- ASHRAE climate zones sort every location into a thermal band numbered 0 to 8 plus a moisture letter of A for moist, B for dry or C for marine, defined in ANSI/ASHRAE Standard 169 and reproduced in the IECC and ASHRAE Standard 90.1.
- The number comes from cooling degree days base 10 degrees Celsius and heating degree days base 18 degrees Celsius. The letter comes from precipitation, mean temperature and seasonal rainfall pattern.
- The zone silently sets envelope U factors, glazing solar heat gain coefficients, insulation, economizer requirements, high limit setpoints and equipment baselines. It touches every discipline.
- California runs a second, incompatible map. Title 24 Part 6 uses sixteen California climate zones defined by the California Energy Commission, assigned by building location.
- The numbers do not correspond. California climate zone 3 is the Bay Area. ASHRAE climate zone 3 runs from Georgia to southern California. Reading the wrong one produces a design that fails plan check.
- Los Angeles is ASHRAE 3B but spans California climate zones 6, 8 and 9. Fremont and Camarillo are both ASHRAE 3C but sit in California climate zones 3 and 6.
- What Are ASHRAE Climate Zones?
- How a Climate Zone Is Derived
- The Zone Numbers and the Moisture Letters
- Which Codes and Projects Read Which Map
- What the Zone Asks of Each Building System
- California’s Sixteen Climate Zones
- Healthcare, Laboratories, Data Centres and Aviation
- What Changes as You Move Between Zones
- Verifying the Zone and Getting It Into the Model
- Where Budlong Works
- Related Reading
- Frequently Asked Questions
A climate zone is a single character on a drawing set, and it determines how much insulation goes in the roof, how dark the glass can be, whether an economizer is mandatory, how big the cooling plant needs to be and whether the building needs a dedicated way to remove moisture from the air. Nobody argues about it in a design meeting, which is exactly why it is expensive to get wrong. A wrong zone assumption is not caught by clash detection or peer review. It is caught at plan check.
California makes this harder than any other state, because it runs two parallel climate zone systems that use overlapping numbers to mean completely different things. This article explains both, and sets out exactly where they collide. It is written for owners, developers and project managers who need to interrogate an engineer’s assumptions rather than reproduce them. Budlong delivers mechanical engineering services across the full spread of California climates, from Humboldt fog to the Coachella Valley, and the range of that work is set out on our expertise overview.
1. What Are ASHRAE Climate Zones?
ASHRAE climate zones reduce the climate at any location on earth to two characters: a number from 0 to 8 for how hot or cold it is, and a letter of A, B or C for how wet, dry or maritime it is. Los Angeles is 3B. Seattle is 4C. Chicago is 5A. Fairbanks is 8.
The system exists because energy codes have to be written once and applied everywhere. A roof insulation requirement that makes sense in Minneapolis is wasted money in Miami. Rather than write fifty state codes, the profession agreed one map and tabulated every requirement against it.
The definitions live in ANSI/ASHRAE Standard 169, Climatic Data for Building Design Standards, maintained by ASHRAE. It is not a design standard in its own right but the reference other standards point at. ASHRAE Standard 90.1 uses it to index every prescriptive requirement, the IECC reproduces the same map, and Standards 62.1 and 55 both draw on it. Once a location has a zone, dozens of downstream decisions are already partly made.
Write both climate zones on the basis of design and get them acknowledged in writing at the end of schematic design. When a zone error surfaces later, the argument is never about what the code says. It is about who assumed what, and when.
2. How a Climate Zone Is Derived
The zone is not a matter of judgement. It is calculated from measured weather data, and the arithmetic is public.
The number comes from degree days
Two quantities do the work. Cooling degree days base 10 degrees Celsius, written CDD10, accumulate the amount by which daily mean temperature exceeds 10 degrees Celsius across the year. Heating degree days base 18 degrees Celsius, written HDD18, accumulate the amount by which it falls below 18 degrees Celsius. The thresholds are set out in the standard and reproduced in the IECC.
