Adrian Jones’ article Are SIPs Necessary (Timber Framing, March 2011) offers one builder’s perspective on the drawbacks of using structural insulated panels (SIP) to enclose timber frame structures.  Although Jones makes many good points, much of the data he provides is grossly inaccurate.  In an effort to help timber framers accurately determine the best enclosure system for their projects, the SIP industry would like to present the following data on the energy savings, labor savings, and resource efficiency of SIP construction. 

Energy Savings

Much of Jones’ argument against SIPs is based on his own calculations that show energy savings of only 1.6 percent by using SIPs instead of 2×6 framing at 24 in. o.c. with open-cell spray foam insulation.  Without seeing the exact calculations, it is impossible to determine how he arrived at that number, but other research conducted by the Department of Energy suggests Jones’ claims are off by a factor of 10.

In the 1990’s, the Department of Energy’s Oak Ridge National Laboratory began developing the concept of whole-wall R-value by constructing and testing entire wall sections for thermal performance.  When they tested 2×6 walls with studs 24 in. o.c., fiberglass insulation rated at R-19 actually performed at R-13.7.  That is a 28 percent drop due to thermal bridging.  Of all the 4-inch and 6-inch SIP wall assemblies tested, the maximum decrease was around 6 percent[1]. 

Open-cell spray foam fared better than fiberglass insulation but still experienced an 18 percent drop in R-value in 2×4 cavity with studs at 16 in. o.c.  Oak Ridge has compiled this information into a free, online calculator that builders can use to make evidence-based decisions regarding wall system performance.

A second study by Building Science Corporation for the Department of Energy’s Building America program examined the performance of 14 high R-value wall assemblies.[2]  Unlike the Oak Ridge data based on physical testing, this analysis was done using Therm 5.2, a software program developed by Lawrence Berkeley National Laboratory to examine heat transfer through building components.[3]  Although Jones claims that thermal bridging “is not the demon that SIP manufacturers would have you believe,” the report found that the assembly Jones is recommending (2×6 wall with studs at 24 in. o.c. and 0.5 pcf open-cell spray foam insulation) experienced a drop in R-value of 21 percent due to thermal bridging, arriving at a whole wall R-value of R-16.5.  Their examination of a 4 in. SIP wall showed a drop of only 6 percent, consistent with the Oak Ridge findings. 

Equally disturbing is the lack of discussion regarding air infiltration.  It is not clear whether Jones omitted this information because he felt air infiltration is not a factor in determining energy savings or because he assumed that his wall system and the SIP wall system were equal in terms of air leakage.  In either case he is incorrect. 

Ongoing research by Building Science Corporation estimates that air leakage accounts for 30 to 50 percent of energy loss in a home[4].  For this reason, blower door tests have become a crucial part of ENERGY STAR, Passive House, and even the 2012 IECC. 

Spray foam insulation can do an excellent job of air sealing if it is installed correctly.  However, cavity wall assemblies have difficulty reaching the performance levels of continuous insulation systems like SIPs and ICFs.  

This was demonstrated by the Zero Energy Building Research Alliance, a residential building research coalition that includes the Tennessee Valley Authority, the Department of Energy and Oak Ridge National Laboratory.  Working with a local builder, the alliance constructed two identical homes: same floor plan, same design, same windows.  One home was constructed with 6 in. SIPs with an EPS core.  The second home used 2×6 framing at 24 in. o.c., with the “flash and batt” method of applying ½ in. of spray foam for air sealing and R-19 fiberglass batts for insulation. 

When a blower door test was conducted, the SIP home tested at 0.74 ACH50, 40 percent better than the flash and batt home[5].  The SIP required 20 percent less energy to heat, despite having an R-35 SIP roof compared to R-50 roof insulation in the flash and batt home.

Structural testing of SIPs over timber frames by Rob Erikson and Dick Schmidt (Timber Framing, June 2002) notes that SIPs add stiffness and lateral load resistance to timber frame structures, offering the potential to reduce cost by eliminating the required amount of knees braces and other structural members.

Similar lateral resistance testing for wood-frame walls attached to timber frames has yet to be done, so engineers are at a significant disadvantage when attempting to properly engineer such structures. 

Another commonly overlooked cost in construction is the overhead cost related to longer build times. Since construction loans run for the duration of the build and business overhead costs also continue through the build cycle, the savings by finishing a few weeks early can be significant.

Time and labor savings

In 2007, chemical company BASF commissioned R.S. Means to conduct a time and motion study on the speed of SIP construction.   R.S. Means is a division of Reed Construction Data, the leading supplier of construction cost information in North America.  Engineers from R.S. Means observed the construction of a two-story, 2,300 sq. ft. home by an experienced SIP framing crew.  Their results showed 55 percent labor savings over conventional framing[6]. 

The study also found that the electrical subcontractor completed the rough wiring 11 percent faster on the SIP home than a conventionally framed home.  In many cases, the SIP installer can reduce electrical costs down by meeting with the electrician prior to installing the panels to make sure they understand how to wire SIP homes.  Installers can also assist by ensuring the correct chases are marked and maintained between panels.  With the proper planning, electrical work on a SIP home is less time consuming than a conventionally framed structure.

Waste

Factory waste is dealt with differently by each SIP manufacturer, but most manufacturers are making efforts to minimize the amount of waste generated and recycle as much as possible.  It is in the manufacturer’s best interest to maximize their yield, and this is made possible by producing a variety of panel sizes and using design optimization to reduce fabrication waste.  Large pieces of scrap are kept for use as dormer cheeks or small panels underneath windows on later projects.

With unusable EPS panel scraps, it has become standard practice for SIP manufacturers to separate the foam core from the OSB.  The foam is then ground up and either recycled into lower-grade EPS products or used as bulk fill insulation. 

Manufacturers are exploring ways to deal with scrap OSB, including grinding it up for livestock bedding.  Another option emerging is using ground SIP scraps as a component of lightweight growing medium for vegetated roofs.  Some manufacturers offer their scraps to the public.

Saying that scrap lumber can be used as firewood ignores the immense environmental impact created by harvesting, processing and transporting that lumber to the jobsite.  Two recent studies have examined the environmental impact of SIPs over the product’s lifecycle, one conducted by BASF and a second Life Cycle Analysis on SIP walls conducted by Franklin Associates, a well-known lifecycle analysis consulting firm.  Both studies show that SIPs have a net positive impact on the environment by preventing greenhouse gas emissions through reduced heating and cooling costs.  And both studies show that SIPs outperform wood framing and fiberglass insulation when these environmental factors are considered. 

Conclusion

The SIP industry recognizes that SIPs are by no means “necessary” or the only option when it comes to enclosing timber frame structures.  In some situations, SIPs will not be the best option because of cost constraints, site accessibility, product availability or a number of other reasons.  But an honest examination of enclosure systems needs to look at the research that has been conducted on SIPs and other enclosure systems so that conclusions can be drawn from verifiable data.

Contrary to the Mr. Jones’ belief that the nation’s building scientists are still struggling to figure out Microsoft Excel, the examination into the most cost effective methods of energy-efficient construction is more active than ever.  There are a number of free tools and software programs available online from the Department of Energy that allow builders to conduct their own energy analysis.  And millions of dollars of research on energy-efficient construction (such as the studies cited in this article) is part of the public domain and easily accessed online. 

With these tools in hand, we encourage timber framers to work with a certified HERS rater or energy consultant and find the right enclosure system for their clients and their business model. 

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