PPP Notes - Climates and Foundations

CLIMATIC FOUNDATION ISSUES

 (These notes are compiled from AGS)

DESIGNING FOR COLD CLIMATES

 

• Cold and unheated climate conditions occur over the northern half of the United States and in mountainous regions.
• These conditions can be generally quantified as where the frost depth is 12 in. or greater.
• Designing foundations for these conditions is treated in a more typical manner, such as: providing a foundation below the frost depth, including a basement, and providing insulation on the exterior to reduce the chances of cold ground temperatures reaching the structure.

 

FROST ISSUES

 

• Detrimental frost action in soils is obviously limited to those areas of the United States where subfreezing temperatures occur on a regular basis and for extended periods of time.
• “Frost action” is the lateral or vertical movement of structures supported on or in the soil.
• Frozen soil is, in itself, not necessarily detrimental to the supported structures. 
• It becomes detrimental when, through the growth of ice lenses, the soil and whatever is resting on the soil above the ice lenses, heaves upward.
• This causes foundations and the structures supported by the foundations to distort and suffer distress.
• Other common problems are the heaving of sidewalks, pavements, steps, retaining walls, fence poles, and architectural features.
• The depth of frost penetration is directly related to the intensity and duration of the freezing conditions, a measure that is termed the freezing degree-day index.
• In milder climates in the United States, the local building codes might stipulate a frost protection depth for foundations of 12 in.
• In the northern portions of the United States, the frost protection depth might be 42 to 60 in. as required by local building codes.
• These guidelines are usually conservative, but there are situations where deeper frost protection depths are warranted.
• If the emergency entrance to a hospital is on the north side of the hospital, where the sun never warms the pavement adjacent to the building, and the pavement is kept 100 percent snow-free for safety reasons, then the frost penetration can easily.
• Carefully evaluate exposure conditions to see if a special condition exists.
• Grass and snow are very effective insulators for the ground below.
• Avoid the use of sloping exterior faces on grade beams or foundations that give the freezing forces something to push against when the frost heave situation develops.

 

DESIGNING FOR HOT, ARID CLIMATES

 

• Though classified as arid and overheated, severe desert climates in the United States typically have four distinct periods for determining comfort strategies.
• The hot dry season, occurring in late spring, early summer, and early fall, has dry, clear atmospheres that provide high insulation levels, high daytime air temperatures, very high sol-air temperatures, and large thermal radiation at night, producing a 30 to 40 degree F daily range.  Night temperatures may fall below the comfort limits and are useful for cooling.  Low humidity allows effective evaporative cooling.
• The hot humid season occurs in July and August.  In addition to high insulation, it is characterized by high dew point temperatures (above 55 degrees F), reducing the usefulness of evaporative cooling for comfort conditioning.  Cloudiness and haze prevent nighttime thermal reradiation, resulting in only a 20 degrees F or less daily range.  Lowest nighttime temperatures are frequently higher than the comfort limits.  Thus, refrigeration or dehumidification may be needed to meet comfort standards.
• The winter season typically has clear skies, cold nights, very low dew point temperatures, a daily range of nearly 40 degrees F, and the opportunity for passively meeting all heating requirements from isolation.
• The transitional or thermal sailing season occurs before and after the winter season and requires no intervention by environmental control systems.  The passive features of the building can extend this season.  Other desert climates have similar seasons but in different proportions and at cooler scales.

 

CONSTRUCTION DETAILS

 

• Capitalize on conditions climatic conditions by incorporating construction practices that respond in beneficial ways to the environment, including:
• Insulate coolant and refrigerant pipes from remote evaporative towers and condensers for their entire length.
• In hot locations, use roof construction similar to the cold climate roof details.
• Do not use exposed wood (especially in small cross sections) and many plastics, as they deteriorate from excessive heat and high ultraviolet exposure.
• Although vapor retarders may not be critical to control condensation, implement them as a building wrap or wind shield, both to control dust penetration and to avoid convective leaks from high temperature differentials.
• Avoid thermal bridges such as extensive cantilevered slabs.
• Radiant barriers and details appropriate to humid overheated climates are at least as effective as vapor retarders, but avoid holes in assembly where convection would leak their thermal advantage.
• Ventilate building skin (attic or roof, walls) to relieve sol-air heat transfer.

 

DESIGNING FOR HUMID CLIMATES

 

• Humid, overheated conditions are most severe along the Gulf Coast, but occur across the entire southeastern United States.
• Atmospheric moisture limits radiation exchange, resulting in daily temperature ranges less than 20 degrees F. 
• High insulation gives first priority to shading.
• Much of the overheated period is only a few degrees above comfort limits, so air movement can cool the body.
• Ground temperatures are generally too high for the Earth to be useful as a heat sink, although slab-on-grade floor mass is useful.
• The strategies are to resist solar and conductive heat gains and to take best advantage of ventilation.