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Project

Florida Solar Cracker House

Credits: Elizabeth Seiberling and Randy Cullom

This document describes an energy efficient, solar-powered home that was built near Interlachen, Florida. The original owners/designers lived in the house from mid-1998 until mid-2006, and describe some of the successes and failures and some of the many things they learned along the way. The original plan was to use no fossil fuels at all. They relaxed that ideal a bit, and have some propane appliances as convenience and backup systems. Total use of propane was about 30-35 gallons per year, for which they paid about $100, including delivery. They also had a backup Honda gas generator, which was rarely used. The house is not connected to the utility grid. It has a Clivus Multrum composting toilet and is not connected to a public sewer or septic tank. Although well water is used, a 3000-gallon cistern can be used to collect rainwater for all household water needs. It was to demonstrate that humans can live in a comfortable and pleasing home while attempting to minimize their negative impact on the earth. The design and building of the home was an exciting challenge that was in progress for over 10 years. (The house has been sold by the original owners.)

 

Florida Solar Cracker House Breezeway

View from just inside the bedroom, looking through the breezeway into the kitchen ©2006 Elizabeth Seiberling

The house is near the middle of 60 acres of wooded land 50 miles south, southwest of Jacksonville. North Florida is fairly flat, but ecosystems can change dramatically with elevation changes of a few feet. The property shares a 1/2 mile border with two small lakes, and at one part of the interface is a pitcher plant bog/wetlands community. On the north border of the property is a low bayhead area. Approximately 15 acres comprises a longleaf pine/wiregrass/turkey oak ecosystem. Please click here to see some more pictures. The goal was to make the land as accessible as possible to wildlife, including black bear, bald eagles, osprey, swallowed-tailed kites, anhinga, green herons, great blue herons, pileated, red-bellied and red- headed woodpeckers, red-shouldered hawks, wild turkeys, a pair of screech owls and eastern bluebirds nesting in birdboxes, flying squirrels, Sherman's fox squirrels, grey squirrels, grey fox, gopher tortoise, box turtles, water turtles, alligators, otter, deer, numerous snakes, lizards, frogs, and, of course, insects.

The house design is based on the "Cracker" architectural style typical of early settlers in Florida, and incorporates many energy conserving features that are tailored to the hot, humid climate found in the southeastern United States. These include cupolas, large roof overhangs, porches on the east and west sides, ventilation under the house, a kitchen and bathroom isolated from the living room and bedroom.

Electric power is provided entirely by photovoltaics. We have 1.2 kilowatts of Siemens PV modules on three Zomeworks trackers. The power system and sizing is described in detail in the chapter on Electric Power.

The Cracker Heritage
Often, the best models for energy efficient design and examples of natural, local building materials come from the very oldest homes in our region; from the homes of early settlers and even from the original homes of the indigenous people of our region. The recent resurgence of interest in earth-bermed (earthship) construction, mainly in the southwest and straw bale construction in the plains states are two good examples of looking to history for guidance in construction. In northern Florida and southern Georgia, the early settlers were called "Crackers", a reference to their practice of cracking corn to make meal, and the cracking sound of their whips used to drive cattle.

Cracker home design was uniquely suited to the local climate and these backwoods country settlers made use of abundant native forests of Southern Yellow Pine in their construction. Pine heartwood, now rare in the age of clearcutting and timber plantations, is virtually indestructable by pests or decay, and was used even for foundation blocks placed directly on the ground. This picture shows a typical cracker style house in north Florida. Notice the wrap-around porches and hip-style tin roof. In the back is a detached kitchen. The house sits on blocks about two feet off of the ground. What is the climate of north Florida? About half of the year, from mid-April to mid-October is generally hot and humid. Daytime high temperatures are normally about 90 F and nighttime lows are in the low 70's during May through September, and slightly cooler in April and October. Mostly, the months from mid-February to mid-April and mid-October to late November are delightful, with cool nights and warm, but not hot, days, and low to moderate humidity. December and January can be cold, with several to about a dozen freezing spells.

Rainfall is more uniform throughout the year in northern Florida than in the south, and this has to do with the mechanism of rainfall. Cool season rainfall (meaning the 6 months from mid-October until mid-April), from 2" to 4" per month, comes from advancing cold fronts, which weaken as they move south. During the hot months, the land heats up as the day advances, causing the humid air mass over the land to rise. This brings in air from the ocean and Gulf, a so-called sea breeze. As the moist air rises, it encounters lower pressure and expands, which causes the air to cool. Because cooler air can hold less water, moisture condenses to form clouds and eventually, rain. This "convective rain" is more commonplace in southern Florida, where the sun is more intense. In northern Florida the average rainfall is about 7" per month during June, July and August. Rainfall in the summer is highly variable. We have been recording rainfall at our house for the last five years, and logged 21" in June of 1994, with 1" for May of 1993! The windspeed is generally low in the central peninsula. It is too far inland for a substantial sea breeze, and except for short-lived storms or an occasional hurricane in the area, winds are normally from still to 8 mph.

In researching passive solar design, we found that most available books and references consider primarily (or only) solar heating, whereas we were almost entirely concerned with passive features to enhance cooling. An excellent resource that we found for passive solar design in hot, humid climates is the Florida Solar Energy Center. The FSEC is the energy research institute of the State of Florida. The center is nationally recognized for comprehensive programs in solar energy and energy efficiency and is administered by the University of Central Florida. Numerous publications and pamphlets are available through FSEC for a very small fee. Also, their Building Design Assistance Center (BDAC) offers free design assistance (which we have taken advantage of). Another resource that had a large influence on the design of our house was a book by University of Florida Professor Ronald W. Haase entitled "Classic Cracker - Florida's wood-frame vernacular architecture". In particular, a design featured in that book by architect Edward J. Seibert, was the inspiration for our house.

