Passive House is a set of methods for creating resource-efficient, extremely low-energy use buildings. The concept began with super-insulated structures, which gathered interest and attention during the energy crisis of the late 1970s and ‘80s. It was further developed in Germany (where it is called Passivhaus) by a scientist named Dr. Wolfgang Feist, to become a comprehensive energy analysis tool, intended to raise the standard of quality for construction. It was brought to the US by Katrina Klingenberg, a German architect and educator.
Choices at every step of the design process and every level of scale are evaluated for their ability to increase efficiency. During the initial design phase, the performance of a proposed scheme is simulated using a computer model, which will account for the following as part of a total energy analysis:
- Detailed floor, wall, and roof assemblies
- Specific climate data and external solar gains
- Internal heating loads from occupants and equipment
- Renewable energy sources
Data from this analysis is used to adjust the scheme for the next design phase, and this process is reiterated until the simulation shows that the building will meet the Passive House standard.
During the construction phase, care is taken to ensure that the critical design elements suggested by the simulation are built as specified. Passive House projects do not require high-tech solutions, but do require tight collaboration between designers who are knowledgeable and thoughtful, and builders who keep a close eye on quality control. Important considerations are:
- Careful selection and proper installation of windows
- Air-sealing of the exterior boundary
- Designing the ventilation system in conjunction with the heating system
- Coordination between the various sub-contractors to ensure there are no conflicts between trades
Everyone involved in the project must work together to achieve these goals.
When construction is complete, the performance of the building is tested to confirm that it meets Passive House standards. Only then can the building be Passive House-certified. Among these evaluations are multiple blower door tests, to ensure air-tightness, and third-party verification that the insulation is properly installed.
Building a “Passive House” costs more than building a house that meets only the minimum code requirements, but the return on investment is worthwhile for a number of reasons. Passive Houses reduce energy consumption to the lowest levels possible today, saving money and natural resources. At the same time, they provide a higher level of comfort and indoor air quality than what is found in a typical house. Fluctuations in temperature are significantly reduced, a continuous supply of fresh air is provided, and moisture intrusion into the walls (often accompanied by mold problems) is greatly diminished.
There are many other programs devoted to ecologically responsible building. For example, the LEED certification standard promotes sustainable practices through an emphasis on prescribed materials and construction methods. Passive House, by contrast, sets an extraordinarily high standard of energy performance, and provides those trained in the program, with the tools to meet this standard. The quality of construction must be verified as part of a certification process. There is no “prescription” or defined set of steps. Any construction process can be used, as long as the required result is achieved. Methods will vary depending on climate, ease of construction, materials used, equipment availability, and many other factors, but the aim is always to end up with a comfortable, energy-efficient dwelling.
The Passive House principles are my foundation for creating sustainable architecture, though do not make up the entirety of my approach. I also believe strongly in the importance of choosing materials and building methods in a way that considers their impact on the environment with respect to efficiency, reuse, conservation, and energy independence.
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Passive House Fundamentals:
Super-Insulation – Assemblies may need twice the code-minimum insulation values. Inclusion of high-performance windows (typically triple-glazed) may be required.
Minimization of Thermal Bridging – Insulation must run continuously around the perimeter, uninterrupted by studs or other elements.
Sealed Building Envelope reduces air drafts and migration of moisture into the exterior assemblies.
Balanced Ventilation System to ensure sufficient fresh air circulation, and remove accumulated moisture and pollutants. Adding a heat recovery ventilator (HRV) to the system will minimize heat loss.
Energy-Efficient Equipment, Appliances, and Lighting to reduce total energy needs.
Form & Orientation – The ideal shape for an efficient insulated building is a compact one, with a low ratio of surface area to volume. When possible, the building should be placed on its site to optimize solar orientation and shading. Passive solar gain is an important consideration, though not a critical constraint.
Ease of Use – Operation and maintenance of a building should be straightforward and user-friendly, in order to make it as easy as possible for the owners to keep it in good shape for decades to come.