| Zone | Name | Thermal Criteria (SI) | Representative US Locations |
|---|---|---|---|
| 0 | Extremely Hot | 6000 < CDD10 | None in the United States |
| 1 | Very Hot | 5000 < CDD10 ≤ 6000 | Miami, Honolulu, Key West |
| 2 | Hot | 3500 < CDD10 ≤ 5000 | Houston and New Orleans (2A), Phoenix and Tucson (2B) |
| 3 | Warm | 2500 < CDD10 ≤ 3500 and HDD18 ≤ 3000 | Atlanta and Memphis (3A), Los Angeles and Las Vegas (3B), San Francisco and coastal California (3C) |
| 4 | Mixed | CDD10 ≤ 2500 and HDD18 ≤ 3000 | Baltimore and Washington DC (4A), Albuquerque (4B), Seattle and Portland (4C) |
| 5 | Cool | CDD10 < 3500 and 3000 < HDD18 ≤ 4000 | Chicago and Boston (5A), Denver and Boulder (5B) |
| 6 | Cold | 4000 < HDD18 ≤ 5000 | Minneapolis and Buffalo (6A), Helena and Great Falls (6B) |
| 7 | Very Cold | 5000 < HDD18 ≤ 7000 | Duluth, International Falls, Anchorage |
| 8 | Subarctic and Arctic | 7000 < HDD18 | Fairbanks, Nome, Utqiagvik |
Zone 3 carries an extra rule. A location that would otherwise land in zone 3 or 4 on cooling degree days alone is pushed into the marine subzone if it meets the marine test below and its heating degree days are low enough. That is how San Francisco ends up 3C.
The letter comes from precipitation
The moisture designation is decided in strict order. Marine first, dry second, moist is what is left.
Marine, the C designation
A location is marine only if it meets all four of the following: mean temperature of the coldest month between minus 3 and 18 degrees Celsius; warmest month mean below 22 degrees Celsius; at least four months with mean temperatures above 10 degrees Celsius; and a dry summer, meaning the wettest month in the cold season carries at least three times the precipitation of the driest month in the rest of the year. In the northern hemisphere the cold season is October through March. That fourth criterion is why the entire California coast is marine and the Atlantic coast at the same latitude is not.
Dry, the B designation
A non marine location is dry when annual precipitation falls below a threshold that scales with mean annual temperature. In SI terms the base form is annual precipitation in centimetres being less than twice the sum of mean annual temperature in degrees Celsius plus seven, adjusted for whether most rain arrives in the high sun or low sun period. Warmer places need more rain to escape the dry classification, which is why Atlanta and Los Angeles share a thermal band but carry different letters.
Moist, the A designation
Everything that is neither marine nor dry is moist. Standard 169 also identifies a warm humid subset, defined by hours above given wet bulb thresholds during the warmest six consecutive months, which triggers the more demanding dehumidification and vapour control provisions in the humid southeast.
📖 Also Read: HVAC Load Calculation Methods for how climate data feeds the peak load and the annual energy picture differently.
3. The Zone Numbers and the Moisture Letters
Three points are worth knowing before you argue about a zone assignment.
Not every zone has every letter
Zones 0 through 6 subdivide by moisture. Zones 7 and 8 do not, because at those heating loads the moisture regime stops changing the design meaningfully. Zone 5C covers a small strip of the Pacific Northwest, and zone 0 was added for extremely hot international climates, with no United States location falling into it.
The letter matters as much as the number
Owners fixate on the number and ignore the letter, which is backwards for anything with a significant ventilation load. The gap between 3A Atlanta and 3B Los Angeles is not a temperature difference of consequence. It is the difference between a building that needs an explicit strategy for removing moisture from outdoor air and one that does not. Latent load drives coil selection, reheat energy, condensate management and mould risk.
The map is assigned by county, not by ZIP code
In the United States, the IECC and ASHRAE Standard 90.1 assign climate zones at county level. Los Angeles County is 3B. Ventura County is 3C. Alameda County is 3C. Imperial County is 2B. A whole county carries one designation regardless of elevation or distance from the ocean, which is a coarse instrument in a state where a forty minute drive changes the weather completely. That coarseness is what California set out to solve with its own map.
A county wide zone hides real variation. Alameda County is uniformly ASHRAE 3C. Under the California map, Fremont sits in California climate zone 3 while Pleasanton, eighteen miles inland in the same county, sits in California climate zone 12. Those zones do not carry the same prescriptive envelope or equipment requirements. The national map cannot see the difference. The state map can.
4. Which Codes and Projects Read Which Map
The practical question is not which map is better. It is which map the document in front of you is written against.