Basic Psychrometrics
Psychrometric charts show in a simple graphical form the state of an air mass in terms of its water content, temperature and energy. They can be invaluable in helping to understand how to passively cool a house. Most people understand that hot air can hold more moisture than cold air, but from a psychrometric chart, one can predict exactly how the relative humidity (RH) of the air in a closed room will decrease if its temperature increases. For example, suppose the outside temperature and relative humidity at 6 am are 70 F and 100%, respectively, (typical summertime overnight conditions in north Florida). Let's assume the air inside of your house is in the same condition as the outside air because the house has been well-ventilated overnight. Now, you shut the windows and capture that air in an leak-free house. As the day wears on and the outside conditions deterioriate to 90 F and 90% RH (also typical conditions in north Florida), let's say the air temperature inside the house rises to 80 F due to heat conduction through the walls and roof. The psychrometric chart tells you that the RH of the air at that point has fallen to about 72%. Statistical studies of human comfort have been performed that indicate certain zones of temperature and RH in which most people feel comfortable. In our example above, 80 F and 72% RH is outside the comfort zone if the air is still. However, even in moderate airflow, such as that provided by a ceiling fan, those conditions would be well within the comfort zone. Thus, by simply manipulating ventilation and airflow, and minimizing the introduction of heat and water into the house, the house can be kept comfortable. The example above does not consider the increase in temperature and water content of the air caused by human occupants, opening doors, etc. Also, the comfort zones refer to people sitting at rest, and someone sweeping the floor may not be comfortable in those conditions. Also, fans are not considered passive devices, although their power consumption (typically about 50 Watts for a ceiling fan) is tiny compared to an air conditioner. Still, the example serves to point out some important principles. First, a house should be designed so it is easy to exchange the inside air for outside air when the conditions are right. Large, open rooms with a minimum of walls, ceilings that slope upward with ventiliation at the top, and vents near the floor facilitate this. Second, sources of water vapor and heat should, as much as possible, be located outside of the living area. Boiling water on the stove, or a shower can fill the house with warm water vapor, which takes much energy to remove. Third, the house should be leak tight. Sliding doors and windows, electrical outlets, and ducts can be large sources of air leaks. If conditions in the house become uncomfortable, it is important to be able to draw in air from outside that is as cool as possible. Again, having a vent near the floor surrounded by a large, shaded area is very helpful.

The house is divided into three units. At the east and west ends are the kitchen/bathroom and livingroom/bedroom, respectively, and these units are joined by a room in the middle, labeled dining room. The intent of the "dining room" was to act as the breezeway of the traditional dog-trot houseform described by Haase and seen in many early Florida cracker homes. The function of this breezeway in our house will be to isolate the part of the house where heat and water vapor are generated (kitchen and bath) from the "living quarters" during the hot months, another characteristic of the traditional cracker house. Our house, as built, differs from the drawing here in that there are no doors on the north or south sides of the dining room area, but, instead, large, 6 foot wide, casement windows on each side. In addition, the connection to the kitchen area is 8 feet wide and completely open, with no doors. The rationale for these changes was to save money spent on expensive doors and to provide more inside space in the house. Thus, our house, as built, is somewhat less like the traditional cracker dog-trot house, but functionally, the dining room still serves to isolate the hot, humid kitchen and bath areas from the remaining living space.

Ceiling fans draw hot, humid air up into the cupolas and out through the cupola windows.Outside the house, covering the entire east and west walls, are screened porches. Typically, east and west-facing walls are difficult to shade because of the large range of sun angles they are exposed to during the day. A good solution is to cover these walls with porches. The south-facing wall is easier to shade with roof overhangs, because, in the summer, the sun is always at a high angle to the south walls. Part of the south side of the house will be shaded by the greenhouse (not yet built), which will have internal shades on the glass. How the roof overhang works to shade the south walls and the importance of shading the walls in preventing heat conduction into the house will be discussed in roof and wall design in the next section.

Structure and Frame
The construction is post and beam style, but without the mortise and tenon joints. We opted for bolted connections on the timbers, for ease and simplicity of construction. The walls and ceiling are entirely outside the frame. All wood used in the house, except the plywood subfloor was cut by us on our sawmill.

The foundation of the house consists of two concrete slabs, 36'x36', with concrete block pillars supporting the subfloor. The floor is about 9' above grade. The reasons for the elevation are several. First, the 100 year floodplane is about 4' above grade at the house site, and code requires (wisely) that the first floor be above that level. Once the house has to be up 4', one might as well raise it a few more feet, and make usable space below the house. This is a good idea for ventilation purposes, and makes for a nice view of the lake 200' away and the surrounding forest. The concrete slab and pillars, which are filled with concrete, provide for a substantial thermal mass below the house. We will experiment with partially closing off this area, so that cool air can be drawn into the house from this "basement". The two-story greenhouse will share a wall with the cistern, which provides a temperature-moderating thermal mass. The greenhouse will provide passive solar heating in the winter, being open to the kitchen, but the most important function of the greenhouse will be for food production. Along with a 2500 square foot organic garden, and many native perennial fruit trees, we hope the greenhouse will help us produce a large portion of the food we eat. We are no longer vegetarians, and have begun to include some wild caught fish and some locally and humainly produced meats and dairy products into our diet. However, we have maintained a simple diet as free as possible from processed foods, and we think it is a realistic goal to produce much of our own foods. The greenhouse should allow us to grow more foods over winter, and start seedlings early for the main garden.


Resources

Florida Solar Energy Center

Florida Solar Cracker House Website

Passive Solar Design (Florida)