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 18°C. More degree-days means more heating.

What it means

A day averaging 8°C adds 10 heating degree-days; a day at or above 18°C adds none. Yearly totals track how much heating a house needs, which is why building codes, climate zones and bill analysis use them. Canadian totals use °C, so they look smaller than US figures for the same climate (multiply 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 energy advisor’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 ↗
Canada's code climate zones	Zone 4 under 3,000 HDD (°C) up to zone 8 at 7,000 or more	climatedata.ca ↗

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 18°C. More degree-days means more cooling.
NBC climate zone (Canada) Canada's building code climate zones, set by heating degree days: zone 4 (mildest) to zone 8 (coldest), with zone 7 split into 7A and 7B.
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.

← Every term, A to Z