SIPs Below Ground: Delivering Comfort Where It’s Least Expected

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SIPs below grade? You bet.

The job: Learn the correct building practices — and convince the building inspector.

Client Gerald Walsh wants a well-insulated basement in his vacation cabin so that he can comfortably use it as a living room. The local building inspector likes conventional foundations. He’s never seen below-grade structural insulated panels (SIPs), and isn’t inclined to grant a permit for them.


“At first, he just wouldn’t hear of it.”

— Stendel , reflecting on the inspector’s attitude toward below-grade SIPs, an application that is not universally recognized.


“When a building inspector refuses to give a permit for a technology like below-grade SIPs, you have to be ready to dig in,” Stendel says. “Persistence got the cabin finished on time.”

“In this case, it was the determination of the homebuyer, who was also the general contractor, that finally convinced the inspector to give below-grade SIPs a try. Despite the objections of the Orono building inspector, Walsh felt that he should be able to put in a SIPs system that was engineered at the price he was willing to pay, rather than accept a lower quality, less insulated alternative.”

“And he had done his homework. After a great deal of research, Walsh settled on SIPs from Extreme Panel Technologies of Cottonwood, Minnesota, and he wanted to use them on the entire house. Extreme Panels, with whom I work exclusively, referred Walsh to me.”


SIPs are made from a thick layer of foam sandwiched between two layers of oriented strand board (OSB), plywood or fiber cement. For below-grade, Extreme Panels offers 4×8, 4×9, and 4×10 panels with 7-3/8th inches of expanded polystyrene and 5/8ths of an inch of treated plywood on each side.

“The key to the below-grade panels is that at every four feet–at every panel joint–there is a 5-ply, 2×4 treated glulam [glued laminated timber],” Stendel says. “That is basically your stud, which serves as a beam supporting the dirt force against the outside. You stand it vertically to carry a load, just like it would carry a load horizontally.”

“The panels are set on a base of washed rock. This helps address moisture infiltration as the layers of granular rock drain the water away from the below-grade panels. Typically, you want to see at least a foot of washed rock below the floor and wall panels. Panels are sealed with panel sealant and expandable foam, then fastened with stainless steel fasteners. The exterior panel joints are covered with panel seal tape and the entire exterior is wrapped with two layers of black 6-mil poly. Backfilling is also done with washed rock to provide drainage and decrease the effects of hydraulic pressure typical in heavier soils.”

“And while the rest of the structure is conventional OSB panels, we use plywood on basements because you are working with a product that has to be approved for below-grade construction. Plywood has been proven in below-grade applications for a long time through permanent wood foundations. All we have done is taken the permanent wood foundation and turned it into a SIP, just like we have done with stick framing on the upper walls. It’s the same concept.”

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