Insulating your foundation to meet new energy standards

By Charles Zave

This past summer, two of my customers inquired how they can insulate their basements to meet soon-to-be net-zero building codes. As a foundation specialist, I knew the procedures needed to retrofit foundations so that the heating requirements are minimized.

At first, the techniques may seem simple. You apply 8 inches (R 30) of below-grade polystyrene or Styrofoam on the exterior of the wall, and the results will manifest. Of course, nothing can be further from the truth. The real work requires adapting procedures to individual foundations. The procedures depend on the foundation’s age and structural integrity. Here is a brief outline of what to look for and what processes to use.

Poured concrete foundation

Let us first look at the easiest foundations to treat and insulate. The most important thing: there cannot be any holes, no matter how small, where cool air or moisture can penetrate. For a poured foundation with no surface degradation, the newly excavated foundation should be cleaned of all debris, then apply two sealing coats of a tar base plastic cement. It is essential that the top of the footing be cleaned of all tar residue one inch from the joint between the footing and the foundation wall. The reason will be apparent in a moment.

The second step requires that the ship-lock insulation be applied so that the finish height is exactly level to the top of the foundation. Insulation glue should be applied between each ship-lock joint between the polystyrene pieces to ensure a tight air seal. The polystyrene is then bolted to the foundation using either expansion bolts or tapcons. I prefer tapcons since the smaller diameter means less cold will be transferred along the metal surface to the interior foundation wall. As mentioned, it is very important that the top of the polystyrene is at the exact precise height. This, of course, means there may be small gaps under the insulation above the footing. Because there is now no tar, these gaps can be filled with a 100-per-cent waterproof hydraulic rebuilding cement like MeadowPatch 5.

Once the insulation is installed, it only has to be capped before a protective coating is put on. On homes with siding, metal flashing can be used. On a brick home, a specially designed triangular cap is glued and bolted on top of the new insulation. The angle of the triangular top piece allows the rainwater to roll away from the brick to prevent moisture penetration. The triangular top finish should cover the two bottom rows of brick to help prevent air passing through the wall at the point between the foundation and the above wall. Afterwards, the surface of the insulation should be roughened with a brush before applying a coat of Thermoshield. This is the bonding agent between the polystyrene and the parging mix that will protect the insulation against damage and loss of R value through exposure to UV light.

Rubble foundation

Other foundation walls such as rubble or stone require more intensive preparation before the above procedures can be applied. In the case of rubble, the wall must be cleaned of all loose and structurally compromised debris. This often leaves considerable cavities in the walls. A smooth concrete skin is then applied to the exterior of the wall. After forming, a flow cement is used to make sure that all areas are filled and covered. The same techniques as for poured foundations can then be applied. Because there is no footing under most rubble or stone foundations, a small concrete bench can be poured to sit the insulation on (it will also act to prevent water penetration under the wall), and the base of the insulation should be snugly bolted to the wall. The weight of the dirt will also keep the insulation pressed up against the new skin so that moisture does not penetrate.

Stone foundation

Finally, we come to the stone foundation. Like the rubble wall, all degraded joints between the stone must be cleaned of debris and packed with S type mortar. Forms are built so that the exterior wall is plumb and level. A flow cement is then poured from bottom to top to prepare the uneven surface for the new insulation. This unfortunately means that the stone look of the house will be no longer be apparent. For some customers, losing that look might not be worth the benefits of better insulation.

Charles Zave is a specialist in structural repair. He can be reached at Charleszave65@gmail.com or by text or phone at 613-915-8377.