Reduced Mechanical System – Incorporating all of the above ideas will contribute to a decreased mechanical footprint for the building. The lower expense of a smaller mechanical system will help to offset the increased budget for a better thermal envelope and airtight construction.
Passive House Planning Package (PHPP)
PHPP is a detailed spreadsheet analysis that accounts for detailed building assemblies, mechanical equipment, appliances, occupant usage, specific energy sources, local climate data and solar radiation, to provide the overall energy performance of a building.
There is a reliance on building-envelope efficiency, high levels of insulation, and passive heat gains from internal usage, rather than “active” renewable energy generation.
The super-insulated house prevents huge heat gains and losses, so temperatures do not fluctuate rapidly. Exterior building elements must have a U-value below 0.026 BTU/hr.ft²°F (0.15 W/(m²K) in order to meet the primary energy requirements. Thermal mass can influence comfort levels, as more mass will slow losses.
The location and detailing of the envelope are critical to reduce thermal bridges, and the relationship of U-value to interior surface temperature must be small enough to ensure thermal comfort and minimize potential condensation in wall. Heat transfer coefficients (U-values) for exterior walls, floor slabs, and roofs range from 0.1 to 0.15 W/(m²K).
All glazing must have U-values below 0.8 W/(m²K) (0.141 BTU/hr.ft²°F) and a high total solar energy transmittance (SHGC, fraction of incident solar radiation) of at least 50% to achieve net heat gains in winter. Effective passive solar glazing should be within 30 degrees of south. Exact glazing, spacer and frame specifications and installation procedure are important factors in the energy analysis.
Frequency of overheating: maximum 10% (temperature is over 77°F) which may require additional summer shading.
It may be possible to attain thermal comfort by conditioning the air delivered through the heat recovery ventilation system, and thereby minimizing the mechanical equipment. The capital savings from omitting the central heating system allow this same money to be reinvested in the building envelope (i.e., with good design, the envelope can be improved at no additional capital cost). 300 Watts per person can be delivered by a fresh air heating system, independent of climate. If heat load exceeds this value then an alternate source of heat is required. 52C or 125.6F max delivered with 82 cfm max.
Fresh air changes to be a minimum of 0.3 Air Changes per Hour (ACH), based on volume, per ASHRAE standards. The ventilator system must be designed to distribute heat evenly throughout the house, with the highest energy recovery efficiency > 75%, and must also have minimal electricity consumption (0.45 W/m³ supply air volume). Filter to be high quality ≥F7 (MERV13). Fan load: maximum 0.76 W/cfm (<0.45 Wh/m³)
Fixtures and appliances can also contribute to internal heat gains and electricity, and must be accounted for. Using higher equipment efficiency and distribution efficiency will reduce total power requirements.
Annual Heating: 4.75kbtu/ft²/year (15 kWh/m²/year) or less. This target was developed based upon whole systems design and life cycle analysis.
Primary Energy (PE): 38kbtu/ft²/year (120 kWh/m²/year) or less. Amount of non-renewable energy (off-site), as well as losses from distribution, conversion, and delivery (takes into account energy factor) for heating, domestic hot water, and auxiliary electricity. Energy efficiency is important because there is a PE max allowed, and electric use has a high Primary Energy factor of 2.7 kWh/kWh multiplied into total usage.
Airtightness: Achieve a blower door test of 0.6ACH (air changes per hour) @ 50Pa. This is to ensure thermal comfort by reducing heat loss through gaps and voids, and the point where the risk of moisture transported into walls via air movement in the walls is “reduced to a safe level.”
Peak Heat Load: ≤ 3.2 Btu/sf (≤ 10 W/m²), so that the house can maintain 50°F without heating.
In addition, the following are recommendations, varying with climate:
- Window U-Value ≤ 0.8 W/(m²K)
- Ventilation system with heat recovery with ≥ 75% efficiency with low electricity consumption at 0.45 Wh/m³
- Thermal Bridge Free Construction ≤ 0.01 W/(m²K)