ASHRAE Standard 90.1 and the IECC
Both index every prescriptive requirement to the ASHRAE zone. Outside California, most state energy codes are the IECC with amendments or ASHRAE 90.1 adopted directly, so the ASHRAE zone is the operative one. The U.S. Department of Energy Building Energy Codes Program tracks which edition each state has adopted, which matters because zone boundaries and requirements have both shifted between editions.
Title 24 Part 6
California’s energy code does not use the ASHRAE map for compliance at all. Every prescriptive envelope table, equipment requirement and compliance calculation in Part 6 is indexed to the California zone number. Submitting a Part 6 package that references an ASHRAE zone is a plan check correction, not a debate.
LEED and voluntary programmes
LEED energy performance is demonstrated against an ASHRAE Standard 90.1 Appendix G baseline, and that baseline is configured by ASHRAE climate zone. A California project certifying through the U.S. Green Building Council therefore needs both designations: the California zone to pass Part 6, and the ASHRAE zone to build the Appendix G baseline.
Other places the ASHRAE zone shows up
Federal projects, national corporate design standards, ASHRAE 62.1 and 55, cool roof and vapour retarder guidance, and most manufacturer selection tools all read the ASHRAE zone. The zone follows the standard, not the state line.
📖 Also Read: LEED MEP Services Explained for how a single energy model can serve both the Title 24 submission and the LEED energy credit.
5. What the Zone Asks of Each Building System
The zone is not an HVAC input. It reaches into every discipline, before anyone has drawn a duct.
Envelope and roofing
Opaque assembly insulation, fenestration U factor and solar heat gain coefficient, window to wall ratio limits and air leakage requirements are all tabulated by zone. Cool roof provisions show how sharply the zone bites. Under the California Energy Code, prescriptive cool roof requirements for new multifamily low slope roofs apply in California climate zones 2, 4 and 6 through 15, and not in the cool coastal and mountain zones. The same roof assembly is mandatory in one city and irrelevant fifty miles away.
Mechanical equipment
The zone sets the ratio of heating hours to cooling hours, and therefore whether the plant is sized around a cooling peak, a heating peak or both. It sets minimum equipment efficiencies in the prescriptive tables. In cold zones it drives heat pump balance point selection and supplementary heat. In hot dry zones it opens the door to evaporative strategies that are useless in a humid climate. Accurate load calculations depend on the correct weather file, and a design built on the wrong file is wrong from the first sizing pass.
Economizers and control setpoints
This is where the zone shows up most visibly in a control sequence. ASHRAE Standard 90.1 requires an air or water economizer on cooling systems above a defined capacity threshold in every climate zone except the very hottest, then sets the high limit shutoff strategy by zone. Fixed dry bulb high limit setpoints are lower in humid zones and higher in dry and marine zones, and differential dry bulb control is not permitted in the warmer humid zones because bringing in warm moist air to save sensible cooling is a false economy. The same rooftop unit therefore gets a different control sequence in Atlanta and in Los Angeles, and getting that wrong is a common acceptance testing failure.
Humidity control
In moist zones, latent load is a separate design problem. It drives coil face velocity and rows, reheat strategy, dedicated outdoor air systems and vapour retarder placement. In dry and marine zones it largely disappears, which is why a design imported from a national standard often carries dehumidification hardware California buildings do not need.
Service water heating, lighting and solar
Water heating baselines and pipe insulation requirements are zone dependent. So are exterior lighting power allowances, which vary by lighting zone rather than climate zone and are frequently confused with it. Solar sizing follows the solar resource at the location, so photovoltaic design and envelope design must share the same climate assumptions, and architectural lighting design is affected because daylight availability and solar heat gain limits trade against each other.
Not Sure Which Climate Zone Your Site Actually Falls In?
Boundary sites, multi zone ZIP codes and projects carrying both a Title 24 and a LEED scope need the answer settled before the envelope is priced. A conversation at concept stage costs a fraction of a redesign.
6. California’s Sixteen Climate Zones
California operates its own climate zone map, defined by the California Energy Commission, because the state has a range of climates a county level national map cannot resolve. The Commission’s own framing is that California’s climatic diversity is not seen in other states and that the Energy Code accounts for it through sixteen zones.
