Heat will flow out of a building fastest via the easiest path, passing quickly through a material that has a higher thermal conductivity—a thermal bridge. Thermal bridges can significantly increase heat losses, which can create areas in or on the walls that are cooler than their surroundings. In the worst-case scenario, this can cause moisture problems—when warm, moist air condenses on a cooler surface.
Thermal bridges occur at envelope edges, corners, connections, and penetrations. A bridge can be as simple as a wall stud with higher thermal conductivity than the surrounding insulated wall or as unsuspected as a balcony slab that is not thermally isolated from an interior concrete floor. Without a thermal break, the balcony will act as a very large cooling fin, sucking heat out of the house in the winter.
In a PH, there are few or no thermal bridges. When the thermal bridge coefficient, an indicator of the extra heat loss caused by a thermal bridge, is less than 0.01 watts per meter per Kelvin (W/m-K), the detail or wall assembly is said to be thermal-bridge-free. Heat loss through this detail is negligible, and interior temperatures are sufficiently stabilized to eliminate moisture problems. It is critical for the PH designer and builder to plan for reducing or eliminating thermal breaks by limiting penetrations, and by using heat transfer-resistant materials. Thermographic imaging can be used to determine how effective the elimination of thermal bridges has been.
Airtight construction helps the performance of a building by reducing or eliminating drafts—hot or cold—thereby reducing the need for space conditioning. This also helps to prevent warm, moist air from penetrating the structure, condensing inside the wall, and causing structural damage.
Airtight construction is achieved by wrapping an intact, continuous layer of airtight materials around the entire building envelope. Special care must be taken to ensure continuity of this layer around windows, doors, penetrations, and all joints between the roof, walls, and floors. Insulation materials are generally not airtight; the materials used to create an intact airtight layer include a combination of various membranes, tapes, plasters, glues, shields, and gaskets. These materials are becoming increasingly durable, easy to apply, and environmentally sound, which in turn is making it easier for a builder to meet the stringent airtightness requirement of the PH standard.
A home’s airtightness provides a measurable dimension of its construction quality. Testing requires the use of a blower door, which is essentially a large, specialized fan that is sealed into an exterior door frame. The blower door can be used to either depressurize or pressurize a house to a designated pressure so a technician can measure how much air is infiltrating the building through all its gaps and cracks. Specific leaks can be detected during the test either with tracer smoke or by examining thermographic images. It is best to conduct the blower-door test at a point during construction when the airtight layer can still be easily accessed and any leaks can be readily addressed.
PHs are extremely airtight. At a standard test pressure of 50 Pascal (Pa), a PH must allow no more than 0.6 ACH (air changes per hour). PHs built from timber, masonry, and prefabricated elements have all met this standard.
Airtightness does not mean that you can’t ever open the windows. PHs have many operable windows to take full advantage of natural ventilation to help maintain comfortable temperatures.