How much heat is lost through uninsulated ducts in an attic?

Uninsulated ducts in an attic can lose 25-40% of heating and cooling energy before it reaches the living space. An attic at 130F in summer and -20F in winter creates extreme temperature differences across the duct wall. A single-layer sheet metal duct has essentially no insulation value. Standard duct design practice requires minimum R-6 insulation for ducts in unconditioned attics and R-4 for ducts in vented crawlspaces. Adding R-8 insulation to attic ducts typically recovers 15-20% of total system capacity.

What is duct conduction loss vs duct leakage loss?

Duct conduction loss is heat that conducts through the duct wall and insulation into the surrounding unconditioned space. It depends on duct surface area, insulation R-value, and the temperature difference between supply air and the surrounding space. Duct leakage loss is conditioned air that physically escapes through holes, loose joints, and unsealed connections before reaching the supply registers. Both types waste energy, but leakage also reduces airflow to rooms, causing comfort problems. Typical homes lose 20-30% of duct air to leakage.

🌸 Duct Heat Loss

Duct Heat Loss Calculator

Calculate heat loss and gain from duct conduction and air leakage in unconditioned spaces. Covers supply and return ducts in attics, crawlspaces, and garages. Use results to size equipment correctly and justify duct sealing upgrades.

Unit System:

🌡 System Conditions

Supply Air Temperature *

°F

Heating: 100-130°F. Cooling: 50-58°F.

Unconditioned Space Temp *

°F

Attic in winter, crawlspace, garage temp

Indoor Setpoint *

°F

🌸 Duct System Properties

Total Supply Duct Surface Area *

ft²

Estimate: duct length x perimeter. Typical home: 300-600 ft²

Total Return Duct Surface Area

ft²

Duct Insulation R-Value i

R-Value

h·ft²·°F/BTU

Total System Airflow i

CFM

Duct Leakage Rate i

🌸 Duct Heat Loss Results

Loss Component	BTU/hr	% of Conduction Total

Export:

Why duct losses matter for equipment sizing

A room-by-room load calculation calculates the building envelope load. But if your ducts run through an unconditioned attic with 20% leakage and R-4 insulation at 130°F supply air, your system needs to produce significantly more BTU/hr than the building load alone. Adding 15-25% for duct losses to the calculated load gives you the correct equipment sizing load. Ignoring duct losses leads to undersized equipment that runs constantly on design days.

Conduction vs. leakage: two different problems

Conduction loss happens through the duct wall even when ducts are well-sealed. Better insulation fixes it. Leakage loss happens when conditioned air escapes through holes and joints into the unconditioned space. Duct sealing fixes it. Most existing homes have both problems. Duct sealing with mastic compound is the higher-impact fix per dollar because leakage both wastes energy and reduces comfort by starving rooms of airflow. See the duct leakage calculator for detailed leakage analysis.

Standard duct design practice

Properly designed duct systems keep ducts inside the conditioned envelope wherever possible — inside insulated ceilings, interior walls, or conditioned mechanical rooms. Every foot of duct moved inside the building envelope eliminates its conduction and leakage losses entirely. When ducts must run outside the conditioned space, R-8 insulation and airtight construction with mastic sealing at every joint are the standards to meet. Use the duct sizing calculator for full friction-rate-based duct design.

Frequently Asked Questions

How do I measure duct surface area?+

For rectangular ducts, surface area per foot of length = 2 x (width + height) in feet. For round flex duct, surface area per foot = pi x diameter in feet. Measure every run, trunk, and plenum that passes through unconditioned space. A typical 2,000 ft² house has 300-600 ft² of supply duct surface area in the attic or crawlspace. If you don't have exact measurements, use the rule of thumb: 0.15-0.25 ft² of duct surface area per CFM of system airflow as a starting estimate.

What duct leakage rate is acceptable for a new home?+

ENERGY STAR requires total duct leakage below 4 CFM25 per 100 ft² of conditioned floor area, or less than 4% of system airflow. Most Canadian energy codes target similar levels for new construction. Existing homes average 20-30% leakage. A blower-door-assisted duct leakage test (duct blaster test) gives the actual measured leakage. Without a test, use 20-25% for an older home and 10-15% for a newer home as conservative estimates. See the duct leakage calculator for more detail.