For homeowners building on the iconic granite of the Canadian Shield, the prospect of anchoring a house directly to solid bedrock is both appealing and daunting. While it offers unparalleled structural stability, this construction method comes with significant, often overlooked hazards that require careful planning to overcome.
The Invisible Moisture Threat from Cool Rock
Building a foundation on bedrock is structurally excellent, but the first major danger is pervasive moisture. Experienced builder Steve Maxwell, who has built his own home on bedrock, explains that the rock's naturally cool temperature in summer acts as a powerful condenser for basement humidity. When warm, moist outdoor air enters a basement with exposed bedrock, it can cause humidity levels to soar, potentially reaching 100%.
Maxwell emphasizes that simply ventilating the basement with outside air will not solve this problem, as the outside air itself is the source of the moisture. He presents two primary solutions for anyone planning a basement. The first is to fully insulate the bedrock with rigid foam insulation and then pour a concrete floor on top. The second, more straightforward option is to forgo a basement entirely, constructing foundation walls that extend down to the rock without an excavated living space below grade.
Managing Water Flow and Securing the Foundation
If choosing an insulated floor, Maxwell advises using a generous layer of crushed stone beneath the foam insulation. This layer creates a path for groundwater from rain and snowmelt to flow away from the building. He notes from direct experience that water will percolate through soil, hit the impermeable rock layer, and then travel along its slope. This water can seep between the foundation wall and the rock, or even through a solid concrete wall, despite robust waterproofing efforts. To manage this, he recommends directing water to a sump pit equipped with a pump.
The second critical issue is mechanical stability. Even on relatively flat bedrock, Maxwell strongly advocates for pinning the foundation wall to the rock using corrosion-resistant fiberglass rebar. The rebar is set into holes drilled into the granite and secured with epoxy adhesive. A typical specification involves installing a pair of 5/8-inch rebar rods every 24 inches along the length of the foundation wall. This prevents any potential movement over time and is especially crucial on sloped building sites. He recommends having a structural engineer review the final foundation plans to ensure all details are correct.
Solving the Winter Humidity Dilemma
In a related question about home comfort, Maxwell addresses a common Canadian winter woe: the battle between dry air that irritates skin and indoor humidity that causes window condensation and even ice formation, particularly behind closed curtains.
He identifies the root of the problem: many modern windows lack sufficient insulating value. The solution lies in improving the thermal performance of the windows themselves. The more energy-efficient a window is, the less likely it is to cause condensation during cold weather. Better windows allow for higher, more comfortable indoor humidity levels without the nuisance of water dripping or freezing on the glass, ultimately creating a healthier and more pleasant living environment throughout the harsh winter months.
