SIPs and Wood Savings

SIPs and Wood Savings

Congratulations on an overall very good article on the current state of Structural Insulated Panels (SIPs) [EBN

Vol. 7, No. 5, May 1998]. Well-researched articles on building systems are few and far between. I want to add some hopefully helpful thoughts on the subject.

The basic engineering premise of SIPs for buildings was begun in the mid ’30s at the Forest Products Lab in Madison, Wisconsin. The early panels were not insulated and relied on core spacers of an egg-crate design very similar to hollow-core door construction. The original test buildings are still in use in Wisconsin and stand as testimony to the durability of the basic engineering.

New technologies have made SIPs more and more material-efficient. For example, prior to 1980, plywood and waferboard (the predecessor to OSB) were the only commonly available materials for use in SIPs. Both materials were generally limited to 4’x8’ in size and necessitated splines —usually dimension lumber at four-foot centers. It was also common to use a double top plate of dimension lumber in a wall panel and extensive lumber around window and door openings, since this was the normal practice in stud framing. In the early 1980s, OSB became available to manufacturers in sizes up to 8’x28’, thus dramatically decreasing the need for dimension lumber splines. The latest OSB mills are capable of producing OSB in a

continuous process, allowing OSB panel length to be limited only by shipping constraints. Just as important, it allows the OSB to be cut to optimum length for SIP use with much less waste and many fewer splines.

Continuing engineering refinement and testing of SIPs is allowing further reductions in the use of dimension lumber. Some examples:

•reducing top plates to single vs. double;

•using SIPs as window and door headers to reduce dimension lumber and significantly reduce thermal losses;

•extending SIPs to mud sill and to second floor deck level, thus further reducing lumber and thermal loss; and

•using the extreme racking resistance and diaphragm action of SIPs in the engineering of roof and walls to further reduce the use of beams, headers, and posts in wall and roof design.

Most of the comparison studies on the use of wood in SIPs do not recognize the latest technological improvements to the SIP system and, as a result, significantly overstate the amount of wood fiber (and, in particular, dimension lumber) use in SIP building systems. Not all SIP manufacturers have adopted these improvements. The cost of changing processes or details has slowed many in adopting the latest technologies. As SIP use increases, material technologists are developing materials specifically for SIP applications to provide better performance in strength, resource efficiency, environmental compatibility, thermal performance, and labor efficiency. The fundamental physics and engineering of SIPs, which is high performance with relatively little material used in an optimum fashion, will not change. New technology will enable SIPs to perform this function even better. It is still more fundamentally sensible to use petroleum stock to produce high-performance, durable insulation than it is to burn the petroleum to heat and cool poorly insulated structures.

On the insect issue, there is much misunderstanding. There is no evidence that foam insulation in SIPs is more susceptible to carpenter ant or termite attack than other insulating materials. Carpenter ants are known to frequent all types of insulation for nesting and any other easily excavated material, including clothing and other materials stored in attics, closets, etc. Termites behave differently. Termites consume the wood or other cellulose and use the wood both as a food source and a medium to travel to more food. Subterranean termites do not typically nest in insulation; they nest deep in the soil under or around a structure and travel back and forth from the food source.

We generally do not recommend aggressive pesticide treatment programs unless the area is known to present a significant risk. SIP homes should be considered for treatment in the same fashion that any other structure should be in higher-risk areas. Northern climates generally are much less prone to termite problems. The unfortunate assumption often made in using treated foams is that they provide the entire structure and its furnishings with protection. This is far from the case. In fact, in most situations, treated foam in SIPs does little to deter these insects from their normal paths into a structure. The wood in the termite’s case and human food in the carpenter ant’s case are the food sources these insects seek. Insulating materials (including almost all types of insulation) are only one of many pathways they will use to get to the food source.

Frank B. Baker, President

Great Lakes Insulspan

Blissfield, Michigan