The truth is, Nature wants to turn buildings into mulch, and her primary tool for doing so is water. Water plus oxygen plus wood equals rot, at least at the mild temperatures of the Pacific NW. And rot means building failure, aka mulch.
So the key to ensuring building durability is to keep building assemblies dry enough to prevent rot and to facilitate the drying of assemblies whenever they do get wet. Our buildings need to manage bulk water (rain, mostly), airborne water vapor, vapor transmission at the molecular level (across assemblies from areas of high vapor concentrations to low vapor concentration), and condensation.
The old way of doing all this was to build walls full of air leaks, clad them well, put on a good roof, and blast the building with heat from a fireplace, boiler, or furnace throughout the winter. Even with this blast of internal heat, the leaky walls might accumulate moisture during the winter in our Pacific NW climate, but those walls would dry out during the summer: all that air and heat passing through them did the trick. It was a resilient approach. Unfortunately it is also wasteful of energy, uncomfortable, and leads to poor indoor air.
So how do high performance buildings ensure durability? A prerequisite is to complete the craft necessary to eliminate bulk water intrusion, of course. With that accomplished, the first priority of high performance building is airtight construction because by controlling the movement of air we also control the movement of moisture and heat. We know that one of the quickest ways to drive moisture into a wall is through an air leak. Warm interior air carries water vapor with it into the wall assembly, and if it hits a cold surface, that vapor can condense into liquid water and wreak havoc. Airtight construction stops air movement – and therefore the movement of airborne vapor – into building assemblies.
The next step in high performance building is thermal bridge-free construction. Because thermal bridges (any building element that cuts across and bypasses a building’s insulation or “thermal envelope”) create cold penetrations through otherwise warm parts of building assemblies they can become focal points of condensation, moisture build-up, and rot. By eliminating or mitigating these thermal bridges, high performance building removes these condensation-inducing cold points in building assemblies.
With airtightness and thermal bridge-free detailing dialed in, we then turn our attention to managing vapor drive, the movement of vapor molecules from areas of high concentration to areas of low concentration. As we do this, we keep close tabs on the dew point (a function of heat and humidity) to ensure that condensation doesn’t occur where it can do damage. To guide our work, we harness the power of thermal and hygrothermal modeling software, charting the behavior of moisture and heat through assemblies over time. Generally speaking, in the Pacific NW, our hygrothermal analysis leads to wall assemblies that are “vapor open” in both directions, into and out of the building. If, for some reason, moisture concentrations build up, that moisture can then readily escape the assembly.
To increase the drying capacity of high performance wall assemblies, we add ventilated rain screens. The ventilated cavity behind the cladding facilitates lots of airflow (measured in the tens to hundreds of air changes per hour) across the face of the wall assembly proper, dramatically increasing the wall’s drying capacity and, therefore, resiliency. This air movement also allows the cladding itself to dry thoroughly, increasing its lifespan and reducing the maintenance requirements of the building exterior.
This high performance approach to building durability is informed by decades of building science insight. By definition, super-insulated building envelopes limit the flow of heat across them, which, all things equal, would inhibit their ability to cope with moisture. But by stopping airborne vapor intrusion, eliminating condensation-inducing thermal bridges, managing vapor drive, and augmenting drying capacity with ventilated rain screens, the durability of high performance buildings exceeds that of the old, leaky approach to construction. This is why the US Department of Energy rates Passive House buildings as so much more durable than conventional ones.
Nature’s desire to turn our buildings into mulch is foiled by the holistic management of air, moisture, and heat by high performance buildings.
Featured image is of Madrona Passive House.Back to Field Notes