Passive Solar Cooling a Home in a Hot Humid Climate
Part of our 850-Page eBook + 3,000 presentation slides on our ZED E-Book /CD-ROM
Across America, today’s buildings consume 39 percent of our nation’s primary energy and 70 percent of our electricity. This is more energy than any other sector of the U.S. economy, including transportation AND industry. The air conditioning electricity requirement for hot humid states like Florida is about 10 percent more than the electricity consumed in the rest of America. Sadly, Florida unwisely produces about 40% of its electricity from dirty coal and 40% from expensive imported petroleum – giving Florida very unhealthy (deadly) air pollution, and huge volumes of greenhouse gas emissions, which contribute to man-made Global Warming, which provides thermal energy for deadly Florida hurricanes.
Eliminating conventional power-company air-conditioning in hot humid climates is both extremely important, AND a challenging technology education issue for environment-damaging business-as-usual architects, builders, and home buyers. This material is therefore a very important portion of Zero Energy Design®’s goal for Abundant Energy In Harmony With Nature.
Many so-called “energy experts” have incorrectly stated: “Passive solar systems do NOT work in hot humid climates like Florida.” This statement is partially correct IF you constrain your simplistic thinking to conventional single-shell houses, but this ignorance is quite false IF you understand the powerful science documented by Zero Energy Design®.
We hope that the reader will remain open to the practical cost-effective easy innovative implementation that we recommend herein. The ZED “Thermal Buffer Zone architectural design pattern” creates a whole new environment of exciting opportunities for using solar energy and state-of-the-art Zero Energy Design® to eliminate most expensive conventional summer cooling requirements in hot humid climates.
How the ZED Passive Solar Thermal Buffer Zone works in Summer is very different from the way it operates in the winter. Instead of a totally-sealed attic, in the summer we open large peak roof vents to exhaust thousands of cubic feet per minute of the warmest air from the top of the TBZ. 2,000 cfm is the MINIMUM roof exhaust rate on most Thermal Buffer Zone homes in areas with a significant cooling requirement.
The TBZ roof vents can be operated manually from inside the insulated attic space (twice a year – spring and fall), or they can be automated with an intelligent minimum-cost sensor-based “enthalpy” control system (discussed in Chapter Two) for more precise hourly temperature and humidity control on larger homes.
The operating cost for the roof vent motors and control system is low. A small amount of electricity is required to quickly open or close any vent, and then no further power is necessary to hold them in the open or closed position. The power to drive the high-volume upward airflow comes mostly from natural convection – warmer less-dense air rising like a hot air balloon. If there is a breeze outside from any direction, it can enhance the exhaust airflow rate by creating a slight vacuum
Our large 1979 indoor swimming pool home had over 7,000 cfm roof vent capacity, in a climate that peaked at 110 degrees Fahrenheit. It continuously exhausted hundreds of thousands of BTU’s of warm air 24-hours a day in the hottest part of the summer.
If a Thermal Buffer Zone home includes a swimming pool, swim spa, hot tub, koi pond, waterfall, etc. in the TBZ solarium, dehumidification ventilation becomes a design issue that must be carefully dealt with. Heated pools MUST be covered when not in use to minimize undesirable large-volume water evaporation. The pool cover can be a movable floor, to increase usable sunspace area (a dance floor, sports court, etc.).
A properly ventilated TBZ significantly REDUCES humidity in the summer. As a large volume of TBZ air rises by convection, it gradually becomes warmer and less dense. This increases its capacity to hold water vapor, which it then draws out of the house through the high-volume roof vents, reducing the absolute humidity in the TBZ.
When the high-volume TBZ roof vents are open, warm air rises toward the top of the TBZ like a hot air balloon through the south AND north sides of the TBZ. Instead of a natural convection circular up-and-down winter airflow LOOP, ALL SUMMER CONVECTION AIRFLOW IS UPWARD AT ALL POINTS. There is NOTHING that causes TBZ air to fall downward from the top of the home in the summer with the roof vents open. The density of TBZ air decreases as it rises, and continuously draws more heat and humidity out of the thermal buffer zone “automagically.”
If ZED-recommended cooler, less-humid TBZ replacement is introduced near the center of the crawlspace with unrestricted airflow in all directions, then the upward airflow rate through the north and south sides of the TBZ will be “automagically” determined by north / south / east / west temperature (and thus air pressure) differentials during different hours of the day. This is far superior to conventional central air conditioner systems, which always deliver the same airflow rate to all rooms, regardless of the fact that eastern rooms need more cool air in the summer morning, and western rooms need more cool air on hot afternoons.
