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Center for Sustainable Agriculture
COMMON TERMINOLOGY OF ENERGY EFFICIENT BUILDING DESIGN
The U-factor is a measure of thermal conductance, or how easily a material (or system) allows heat to pass through it. The U-factor is defined in the U.S. by the number of British thermal units (BTU) that pass through one square foot of a material or combination of materials (called a system) per hour to create a one degree Fahrenheit temperature difference between the two sides of the material. In most other countries U-factor is defined in terms of Watts per square meter per Kelvin [W/(m2*K)]. To convert metric (SI) U-factors to British (IP) U-factors divide by 5.678; to convert the other way, simply multiply by 5.678.
*1 BTU=253 calories=1060 Joules
*thermal conductivity of various materials: brick 0.84 J/sqm degC, glass 0.84, water 0.56, cork 0.042, wood 0.04, air 0.023 (a straw bale would act as a "system" comprised of wood and air.)
The R-value is the inverse of U-factor (R = 1 / U). To convert IP R-values to metric R-values, multiply by 0.1761.
*R value of typical straw bale is 1.45 to 3.15 per inch thickness
*Typical studwalls suffer degradation of R values as a system, not typical of straw bale walls
*R=1/U so an R value of 14 = U value of 0.0714 and R value of 35 = U value of 0.0286. Therefore; a standard construction 2x6 studwall insulated with R19 fiberglass and a working R value of R14 has 2.5X the Thermal Conductivity of an R35 straw bale wall unit! In other words, 2 1/2X as much heat can flow through an R14 stud and fiberglass insulated wall than a typical 23" straw bale wall each hour!
Thermal lag is a very important insulating property which is not reflected in the R or U value calculation (if such measurements are made after a steady state condition was reached.) However, some materials (such as straw bales) act as complex systems and introduce a Thermal Lag which delays the formation of "channels" of conduction up to 12 hours or more. Obviously, in circumstances where the outside temperature fluctuates dramatically between day and night, high thermal lag can give great additional insulating value not necessarily reflected in R or U value measurements. And, therefore, if dramatic daily swings in temperature are a factor, thermal lag measurements are at least as significant as R or U value measurements and should be considered.
*In their analysis of the Real Goods Living Center in Hopland, California, three UC Berkeley grad students (Carter, Jain and Hou; 1996) determined the thermal lag (the time it takes for a “pulse” of heat to travel through the wall) was about 12 hours whereas the standard studwall takes 20 minutes to a couple hours.
Direct gain is the term used for a surface which absorbs and transmits heat to the living space. High thermal mass and high conductivity are typical of these types of materials. PCMs or Phase Change Materials can be employed as well which give off stored heat when changing from a liquid to a solid (certain types of salts for instance.)
DIRECT GAIN THERMAL CONDUCTIVITY CONSIDERATIONS
In devising the most energy efficient home, it is short sighted to rely completely on materials with a low thermal conductivity. Indeed, materials with a high thermal conductivity and heat storage ability like brick and stone can be utilized as direct gain materials to channel heat from sunlight into the interior living space either in conjuction with a window wall as a trombe wall or as a conductive outer wall. If used as a conductive outer wall or even ceiling, then it must be used as part of a two wall system incorporating a duct space and an insulated inner wall - a double envelope, or covered externally with a heavy insulating material when not absorbing sunlight.
Rock beds or stone walls or floors can act as thermal storage where heated air is ducted to them for release when needed later.
Green Home Building- a fairly comprehensive site giving a broad, albeit basic, overview of many "primative" alternatives to conventional construction techniques. Readers seeking more information are directed to available publications and a plethora of linked sites.
Straw Bale Central- offers a broad range of books and videos on the subject of straw bale homebuilding, cordwood homebuilding, composting toilets, other green living alternatives.
Shelter Publications Online- Books About Shelter, Building, Architecture, and the Home Arts. Home of the classic "Shelter" book.
Tipis-Tepees-Teepees- A great Tipi website with research and many photos.
Developed by Linda Holley and dedicated to the Laubins, who spent their lives studying Native American culture.