the time the heat has made that journey, diurnal (daily) temperature swings are driving the heat the other way in the wall. This means that a straw bale wall can give you the real-world impact of an R-50 wall, even though it is really only R-30.
— Nehemiah Stone built his own straw bale house in Penryn, California. The explanation above is adapted from his straw building list serve and presentations on thermal performance of straw bale homes.
Efficiency Benefits
Whether it’s a concern for the environment or for the bottom line of our monthly heating and cooling bills, the high level of energy efficiency achieved by straw bale homes is often the foremost reason for choosing straw bales over other building materials. The enviable energy efficiency is due to the good insulating properties of straw bales. The role of insulation is to minimize temperature loss or gain and therefore the amount of energy consumed to maintain a desired temperature. Through a combination of thickness, the amount of air they entrap, and the fairly low conductivity of straw itself, straw bales offer insulation values that can exceed those of modern, well-insulated, frame-walled homes.
Energy Consumption Comparison
The Canada Mortgage and Housing Corporation funded a study overseen by British Columbia bale builder Habib Gonzalez. Using energy consumption data from BC bale homes, they were compared to equivalent frame-walled homes via computer modeling. The following is an excerpt from that report:
While straw bale houses have a theoretical energy saving advantage over conventional houses, there is little good data on how they actually perform. This survey attempted to provide a first cut at comparing the space-heating energy consumption of straw bale homes and conventional homes.
Many straw bale homes are wholly or partially heated with wood-burning appliances. As wood consumption is difficult to measure accurately, the 11 houses in this survey used other fuel sources — gas, oil, electricity.
Most surveys of this type compare the measured houses to “control” houses of the same size, construction quality, occupancy, etc. Control houses for this study were too hard to locate, given the diversity of straw bale house design and the use of slab-on-grade foundations. Only 3 of the 11 study homes had full or walkout basements. Instead of actual control houses, the energy use of the conventional houses was modeled using HOT2000 software. The measured space-heating consumption of the straw bale houses was compared to the modeled energy consumption of conventional 2001 British Columbia (BC) building code houses of the same dimensions as the straw houses.
1.2 CMHC Energy Consumption Comparison.
Research Program. The contractor located 11 straw bale houses that used measurable fuel types. He visited the houses, measured floor areas, windows, and doors, and examined the energy bills. From the bills, he was able to extract the energy used only for house space-heating, by subtracting the energy consumption of appliances, lighting, water heating, etc. Using the measurements of these houses, he created simulated houses built to current BC building codes and having 2-by-6 walls. All interior floor dimensions, floor insulation (if any), window dimensions, attic insulation, solar exposure, etc., were the same in the actual straw bale houses and the simulated conventional houses. The simulations used reinforced vinyl double glazed-windows with a half-inch air gap and insulated spacers, according to common BC construction practice, even if the windows of the straw bale houses were of a lesser quality. When the study home included windows with high-efficiency elements, such as low-E tints and argon gas, the modeled home windows matched these details. The simulation program used (HOT2000) has had wide application in the Natural Resources Canada (NRCan) R2000 program and in the Energuide for Houses retrofit program. It has been extensively tested and is typically within ten percent of measured data on individual houses, perhaps with a small bias to over-predict energy usage. When a number of houses are averaged, the resulting mean should be close to the truth. In cases where the same hot water heater was used for space heating and domestic hot water use (dishes, showering, etc.), the modeling of these houses (and the energy usage in Figure 1.2) included simulated hot water usage as well.
One qualifier: BC building code requires some form of mechanical ventilation be installed. The occupants of the straw bale houses may or may not have used the ventilation systems. In the simulated houses, an air change rate of 0.2 air changes per hour (ACPH) was used in the simulations to reflect both the natural infiltration rate and whatever use of mechanical ventilation. This is relatively low but still may be higher than the ventilation rates actually experienced in the straw bale homes. Similarly, the modeled house air leakage was set at 4.5 air changes per hour at 50 pascals (Pa), typical for new BC stock. There were no air-tightness tests of the straw bale houses. The 1.5-inch thick stucco skins on the inside and outside of the straw generally provide good air barriers (and add to the thermal mass of the wall construction).
Findings. House space-heating consumptions are listed in Figure 1.2. Electrical houses had their kWh readings changed to gigaJoules (GJ) to provide an easier comparison to houses using other fuels. The energy use listed is usually for space-heating only, with some exceptions, as described above.
The straw bale houses used over 20 percent less space heating energy when compared to the modeled conventional houses. Some of this may be due to under-ventilation of the straw bale houses and a small tendency for the model to over-predict energy consumption in the conventional houses. However, the size of the savings and the consistency (9 of 11 houses) indicates that the straw bale houses in this survey require significantly less space-heating energy than comparable conventional houses.
A version of this report appeared in The Last Straw, no. 41.
Habib John Gonzalez is the owner/operator of Sustainable Works, a British Columbia-based outfit offering straw bale home building services, plaster spraying, consulting, and workshops. Contact: Sustainable Works, 615B Cedar Street, Nelson, BC, Canada V1L 2C4, tel/fax 250.352.3731, <[email protected]>.
The Canada Mortgage and Housing Corporation (CMHC) has funded a number of straw bale construction studies. The final version of this report (and others) can be downloaded at <http://www.cmhc-schl.gc.ca/publications/en/.rh-pr/tech/02-115e.pdf >.
The H2K modeling software has been developed for the R-2000 housing system. It is an extensive program covering a wide variety of housing configurations. The software can be reviewed at <http://buildingsgroup.nrcan.gc.ca.html>.
R-What?
Insulation values are most commonly expressed as R-values, a measurement that denotes the ability of a material to resist the flow of heat. R-values for residential wall systems typically range between R-12 and R-20, depending on climatic conditions, building code regulations, and type of insulation. Plastered bale walls have R-values ranging from R-30 to R-50, depending on their width and orientation.
No Gaps, No Leaks
A well-built bale wall creates an unbroken wall of high insulation. In a traditional frame wall, the space between the studs might be insulated to R- 12 or R-20, but the wooden studs themselves only offer approximately R-1 per inch, or R-5.5 for a common 2-by-6-inch stud. The thermal efficiency of the building is broken by these regular “cold bridges.” Infrared photographs of frame homes taken on a cold day will show the outlines of the studs as cold strips on the interior wall surface. Problems can also arise in frame walls with settling and improper installation of various insulation materials, creating