Automatic self-regulation is one of the most powerful assets of any intelligent natural-convection airflow design, summer or winter. In essence, in the summer, the hottest exterior walls of the Thermal Buffer Zone architectural design pattern automatically receive the largest volume of cooler, less-humid replacement airflow, with no electrical control systems to make it happen effortlessly, using free Mother Nature Airflow Intelligence.
This is EXTREMELY IMPORTANT. It can be implemented in many clever ways. In a single-story house, higher ceilings can be used. Vents can be hidden in spaces above kitchen cabinets, lowered hall ceilings, false vaulted ceilings, etc.
The fairly obvious key to making the TBZ summer convection exhaust work effectively is the quality of the replacement air that is introduced into the TBZ ONLY at the base of the upward convection airflow path (near the center of the crawlspace).
A ZED Thermal Buffer Zone thermal envelope is totally sealed in the winter. When we open the roof vents on hot days, we do NOT want to take in hot outside air at any point. If the TBZ is 85 degrees, we do NOT want to bring in 95-degree humid air! We NEVER have open soffit (roof overhang eve) vents. This point is sealed all year round and cannot ever be opened.
The ZED TBZ solarium does have doors that can optionally be opened when the outside temperature, humidity and air quality are nice (mold and pollen are low, etc.). BUT, it is strongly advised that the solarium doors and windows be left closed most of the time, ESPECIALLY when daytime temperatures are above 72o F, or outside mold/pollen/allergens are high (listen to your local weather report).
In almost all poorly-design conventional homes, the attic airspace becomes MUCH hotter than the peak outside air temperature in the summer. This is caused by dark roofs that absorb millions of BTU’s of high-summer-sun solar heat gain, a lack of roof decking radiant barrier system to block radiation from the roof deck into the hot attic, insufficient (or NO) conductive roof insulation, and no way to introduce replacement air that is cooler than outside peak temperature air.
When an attic is hotter than the outside air temperature, it is thought to be a “good” idea to bring hot outside air into a conventional attic to cool it down. If the outside air temperature is 95o F, and your stupid conventional attic is 145o F, then exhausting the 145o air and replacing it with 95o air can reduce your unnecessarily high air conditioning utility bills, somewhat.
In stark contrast, bringing in hot humid outside air turns out to be a very BAD thing to do in a well-designed ZED Thermal Buffer Zone TBZ. The combination of our white cool roof, radiant barrier roof decking, and adequate roof conductive insulation, should create a roof that SHADES YOUR ATTIC AND MAKES IT COOLER THAN THE OUTSIDE AIR TEMPERATURE. Conventional architects and builders haven’t a clue that this is even possible, and they refuse to take ten minutes to learn how to make it happen. The result is that our non-learning nation assumes that the attic HAS to be hotter than the air outside, WHICH IS TOTALLY FALSE SCIENTIFICALLY!
If the outside air temperature is 95o F, and your Thermal Buffer Zone attic TBZ is less than 90o, then it would be just plain stupid to bring 95o air into your attic, which would make the attic warmer and increase your cooling energy requirement! This is a VERY FOREIGN CONCEPT to conventional architects, and business-as-usual builders, who have no training or interest in designing or building Zero Energy Homes. (They pay high energy bills themselves, so why shouldn’t their clients? – How stupid is THAT?) The U.S. Department of Energy was created by Jimmy Carter in 1977 to EDUCATE non-learning architects and builders about how to avoid energy bills, but the simple scientific knowledge has always fallen on non-learning, intentionally-deaf ears.
For the ZED Thermal Buffer Zone architectural design pattern to work properly in the summer (2 small Delta T’s instead of one large one), the temperature (and humidity) of the TBZ MUST BE BETWEEN THE OUTSIDE PEAK AIR TEMPERATURE, AND THE INTERIOR LIVING QUARTERS TEMPERATURE. The science discussed below is non-trivial, but the construction turns out to be simple (although unconventional).
If we exhaust thousands of cfm of warm humid TBZ air out of our large capacity roof vents, then we MUST find a way to bring in replacement air at the base (near the center of the crawlspace) that is both COOLER AND LESS HUMID THAN THE PEAK OUTSIDE AIR TEMPERATURE AND HUMIDITY.
