Designs That Work

Our Hygro-Thermal Regions

Climate-specific design and construction of high performance homes is a cornerstone of all BSC work. We recently modified both our criteria and our North American map for the hygro-thermal regions. The changes are not drastic but they are important because they make our criteria and map align with the International Energy Conservation Code (IECC) Climate Zones as developed by the Department of Energy. Whenever the building science community and the code community get on (literally) the same page, this is good news for builders of any homes, but particularly those that build climate-tuned, high performance homes.

Here is how the two align:

Don?t forget that it is always the conditions that you actually experience in your area that determine the appropriate building design and construction details. For more detailed information on the criteria that make up the BSC regions and the DOE zones, see the map below.

Subarctic/Arctic¹

A subarctic and arctic climate is defined as a region with approximately 12,600 heating degree days (65?F basis)² or greater. On the IECC/IRC climate map, it is zone 8.

Very Cold

A very cold climate is defined as a region with approximately 9,000 heating degree days (65?F basis)³ or greater and less than approximately 12,600 heating degree days (65?F basis).

On the IECC/IRC climate map, it is zone 7 (notice the zone 7 areas in WY and CO).

[Old: Severe Cold - A severe cold climate is defined as a region with approximately 8,000 heating degree days or greater.]

Cold

A cold climate is defined as a region with approximately 5,400 heating degree days (65?F basis)⁴ or greater and less than approximately 9,000 heating degree days (65?F basis)⁵ .

On the IECC/IRC climate map, it is zones 5 and 6.

[Old: Cold - A cold climate is defined as a region with approximately 4,500 heating degree days or greater and less than approximately 8,000 heating degree days.]

Mixed-Humid

A mixed-humid climate is defined as a region that receives more than 20 inches (50 cm) of annual precipitation, has approximately 5,400 heating degree days (65?F basis)⁶ or less, and where the average monthly outdoor temperature drops below 45?F (7?C) during the winter months.

On the IECC/IRC climate map, it is zone 3A (the portion above the white ?warm-humid? line) and zone 4A (the area east of the bold line running through the middle of TX).

[Old: Mixed-Humid - A mixed-humid climate is defined as a region that receives more than 20 inches of annual precipitation, has approximately 4,500 heating degree days or less and where the average monthly outdoor temperature drops below 45?F during the winter months.]

Hot-Humid

A hot-humid climate is defined as a region that receives more than 20 inches (50 cm) of annual precipitation and where one or both of the following occur:

• ? a 67?F (19.5?C) or higher wet bulb temperature for 3,000 or more hours during the warmest six consecutive months of the year; or
• ? a 73?F (23?C) or higher wet bulb temperature for 1,500 or more hours during the warmest six consecutive months of the year. ?

? These last two criteria are identical to those used in the ASHRAE definition of warm-humid climates and are very closely aligned with a region where the monthly average outdoor temperature remains above 45?F (7?C) throughout the year.

On the IECC/IRC climate map, it is zones 1A and 2A and the portion of 3A below the white "warm-humid" line.

[Old: Hot-Humid - A hot-humid climate is defined as a region that receives more than 20 inches of annual precipitation and where the monthly average outdoor temperature remains above 45?F throughout the year.]

Hot-Dry and Mixed-Dry

A hot-dry climate is defined as a region that receives less than 20 inches (50 cm) of annual precipitation and where the monthly average outdoor temperature remains above 45?F (7?C) throughout the year.

A mixed-dry climate is defined as a region that receives less than 20 inches (50 cm) of annual precipitation, has approximately 5,400 heating degree days (65? F basis) or less, and where the average monthly outdoor temperature drops below 45?F (7?C) during the winter months*.

On the IECC/IRC climate map, this is zones 2B, 3B, and 4B.

[Old: Hot-Dry/Mixed-Dry - A hot-dry climate is defined as a region that receives less than 20 inches of annual precipitation and where the monthly average outdoor temperature remains above 45?F throughout the year.  A mixed-dry climate is defined as a region that receives less than 20 inches of annual precipitation, has approximately 4,500 heating degree days or less and where the monthly average outdoor temperature drops below 45?F during the winter months.]

Marine⁷

A marine climate meets all of the following criteria:

• A mean temperature of coldest month between 27?F (?3?C) and 65?F (18?C)
• A warmest month mean of less than 72?F (22?C)
• At least four months with mean temperatures over 50?F (10?C)
• A dry season in summer. The month with the heaviest precipitation in the cold season has at least three times as much precipitation as the month with the least precipitation in the rest of the year. The cold season is October through March in the Northern Hemisphere and April through September in the Southern Hemisphere.

On the IECC/IRC climate map, this is zones 4C and 3C.

¹ BSC has not yet developed climate-specific building science guidance for the new Subarctic/Arctic region. It will be developed and made available during the first half of 2004.
² Celsius: 7,000 heating degree days (18?C basis)
³ Celsius: 5,000 heating degree days (18?C basis)
⁴ Celsius: 3,000 heating degree days (18?C basis)
⁵ Celsius: 5,000 heating degree days (18?C basis)
⁶ Celsius: 3,000 heating degree days (18?C basis)
⁷ BSC has not yet developed climate-specific building science guidance for the new Marine region. In the interim, guidance provided in the HTW Mixed-Humid climate can be applied.

* This coldest-month of the year average temperature is critical in the design of wall and roof assemblies in terms of managing for the temperature of the first condensing surface. On the maps and within climate zones, comprehensive reconciliation of this criterion with degree-day criteria has not yet been accomplished. Check the appropriate EEBA Builder's Guide and climate information for your specific location for final determination of how this criterion can affect your design and construction.