
A Slab Appeals to Potential Buyers
The FHB House project, designed for the affordable market in Wilder, Vermont, aims to achieve a net-zero energy designation while focusing on cost-effectiveness and universal design principles. This involves a strategic balance between investing in photovoltaic (PV) arrays and optimizing the building envelope. For this production-built, easily duplicable house, the team prioritized reducing construction costs and duration by opting for a slab foundation, a decision that also caters to its target demographic of older, downsizing homeowners seeking single-level living without subterranean spaces.
The foundation system for the FHB House utilizes traditional footing and stem-wall construction, extending below the frost line. The interior of these stem walls is insulated with 2 inches of Amvic Envirosheet. A critical component of the slab's thermal performance is the Amvic SilveRboard subslab insulation, providing an R-value of 5 per inch and a compressive strength of 35 psi. This expanded polystyrene (EPS) foam is laminated on both faces with polypropylene film, enhancing its strength and durability to minimize breakage and cracking during construction. The EPS insulation is free of CFCs and HCFCs and is available in various compressive strengths to suit different structural requirements.
Historically, the Biebels, the builders involved, have explored the use of ICF (Insulated Concrete Forms) for stem walls. However, they returned to formed stem walls after considering the additional costs associated with protecting the exterior foam of ICFs. For the slab insulation itself, they specified 4 inches of Amvic SilveRboard, achieving an R-20 insulation value. Prior to the concrete pour, a 15-mil vapor barrier was installed over the insulation. All seams of the vapor barrier were overlapped and meticulously sealed with tape, as were any penetrations through the slab. At the edges of the slab, the vapor barrier was extended up the side and over the top of the stem wall, then draped long. Following the erection of the walls, this vapor barrier was taped securely to the exterior face of the wall sheathing, ensuring a continuous moisture barrier.
This approach to the slab foundation demonstrates a concerted effort to balance energy efficiency, construction costs, and market appeal. The choice of a slab-on-grade not only streamlines the construction process and reduces material usage but also aligns with the accessibility needs of potential buyers. The robust insulation and vapor barrier detailing are crucial for achieving the net-zero-ready goal and maintaining a comfortable, energy-efficient interior environment. The selection of specific insulation products like Amvic SilveRboard highlights a commitment to high-performance, durable materials in the pursuit of affordable and sustainable housing.
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