Building Science - Heat Transfer, Air Sealing & Duct Leakage
Heat transfer mechanisms (conduction, convection, radiation), building envelope air sealing, blower door testing, duct leakage testing, and insulation effectiveness.
- Explain the three mechanisms of heat transfer and how each affects building energy performance
- Identify common air leakage pathways in residential buildings and their solutions
- Interpret blower door test results and duct leakage test results
- Apply building science principles to HVAC system design and troubleshooting
Lesson 1
Heat Transfer Mechanisms & Building Envelopes
The Three Mechanisms of Heat Transfer
Heat always moves from hot to cold. Understanding how heat moves through a building is fundamental to designing, installing, and troubleshooting HVAC systems. There are three mechanisms:
Conduction - Heat transfer through solid materials by direct molecular contact. Heat conducts through walls, ceilings, floors, windows, and doors from the warm side to the cool side. The rate of conduction depends on the material's thermal conductivity (how easily heat passes through it), the thickness of the material, and the temperature difference across it.
Insulation works by reducing conductive heat transfer. Materials with low thermal conductivity (fiberglass, cellulose, foam) slow the flow of heat. The effectiveness of insulation is measured by its R-value - the resistance to heat flow per inch of thickness. Higher R-values mean better insulation.
Convection - Heat transfer through the movement of fluids (air or water). In buildings, convection occurs when warm air rises and cool air falls (natural convection) and when fans or wind force air movement (forced convection). Air leakage through the building envelope is the most significant convection heat transfer pathway. Warm indoor air escaping through cracks and gaps in winter carries far more heat than conduction through well-insulated walls.
Radiation - Heat transfer through electromagnetic waves. Every object above absolute zero emits thermal radiation. The sun heats buildings through radiation. Hot roofs radiate heat downward into attic spaces. Radiant barriers and low-emissivity (low-E) window coatings reduce radiative heat transfer.
R-Value and U-Value
R-value measures resistance to heat flow. Higher R = better insulation. R-values are additive - a wall with R-13 insulation, R-1 drywall, R-0.5 siding, and R-0.7 air films has a total wall R-value of approximately R-15.
U-value (U-factor) measures the rate of heat flow. It is the inverse of R-value: U = 1/R. Lower U = better insulation. Windows are rated by U-factor because they have relatively low R-values. A double-pane low-E window might have a U-factor of 0.30 (equivalent to about R-3.3).
The Building Envelope
The building envelope is the boundary between conditioned (indoor) space and unconditioned (outdoor or buffer) space. It includes walls, ceiling/roof, floor, windows, and doors. The envelope's job is to control heat flow, air flow, moisture flow, and vapor flow.
A well-performing envelope has:
- Adequate insulation (R-value) to reduce conductive heat transfer
- Continuous air barrier to reduce convective heat transfer (air leakage)
- Proper vapor management to prevent condensation within wall and roof assemblies
- Appropriate solar control (shading, low-E glass) to manage radiative heat gain
Air Sealing Before Insulation
Adding insulation to a leaky building is like putting on a sweater with holes in it. Air sealing should always come before or simultaneously with insulation upgrades. Air leakage can carry 10 to 50 times more heat than conduction through the same area of wall. Sealing the top plates, electrical penetrations, plumbing chases, and recessed lights in the attic floor is typically more cost-effective than adding more insulation on top of existing adequate levels.
Heat transfers by conduction (through materials), convection (through air movement), and radiation (through electromagnetic waves). Air leakage accounts for 25 to 40% of heat loss in a typical home - more than any other single pathway. R-value measures insulation effectiveness (higher = better). Air sealing should precede or accompany insulation upgrades because air leakage transfers far more heat than conduction through insulated assemblies.