of that first floor or garden unit and cannot get the association to fix the bigger issues.
3. Tighten up ground-level leaks (windows, doors, and penetrations).
And here’s the key: these aren’t just fixes for drafty first floors. Everyone in the building benefits when the very top of the structure is properly sealed and insulated. However, it does take cooperation, and sometimes, that is not possible.
Why the Problem Hits Hard in Late October
This isn’t just theory. In Chicago, the first night when temperatures drop to freezing—32°F or below—usually occurs between October 27 and November 5. That’s when homeowners first notice cold floors, basement drafts, and the heating system working overtime.
By early November, the stack effect is in full swing: warm air escapes through the roof, cold air gets pulled in from below, and heating bills start climbing. To prevent it, the time to act is before Halloween, not after Thanksgiving.
The Stack Effect in Action
Whether you live in a two-flat, three-flat, four-flat, townhome, condo, or multi-story single-family home, the same physics apply. The stack effect is a simple yet powerful phenomenon: warm air rises, escapes through the top of the building, and draws cold air in from the bottom.
This is why the first-floor, garden, or basement level always feels cold—not just because of bad luck, but because of heat loss through the roof and the suction of the stack effect, drawing in frigid air from below. The more warm air escapes from the top, the more cold air is pulled in from the bottom to replace it.
In Chicago’s older housing stock, where insulation and air-sealing are minimal, the entire structure behaves like a vertical chimney.
The Three Kinds of Heat Transfer
To understand both the winter stack effect and summer roof overheating, remember there are three main ways heat moves through a building:
1. Air Leakage (Infiltration/Exfiltration) – Uncontrolled airflow into and out of a building. In winter, warm air leaks out at the top and cold air gets sucked in at the bottom. This is the main driver of the stack effect.
2. Conduction (Through Solids) – Heat travels through solid materials like roof decking, plaster, drywall, and floor joists. In winter, that’s warmth leaking through ceilings and walls; in summer, it’s roof heat transferring down into living spaces.
3. Radiation (Energy in All Directions) – Hot surfaces emit energy equally in every direction. The sun heats your roof, which then radiates heat down through the attic and ceiling. Radiation doesn’t rise or fall; it moves in straight lines outward from hot surfaces—if it didn’t, the sun would have to be beneath us to keep us warm.
A Quick Proof of the Stack Effect’s Power
If you want to see how powerful the stack effect can get, look at Chicago’s high-rises. In the middle of winter, the pressure difference between top and bottom can become so strong that lobby doors are nearly impossible to open. That’s why skyscrapers use revolving doors and vestibules—not just for style, but to break the suction of moving air.
Your two-flat, three-flat, townhome, or condo doesn’t generate that level of pressure, but the same physics apply. The suction that traps lobby doors in a high-rise is the same force pulling freezing air into your basement or first-floor unit.
The Three Most Important Fixes
Now that you understand the causes, here’s how to stop them:
1. Seal and Insulate the Roofline – If the top leaks, the stack effect never stops. During renovation, apply closed-cell spray foam directly to the underside of the roof deck. This both seals and insulates, stopping warm air loss in winter and blocking radiant and conductive heat gain in summer. Without renovation, dense-pack cellulose blown into attic cavities slows airflow and adds insulation.
Everyone in the building benefits when air-sealing and insulation are installed at the very top of the structure—above the highest living unit, whether it’s a third, fourth, or even fifth floor. This is the most important step in our work, because controlling the roofline is what stabilizes comfort and efficiency for the entire building.
2. Air-Seal and Insulate the First-Floor or Basement Ceiling – The bottom of the building acts as the intake. If cold air pours in here, the stack effect will continue. Spray foam expands to seal cracks and penetrations while adding high R-value insulation. Dense-pack cellulose reduces airflow and heat loss if spray foam isn’t possible.
3. Control Infiltration at Lower Openings – Even with insulation in place, ground-level leaks can reintroduce cold air. Replace old basement windows and exterior doors, seal around plumbing and electrical penetrations, and add door sweeps or caulk where possible.
Pulling It Together
By addressing the top, bottom, and openings, you’re stopping all three kinds of heat transfer at once: air leakage is sealed, conduction is slowed, and radiation is blocked.
The result is the same whether you own a two-flat in Logan Square, a townhome in Lakeview, or a multi-story condo in Lincoln Park:
– Garden and first-floor units stay warm in January.
– Top floors stay cooler in July.
– Heating and cooling bills go down across the board.
And with Chicago’s first freeze typically arriving by Halloween, there’s no better time to act than right now.
Conclusion
The stack effect explains why two-flats, three-flats, four-flats, townhomes, condos, and multi-story single-family homes with little or no air-sealing and insulation are uncomfortable year-round. The first-floor, garden, or basement is cold due to heat loss and the stack effect, while upper levels bake in summer from radiant and conductive roof heat.
The three best ways to fight back are:
1. Seal and insulate the roofline (the single most important fix for the entire structure).
2. Air-seal and insulate the first-floor or basement ceiling.
3. Tighten ground-level openings and penetrations.
When the top is sealed, the bottom stabilized, and airflow controlled, your home stops behaving like a chimney. The result: steady comfort, lower bills, and a structure ready for both Chicago winters and summers—starting before that first freeze hits in late October.
With ten years of experience selling spray foam insulation in Chicago, Tom Decker is THE person to call and the Chicago Green Insulation is the organization to hire when you are looking for top notch quality and performance as well as someone who can deal with the needs of code officials, home owners and general contractors. Call the others in Chicago, if you are interested in the cheapest price, call Chicago Green Insulation if you are interested in using your dollars to make Chicago a better city for all of us!
Chicago Green Insulation
Chicago Green Insulation
Chicago Green Insulation
Chicago Green Insulation
Chicago Green Insulation