The science behind the Passive House Standard starts with modeling heat flow. Then balancing the modeled flows, and finally achieving a set of benchmarks. This detailed heat flow model will exceed the energy efficiency of the BC Step Code #5, and do so at no additional costs. This is achieved through balancing in the modeling.
Five systems of the building get modeled when determining the annual energy consumption needed to maintain a comfortable interior temperature. Four areas concern the thermal envelope, and the fifth conditions the interior air (generally heating in Canada). Modeling the thermal envelope involves calculating heat flow by way of the following processes:
- conduction through exterior surfaces – which includes the exterior walls, the basement floor or slab, and the roof
- conduction through the windows – the most expensive of these four
- conduction through thermal bridges – which occur at all the corners, and anywhere something pokes through the thermal envelope, like a beam or joist.
- convection through gaps in the air barrier that leads to uncontrolled ventilation.
Buildings modeled to this standard have been shown to retain heat during power outages
Exterior Surfaces
Thermal conductivity is a property that every material possesses. The thermal conductivity and the thicknesses of the materials are used to calculate the heat loss. For example, an exterior wall may have (from the inside to the outside) drywall, studs (soft wood), insulation (many, many different kinds are available), plywood.
Windows
Because windows are the most expensive components of the thermal envelope, care must be taken to only use them when they have a purpose. Lighting the room or providing egress are common purposes. Floor to ceiling windows are discouraged because the bottom third (near the floor) does provide usable light.
Many manufacturers will independently verify the thermal properties of their windows through a testing lab. With the proper testing in the appropriate lab, these windows would be “Passive House Certified” (PHC). Using PHC windows makes the energy modeling much easier. Uncertified windows can be used, but values for all the necessary physical characteristics need to be available from the manufacturer. Often times, window manufacturers are not used to providing that information.
Thermal Bridges
Thermal bridges occur where ever the materials in the exterior surface changes. For example, For example, there is more wood used (and less insulation) at the corners along the exterior walls. If there is a balcony, the joists will often penetrate the exterior wall. That is a serious thermal bridge that also occurs in a cantilevered second (or higher) story.
Thermal bridges are often ignored in a typical construction project because they’re so small compared to the heat losses through conventional windows and walls. However, a highly energy efficiency building such as a Passive House has much better insulated walls and higher quality windows. In these situations, the heat losses from thermal bridging becomes much more significant.
The values for thermal bridges are analyzed separately from the energy modeling already discussed. Thermal bridges are calculated using a 2D heat flux simulator such as THERM or Flixo. Also, the Passive House Institute has a number of thermal bridging values calculated and available for building features that are common in central Europe.
Air Barrier Gaps
Heat losses through air gaps is probably the most under appreciated form of heat loss. The uncontrolled ventilation is difficult to discover and locate without specialized equipment such as a blower door testing apparatus and smoke pencils. It’s much easier to tackle the uncontrolled ventilation issues in a new construction than a remodel because you can seal things air tight from the ground up.
The Passive House standard includes the strictest ventilation requirements. These requirements are measured in air changes per hour (ACH). Passive house sets the bar at 0.6ACH. What that means is that now more that 60% of the air volume in the house can be exchanged each hour. A typical conventionally build house from the 1980s might have 5.0ACH, and one built earlier in the century night have 10-15ACH.
Compared to BC Step Code
For reference, the BC Step Code (BCSC) for Part 9 buildings sets 2.5ACH for Step 3. Step 3 is the current one for Part 9. The air change threshold for Step 4 of the BCSC is 1.5ACH. The final benchmark of the BCSC is Step 5 which establishes 1.0ACH as the maximum amount of indoor air which can escape in an uncontrolled fashion: nearly twice as much as the Passive House standard.