Why Is My House Drafty

Most houses feel drafty because air is leaking through gaps and holes in the building enclosure. As warm air escapes through the upper parts of the house, outdoor air is pulled in through lower areas such as foundations, rim joists, and wall penetrations. This air movement can create drafts, cold floors, uneven temperatures, moisture problems, and higher heating bills.

If your house feels drafty, one room is always cold, or your heating system never seems to catch up, you're not alone. If, in the summer, you’re running your air conditioner and the lower floor is freezing while the upper floor is sweltering, you’re also not alone (but that’s another article for another day).

One of the most common questions we hear homeowners ask is:

"Why is my house drafty?"

The answer is usually not what most people expect.

In more than 30 years working with builders, renovators, energy advisors, and homeowners, I've seen people spend thousands of dollars solving the wrong problem. New windows, more insulation, bigger heating equipment, and even major renovations often fail to deliver the expected comfort improvements because the real issue was uncontrolled air leakage. I've also seen well-intentioned air sealing projects create moisture and indoor air quality problems when ventilation and moisture management weren't considered as part of the overall strategy. Houses work as systems. Changing one part of the system affects the others.

Many people assume they need:

• More insulation
• New windows
• A bigger furnace
• A heat pump upgrade

Sometimes those improvements help.

But in many homes, the real problem is uncontrolled air leakage.

Air is leaking out of the house and outdoor air is leaking in through cracks, gaps, holes, and joints in the building enclosure. Until those leaks are addressed, comfort problems often persist.

Most houses have dozens of air leaks, but not all leaks have the same impact.
The Air Sealing Priority Guide helps you identify the areas that typically deliver the biggest improvements in comfort, energy performance, and moisture control.
Get the Air Sealing Priority Guide →

Most houses feel drafty because air is moving through the building envelope (the walls, floors, and ceilings of the building), coming through holes and gaps in the materials and components that make up the building itself. Homes constructed from wood frames by humans just don’t fit together as snugly as equipment milled and put together by precision machines.

The amount of air that moves through the building envelope is caused by three key drivers: the stack effect (the vertical movement of air), wind pressure, and the effects of mechanical system vents and exhaust fans.

Depending on where the holes and gaps are the result can include:

• Cold floors
• Drafts near windows
• Drafts near electrical outlets
• Uneven room temperatures
• Window condensation
• High heating bills
• Moisture problems

In many cases, the largest leaks are hidden and you’ll never see them.

While every house is different, the majority of uncontrolled air leakage tends to occur in a handful of predictable locations.

Warm air is more buoyant than cold air, so it rises. As it moves upward, it escapes through:

• Attic hatches
• Pot lights
• Ceiling fixtures
• Plumbing penetrations
• Wiring penetrations
• Exhaust fan housings

Nothing in this world works in isolation, there’s always cause and effect! In this case, because warm air is escaping at the top of the house, replacement air is pulled in elsewhere. Usually from the lower part of the house.

In colder Canadian climates, uncontrolled air leakage into attics is one of the leading causes of attic frost, ice dams, and moisture-related roof problems. Hidden openings between walls and attics often act like chimneys.

These pathways allow warm air to move into attics where it can contribute to:

• Heat loss
• Condensation
• Attic mold
• Ice dams

In some cases, this can lead to attic rain, where moisture condenses inside the attic and drips back into the building. Read more in Forecasting Trouble: Why Attic Rain Is More Than Just Bad Weather.

The rim joist area is one of the most common sources of cold floorsand draft complaints.

This is where the floor structure meets the exterior wall. It can be exposed in an unfinished foundation, but in a 2 or 3 storey house, the rim joist is buried behind insulation, the rest of the floor system and the ceiling finish on the inside, and by the sheathing and the cladding (siding) on the outside. Leaks here often be the cause of cold floors, drafty basements and comfort problems near exterior walls.

