Issues relating to energy and greenhouse gas emissions are high on the political agenda in many countries. And a lot of interesting things are happening in the field of household energy consumption.

A normal Swedish single-family home uses about 24,000 kWh of energy per year. About 25 percent is used for lighting, appliances and home electronics, 19 percent is used to heat water and 56 percent is used for heating.

But heat energy leaks from the house through walls, ceilings, doors, windows, and ventilation, hence the Swedish expression: ‘heating the crows,’ which means keeping the birds warm.

Some initiatives are driven by legislation and subsidies. But the key drivers for many households relate to the economy and increased environmental awareness. As a result, the construction industry faces many challenges in terms of material selection, design solutions and construction methods.

The Swedish company Zenergy has developed a sandwiched wall design that is based on the concept of ‘Structural Insulated Panels’ or SIP. According to acting CEO Leif Thörnvall, SIP offers many environmental advantages over conventional technologies.

They include:

  • Lower energy losses through the walls
  • A tight building shell that reduces the risk of moisture damage in buildings
  • Better control of the indoor climate
  • Rational construction methods with fewer components and shorter construction time

A wall built like a sandwich

Building walls with SIP is nothing new. SIP is a sandwich construction consisting of an insulating foam core between an outer and an inner structural element.

The technique has been used for a long time in the automotive and boat industry and interest in SIP is increasing in the construction industry.

With SIP, it is possible to build houses of varying sizes and designs. In the U.S., hundreds of houses and shopping malls have been built with the SIP method and it is growing in use by about 25 percent per year accounting for about one percent of the construction market.

In Sweden, about a dozen houses and nearly one hundred industrial buildings have been built with SIP. One of the advantages is that the wall elements are produced indoors in a factory and delivered ready to the site. In the case of a smaller building, the entire assembly can be done indoors.

“We see great potential for SIP in Sweden and many other countries,” says Thörnvall. “The wall structures that we developed have particular characteristics in terms of coating and core materials. Our building shell consists of a material that is impervious to moisture and mold and the sandwich construction is completely airtight. The leakage of heated air to the surroundings is therefore low, which is advantageous from an energy perspective.”

Common sandwich structures

In a sandwich construction, thin layers of a relatively strong material are joined together on either side of a thicker layer of light yet strong material. This yields a sheet material that is both lightweight and durable. An example of a sandwich structure would be foam between aluminum plates.

This is common in refrigerators and mobile homes, for example. Another type of design is the ‘honeycomb’ of cardboard between two slices of masonite and is common in interior doors and cabinet doors. Sandwich structures are also used in boats, where a core of hard foam is placed between two layers of fiberglass.

SIP technology was first used in the construction industry back in the 1930s. Since then, many different types of materials have been tested for the outer and inner cover/layer and the insulating core. The most common material combinations consist of OSB panels (Oriented Strand Board, a fiberboard) which is like a sandwich around a core of expanded polystyrene, extruded polystyrene or rigid polyurethane foams. Alternatives to OSB include plywood, steel, aluminum, cement, fiber reinforced plastic and other materials.

Energy savings with SIP

To reduce heat loss in a small house, it is important that the exterior walls, roof and foundation are properly insulated. How well a building is insulated is rated according to the so-called U-value.

This value describes how much heat is lost to the surroundings per square meter, at a one degree difference between indoor and outdoor temperatures. The lower the U-value, the better the insulation.

With traditional insulation materials and methods equivalent to ten centimeters of insulation, one achieves a U-value of around 0.5 W/m2K (Watts per square meter Kelvin). To lower the U-value to 0.1 W/m2K would require 50 cm of insulation of the traditional type, which of course is impratical. With SIP, Zenergy achieved a U-value of 0.16 W/m2K with only 10 cm of insulation.

The combination of materials in a sandwich construction provides nearly twice the insulation value of traditional insulation made of glass wool and mineral wool.

If the walls of all of Sweden’s single-family homes were constructed according to the SIP method, 2.5 TWh of energy could be saved each year. This corresponds to half of the amount of energy currently produced by wind power in Sweden.

(Want to learn more about different techniques that contribute to energy savings and reduced environmental impact? There are many other articles on environmental benefits under the heading ‘Buildings,’ top left)

The article was published in November 2011