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Spray-foam insulation has become extremely popular for both new homes and old ones too.  Its energy efficiency ratings are better known than some of the health and environmental safety risks.  Spray foam is a petrochemical based substance that often contains benzene and toluene.  Some spray foams are safer than others, but at this point we don’t endorse the use of any of them.  On one job that we did for the Stowe land trust, an uninsulated ceiling in a timbered structure had been coated with spray foam.  The foam concealed water damage from a leaky roof and held moisture that destroyed all of the rafters and a plate.  Once spray foam is applied directly to timbers the patina and historic markings suffer irreversible damage.  The properties of spray foam insulation are inappropriate for use with historic structures.  In another circumstance, a homeowner that we built a frame for chose to insulate with spray-foam.  One of their young children developed health problems that were eventually traced to an incorrect mixture of the spray-foam containing toxic materials not suitable for residential applications.

The first rule of historic preservation is ‘do no harm’.  Modifications or alterations to historic structures should always be done with attentiveness to ensure that they are reversible and to maintain as much of the original integrity and detail as possible for future generations.  One simple technology that we have come to favor for insulating both new and old structures is the Larsen truss.  The basic idea of the Larsen truss is a rigid, lightweight envelope that encases a structure.  The light framing provides an insulation cavity without the thermal gaps found in modern stud framing.  There are numerous, viable insulation types that can be used with this system.  The depth of the insulation cavity can be chosen to accommodate the desired ‘R’ value of a specific insulation material.  The Larsen truss system can be done  cost effectively and helps to support the local economy through the use of local materials and labor.

Although many of the properties of the Larsen truss system may seem similar to those of SIPS (structurally insulated panels) there are some distinct differences.  Many manufacturers of SIPS have modified their formulas to lower the amount of VOCs (volatile organic compounds) and use OSB (oriented strand board) that has ‘only trace amounts of formaldehyde’ in the adhesives.  We feel that there is no room in good building practices for any amount of these toxic substances.  While the amount of volatile chemicals in the finished product may be minimized, the production of these panels is still chemically intensive and unnecessary.  History has shown us other building and insulation products that were thought to be flawless when first brought to market, that turned out not to be so environmentally friendly in the end.  Aside from associated health and ecological concerns, we have our reservations about the structural integrity and longevity of SIPs.   Moisture or water leakage can cause irreversible swelling in OSB, particularly at the edges.  We always try to consider what kind of future maintenance scenarios might arise when choosing materials and techniques.  Although SIPS have been shown to hold up well under ideal conditions, we try to plan for potential neglect.  Since the SIP’s are a unit, the failure of any part of the sip may require the entire panel to be removed.  This would mean disturbing the, often delicate, interior finish details as well as pluming and electric.  The use of a Larsen truss system offers the option of replacing insulation materials in the future without disturbing utilities or interior finishes.