Structural Insulated Panels

Posted September 19, 2008

I consider Structural Insulated Panels (SIP) to be one of the best options for an environmentally friendly home.  This is because they form a wall that has little or no air infiltration, use less lumber than conventional framing, and in some cases the foam can be made from renewable resources.  There have even been SIPs made using straw as the insulating medium.  Also, since they are built in a factory, there is little jobsite scrap and what scrap that is produced at the factory can be easily recycled.  The SIPs can be manufactured in a variety of thicknesses, with 6 inch being the most common.  I would recommend going with 8″ for a R32 wall.

SIPs usually made of two layers of Oriented Strand Board (OSB) separated by a layer of foam insulation.  The foam can be Expanded Polystyrene (EPS), which is a foam made of small beads fused together, Extruded Polystyrene (XPS), commonly a blue or pink solid foam board, or Polyurethane foam, a solid white or beige foam.  The SIPs are made to order in a factory and then shipped to the building site.  The sections can be anywhere from 4 feet to 24 feet in width.   While the wider sections provide a better insulate wall (since there are no joints), they require the use of a crane and a larger building crew for installation.  The SIP sections are joined together during installation using a spline, often made of lumber and then sealed with low expanding foam, specialized mastic and/or SIP tape.  SIPs can be used to build not only the walls of a building, but can also be used to build the roof of the building, however, extreme care must be taken to make sure the joints are completely sealed, since if air can pass through them, it will carry moisture that can condense and produce an environment conducive to the growth of mould.  This has been the cause of structural failures in the past, particularly in very cold climates.

A Do It Yourselfer can build with SIPs, but you would have to make sure to get smaller panels that can be handled by one or two people.  The cost will be higher than building with standard framing, but will result in a better sealed home.  If you are contracting the work out, or doing your own general contracting, SIPs would be a good way to go since the labour costs would be less than for framing and insulating.

Some disadvantages of SIPs are that you have to be quite exact in your measurements when building the foundation, as the SIPs are cut to fit the plan, and field modifications require the use of specialized equipment.  The other issue is that SIPs don’t have a lot of thermal mass, but this can be offset by designing extra mass into the interior of the house.

Posted under Techniques

Straw Bale Construction

Posted September 15, 2008

Straw bale building is an alternative construction technique that uses a sustainable material for the construction of very well insulated, energy-efficient building.  Straw bale buildings are constructed using agricultural waste that has been compressed into bales.  The most commonly used size of bales are 36″x18″x14″ and are stacked in a brick pattern to create strength.  The straw bales can be stacked to produce a wall that is either 18″ thick (straw is horizontal) or 14″ wall (straw is vertical).   The straw bales can be used as the main structural support for the building (usually limited to 1 story) or can be used as infill in an open frame, made of either standard framing materials or by timber frame techniques.  After the straw bales are stacked, they are coated on both sides of the wall with a plaster.  The plaster can vary from a portland cement plaster to a natural clay plaster, all of which have their own advantages and disadvantages.   The combination of the straw bales and the plaster creates a site built Structured Insulated Panel (SIP), in which the combination and attachement of a number of relatively weak materials results in a very strong structural wall.

The advantages of building with straw bales are numerous.  The biggest advantage is that it is using a waste product for the majority of the structure, reducing the need to cut down trees to create the building, and not using as much fossil fuel as would be used for Insulated Concrete Forms (ICF).  Another advantage is the insulating properties of the straw, with a straw bale wall have an R value between 28 and 36, depending on the orientation of the bales.  In a load-bearing straw bale wall there are also no thermal bridges.  Another advantage of the technique is that it results in better air quality in the building due to the walls being able to pass moisture vapour through them.  Tests have shown that a straw bale wall coated in plaster has a high fire resistance and after exposure to a flame, the bales suffered little to no damage.  Also, due to the plaster coating, the bales are not prone to insect or rodent infestations.  The plaster, being a continuous seal, also reduces the amount of air infiltration through the walls and also acts as a thermal mass, storing heat during the day and releasing it at night.  Straw bale building is also a very straightforward process and is very accessible to Do It Yourselfers, allowing people to build their own homes and save a considerable amount of money.

The main disadvantage of straw bale building is that, being an organic material, it is susceptible to moisture damage and mould.  If water gets into the wall, mould can grow and damage the structure.  Also being fairly unfamiliar to some building inspectors, there may be some resistance to allowing a straw bale structure to be built.  With time, however, this is becoming less of a problem as inspectors become more familiar with the technique.

There are a number of resources for more information about straw bale building, including a number of websites, books, and short courses.  If an individual is not interested in doing the work themselves, there are also a number of contractors specializing in straw bale building techniques.

Posted under Techniques