SIPs consist of two layers of plywood or OSB that “sandwich” an inner core of insulating rigid foam. Panels are spliced together using splines, consisting of strips of OSB, SIPs, or 2x4 or 2x6 lumber. The seams where one panel joins another are susceptible to air leakage. To form an air tight bond, spray foam or caulk is applied to the seams before the panels are connected. Many SIP manufacturers will provide the caulk and instructions. The wall-floor, wall-wall, and wall-roof seams can each require as many as six beads of caulk, and the roof ridge seam can require up to 8 beads of caulk.
After caulking, the panels are fit together to assemble the structure. To ensure that joints lock tightly together, a belt winch can be used to pull wall assemblies together; this is especially helpful with larger panels. Before drywalling, the seams can be covered with peel and stick tape as a second layer of protection against air leakage. Before installing drywall is also an ideal time to test the air tightness of the seams with a blower door test and smoke pencil to visually identify the location of any leaks.
Air barrier effectiveness is measured at the whole-house level. High-performance branding programs and the 2009 International Energy Conservation Code (IECC) require that builders meet specified infiltration rates at the whole-house level. See the “compliance” tab for these specified infiltration rates.
For more information on SIP installation, see Structural Insulated Panels.
How to Seal SIP Seams
1. Connect the panels with splines. The three most common splines are surface splines, insulated splines, and structural spines (see Figures 1, 2, and 3). To minimize thermal bridging, only use structural splines when needed to carry the structural load. Splines can be installed on one side of the panel in the factory; this can save assembly time at the site.
Figure 1 - A surface spline reduces thermal bridging much more than a structural spline at SIP panel seams.
Figure 2 - An insulated spline is another option for avoiding thermal bridging at SIP panel seams.
Figure 3 - A structural spline made of a solid 2x is used where needed to meet structural load requirements at SIP panel seams.
2. Caulk on each side of the spline. Use caulk from the manufacturer, if provided. Follow the caulking schedule required by the SIP manufacturer for the number of beads of caulk to use at each type of seam. Make sure the beads are continuous. Consider using a power caulker; even in a small (1,200 sq. ft.) home, the amount of caulk required can total over 5,000 lineal feet of caulk.
3. Assemble the walls and roof. Use a fork lift and crane to place panels. Use lift plates and a belt winch (available from the manufacturer) to pull panels together tightly, if needed (Figure 4).
Figure 4 - Lifting plates attached to the wall provide good bracing to tighten up SIP panel seams.
4. Install peel-and-stick tape at panel-to-panel seams and at the ridge and wall-roof interface (See Figure 5).
Figure 5 - Peel-and-stick panel tape provides added assurance that SIP panel seams will remain airtight.
5. Prior to drywalling, conduct a blower door test and use a smoke pencil to ensure that panel seams are tight (Figure 6).
Figure 6 - Use a smoke pencil to check for air leaks at SIP panel seams, especially along the ridge beam.