High thermal mass. Poor insulation difficult to add unless lined externally, as shown in the image on the previous page of a rammed earth building being externally insulated and lined.
Timber Frame Construction at a New Level
Minimal transport energy when used on remote sites. Minimal manufacturing process impact. Very durable but requires some maintenance where used externally reapplication of waterproofing. Average to high site impact, depending on footing system. High cost see Rammed earth. The most common form of low mass wall construction uses lightweight timber or steel framing as the structural support system for non-structural cladding and linings such as fibre cement, plywood and steel. Insulated lightweight walls reduce heat loss and can have minimal embodied energy, depending on the cladding material used.
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Fibre cement sheet, plywood and other sheet cladding systems have low embodied energy and generally low environmental impact. They are very durable — although maintenance is required for any painted surface see Embodied energy. Lightweight timber — Low to medium embodied energy. Medium to high insulation values. High maintenance unless protected from weather and termites. Suited to off-site and on-site fabrication. Relatively low transport costs see Lightweight framing.
Structural insulated panels — SIPs consist of an insulating layer of rigid insulation material sandwiched between two structural skins of sheet metal, plywood, fibre cement or engineered timber. These systems usually achieve high levels of structural efficiency with high insulation levels. Many now use environmentally preferred materials. One Australian SIP system, for example, uses panels made from forestry waste through a carbon zero manufacturing process.
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While already an environmentally preferred product, its sustainability performance could be further improved through use of recycled cellulose paper insulation as a substitute for the rigid foam insulation material. The range of SIP products is growing rapidly. These systems can be particularly effective because they position the mass where it is most useful and can use environmentally preferred materials to provide insulation and structural integrity. Log walls — Low mass systems include log wall construction. While a broad range of systems is used, they generally achieve good insulation and have low environmental impact when logs are sustainably sourced.
Cladding — Insulated lightweight walls reduce heat loss and can have minimal embodied energy, depending on the cladding material used.
They are very durable, although maintenance is required for any painted surface see Embodied energy. These systems fit between high and low mass with either moderate density, such as AAC where high mass concrete is used to trap tiny no mass air bubbles, or a combination of high and low mass, like straw bale where straw is low mass and the render finish is high mass.
Autoclaved aerated concrete, or AAC, contains closed air pockets that make it lightweight and fairly energy efficient. AAC block — Medium to low embodied energy, fair thermal mass, fair insulation, average durability depending on finishes. Maintenance required depends on finish but these blocks are prone to impact damage. They have low processing impacts and moderate transport requirements see Autoclaved aerated concrete. Concrete block — Block walls have lower embodied energy than concrete or brick because they are hollow and contain less concrete per square metre.
However, when filled with concrete they can equal or exceed the embodied energy of brick. Fly ash blocks further reduce embodied energy. They have good thermal mass when filled with concrete, but low insulation values which is difficult to add unless lined externally. Not easily recycled because they have insufficient strength for reuse as aggregate for concrete. Can be crushed as gravel or fill. Average cost. Mud brick adobe — Lowest embodied energy if sourced locally , high thermal mass, poor insulation difficult to add unless lined externally , suited to remote sites.
High labour content. No manufacturing impact. Low site impact. Low cost if labour is not included owner built. Requires regular waterproofing in exposed locations see Mud brick.
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Straw bale — Low embodied energy some additional embodied energy and materials in extra width footings and slabs. Low—medium thermal mass depending on render thickness. Extremely high insulation, excellent thermal performance, and high level renewable material content. Long term durability is unproven in Australia and maintenance levels are variable. Bales must be compressed well to minimise settlement and movement.
Cost varies from average to high see Straw bale. Panel systems — Sandwich panels have varying embodied energy depending on surface materials and insulation. Other lightweight panel systems such as straw board and recycled paper products have low thermal mass, high insulation levels and very low embodied energy. They respond rapidly to heating and cooling and are ideally used with a high mass concrete slab floor. The recycled content of many commonly available systems is high. Reuse potential is good, waste rates are low and transport costs are low. Construction cost varies from high to average.
Panel system with fibre cement outer linings with expanded polystyrene studs and concrete core fill.
The concrete fill adds thermal mass and an outer layer of insulation yields excellent all round thermal performance. Log wall construction — a low mass system — is one of the oldest methods of building, dating back to prehistoric times. It developed as a natural consequence of having a plentiful supply of tall, straight timber that could be relatively easily cut and worked and turned into building components.
It is historically associated with countries and regions with tall pines and similarly straight-trunked trees.
It appears to have first developed in northern Europe and spread with European colonisation, notably to North America where the indigenous pine forests provided plentiful timber suited to the method. Australian log homes use solid timber logs. At least one supplier uses imported Scots pine and Norway spruce, laminated when wider logs are needed; others use Australian white cypress or Monterey pine Pinus radiata , which is native to the central coast of California but widely grown in Australia as a plantation tree.
Log walls that retain the natural shape of the original timbers can have a quite rustic appearance but those made with machine finished logs can present a very smooth, much more formal appearance. It is usual to finish the timbers with oils or coatings that allow the warm natural colour of the wood to show through. In true log wall construction the horizontally laid logs are load bearing and the roof is constructed from substantial, solid timber members, often using traditional joints and details.
Other variants include using large diameter logs to build post and beam frames that are filled with lighter stud-framed construction. In the least authentic and most economical form, the logs are an applied veneer and the structure is timber frame with the roof being made from lighter, smaller section timbers in a similar manner to conventional brick veneer houses. There are two main ways of joining solid timber logs horizontally: scribing the logs lengthwise to fit snugly onto each other, often incorporating grooves to improve weather sealing; and fitting the logs on top of one another and sealing the air gaps with caulking or foam backing rods.
Log walls may be finished with or without the logs extending past the corners, creating a stronger and more resilient structure if they do as this exploits the structural benefits of the interlocking joints. Log walls have moderate to good thermal insulation with thermal mass characteristics that become more significant as the wall thickness increases. However, log walls do possess a similar acoustic insulation capability to straw bale.
In the same way that a large log takes a considerable time to burn through in a wood stove, so large diameter logs used in solid log wall construction do not burn easily and, depending on overall wall thickness and whether the construction is scribed and interlocked or sealed with caulking , overall fire resistance can satisfy bushfire requirements. Log walls are subject to the same vermin and hazards as other timber building systems and are protected in the same way with appropriate mechanical or chemical barriers.
Log timber is non-toxic. Oils, stains, varnishes, rot protection and other finishes should be checked for toxicity before specification for application to the timber. Most log wall manufacturers claim to use plantation timbers. As log walls use timber that has had minimal processing, the overall environmental impacts have the potential to be significantly less than for conventional construction.
Log wall companies in Tasmania and the south-east of Australia are able to deliver projects anywhere in the country. The cost varies considerably according to location and the kind of log wall adopted, with fully scribed and notched solid log walls being the most expensive. It is not unusual for firms to preassemble log houses to test their buildability before site delivery. Roof systems are unable to improve thermal performance in thermal mass terms unless they can be exposed internally and insulated externally.
Because ceiling level insulation is critical, exposed roof mass is unusual except in multi-level homes or apartments.
Earth covered construction — A high mass roof system capable of delivering highest thermal performance.