Tempered Solar Home
Most people driving past this home wouldn't give it a second look, much less recognize it as a "Solar" home, but in fact, it is, and not just because "Every Home is a Solar Home". This is an example of a Sun Tempered design. The home was built in El Paso County by a local non-profit, Mission Valley Homes, as part of their commitment to fulfilling the need for affordable housing. At just over 1,000 sq. ft., the original floor plan provided by the City of El Paso was left unchanged, but thanks to the Energy Center at the University of Texas at El Paso, that's where conventional construction ended. With funding provided by the State of Texas, State Energy Conservation Office, the Energy Center's Passive Solar Homes Project works with non-profit housing groups to not only include solar in their homes, but also improve energy efficiency.
The project is the brain child of Energy Center Program Coordinator (and EPSEA member) Steve Cook. Steve recognized the disproportionate amount of income that energy costs extract from the very folks who can least afford it. Although the design changes implemented do increase the initial cost of the home, a quick study of life cycle costs show a return on the "energy investment" from the first year.
This is a great example of how a little solar goes a long way. There is no added thermal mass in the home and most of the floor is carpeted. The solar design features together with the added insulation package reduce the need for both heating and cooling and the homeowners are comfortable.
Orientation, Orientation, Orientation!
The orientation of the building lot was near perfect, making this home an ideal candidate for solar. The long wall of the house runs east to west.
Sun tempering does not mean you must cover the south wall of your home with glass! Move 7% of the glass to the south wall. We say "move" because there's probably enough glass area in the existing plan, it's just not located where you want it to do the most good. Remember that it's 7% net.
Glass areas on the east, west, and north walls were reduced. Decreasing the amount of glass on these walls reduces both the heating and cooling loads.
Sole plate insulation was put in place before standing up the frame walls.
Prior to installing interior insulation, all plumbing and electrical penetrations were sealed using either expanding foam insulation or caulking. All rough openings around doors and windows were also sealed using the same materials as well as using strips of fiberglass insulation where needed.
The furnace closet plenum was sheathed and all walls surrounding the furnace closet, both interior and exterior, were insulated.
Insulation: Code minimum in El Paso (the very least you must do) calls for R-22 ceilings and R-11 walls with no requirement for perimeter slab insulation.
Under the Energy Center's program, the attic insulation was upgraded to R-30, the walls to R-13 and the exterior impregnated sheathing was replaced with R-4 expended polystyrene. The perimeter slab insulation was 1" R-5 extruded polystyrene. All windows are double-glazed (insulated).
The cost for all of these changes have averaged less than $1500. The actual added cost per sq. ft. will decrease with larger homes. If you were to build a 2,000 sq. ft. home, wouldn't an added $1/sq. ft. be a great investment?
Beyond Sun Tempering
Thermal mass materials have the ability to conduct and store energy, both heat and cold, and to release that energy back into the living space when it's needed.
Heat always moves to colder surfaces. In the solar home, the free solar energy first heats up the air. Since the mass floors and walls are cooler, the heat is absorbed and conducted into these materials. Later, when the sun has set and the room air temperature falls, it will reach a point where the mass materials are warmer than the room air temperature. Since heat seeks out cold, the stored energy will now return to the room. The more mass in the home, the more energy that can be stored.
The amount of south facing glass to be installed is related to the amount of thermal mass in the home, and the reverse is also true. In the sun tempered design, the home does not have the mass needed to absorb the amount of solar energy delivered when glass amounts exceed 7%. With slab construction, the mass is built in, but when carpeted, it can't work for you. When you tile your slab, you have added thermal mass and you can increase the amount of south glass accordingly.
The Direct Gain System: Glass and Mass
The direct gain system is the easiest and most cost-effective way to use solar energy. The structure itself is the solar system. The south windows are the collectors. The walls and floor are the storage (mass). Orientate the structure and windows as close to true south as possible.
- The ideal thickness for mass materials is 4 to 5 inches.
- Use mass materials in the construction, for floors and walls. (e.g. adobe, concrete, brick, rock)
- Water is the best mass, storing far more energy than other materials, but it's not structural.
- A masonry fireplace adds thermal mass, but should be located on an interior wall.
- Insulate the exterior of your walls, keeping the masonry inside, protected from outside temperature extremes.
- Provide for night time insulation on large glass areas. (e.g. insulating curtains, moveable insulation, shutters)
- Provide adequate overhangs on the south side to prevent direct gain during the cooling season.
- Use light colors on low mass construction. (ceilings and partition walls)
Glass to Mass Ratios
Each design starts with 7% south glazing net. To increase beyond 7%, we must also add thermal mass, usually starting with floor mass and then walls. An additional 1 sq. ft. of south glass may be added for every:
- 5.5 sq. ft. of sunlit thermal mass floor*
- 40 sq. ft. of floor not in direct sunshine
- 8.3 sq. ft. of thermal mass wall
* The maximum amount of sunlit floor is 1.5 times the south window area.
The recommended maximum amount of south glass for direct gain is 12-15%.