How the California zones are assigned
The zone is determined by the physical location of the building. The Commission publishes an address based lookup tool and a ZIP code list, and the ZIP list is what most teams use day to day. Where a single ZIP code spans more than one climate region, the local jurisdiction may designate which zone applies. Both tools, plus GIS data for checking a site against the boundary directly, sit on the Commission’s climate zone tool page.
The sixteen zones and where they land
| CA Zone | Character | Example Locations | ASHRAE Zone for the Same Area |
|---|---|---|---|
| 1 | North coast, cool and foggy | Eureka, Arcata | 4C |
| 2 | Inland north coast valleys | Santa Rosa, Napa, San Rafael | 3C |
| 3 | Central coast and inner Bay | San Francisco, Oakland, Berkeley, Fremont, Hayward, San Mateo, Santa Cruz, Monterey | 3C |
| 4 | South Bay and inland peninsula | San Jose, Palo Alto, Milpitas | 3C |
| 5 | Central coast | Santa Maria, San Luis Obispo | 3C |
| 6 | South coast, marine influenced | Torrance, Long Beach, Santa Monica, Venice, Oxnard, Camarillo, Santa Barbara, Ventura | 3B in LA County, 3C in Ventura and Santa Barbara |
| 7 | San Diego coastal | San Diego | 3B |
| 8 | Inland Orange County and south LA basin | Fullerton, Anaheim, Irvine, South Los Angeles | 3B |
| 9 | LA basin and foothills | Downtown Los Angeles, Hollywood, Westwood, Glendale, Burbank, Pasadena, Thousand Oaks, Simi Valley | 3B in LA County, 3C in Ventura County |
| 10 | Inland Empire | Riverside, San Bernardino | 3B |
| 11 | Northern Central Valley | Red Bluff, Redding, Chico | 3B |
| 12 | Central Valley and inland Bay | Sacramento, Stockton, Modesto, Livermore, Pleasanton, Concord | 3B in the Valley, 3C in Alameda and Contra Costa |
| 13 | Southern Central Valley | Fresno, Bakersfield | 3B |
| 14 | High desert | Palmdale, Barstow | 3B |
| 15 | Low desert | Palm Springs, El Centro | 3B in Riverside County, 2B in Imperial County |
| 16 | Mountains and far northeast | Truckee, Mammoth Lakes, Alturas | 5B or 6B depending on county |
Read the right hand column carefully. It is not a translation table. The relationship between the two systems is many to many and it is not stable within a single county. Alameda County is entirely ASHRAE 3C, yet contains California climate zones 3 and 12. Ventura County is entirely ASHRAE 3C, yet contains California climate zones 6 and 9. Los Angeles County is entirely ASHRAE 3B, yet contains California climate zones 6, 8, 9, 10, 14 and 16.
Three projects, three answers
A project in Los Angeles
A developer with three sites in Los Angeles does not have one climate zone. A building in Venice or near the airport is California climate zone 6, marine influenced, with mild cooling loads and a heavy economizer share. One in South Los Angeles is zone 8. One downtown, in Hollywood or in Glendale is zone 9, hotter in summer and with a materially different prescriptive envelope. All three are ASHRAE 3B for LEED purposes. Carrying an envelope specification between sites is the most common avoidable plan check correction on multi site portfolios.
A project in Fremont
Fremont is California climate zone 3, the inner Bay marine zone. Cooling loads are modest, heating is meaningful, economizer hours are abundant, and the value of high performance glazing is dominated by winter heat loss and comfort rather than peak solar gain. It is also ASHRAE 3C, so a LEED baseline follows the marine column in ASHRAE 90.1.
A project in Camarillo
Camarillo is California climate zone 6, the south coast marine zone, and ASHRAE 3C. It shares its ASHRAE designation with Fremont and shares nothing else. Summer cooling is more significant, the heating season shorter, and the prescriptive envelope and cool roof provisions in zone 6 are not those in zone 3. Thousand Oaks and Simi Valley, twenty miles away in the same county and the same ASHRAE zone, are California climate zone 9.
The two systems share numbers and share nothing else. California climate zone 3 is San Francisco and the inner Bay. ASHRAE climate zone 3 is a warm band running from Georgia through Texas to southern California. When a consultant says a project is in climate zone 3, the only safe response is to ask which map they are reading.
📖 Also Read: Title 24 Compliance Guide for how the climate zone flows through the Part 6 documentation package.