The bottom of the house is just as important as the top. Where the foundation wall meets the framed wall is one of the main places outdoor air enters a house. When air leaks in at the bottom, warm indoor air is pushed out through leaks at the top (remember those attic hatches, ceiling penetrations, top plates and attic bypasses?). This is one reason foundation air sealing can have a surprisingly large impact on comfort and energy performance.

Every pipe, vent, duct, and cable that passes through the enclosure creates potential leakage paths. Holes are drilled, services are installed, sealing might happen. These areas are frequently overlooked.

One of the most important concepts in building science is something called the stack effect.

Warm air is lighter than cold air. That means it rises. (Nerd alert: heat doesn’t rise, it travels in all directions at once towards any and all cooler surfaces)

During heating season:

• Warm indoor air rises.
• Air leaks out through the upper parts of the house.
• Outdoor air gets pulled in through the lower parts of the house.

Think of your house as a giant chimney. A giant chimney stack.

Want to understand why warm air rises through a house and why leaks at the top matter so much? Read more in What Everybody Needs to Know About Building Science – Part 1.

The larger the leaks at the top, the more air gets pulled through the rest of the building.

This is why:

• Attics matter so much.
• Ceiling penetrations matter so much.
• Basement leaks matter so much.

The top and bottom of the house often have a greater impact than the walls.

Insulation does one job really well. Some types of insulation do the job better than others. Not all insulation materials stop drafts.

So the answer to ‘do I need more’ is: not necessarily. And certainly not before air sealing.

This is one of the most common renovation mistakes.

Insulation and air sealing do different jobs.

Insulation: Slows heat flow.

Air Sealing: Controls air movement.

Not sure what the difference is between an air barrier and a vapour barrier? Read our Quick Primer: Air Barrier vs Vapour Barrier.

You can add insulation and still have a drafty house. If air is moving through the enclosure, comfort problems often remain. Addressing major air leaks first can often provide better results than simply adding more insulation.

Yes, and this is where it’s important to understand what is going on with the building envelope. Air leakage and moisture problems are closely connected.

Learn more about how moisture moves through buildings and why managing it correctly is critical in our Moisture Management Explained guide.

Warm indoor air carries moisture. When that air leaks into cold building assemblies, condensation can occur. When condensation occurs, there’s going to be trouble if it can’t dry out.

This can contribute to:

• Attic mold
• Window condensation
• Moisture accumulation
• Material deterioration
• Indoor air quality issues

Many homeowners discover moisture problems long before they realize air leakage is involved.

You may have air leakage issues if you notice:

• Drafts around windows or doors
• Cold floors
• Uneven temperatures
• One room consistently colder than others
• Window condensation in winter
• Attic frost
• Attic mold
• High heating bills
• Musty smells
• Excessively dry indoor air during winter

The more symptoms you recognize, the more likely air leakage is contributing to the problem.

A blower door test is the best way to identify and measure air leakage.

However, homeowners can often identify obvious problems by checking:

• Attic hatches
• Ceiling penetrations
• Basement rim joists
• Foundation connections
• Mechanical penetrations

Understanding where houses typically leak is often the first step.

Most houses have dozens of air leaks. The challenge isn't finding them—it's figuring out which ones have the biggest impact on comfort, energy bills, and moisture problems.

The challenge is not finding one leak.

The challenge is knowing which leaks matter most.

Many homeowners spend time sealing minor gaps while overlooking the areas responsible for most of the air movement. The ones that actually slow down the stack effect and reduce drafts, energy loss and moisture transfer.

That's why we created the:

The guide identifies:

• The first five places to air seal in almost any house
• Why those locations matter
• What to look for
• What materials are commonly used
• How air leakage affects moisture and ventilation
• What to tackle before insulation, renovations, or HVAC upgrades

Building Science & The Stack Effect, Part 1

Moisture Management Explained

Air Barrier vs. Vapour Barrier

Building Science Radio

Shawna Henderson has worked in building science, energy efficiency, home performance, and construction training since 1991. She is the CEO of Blue House Energy, an online education provider that has trained more than 10,000 building professionals across North America.