Lightweight construction

At the early design stage of NxtGen Houses we wanted to create a lightweight construction/building system.

The combinations of materials used to build the main elements of our homes such as, roof, walls and floor, are referred to as construction elements. Preliminary decisions about construction are made during the early design stages. Important factors influencing the development of our residential construction systems are:

• availability of skills and materials
• durability compared to intended life span
• life cycle cost effectiveness
• life cycle energy consumption
• role in improving thermal performance
• source and environmental impact of all component materials and processes
• maintenance requirements
• adaptability and reuse or recycling potential
• transport modes required for components

Lightweight systems
Lightweight steel construction has more embodied energy than timber but is highly recyclable at the end of its life. Steel has greater durability, is stable, termite-proof and has lower transport costs. Additional strength can be achieved by using thicker gauge steel or additional folds or bends in the cross-section. Steel is an excellent conductor and is by far the greatest source of thermal bridging. Therefore our steel structured elements are enclosed by non-flammable strips of sheet material and on connections we use thermal breaks. To allow for highly efficient use of materials, and reduce weight we use engineered timber bearers and joists. Other advantages of lightweight structures are:

• generally lower embodied energy
• lower total life cycle energy use, particularly where the diurnal (day-night) range is low
• rapid respond to external temperature changes or heating and cooling input
• significant benefits in temperate by cooling rapidly at night

We use a combination of engineered timber and steel structural design solutions in order to maximize structural capacity whilst minimizing weight and materials use.

Mass content
An important point of differentiation between construction systems is their mass content. Mass can only contribute to thermal performance when it is exposed internally and insulated externally. High and low mass materials make different thermal performance contributions depending on:

• the climate zone they are used in
• where they are used (internally or externally)
• availability or access to passive heating or cooling

Our hot swappable wall and open floor systems provide vertical and horizontal separation between dwellings resulting in cost effective, low mass housing solutions. Phase change material (PCM) is used to increase thermal storage capacity.

Footing systems
Footings are the structures that transfer the weight of a building to the foundation material. Footing systems are designed to suit adequate tie-down for the building structure. Lightweight construction systems have the lowest site impact and embodied energy. A broad range of lightweight steel footing systems is available including screw piles, which reduce excavation and site impact.

Low mass floors
When designed and built for deconstruction (e.g. screwed, not glued), flooring has a high potential for reuse at the end of its life. Lightweight floor frames are easy accessible for services and are designed to support internal and external wall, floor and roof loads.

Low mass walls
Our low mass wall construction system uses lightweight timber or steel framing as the structural support for non-structural cladding. Highly insulated, low mass houses can respond rapidly and efficiently to auxiliary heating and cooling. Our steel skeleton structures lend themselves to the creation of buildings with diverse openings for passive solar heating, natural light and ventilation.

Panel systems
Our lightweight hot swappable twin-panel systems have low thermal mass, high insulation levels, very low embodied energy and respond rapidly to heating and cooling. Increased insulation levels may be more cost effectively achieved by adding an extra layer of insulation, which has the added benefit of limiting conducted heat loss via the framing system, known as thermal bridging.

Low mass roof systems
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.

Sources:
Chris Reardon – Sustainable Building Consultant, Australia
Prof. Dr. Ir. Jos J.N. Lichtenberg – Slim bouwen, a strategy for innovation