Straw bales provide a thicker wall that is sealed in with plaster, which gives an R value in a range of 25-35+ compared to the typical stick frame home that has an 18-20 R value. This range is dependent on whether the straw bale is stacked on edge or flat and the type of plaster coating that is applied to the surface.
Building Performance is affected by a lot more than the R value of a wall such as varying climate conditions, internal heat gains (heat of people and appliances ) and humidity. Thermal mass and surface emissivity also affects the thermal performance of a wall. The actual R value for a straw bale wall system will vary with a number of factors including the type of straw, moisture content of the straw, density and orientation of the straw, presence and size of other wall elements, type and thickness of finish applied, as well as other factors. Other factors can include size, material and spacing of studs (or other structural frame), sealing around wall outlets and switch plates, sealing at junction of wall and floor, and fill of the insulation to the top of wall cavities (no gaps).
The R value is only one piece of information that enables one to estimate the heal loss and heat gain through a wall – it is NOT an absolute measure of how energy efficient a building is.
A conventional R 19 rated standard wood frame wall with fibreglass insulation, plastic vapour barrier with inconsistent sealing at edges and punctures more typical rates as R 14, whereas an R 30 straw bale wall has NO gaps at edges or punctures and reads consistently at R 30 and no less.
R 39 – R 52 for two string straw bales = R 2.38 to 3.15 per inch [14” = R 33 – R 49]
R U Acton, 1994
R 44 for 16.5” two string straw bales
Watts, Wildkie, Thopmson, Corson CMHC, 1995
R value 28.4 for 18.4” thick straw bale wall R = 1.48/inch [14” = 20.72]
N Stone, 1997
R 33 for bales on edge
J Christian ORNL & D Eisenberg, 1998
R 27.5 for two string 19” straw bales, plastered both sides R = R 1.45/inch [ 14” = 20.3]