Skin-load vs internal-load buildings; HDD and CDD

One-line orientation

Building loads fall into two main groups: skin loads and internal loads. Skin loads come from the envelope and outdoor climate. Internal loads come from occupants, lights, and equipment. Degree days describe how demanding a climate is for heating or cooling over a season.

Key points

• Skin-load-dominated building: the dominant source of heat gain or loss is the building envelope reacting to external climate — solar radiation, wind, and conduction through walls, roofs, and glazing. Examples: small residences, warehouses, buildings with high window-to-wall ratio, and any compact building where the skin area is large relative to floor area.
• Internal-load-dominated building: heat generated inside the building — by occupants, lighting, computers, servers, and equipment — outweighs the envelope’s contribution. The HVAC system must remove this heat even in cold weather. Examples: open office floors, data centers, large retail stores, hospital surgery suites.
• Heating Degree Days (HDD): measure how much (in degrees) and how long (in days) outdoor temperature falls below the base temperature of 65°F (≈18.3°C). Formula: HDD = 65°F − Average Daily Temperature (for each day where avg < 65°F; zero otherwise). Sum daily HDD over a year for annual HDD.
• Cooling Degree Days (CDD): measure how much and how long outdoor temperature is above 65°F. Formula: CDD = Average Daily Temperature − 65°F (for each day where avg > 65°F).
• What degree days mean: annual HDD and CDD are rough seasonal climate and energy indicators. Use them to compare climates and estimate seasonal energy or fuel use; they do not equal a building’s total energy demand. Size equipment from a peak design-load calculation instead.

Degree days: below 65°F heats, above 65°F cools

Daily gaps to the 65°F base sum into annual HDD / CDD; open dots mark the crossover points.

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Key

• open dot = curve crosses the 65°F base; accrual flips
• shaded area = Σ daily = annual HDD / CDD total

Confusions / comparison

	Skin-load-dominated	Internal-load-dominated
Dominant heat source/sink	Envelope (outdoor climate, solar)	Occupants, lights, equipment
Typical building types	Houses, warehouses, small commercial	Offices, data centers, large retail, hospitals
HVAC implication	System must track outdoor conditions closely	Cooling needed year-round; less seasonal swing
High SHGC glazing benefit	Yes, in cold climate (south-facing passive solar)	Less relevant — internal gains already supply heat
Envelope insulation priority	High	Moderate (internal gains may dominate anyway)

Metric	Formula	What counts	Accumulates when
HDD	65°F − Avg Daily Temp	degrees below 65°F	Outdoor avg < 65°F
CDD	Avg Daily Temp − 65°F	degrees above 65°F	Outdoor avg > 65°F
Annual HDD	Sum of daily HDD over 12 months	total heating demand	Climate is cold
Annual CDD	Sum of daily CDD over 12 months	total cooling demand	Climate is hot

Related

→ Thermal: Solar geometry & SHGC (skin-load buildings most sensitive to SHGC choice) · Thermal mass (mass moderates temperature swings, reducing peak degree-day impact on the envelope) · HVAC: Balance temperature (the outdoor temperature where internal and solar gains equal building heat loss — related concept to the 65°F base) · Code: energy code (ASHRAE 90.1 climate zones based on HDD/CDD bands).