Mixed-Humid Climate Zone

This map shows the Mixed-Humid Climate which reaches from the coasts Maryland through North Carolina and sprawls to cover most of Kansas and Oklahoma.

The mixed-humid climate presents several challenges for home building.

The mixed-humid climate has moderate conditions much of the time. But weather conditions similar to those found in neighboring climate zones are also frequent. Homes in the mixed-humid climate are faced with a substantial heating season with monthly average outdoor temperatures dropping below 45ºF. And the summers can have soaring humidity.
During the winter heating season, vapor may be driven from the building interior into the walls, roof and floors. But in summer, air conditioning can force vapor flow in the opposite direction, with cold, dry air on the inside, and hot moist air from the outside pushing its way in. Houses in the mixed-humid climate cannot be built as if they are one-way streets for moisture transfer.
The use of full basements and crawlspaces is common in this climate and presents another set of challenges. These design features may bring extra living and storage space, but they also bring their own moisture and temperature management challenges.

Mixed-Humid Climate Best Practices

The mixed-humid climate stretches from New York City to Oklahoma and from the Mid-Atlantic to the Midwest. This climate touches on four other climate zones, hot-humid, hot-dry, mixed-hot and cold. Housing types vary greatly throughout this climate.

In the face of this diversity, no single set of measures will achieve the 30 percent energy savings in space conditioning and water heating that qualifies a home as ENERGY STAR^®-qualified. The principles included in these best practices need to be adjusted for different circumstances. A building scientist, such as a home energy rating professional, can help homeowners determine which combination of best practices is most appropriate.

These best practices are derived from Building America's research on tens of thousands of homes. Building scientists have tried and tested these measures on actual homes in the field. While not every measure will be right for a specific home, achieving high-efficiency performance and a healthy.

Special Considerations for the Mixed-Humid Climate Zone

Site Design

In the mixed-humid climate, planners should do all they can to orient buildings to use the sun for daylighting and solar gain. Site planners have two important tools to help manage solar heat gain: lot orientation and shade trees.

Foundations

Slabs, crawlspaces, and basements are all found in the mixed-humid climate. Building foundations should be designed and constructed to prevent the entry of moisture and soil gases such as radon.

Slabs and Basement Floors

Slabs in this climate should be insulated at the perimeter with one inch of borate-treated foam board insulation or rigid glass fiber insulation.

Walls

Wood Frame Walls

Best practice for frame wall construction involves advanced framing techniques. However, more traditional wall construction techniques can also achieve 30 percent space conditioning energy savings in the hot and humid climate zone. More information on gaining greater efficiency using advanced framing can be found at the Building Science Consulting Web site. If advanced framing is to be used, a detailed plan should be developed showing framing placement.

External walls with 2x4 framing are adequate in the mixed-humid climate. These walls should include the following features:

Exterior sheathing, preferably insulating sheathing with joints taped to provide a water and air barrier. Use insulating sheathing that does not have a film facing.
R-13 friction-fit unfaced fiberglass insulation, Kraft-faced fiberglass insulation, or blown-in cellulose insulation.
Unfaced insulation on frame walls between the garage and the conditioned space, including bonus rooms.
Rim joists that have kraft-faced R-19 friction-fit batt insulation cut to fit.
Foam-sealed or caulked top-plate and exterior wall penetrations.
Sealed gypsum board to control air leakage through the walls, especially in penetrations to garages and porches, and where the walls meet the ceiling.

Structural Moisture Control

Carefully install insulation and air barriers to avoid ice dams. Ice dams may be a problem in colder portions of the mixed-humid climate. When roof snow melt refreezes it can build up ice that blocks water flow. Newly melted snow adds to the ice dam and eventually can cause water to back up under shingles and flashing and can cause safety hazards with suddenly released snow and ice. Snow could be melted by warm air leaking from the house into the attic. The control of outwardly leaking air is the most effective means of stopping ice dams. This can be accomplished by the installation of air barriers at the ceiling. Well insulated roof areas also help to keep the surface of the roof cool to avoid the melt-freeze cycle.

Some snowmelt is unavoidable and in transitional weather, rain may temporarily back up behind ice dams. To minimize leakage, install roof membranes in roof valleys and at eaves.

Insulation

Slab Foundation Insulation

Slabs in the mixed-humid climate may be insulated at the perimeter with borate-treated foam board or rigid glass fiber insulation.

Use only insulation approved for below-grade use. Some code officials may require a gap between exterior insulation and wood foundations elements to provide a termite inspection area. Exterior insulation should be applied from the top of the foundation wall to the bottom of the frost line. Cover the exterior face of the insulation exposed to outside air using material such as flashing, fiber cement board, parging (stucco type material), treated plywood, or membrane material.

A shallow, frost protected slab foundation may be used in areas subject to seasonal ground freezing. With this approach, foundation footings need not be placed below frost depth. However, rigid insulation, approved for below-grade use, must be placed vertically on the exterior of the grade beam, and must be placed to extend away from the foundation horizontally at the base of the grade beam for a distance equivalent to frost depth. Rigid insulation is also needed vertically on the inside of the grade beam, and must extend horizontally under the slab, on top of the gravel capillary break, for two feet. Code officials may require that a structural engineer review and approve specific plans.

Slab perimeters may be insulated on the interior side. This approach requires that rigid insulation be placed between the slab and the foundation wall, and under the slab, as required by local code.

Crawlspace Insulation

Two methods are in use for insulating crawlspaces in the mixed-humid climate. The first, in common use over the last several decades, is to insulate the underside of the building floor, and provide outside air vents in foundation walls. Research has shown that this approach can lead to moisture problems, especially in areas with cold or humid air.

Basement Insulation

Basements are a common foundation system in the mixed-humid climate. Wall insulation in basements is similar to the approaches described for crawlspaces, and basement floors are insulated in ways similar to slabs.

Components and Systems: HVAC

Supplemental Dehumidification

Building America recommends that houses be equipped with a dehumidification system in the mixed-humid climate or designed so that a dehumidifier can be easily added if needed.

In mixed-humid climates, high exterior humidity may occasionally require dehumidification to maintain reasonable interior moisture levels. Moisture may also be controlled with exhaust ventilation in bathrooms and kitchens. Some Building America teams recommend that a dehumidification system serve the insulated basement area.

ENERGY STAR qualifies the energy efficiency of dehumidifiers in terms of liters of water removed per kilowatt-hour of energy consumed. To earn the ENERGY STAR label, dehumidifiers must fall within the range of greater than or equal to 1.20 to 1.50 L/kWh for standard capacity units. The requirement for high-capacity units is greater than or equal to 2.25 L/kWh. Lists of all ENERGY STAR-rated appliances can be found at the ENERGY STAR Web site.