7. Healthcare, Laboratories, Data Centres and Aviation
Four building types deserve separate treatment, because in each the zone interacts with a constraint that overrides ordinary energy logic.
Healthcare
Hospitals and outpatient facilities carry ventilation rates, filtration levels and pressure relationships that exist for infection control rather than energy efficiency. The climate zone relaxes none of them. What it changes is the cost of meeting them, because the energy penalty of once through air scales with the difference between outdoor and supply conditions. Our healthcare facilities work routinely involves reconciling a national health system standard written against ASHRAE zones with a project that has to comply in a California zone.
Laboratories
Laboratory exhaust volumes are set by containment, not by climate. The zone determines what it costs to condition that air and whether energy recovery is worth the pressure drop and the cross contamination risk. In marine California zones sensible only recovery can be marginal. In the Central Valley and desert zones the economics flip. The 2025 California Energy Code removed the long standing laboratory exemption, which makes this a compliance matter rather than an optimisation exercise for our laboratory and industrial clients.
Data centres and mission critical
Nowhere does the climate zone change the design more. Free cooling hours are the entire argument for air side and water side economizers, and the hours available are a direct function of the zone’s dry bulb and wet bulb profile. A facility in California climate zone 3 or 4 can run economizer cooling for a very large fraction of the year. The same design in zone 13 or 15 cannot, and the plant, electrical service and water budget all change accordingly. See our mission critical capability, and chilled water system design for how the plant side resolves.
Education and aviation
Schools carry a heavy occupancy driven ventilation load in a building largely unoccupied through the hottest months, which makes the zone’s shoulder season behaviour more important than its peak. Terminals combine enormous glazed areas with high outdoor air rates and long operating hours, so both the number and the letter matter. Our education work spans districts in six California climate zones, and no two carry the same prescriptive package.
8. What Changes as You Move Between Zones
The table below sets out how the design shifts as you move across the map. The requirement values themselves are tabulated in the applicable code and change between editions, so what follows describes direction and emphasis rather than substituting for the code tables.
| Design Element | Hot Dry (ASHRAE 2B/3B, CA 13 to 15) | Marine (ASHRAE 3C/4C, CA 1 to 6) | Mixed to Cold (ASHRAE 4A to 6A, CA 12 and 16) |
|---|---|---|---|
| Roof and wall insulation | Moderate. Driven by solar gain and attic temperature more than by conduction. | Lowest of the three. Small annual delta between indoor and outdoor conditions. | Highest. Conduction loss dominates and thermal bridging becomes a real line item. |
| Glazing solar heat gain coefficient | Tightest. Low SHGC is the primary lever on cooling peak. | Moderate. Balanced against daylight and winter solar benefit. | Loosest. Winter solar gain is useful and the limit relaxes. |
| Glazing U factor | Least demanding. Conduction is a small share of the load. | Moderate. Comfort and condensation at the glass drive the decision. | Most demanding. Triple glazing and warm edge spacers become defensible. |
| Cool roof | Required and highly effective. | Often not required in the mildest zones and of limited benefit. | Required in some zones, and the winter heating penalty enters the calculation. |
| Air side economizer | Required above the capacity threshold. High limit setpoints sit at the upper end. | Required and extremely valuable. Free cooling for a large share of the year. | Required. Valuable but heating season control and freeze protection become the design focus. |
| Humidity control | Rarely a separate design problem. Latent load is small. | Generally not a separate design problem, though coastal fog affects condensation risk. | Depends on the moisture letter. In A zones dedicated dehumidification is a distinct system. |
| Cooling equipment | Largest plant. Evaporative and indirect evaporative strategies viable. Water budget matters. | Smallest plant. Part load efficiency and turndown matter more than peak capacity. | Moderate plant, heavily part loaded. Oversizing penalties are severe. |
| Heating equipment | Minimal. Often a reheat and morning warm up function only. | Modest but continuous. Heat pumps operate near ideal conditions all year. | Significant. Heat pump balance point, defrost and supplementary heat all need resolving. |
| Ventilation strategy | Energy recovery valuable. Sensible recovery sufficient. | Recovery often marginal. Economizer does most of the work. | Recovery valuable. Total energy recovery preferred where the moisture letter is A. |
| Preferred system type | Packaged rooftop or central chilled water with high efficiency compressors and evaporative pre cooling. | Heat pump based systems, variable refrigerant flow with dedicated outdoor air, four pipe fan coils with air to water heat pumps. | Variable air volume with hydronic heating, or heat pumps with supplementary capacity sized to the balance point. |
Two things fall out of this table. The marine column is where California mostly lives, and it is where variable refrigerant flow and heat pump plant perform best, which is one reason the California Energy Code has moved its baselines in that direction. And the difference between the columns is not marginal: it changes plant room area, electrical service size, roof structural load and the shape of the facade. Deciding system type before the zone is confirmed is a way of committing to a design that may not be legal.
