What is CFM25 duct leakage?

CFM25 is the airflow through duct leaks when the duct system is pressurized to 25 pascals (0.10 IWG) above indoor pressure during a duct blaster test. It is the standard metric for measuring duct airtightness in North America. ENERGY STAR requires total duct leakage below 4 CFM25 per 100 square feet of conditioned floor area. A typical existing Canadian home has duct leakage of 150-400 CFM25, meaning 15-30% of system airflow escapes before reaching the living space.

How much energy do leaky ducts waste?

Duct leakage typically wastes 20-30% of heating and cooling energy in Canadian homes. A 200 CFM25 leakage rate in a house with a 1,200 CFM furnace means 17% of conditioned air escapes into the attic or crawlspace. At $0.50/m³ for natural gas and 4,000 heating degree days, this wastes approximately $300-500 per year in heating energy alone. Sealing ducts with mastic compound is one of the highest ROI energy upgrades available, with simple paybacks of 2-4 years in most Canadian climates.

💧 Duct Leakage

Duct Leakage Calculator

Enter your duct blaster test result or estimated leakage rate to calculate energy losses, compliance status against ENERGY STAR and Canadian code requirements, and annual dollar cost of unsealed ducts. Shows you exactly what duct sealing is worth.

Unit System:

🏠 Building & System Data

Conditioned Floor Area *

ft²

System Design Airflow *

CFM

Building Type

💧 Leakage Measurement

Measured CFM25 Leakage i

CFM25

OR Estimated Leakage % i

Where Ducts Are Located

💵 Energy Cost Inputs (optional)

Heating Degree Days i

°F·days

Natural Gas Cost

$/m³

Furnace Efficiency (AFUE)

💧 Duct Leakage Results

Metric	Value	Note

Export:

Leaky ducts: a problem most Canadian homeowners don't know they have

Most homes in Canada have duct systems that leak 20-30% of conditioned air before it reaches living spaces. This means your furnace or AC runs significantly longer than needed, rooms far from the air handler are always uncomfortable, and you are paying 25-40% more in heating and cooling costs than a sealed system would require. The insulation, windows, and furnace upgrade conversations all happen while ducts — the actual delivery system — are ignored. A duct blaster test costs $150-200 and is the single most useful diagnostic tool for understanding where your energy dollars are going.

Mastic sealant vs. duct tape: why it matters

The grey tape sold as "duct tape" fails within 2-5 years due to heat cycling and movement. Almost every home built before 2000 that used duct tape on joints now has unsealed ducts, even if they looked fine at installation. The correct material is mastic sealant — a thick paste applied with a brush to all joints, seams, and connections. Mastic lasts the life of the duct system. It costs $5-10 per tube and a contractor can seal a typical residential duct system in 2-4 hours. The ROI is better than any insulation or window upgrade in most cases.

When duct sealing can downsize your equipment

If you have 25% duct leakage and are replacing your furnace, sealing ducts first can reduce your calculated heat load by 8-12% (the portion of leakage that escapes unconditioned space). This may allow one size smaller furnace, saving $500-1,500 on equipment. Always seal ducts before calculating replacement equipment size. Use the load comparison tool to quantify the load reduction before and after sealing.

Frequently Asked Questions

What CFM25 reading means my ducts are too leaky?+

ENERGY STAR Canada requires total duct leakage below 4 CFM25 per 100 ft² of conditioned floor area. For a 2,000 ft² home, the limit is 80 CFM25. Most existing Canadian homes test at 150-400 CFM25. If your reading is above the ENERGY STAR limit, duct sealing is clearly justified. Even if you are not pursuing ENERGY STAR certification, any reading above 100 CFM25 in a 2,000 ft² home means you are losing meaningful energy. Leakage to outside the conditioned envelope (attic, crawlspace) is far more costly than leakage within the conditioned space, because conditioned-space leakage still stays in the house.

Can I seal my own ducts, or do I need a contractor?+

Accessible duct joints in a finished basement or mechanical room are straightforward DIY work — buy mastic sealant and a brush, wear gloves, and coat every visible joint, seam, and connection. The air handler cabinet seams and supply plenum connections are the highest-priority targets. Ducts in attics or crawlspaces are harder to access but still DIY-feasible. Ducts inside finished walls and ceilings require a contractor with an Aeroseal system (pressurized sealant injection). For a full ENERGY STAR duct test and verification, you need a certified energy auditor with a duct blaster. Many provincial rebate programs in Canada (Ontario Enbridge, BC Fortis, Alberta ATCO) offer incentives for certified duct sealing work — check the rebate calculator for your province.