- February 05 2012
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Should SIP’s (Structurally Insulated Panels) be used in Tahoe Truckee area?
Project description: 1,700 square-foot, single-family residence with 600 square-foot garage in Martis Camp
Engineer: David K. Zachary, PE of Zachary EngineeringZACHARY ENGINEERING
Contractor: Jim Riley of Truckee Custom HomesTRUCKEE CUSTOM HOMES
With SIPS, like with any other structural system, you have to proceed with caution and with some degree all the options and information on the table. When deciding between SIP’s and conventional framing, it’s not a clear cut easy choice for every project, especially in this climate. Some projects that lend themselves to SIP’s have rooms that are relatively small with reasonable distances between bearing walls. Others might be large rooms with simple shed roofs and exposed timber or steel framing to support the SIP’s. Projects that lean toward conventional framing are big spaces like grand great rooms that are 24’ x 48’ and rooms with a lot of dormers and or geometric complexity. Trying to use SIP’s in these areas often requires a great deal of architectural and structural gymnastics, which can increase costs thru-out the project. In this climate, spans and complexity can be beyond the limits of a SIP’s. This home at the size that it is, 1600 sq. ft. at the ground floor, and having an open room consisting of living, dining, kitchen and great room with exposed timbers @ < 10’ o.c., made the SIP’s option pretty straight forward.
In conventional framing and insulation there are typically areas such as conduits or blocking where insulation material cannot be blown in, leaving air gaps that can translate heat. Similar if not better R-values can be obtained via blown in closed and open cell foam and standard framing, but you end up with a thermal bridge every 16” o.c., whereas with a SIP’s you typically have a thermal bridge every 48” o.c., which is significantly better. This can easily be seen on the roof when we get a light snow or frost overnight and the snow or frost melts at the thermal bridging first, exposing the rafter locations and sometimes even the can lights. In this project, we kept all lights and conduits outside the SIP’s system, creating a much better thermal bridge between the interior and exterior.
One of the advantages to using SIP’s on this project was the erection speed. The whole roof came together in a day and a half, and we ended up with an insulated complete sealed structural diaphragm that was done and ready for roofing in 8-12 hours of framing time, which is very fast. Another advantage is that the SIP panel foam is integral with the material, there is no installer error, excellent quality control and the cavity is completely filled. So you can sleep at night knowing that you have reduced thermal bridging, created a much tighter air infiltration system and even potentially reduced ice dams with the SIP’s system.
This project utilized R-Control panels, which produces load design charts that can support SIPs for short spans (<=10’) with either I-Beam splines or PSL splines for low snow load properties. The problem occurs at the overhang which has a 1.5* (or more) the roof snow load. None of the R-Control charts will support this extreme load. It is then up to the engineer to evaluate the panels based on panel properties and data gathered via third party testing agencies.
The panel at the overhang begins to fail via transverse loading between the 4’-0” splines or perpendicular to the splines, requiring the engineer to evaluate the SIPs outside the load charts. Determining the capacity of a panel perpendicular to the splines is based on testing values by third party testing agencies that are reluctant to release results, give very conservative factors, and/or have not tested the required scenario. It is then up to the engineer to interrupt the factors and design the panel based on research they did not do. This scenario increases the engineer’s liability and decreases his or her level of comfort when we have extreme winters like the winter of 2011. Being conservative in this scenario makes sense, but can add cost that may make the SIP panel a non-cost effective framing member.
We were able to make this project work by doubling the splines at all overhangs to not only support the overhang snow load, but also the uplift from wind at the large overhangs. This was accomplished via many phone conversations with both the manufacture and several different third party testing agency/engineers. It turned out that shear and deflection controlled at the overhangs.
I was approached by my client to use SIP panels on a Green Point Rated home here in Truckee, California. SIP’s intrigued me since I had read so much about the product and had not had the opportunity to use the panels in the past.
Before the decision was made to go ahead with the SIPs, I was asked to give a comparison on what the cost would be for a conventionally famed roof vs. an SIP panel roof. This particular roof was a simple low slope 9 (2:12), shed roof. My estimation came in that the roofs would be close to the same in cost but the SIP panels would provide a roof structure in about one quarter on the time for the given roof, which could mean a lot when you live in snow country and a larger winter storm is bearing down on your area. My estimation relied heavily on the SIP fabricators input on how much labor he felt it would take a non-experienced SIP installer, myself and crew, to complete the roof system. Once the roof was complete, I was able to thoroughly compare what the actuals turned out to be and really evaluate what the differences were.
When it came to the framing/installation labor costs for both roofs, including the crane for the SIP’s, the panels beat the conventional roof by roughly 30%. When it came to the material, the conventional roof beat the SIPs by roughly 45%. This included the use of a hybrid combination of closed cell foam and batt installation to meet the SIP R-value and create a non-vented roof assembly. This large difference in material cost is partly due to the very expensive PSLs that were required as splines, and for the eaves the panels needed to meet our extreme snow loads. My material comparison used 1- 3/4 x 11- 7/8” LVLs as my rafters.
My conclusion is that the SIP panels are an excellent product. They do provide a good air barrier and they are in installed faster than a conventional roof can be framed. Using them in high snow load country does add a substantial cost to the panels due to the engineered structural members needed to reduce deflection. Overall, the cost of using the SIP panels was roughly 20% more than the conventional shed roof on this particular project. Also, with the recent introduction of closed cell foam in our area, I do question whether SIPs still stand ahead of the curve in the arena of creating a high R-value and low air infiltration building shell. Closed cell foam creates an excellent air barrier when used correctly and gives an excellent R-value per inch. Another thought for energy efficiency of the conventional roof is that for an additional 5-10% of the overall cost, the R-value could be increased by 15-30%, from and R-43 to a R-54 or R-60 respectively.