Glossary · Energy, bills & ratings

Heating degree days (HDD)

Also called: degree days · HDD18 · HDD65

A measure of how cold a place is: each day adds how far its average temperature falls below 65°F. More degree days means more heating.

What it means

A day averaging 50°F adds 15 heating degree days; a day at or above 65°F adds none. Yearly totals track how much heating a house needs, which is why climate zones and bill analysis use them. US totals use °F; Canadian figures use °C and look smaller (multiply them by 1.8 to compare).

Why it matters in an audit

Dividing heating energy use by HDD lets an auditor compare winters fairly and measure the real effect of upgrades.

Typical numbers

Reference values with their sources. Your auditor’s report gives the figure for your own house.

What	Value	Source
Bases	18°C in Canada (ECCC); 65°F in the US (EIA)	climate.weather.gc.ca ↗
Worked example (EIA)	High 33°F, low 25°F: mean 29°F, so 36 HDD	eia.gov ↗
US Building America regions	Cold 5,400–9,000 HDD (°F); very cold 9,000–12,600; subarctic 12,600 or more	energy.gov ↗

Sources

Learn more here

Related terms

Cooling degree days (CDD) A measure of how hot a place is: each day adds how far its average temperature rises above 65°F. More degree days means more air conditioning.
IECC climate zone The US energy code's climate map: zones 0 (hottest) to 8 (coldest), plus a moisture letter — A for moist, B for dry, C for marine.
Heating-dominated vs cooling-dominated climate Whether a place needs more energy for heating or for cooling over a year, which changes which upgrades and equipment matter most.
Weather normalization Adjusting energy use for how cold or hot a period was, using degree-days, so bills from different years or homes can be compared fairly.
Climate normals Thirty-year averages of temperature, degree-days, precipitation and other weather at a station, used as the typical climate for a place.

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