External Heat Gains

Overview

External heat gains represent thermal energy entering conditioned spaces from the outdoor environment through multiple physical mechanisms. These loads originate from conductive heat transfer through the building envelope, solar radiation admitted through fenestration, and convective-diffusive transport via infiltration and ventilation air. External loads exhibit strong dependence on outdoor weather conditions, building orientation, envelope thermal properties, and time of day.

Envelope Transmission Fundamentals

Heat transmission through opaque building envelope components follows Fourier’s law of conduction, with the instantaneous heat flux proportional to the temperature gradient and thermal conductivity of materials. For steady-state analysis, the overall heat transfer coefficient (U-factor) consolidates thermal resistances of all material layers plus interior and exterior surface film resistances into a single parameter with units of BTU/hr-ft²-°F or W/m²-K.

The one-dimensional heat transfer assumption applies reasonably to large, homogeneous wall and roof sections but breaks down at thermal bridges where higher-conductivity materials create multidimensional heat flow paths. Steel studs in wall assemblies, concrete balcony slabs penetrating insulation layers, and window frames create thermal bridges that increase effective assembly U-factors by 10-50% above center-of-cavity values.

Sol-Air Temperature Concept

The sol-air temperature provides a convenient method to account for both convective heat transfer from outdoor air and radiation heat gain from solar absorption on exterior surfaces using a single equivalent temperature. Sol-air temperature equals outdoor air temperature plus the absorbed solar radiation divided by the exterior surface heat transfer coefficient, minus the correction for longwave radiation exchange with the sky and surroundings.

Dark-colored roof surfaces in direct sunlight can reach sol-air temperatures 40-60°F above ambient air temperature during peak solar conditions. This elevated driving temperature substantially increases roof heat gains compared to shaded surfaces. Reflective roofing materials with high solar reflectance (cool roofs) reduce sol-air temperature and resulting heat gains by 15-25 BTU/hr-ft² compared to conventional dark surfaces.

Fenestration Heat Transfer

Windows, skylights, and glazed doors transfer heat through combined conduction, convection, and radiation mechanisms. The fenestration U-factor accounts for conductive-convective heat transfer driven by indoor-outdoor temperature difference, while the solar heat gain coefficient (SHGC) quantifies solar radiation transmitted and absorbed then released inward.

Modern low-emissivity (low-e) coatings significantly reduce radiative heat transfer across glazing cavities while maintaining high visible light transmittance. Spectrally selective low-e coatings preferentially reflect infrared radiation while transmitting visible wavelengths, achieving SHGC values of 0.25-0.40 with visible transmittance above 0.60. Multiple glazing layers with low-conductivity gas fills (argon or krypton) reduce U-factors to 0.20-0.30 BTU/hr-ft²-°F in high-performance residential windows.

Solar Heat Gain Through Glazing

Solar radiation incident on fenestration consists of direct beam, diffuse sky, and ground-reflected components. The transmitted solar radiation depends on sun angle relative to glazing orientation, which varies continuously throughout the day and year. SHGC values typically apply to normal incidence; actual heat gains require integration over all incidence angles accounting for angular dependence of optical properties.

External shading devices including overhangs, fins, and louvers can dramatically reduce solar gains during cooling season while admitting beneficial solar heat during heating season through proper geometry. The projection factor method provides simplified calculation of shading effectiveness for common overhang configurations. Movable shading systems offer seasonal adjustment but require occupant interaction or automated controls to realize energy savings.

Infiltration Heat and Moisture Transfer

Infiltration represents uncontrolled air leakage through cracks, gaps, and penetrations in the building envelope driven by pressure differences from wind, stack effect, and mechanical system operation. The sensible heat transfer equals the volumetric infiltration rate multiplied by air density, specific heat, and indoor-outdoor temperature difference. Latent heat transfer depends on humidity ratio difference and the latent heat of vaporization.

Stack effect pressure differences increase with building height and indoor-outdoor temperature difference according to ΔP = Cρgh(1/To - 1/Ti), where C is a constant, ρ is air density, g is gravitational acceleration, h is vertical distance, and To and Ti are outdoor and indoor absolute temperatures. Tall buildings in cold climates experience stack pressures exceeding 0.5 in w.c., driving substantial infiltration if envelope airtightness is inadequate.

Wind-induced infiltration depends on surface pressure coefficients that vary with building geometry, local terrain, and wind direction. Windward surfaces experience positive pressure while leeward and side surfaces typically see suction. The resulting pressure field drives infiltration flows that vary in magnitude and direction throughout the building envelope.

Ventilation Load Characteristics

Ventilation outdoor air requirements determined by occupancy and space function represent controlled infiltration necessary for indoor air quality maintenance and building pressurization. Unlike infiltration, ventilation air quantities remain relatively constant during occupied periods regardless of outdoor conditions. The resulting load varies only with outdoor temperature and humidity.

In climate zones with significant heating and cooling seasons, ventilation represents a major energy consumption component. The peak ventilation cooling load occurs at design outdoor conditions, while annual ventilation energy consumption depends on climate and operating schedule. Demand-controlled ventilation (DCV) using CO₂ or occupancy sensors reduces ventilation when spaces are partially occupied, providing 20-40% energy savings in applications with variable occupancy.

Ground-Coupled Heat Transfer

Floors on grade and below-grade walls exchange heat with the earth rather than outdoor air. Ground temperature varies seasonally with a phase lag and amplitude decay depending on depth below surface. Annual average ground temperature approximates average annual air temperature, while shallow depths experience temperature swings following outdoor air with several months delay.

Heat transfer to ground follows multidimensional paths that require numerical solution or empirical correlations for accurate calculation. Simplified methods use equivalent U-factors and temperature differences to estimate steady-state heat flow. Below-grade spaces typically experience net heat loss year-round in most climates since ground temperature remains below indoor setpoint, though summer cooling loads may occur in hot climates with minimal below-grade depth.

Load Reduction Strategies

Reducing external cooling loads directly decreases equipment capacity requirements and operating energy consumption. Envelope improvements including increased insulation, high-performance windows, and air sealing reduce both peak loads and annual energy use. Strategic window orientation and shading design minimize solar gains while preserving daylighting benefits and views.

Radiant barriers in attic spaces reduce roof heat gains in hot climates by 20-40% by limiting radiative heat transfer from hot roof deck to attic floor insulation. Ventilated facades and roofs exhaust solar heat gains before entering the building thermal mass. These passive strategies provide permanent load reduction without requiring ongoing maintenance or operational energy consumption.

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