Over the past few years, we have turned to Durisol insulated concrete forms (ICFs) several times. They offer an attractive blend of sustainable features with the convenience of conventional methodology.
For the teachers’ union office project, we needed a short stem wall to raise the walls of the building a suitable height above grade. There is no basement, and the footing is a shallow, frost protected perimeter beam, so the stem wall is only 2 feet tall. Durisol blocks provided us with a solution that worked well for several reasons. The blocks have a high insulation value (R-21 to R-28), are made from a very high percentage of recycled content, and are produced within a reasonable distance of our building site. They form a 5.5-inch concrete wall, using much less concrete than a full foundation wall (typically 8-12 inches wide).
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- R-28 Durisol blocks are laid as the first course of the stem wall. They are dry stacked on the footing with the Roxul insulation on the interior face
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- Horizontal rebar is inserted between courses and pinned in place. Vertical bars are added in each cavity
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- Narrower 12-inch Durisol blocks make up the second course. The 2-inch lip on the inside will support the floor joists
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- Hempcrete is used to fill in the gaps created by butting the Durisol blocks together around the curve
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- The site mixed hempcrete has a very similar texture to the Durisol. One is wood chips mixed in a cement slurry, the other is hemp chips in a lime slurry
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- The cores of the Durisol blocks are filled with concrete
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- The completed stem wall is ready for floor joists and walls to be added
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- A coat of lime-cement plaster is applied to the Durisol blocks as the base of the waterproofing
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- A water-based, no-VOC liquid rubber is painted on the below-grade portion of the wall, and a dimpled drainage layer is added over top
In addition, the blocks come in several widths which for this building meant that we could use the 14-inch wide blocks as the first course and 12-inch wide blocks on the second course, leaving us with a 2-inch lip on which we could rest our floor joists. By keeping the floor joists within the insulated walls, we minimize thermal bridging and simplify air tightness at this important seam.
The Durisol blocks are dry stacked on the footing and on each course, making them very fast to install. They can be cut with a regular circular saw when necessary, and take a wood screw very securely. For our curved foundation, we did not attempt to cut the blocks to match the curve. Rather, we set them so the inside edges were touching on the curve and used a site-mixed hempcrete to fill in the gaps.
A lime-cement plaster is used to coat the exterior of the blocks, and we used a water-based, no-VOC liquid rubber and a 100% recycled content plastic dimple mat to complete the waterproofing layers on the exterior.
While this foundation wall still has relatively high embodied energy (due to its cement content), its energy efficiency, the lack of foam products, durability and the ease of construction make it an excellent option.
