Straw Bale Construction/Characteristics/Passive solar
Thermal mass[edit | edit source]
(Please help expand this section, specifically adding comparative)
Straw bale like all other organic insulation materials are better able to buffer heat than inorganic insulation. Basically this improves thermal comfort within a building, exterior temperature swings are delayed and damped. Having thermal mass is the difference between the interior comfort experienced within a catherderal or in a corrugated shed, either in the summer or winter. The fact that a straw bale wall should be plastered on innner and outer surfaces greatly enhances the heat buffering effect by substantially adding to the thermal mass of the building.
The combination of insulation with sufficient thermal mass creates the high level of comfort experienced in most straw bale buildings. Adding extra thermal mass by even thicker plaster layers on inner surfaces encounters the effects of diminishing returns. Doubling the thickness over 35mm (which seams to be the optimum) increases comfort by an insignificant amount. Depending on the location overheating can take place if there is too much equator facing glass. This can be combatted by adding extra thermal mass. Shading though is far more effective but correctly dimensioning the amount of glasing is even better as it saves the heat loss through the glasing at night or overcast days.
(Passive Solar should be moved to a more appropriate section)
Passive solar refers to buildings designed to maximise the heating and cooling effects of the environment around them. They are called passive because there are none (or few) parts of the design that require energy to operate. The most common technique for passively taking advantage of the environment is maximising solar gain by exposing interior surfaces to the suns warmth and then designing the building to best contain that warmth. At the other end of the scale, where climates are hot and passive cooling is what's needed, one technique is using rising warm air to draw basement cooled air up through a building.
Any building taking advantage of passive solar gain must have well insulated interior surfaces which are exposed to sunlight and have enough mass to store daytime heat and release it at night. How suited a straw bale house is to taking advantage of solar gain depends on the mass (think of thickness) of the inside plaster coating, though some maintain that straw bale constructions are inherently unsuitable for passive solar gain  (although the article seems to neglect the surface plaster). It should be stressed that straw bale homes are not inherently good for passive solar gain, they need to be designed to make use of it, it doesn't just happen. The same is true of any building material or system.
Following are the basic features that distinguish straw bale buildings designed to maximise passive (think of free and sustainable) heating and cooling:
- Limited exterior wall surface with high insulation.
- Equator-facing, East and West Roof overhangs correctly sized to block the summer sun (angle) and still allow the lower winter sun angle to provide heating of interior thermal mass.
- Passive preheating/precooling of external air by drawing through cellers, porches, glass houses and heat exchangers.
Features specific to cold climates
- Large (super insulated low-e) glass surfaces with high orientation for maximum sun exposure of the buildings interior. In strawbale buildings the inside plastered surface of the bales is a great surface for collecting sunlights heat and radiating it slowly back to the inside space. See also w:Solar_gain:solar gain.
- Superinsulated doors, windows and frames. Glazing with low-emissivity glass coatings facing outwards
- Position doors for minimum wind exposure, preferably with an enclosed porch.
- External postbox, not an in-door hole.
- Building envelope air-tightness (see below).
- For extra winter heating the focus is on renewable fuels (plant oils/ charcoal and wood) or sun heated systems (solar collectors or heat pumps).
Features specific to hot climates
- Glass openings (and leisure areas) need to be protected from radiated heat from surrounding object like sun baked sand or earth, outside planting can greatly reduce radiated ground heat.
- Shading and orientation to avoid sun exposure, especially to the buildings interior.
- Position windows where they can make the most efficient use of prevailing wind for cooling and ventilation.
One common source of confusion when talking about 'passive' construction is the term 'breath' which is more accurately known as “vapor permeability”. People talk about straw bale walls breathing, but this has nothing to do with air moving through the wall, it's about moisture moving through the wall. Really it is better to refer to it as moisture permiability. In this way walls that can transport odour filled moisture to the outside contribute to a high air quality, without air moving through the wall.