Hot and Humid

A century ago, people lived in south Mississippi, even in the summertime.  Houses had fireplaces for heat, but folks relied on fans, both electric and hand-powered, for cooling.  There were porches with rocking chairs and swings to make your own breeze.  People flocked to the beach for the summer to take advantage of the afternoon onshore breezes, at the time of day when the heat was most oppressive.

Buildings were constructed with mass and shaded openings to limit direct heat gain, and provide natural ventilation.  In the 1920’s, New Orleans buildings provided comfort about 63% of the hours in a year, and the condition that created the most discomfort was the humidity. 

Not much has changed.  About 13% of the time it’s not really hot enough to use air conditioning, but it is too humid for comfort.  In our slab-on-grade house, these are the days we leave the windows open until the concrete floors get clammy.

This summer, the U.S. has been plagued by unseasonal hot weather across the central area of the country.  More than a dozen cities from Tallahassee to Minneapolis have seen all-time highs exceeding any temperature on record.  The heat index, a combination of air temperature and humidity, reached well into three digits in unanticipated places. Adding to the difficulties, heat waves spawn thunderstorms and high winds, making tornadoes a likely prospect.

The heat attacked places that are usually not affected by humidity: Minneapolis and Cleveland (average summer afternoon humidity = 58%), Dallas (average summer afternoon humidity = 52%), in contrast to New Orleans (average summer afternoon humidity = 67%).  Air conditioning works a lot harder to remove moisture from the air than it requires to cool air, driving energy use higher.

These cities are not designed for this kind of heat, but the weather trend towards greater extremes has become clear, and modifications may be necessary.  There is no model solution to fit every climate, but design can accommodate local climates even as they change.

In humid climates, shading and ventilation are easiest with proper orientation along the east-west axis, a narrow enclosed area to allow cross-ventilation, and wide overhangs on the south side to keep summer sun from reaching the windows.  High ceilings keep warm air away from the inhabited zone. 

These strategies can be applied in any climate, and the building envelope always matters – good insulation, airtightness for when you do run the heat and cooling, and the availability of daylight throughout.  But how can a building or its components change to match the changing climate of the temperate, mixed climates of most of this country?

Prototypes for adaptability were present even in the 1700’s.  The plantation shutter swung wide on cool days to allow light and heat inside, and closed in summers to  deny the heat but allow the breeze.  Houses on raised platforms could sweep cool air from the shaded undercroft up through walls with air cavities and out the top, and be infilled with insulating panels in wintertime (of course, they sheltered livestock there in the old days.)  Keeping deciduous trees close by helps with shading, and the trees’ ability to filter light changes with the seasons.  

Access to the water helped in the pre-AC days, and many people chose to spend summers on the beaches and waterways, isolated from the contagion and plagues of the cities, with space to breathe.  Building in a hot, humid climate demands more space for natural ventilation, with shade trees and at least spittin’ distance from a neighbor’s house.  We don’t need to recreate the architecture of old to reap the benefits of natural ventilation, but we can capture the strategies of a previous age and renew their utility in this age of diminishing resources.