If a site sits within about two miles of a California climate zone boundary, get the assignment confirmed by the authority having jurisdiction in writing before you price the envelope. Boundary sites are where the ZIP code list and the mapped boundary are most likely to disagree, and where the jurisdiction’s designation is the only answer that counts.
📖 Also Read: How to Select an HVAC System for Commercial Buildings for the selection logic that sits on top of the climate zone inputs.
9. Verifying the Zone and Getting It Into the Model
The zone feeds almost every downstream calculation, so it deserves a verification step of its own rather than being inherited from a previous project.
Confirm both designations at the start
Record both zones in the basis of design, with the source for each. The California zone comes from the Commission’s lookup tool or ZIP list, checked against the mapped boundary for anything near an edge. The ASHRAE zone comes from the county table in the applicable edition of ASHRAE 90.1 or the IECC. If the project carries a national design standard, note which system that standard is written against.
Match the weather file to the zone
An energy model is only as good as its weather file. Part 6 compliance software uses the Commission’s reference weather data for the California zone, and a LEED Appendix G model uses a file appropriate to the ASHRAE zone. Those are different files for the same building, and running one model with the wrong file produces results that look plausible and are wrong. Avoiding it is a basic control in energy modelling for MEP design.
Check the zone assumption against the equipment schedule
Read the equipment schedule against the zone and ask whether the selections make sense. Dehumidification hardware in a marine zone, an oversized cooling plant in California climate zone 3, no economizer on a large air handler, or a high limit setpoint copied from a humid climate all signal a design imported rather than engineered.
Carry the zone into commissioning
Zone dependent control sequences need verifying in the field, not just drawing. Economizer high limit setpoints, changeover logic, heat pump balance point and supplementary heat lockout are all commonly wrong at start up. The building commissioning process is where those assumptions are tested against reality, and Title 24 acceptance testing covers a subset of the same ground. Running them together through one commissioning services scope avoids paying twice for the same site visits, and duct leakage testing sits alongside the control tests.
Key Takeaways
- ASHRAE climate zones combine a thermal number from 0 to 8 with a moisture letter of A, B or C, and the definitions sit in ANSI/ASHRAE Standard 169.
- The number is calculated from cooling and heating degree days. The letter is calculated from precipitation, mean temperature and seasonal rainfall pattern.
- Marine is tested first, dry second, moist is what is left. The marine test requires a genuinely dry summer, which is why California’s coast is marine and the Atlantic coast at the same latitude is not.
- The ASHRAE map is assigned by county, which hides real variation inside large counties.
- California operates a separate map of sixteen zones assigned by building location, and Title 24 Part 6 compliance uses only that map.
- The numbers do not correspond between the two systems. Always ask which map a stated climate zone refers to.
- Los Angeles is ASHRAE 3B but spans California climate zones 6, 8 and 9. Fremont and Camarillo are both ASHRAE 3C but sit in California climate zones 3 and 6.
- The zone sets envelope requirements, cool roof triggers, economizer thresholds, humidity strategy and equipment baselines, so it belongs in the basis of design before schematic design closes.
10. Where Budlong Works
California packs sixteen climate zones into one state, and a firm that works in only one of them develops habits that do not travel. Budlong maintains teams across California so that envelope, plant and control assumptions match the climate the building actually sits in, and so local plan check practice is understood before a submission is made. More on our company story page.
Sectors where the climate zone drives the outcome
Healthcare pays for the zone through once through air. Mission critical facilities live or die on free cooling hours. Education carries a ventilation load in a building empty through the hottest months. Multifamily absorbs the envelope and hot water requirements most directly.
11. Related Reading
12. Frequently Asked Questions
What are ASHRAE climate zones in simple terms?
