A house can lose heat without any heat passing through a wall.
Outdoor air simply leaks in.
That uncontrolled air exchange is infiltration, and it can be one of the largest variables in a residential load calculation.
Why infiltration matters
When outdoor air enters the house, the HVAC system has to condition it.
In winter, that means warming cold outdoor air.
In summer, that may mean:
- Cooling hot outdoor air
- Removing moisture from humid outdoor air
So infiltration can contribute to:
- Heating load
- Sensible cooling load
- Latent cooling load
The effect depends heavily on climate.
A leaky house and a tight house are not the same load
Take two homes with identical:
- Floor area
- Insulation
- Windows
- Orientation
If one is significantly leakier, its heating and cooling loads can be higher.
This is why square-foot rules miss so much of the building physics.
Air leakage is invisible on a floor plan.
What creates infiltration?
Common leakage paths include:
- Attic penetrations
- Recessed lights
- Plumbing chases
- Rim joists
- Window and door gaps
- Fireplace assemblies
- Unsealed duct boots
- Knee walls
- Crawlspace connections
- Top plates
- Utility penetrations
Older homes often have many of these.
Newer homes may be much tighter, especially when air sealing is tested during construction.
Blower-door data is better than guessing
A blower-door test measures building leakage under a controlled pressure difference.
The commonly reported result may be:
- CFM50
- ACH50
That test result is not the same as natural infiltration during normal weather.
A proper calculation converts measured leakage into an estimate of natural air exchange using the applicable methodology and factors.
The advantage is that the starting point is measured rather than assumed.
CFM50 vs. ACH50
CFM50
Cubic feet per minute of leakage at 50 Pascals of pressure difference.
ACH50
Air changes per hour at 50 Pascals, normalized by building volume.
ACH50 makes it easier to compare buildings of different sizes.
Neither value should be dropped directly into a normal operating load formula without the proper conversion.
What if you do not have a blower-door test?
Most replacement jobs do not.
You can still run Manual J.
Use a reasonable infiltration assumption based on:
- Age
- Construction quality
- Visible air sealing
- Window and door condition
- Renovation history
- Local building practices
The important thing is not to pretend the assumption is measured data.
Document it.
Why "tight / average / leaky" can still be useful
Categories are less precise than a blower-door test, but they can be practical when no measurement exists.
The problem starts when the category is chosen to manipulate the result.
Marking every house "leaky" to increase tonnage is not conservative engineering. It is just another hidden safety factor.
Infiltration is especially important in cold climates
Heating load from outdoor air roughly follows the temperature difference between inside and outside.
As winter outdoor temperature falls, each cubic foot of infiltrating air requires more heat.
A leaky home in a cold climate can have a substantial portion of its heating load tied to infiltration.
Air sealing can therefore change HVAC sizing—not just energy use.
In humid climates, infiltration adds latent load
Warm humid outdoor air brings water vapor with it.
The AC must remove that moisture.
This is one reason leaky homes can feel clammy even when thermostat temperature looks acceptable.
If infiltration is understated, latent load may be understated.
If it is overstated, the calculation can make humidity demand look much larger than reality.
Do not confuse infiltration with ventilation
Infiltration is uncontrolled leakage.
Ventilation is intentional outdoor air.
Examples of ventilation include:
- ERV
- HRV
- Dedicated fresh-air duct
- Balanced ventilation system
A tight home can have low infiltration and still have a meaningful ventilation load.
Both need to be represented correctly.
What about duct leakage?
Duct leakage can interact with infiltration, especially when ducts are outside conditioned space.
A leaky return can pull hot attic or crawlspace air into the system.
A leaky supply can depressurize the house and encourage more outdoor-air infiltration.
Manual J duct-loss treatment and building infiltration should be handled using the selected methodology rather than piling arbitrary leakage factors on top of each other.
When blower-door data is worth using
Measured leakage is especially valuable when:
- The home is very tight
- The home is very leaky
- Air sealing was recently completed
- An energy program requires testing
- The load is close to an equipment-size threshold
- You are sizing a cold-climate heat pump
- The homeowner is paying for major envelope improvements
If leakage materially affects the equipment decision, measurement can remove a major uncertainty.
How to sanity-check infiltration
If the load looks suspicious, ask:
- Does the leakage assumption fit the age and construction?
- Was building volume entered correctly?
- Was blower-door data entered in the expected units?
- Was ventilation also added separately?
- Is the home actually tighter after renovation?
- Are ducts outside the envelope influencing the result?
Infiltration errors can move both heating and latent cooling significantly.
Bottom line
Air leakage is not a minor detail in Manual J.
It can meaningfully change heating load, cooling load, humidity demand, and heat-pump sizing.
Use measured blower-door data when you have it. When you do not, use a defensible building-tightness assumption and document it.
Load Calc Guru supports both default infiltration modeling and measured blower-door inputs so you can refine the model when better data exists.