Air Sealing SIPs

Durability & Energy Savings

Air sealing SIPs properly provides huge reductions in energy consumption through reduced air infiltration. This is not the only purpose of air sealing, the other is building durability. Air infiltration and exfiltration carry large volumes of water vapor through the envelope’s thickness. In northern climates, air exfiltration allows condensation within the envelope, particularly near the exterior sheathing. Long term water accumulation often leads to expensive and recurring repairs.
Air sealing done properly eliminates both the energy consumption and the durability problems. To meet these needs, the air sealing system must remain robust for decades, be easy to assemble on the job site (under New England’s weather conditions), and be cost-effective. Unfortunately, there doesn’t seem to be one material or solution that meets these needs for all the different types of panel joinery. The following list illustrates the best sealing system for different types of connections

Sealing Details
Connection Materials Sealant
Panel Core Foam-to-Panel Core Foam Spray Foam
Panel Core Foam-to Wood Panel Mastic
Other Foam-to-Wood Panel Mastic
Wood-to-Wood Panel Mastic

Sealing Locations
Location Seal Type
Surface Spline Joints Spray Foam Drill & Fill
Shoe-Plate-Roof edge & RO-Blocking-to-Panel Core Foam Panel Mastic
Structural Splines & I-Joist Splines Panel Mastic +Tape
Structural Corners Panel Mastic + Tape
Timber-Frame Style Corners Spray Foam Gap
Structural Eave and Rake Connections Panel Mastic + Tape
Timber-Frame Style Eave & Rake Connections Spray Foam Gap
Ridge-Hip & Valley Joints with Embedded Lumber Panel Mastic + Tape
Mechanical Penetrations Spray Foam Gap
Chimney & Flue Penetrations Empty Gap

Spray Foam Drill & Fill

Drill & Fill Sequence

Drill & Fill Sequence

The goal with the Drill & Fill method is to create a continuous seal between the two foam panel cores. For best results, ensure that all splines are fastened before air sealing begins.
Drill 1/2” dia. holes into the panel joint, through the spline, and to the foam channel. Drilling beyond the foam channel is not harmful.
A Properly Drill & Filled Roof

A Properly Drill & Filled Roof


Insert the foam gun nozzle into the hole down to the depth drilled. Inject foam into the joint with a 2 to 4 second blast. As the spray foam is being injected, withdraw the foam gun, pausing at the foam channel. The ideal duration of the foam blast depends on temperature and humidity. The goal is to fill the joint with enough foam so, as it finishes expanding, foam backs up out of the hole to form golf ball sized “plugs” on top of each hole.
The best gauge of foam blast duration is to experiment on scrap panel. It is critical that the foam chemistry be kept above 60 degrees F. The panels don’t have to be that warm, but the cans of foam must be. The foam plugs should be left in place for at least 24 hours because the foam in the middle of the panel can take hours to cure.

Spray Foam Gap

For some ridges, hips, valleys, and timber-frame corners, intentional gaps are left between panels to be filled with spray foam. Fill the full depth of the gap with spray foam. Once filled, leave the foam 24 hours to harden before breaking off any protruding foam.

Section View of Spray Foam Gap at Timber-Frame Corner

Section View of Spray Foam Gap at Timber-Frame Corner

Finished Spray Foam Gap at Corner

Finished Spray Foam Gap at Corner

Panel Mastic

Panel mastic is applied in a single, straight bead on the mating surface of one component, often wood, and then the opposing component is installed so that the bead is compressed in between over the whole length of the joint.
Bringing both mating materials into firm contact with the bead of mastic is critical. In cases where conditions prevent this, see the Spray Foam Alternate To Panel Mastic.

Panel Mastic Seal

Panel Mastic Seal

Panel Mastic + Tape

In panel joints that contain full-thickness lumber, we take the belt-&-suspenders approach of using panel mastic and an air sealing tape. Tape should be applied to joints that contain sawn lumber, SCL, and I-joists. The tape should be applied on the warm-side of the joint (the interior side in New England).

I-Joist Spline

I-Joist Spline

Eave Connection

Eave Connection

Structural Spline

Structural Spline

Spray Foam Alternate to Panel Mastic

In cases where panel mastic is called for, but the joint will not fit tightly enough for panel mastic to be practical, spray foam can be used.
Fully assemble and fasten and fasten the connection. Drill 1/2” dia. holes at an angle to access the void space and inject foam using the same basic method as the Spray Foam Drill & Fill method described above. Use enough spray foam to completely fill the void in the connection. Use the size of the foam balls created at the injection Sites as a guide.

Spray Foam Alternate Drill & Fill

Spray Foam Alternate Drill & Fill