The wall section and partial plan below describe our exterior wall assembly in more detail. Note the dual insulative layers that work together to minimize thermal bridges.
Along with insulation, air tightness (weatherization) is the key to reducing heat loss and maintaining an energy efficient and comfortable house. The Passive house standard, which we hope to meet with this building, has extremely stringent requirements regarding air tightness compared to conventional construction: The building must not leak more air than 0.6 times the house volume per hour (n50 ≤ 0.6 / hour) at 50 Pa (N/m²) as tested by a blower door.
Why is air tightness important? Increased comfort, reduced energy use, and building longevity. Comfort in increased by reducing interior temperature fluctuations, protecting from drafts and cold winds, and reducing dust infiltration.
Reduced heat loss occurs when we reduce the convective flow of heat by reducing the amount of air that passes through the exterior envelope. In the US, buildings use one third of all energy consumed and two thirds of all electricity. Due to the high energy usage, they are a major source of the pollution that causes urban air quality problems and pollutants that contribute to climate change. Thus, reduced energy consumption though airtightness results in less pollution and smaller electrical bills.
Airtightness contributes significantly to building longevity because by eliminating air flow through the wall, less moisture can penetrate the building structure and degrade it. Indoor air has a higher water vapour content than outside air. In cold weather, indoor air is cooled while escaping through the wall. As it cools, it cannot retain its water content and condensation will occur within the wall which can lead to serious damage.
The airtight layer (shown in red) is not a single substance or material, but a series of surfaces (the outer surface of the SIPs, the windows, the vapor barrier in the floor, with all joints sealed with spray foam or mastic and securely taped) working together to form a single barrier. In designing the house and its wall system, we made sure to maintain a continuous and clearly defined air barrier without major structural penetrations or ins-and-outs in order to reduce the risk for gaps and weak spots. For this reason the outer surface of the windows aligns with the outer surface of the SIPs, the roof structure was built without overhangs, and the balcony support structure is entirely outboard of the air barrier .
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