ASHRAE climate zones are a classification system that sorts every location on earth into a numbered thermal band from 0 to 8, then adds a moisture letter of A for moist, B for dry or C for marine. The number comes from measured cooling degree days and heating degree days at that location. The letter comes from annual precipitation, mean temperature and the seasonal pattern of rainfall. The definitions live in ANSI/ASHRAE Standard 169, and the same map is reproduced in the International Energy Conservation Code and in ASHRAE Standard 90.1.
How many ASHRAE climate zones are there?
There are nine thermal zones numbered 0 through 8. Zones 0 through 6 subdivide by moisture into A, B and C variants, which is why you see labels such as 3B and 4C. Zones 7 and 8 carry no moisture letter. Zone 0 was added to cover extremely hot international climates and does not occur in the continental United States. In practice the contiguous United States uses roughly seventeen zone and subzone combinations, from 1A in south Florida to 8 in interior Alaska.
What is the difference between ASHRAE climate zones and California climate zones?
They are two different maps built for two different purposes. ASHRAE climate zones are a national and international system of nine numbered thermal bands with moisture letters, used by ASHRAE Standard 90.1, by the International Energy Conservation Code and by LEED energy modelling. California climate zones are a set of sixteen zones defined by the California Energy Commission specifically for Title 24 Part 6 compliance. The numbers do not correspond. California climate zone 3 is the San Francisco Bay area, while ASHRAE climate zone 3 covers a warm band that runs from Georgia to southern California.
Which climate zone system does Title 24 use?
Title 24 Part 6, the California Energy Code, uses the sixteen California climate zones defined by the California Energy Commission. Every prescriptive envelope table, equipment requirement and compliance calculation in Part 6 is indexed to that zone number. ASHRAE climate zones have no standing in a Part 6 submission. A California project that is also pursuing LEED or that is being compared against ASHRAE Standard 90.1 will need both zone designations, because the two exercises read different maps.
How do I find my California climate zone?
The California Energy Commission publishes a ZIP code list and an address based lookup tool, and the zone is formally determined by the physical location of the building rather than by its mailing address. Where a single ZIP code spans more than one climate region, the local jurisdiction may designate which zone applies. For any site near a zone boundary, confirm the assignment with the authority having jurisdiction in writing before the envelope is priced, because a one zone difference can change insulation, glazing and equipment requirements.
What climate zone is Los Angeles in?
It depends which map you are reading and which part of the city you mean. For ASHRAE and IECC purposes, Los Angeles County is climate zone 3B, warm and dry. For Title 24 purposes, the city spans three California climate zones. Coastal ZIP codes around Venice, Santa Monica and the airport fall in California climate zone 6. Much of South and Central Los Angeles falls in zone 8. Downtown, Hollywood, Westwood and the foothill communities including Glendale, Burbank and Pasadena fall in zone 9. Two buildings twelve miles apart can carry different prescriptive requirements.
Do Fremont and Camarillo have the same climate zone?
Under ASHRAE they do, and under Title 24 they do not. Alameda County and Ventura County are both ASHRAE climate zone 3C, the marine subzone. Under the California map, Fremont sits in California climate zone 3 and Camarillo sits in California climate zone 6. The two designs are not interchangeable. This is the clearest illustration of why a project team cannot carry a climate zone assumption from one California city to another, even when both sit on the coast.
How does the climate zone change my HVAC equipment selection?
The zone sets the balance between heating and cooling hours, the value of outside air as free cooling, and whether latent load has to be managed separately from sensible load. In a hot dry zone the design leans on cooling capacity, evaporative strategies and high limit economizer setpoints that allow outside air well above seventy degrees. In a marine zone the cooling plant shrinks, economizer hours dominate the annual energy picture, and heating and ventilation control matter more than peak tonnage. In a moist zone dehumidification becomes a separate design problem that a sensible only system will not solve.
Does a California project need an ASHRAE climate zone at all?
Only if something other than Title 24 Part 6 is in scope. A project pursuing LEED certification runs an energy model against an ASHRAE Standard 90.1 Appendix G baseline, and that baseline is set by ASHRAE climate zone. Federal work, corporate design standards written for a national portfolio, manufacturer selection software and ASHRAE Standard 62.1 and 55 references all use the ASHRAE map. The practical answer is that most commercial California projects of any size end up carrying both designations, and the two should be recorded on the basis of design at the start of the job.