IF the TBZ replacement air is the same temperature as outside air (as in ventilated conventional attics), then we have completely defeated then entire reason for the Thermal Buffer Zone design in the summer. That is why solarium doors and windows MUST be closed whenever the outside air temperature, humidity, and quality (allergens, etc.) are uncomfortable.
The following material in this chapter will explain in detail, in simple terms, how ZED can achieve this very important thermal buffer zone goal, at a very low cost, WITHOUT conventional compressor-based air conditioning systems for the TBZ.
An Earth Temperature Tube Filtered Fresh Air Intake (before being packed in sand and buried). In hot locations where necessary and feasible (such as south-facing hillsides or low-water-table areas) large-diameter underground “cool tubes” can lower or raise the outside replacement air temperature for the sunroom and attic, which are buffer zones between the interior temperature of the home and exterior temperature extremes. When warm outside air passes through the underground cool tube (surrounded by ambient temperature earth), condensation forms on the walls that must be drained off (just like the dehumidification process of an expensive high-energy-consumption conventional air conditioner). This is why hillsides are the best location for the use of cool tubes. ZED NEVER allows ground water, condensation, mold or mildew to accumulate inside a cool tube. This design factor is critical.
Cooler, dryer air from the cool tube is then input to the solarium thermal buffer zone (by natural convection created by turbine vents with no fans, thermostat or electricity). The cool dry replacement air is not added directly into the living quarters interior, unless the owner intentionally leaves the sunroom interior doors open on mild days or nights. Cool tubes are NOT for every house everywhere (there are several critical design issues), but where applicable, they are a nice tool of Zero Energy Design®.
If a Thermal Buffer Zone (TBZ) Home includes a swimming pool, swim spa, hot tub, koi pond, waterfall, etc. in the TBZ solarium, dehumidification ventilation becomes a design issue that must be carefully dealt with. Heated pools MUST be covered when not in use to minimize undesirable large-volume water evaporation. The pool cover can be a movable floor, to increase usable sunspace area (a dance floor, sports).
Never Use Unshaded West Facing Glass – It is a Solar Furnace on hot afternoons (over 3000 BTU / sq.ft. of unwanted heat added to western rooms) Use Plantings to reduce exposure. Plants work by evaporative process which is very good at reducing heat. Remember how much cooler it is under trees.
Never Use Un shaded High Thermal Mass (e.g., concrete) On The West Side – Absorbs afternoon sunlight and then radiates heat into the house long after the sun sets. High thermal mass must be heavily insulated on the OUTSIDE to reduce/prevent undesirable heat transfer. West wall radiant barriers with an air gap are important. Plantings do well here to.
Never (intentionally or unintentionally) draw in unconditioned replacement air that is hotter or more humid than interior air, without some type of heat exchanger / dehumidification solution (heat pipe, etc.) See also Conventional Laundry Room Energy Disaster
Never Ignore The Importance Of Year Round Humidity Control / Regulation. Dry air cools better than humid air. The evaporative process working again but this time on your person.
Never Use A Dark Colored Roof – You should use a highly-reflective white "cool roof" (or roof planting "green roof"). White metal roof is better than asphalt (with a black substrate). See also Home Energy Consumption Reduction
Never Ignore The Importance Of Radiation In Downward Heat Flow – Must have a radiant barrier under your roof deck. A well-designed ZED attic should not be much hotter than the peak outside air temperature. A cooler (or warmer in winter) attic will dramatically reduce energy costs.
Never Ignore The Importance Of Window Overhangs and Shading Devices – Must understand the seasonal path of the sun for your specific location.
Never Operate A Ceiling Fan In A Room When No One Is Present – Ceiling fans can improve evaporative cooling of perspiration and make the skin about 3 degrees cooler, BUT ceiling fans disturb the laminar insulating air layers at the ceiling and around windows and make the room hotter in the summer (like the "wind chill" factor operating in reverse). Ceiling fan usage can thus INCREASE the operating time of your air conditioner system for a given thermostat temperature setting.
We sincerely wish all of our readers an Abundant New Life In Harmony With Nature
Lifelong Learning In An Ever-Expanding Universe Of Endless Possibilities TM
We invite constructive suggestions and collaboration with our new friends
E-Mail To: ZEDmaster@ZeroEnergyDesign.com