Air Leakage Load Calculator
Estimate the HVAC sizing penalty from a drafty building envelope. Start with a base load (BTU/hr), then add a leakage estimate (quick) or blower-door results (advanced).
Calculator · How to use · What the result means · FAQ
Tip: If you don’t have a base load yet, use the BTU calculator first, then paste your BTU/hr result here.
This tool provides a planning estimate of infiltration impact. If you’re choosing equipment, verify with an HVAC pro and a full Manual J.
How to use this calculator
- Get a base load from the BTU calculator.
- Select Quick estimate if you don’t have blower-door numbers.
- Select Blower-door if you have ACH50 or CFM50 from an energy audit.
- Use the adjusted load as a sanity check before picking equipment size.
What the result means
Air leakage (infiltration) adds load because outdoor air must be heated in winter and cooled/dehumidified in summer. If your leakage penalty is high, you often get better comfort and lower operating cost by improving the envelope (air sealing, attic insulation, rim joist sealing) before upsizing equipment.
Next steps: AC size calculator · Heat pump calculator · Furnace size calculator
Frequently asked questions
What is ACH and why does it affect HVAC sizing?
ACH is air changes per hour — how many times the air in a home is replaced by outdoor air. Higher leakage (higher ACH) increases heating and cooling loads because outdoor air must be conditioned. A home with ACHnat of 0.8 can need 20% more heating or cooling capacity than the same home at 0.2 ACHnat.
What is the difference between ACH50 and ACHnat?
ACH50 is measured during a blower-door test at 50 Pascals of pressure difference. ACHnat is an estimate of natural leakage under typical wind and stack effect conditions. Converting ACH50 to ACHnat requires assumptions about the number of stories, wind exposure, and local weather — the blower-door method in this calculator makes these estimates.
Is this a replacement for Manual J?
No. This is a planning estimate that applies a leakage penalty to a base load. A full Manual J accounts for construction assemblies, duct losses, design temperatures, and detailed infiltration modeling. Use this calculator as a sanity check, then validate with an HVAC professional before final equipment selection.
Should I just oversize my equipment if my house is drafty?
Not automatically. Oversizing can cause short-cycling, comfort problems, and poor dehumidification in cooling mode. Often the best ROI is air sealing and insulation first — spray foam, weatherstripping, and outlet gaskets can reduce a leaky home's load by 15–25% before you buy any new equipment.
Can I use this without blower-door results?
Yes. Use the quick tightness estimate. It's less precise, but it helps you understand how much draftiness can swing a sizing decision — especially if you're between equipment sizes.
Why does the blower-door method ask about stories and exposure?
Blower-door testing measures leakage at a fixed pressure (ACH50), but natural infiltration depends on wind and stack effect. Stories and exposure help estimate ACHnat, which is more relevant to day-to-day heating and cooling load.
Should I add the penalty to cooling load too?
Infiltration increases both heating and cooling loads, but the impact varies by climate and humidity. Use this as a planning estimate — if you're close to a sizing breakpoint, a full Manual J is worth it.