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Design Principles

Architectural design has an important effect on the buildings heating and cooling demand. BedZed in UK. (www.suntool.net)
U-values of walls, floor, and roof from 0,07 W/m2K to 0,9 W/m2K
Eleven typical details where problems with air tightness typically exist in an open wooden building system (www.puuinfo.fi).
Both light-weight and heavy buildings have proper thermal mass. In the light-weight building the mass of the concrete floor is sufficient.

Performance
A Passive House bases on performance requirements expressed in terms of heating energy demand and total primary energy consumption. A performance concept leaves it up to the design team to decide how the requirements are fulfilled. However, the very low energy demand can be fulfilled only by applying energy conservation measures that enable significant reduction of the building’s heat losses, household energy use, and utilization of passive solar energy.

Passive Houses can utilize passive solar energy efficiently for heating in winter. In summer however, solar gains especially through large window areas increase the likelihood of overheating and thus appropriate measures to limit solar gains need to be specified. Balconies, optimized overhangs of roof structures and solar shades can prevent overheating. In addition balconies, overhangs, and fixed solar shades minimize the possible risk for condensation on exterior surfaces of windows. Typically the window area is 15-20% of floor area. In order to prevent possible draft in cold climates, windows should not be higher than 1,8 m. In a cold climate windows should not be connected directly to floors for both air tightness and comfort.

Building envelope

  • Wall 0,9 – 0,15 W/m2K
  • Floor 0,08 – 0,15 W/m2K
  • Roof 0,07 – 0,15 W/m2K
  • Window 0,8 – 1,0 W/m2K
  • Mounted window 0,6 – 0,85 W/m2K
  • Door 0,4 – 0,8 W/m2K

The lower end U-values concern detached houses in Northern climates and the high end values Central European row or apartment houses. Main window orientation should ideally be from South-East to South-West for winter time solar utilization. Typical U-values for windows are 0.70 - 0.80 W/m2K. Windows achieving that good U-value are typically three pane windows with two low-emissive surfaces and krypton or argon fill. Also, window frame should have a thermal break. Passive House certification scheme offers a possibility to choose certified products with well established technical properties.

Airtightness
Air tightness of the building envelope is important. Simple and well-defined structures serve for continuity of air barrier layers and air tightness. They also contribute to easiness of insulation, service, window, and door installations and structural details at connections. Especially with wooden or other frame structures pre-fabrication typically makes it easier to build air tight envelopes. The continuity of air barrier in the structural connections needs to be designed and built carefully.

The required building air tightness or n50 value is 0.6 ach (air changes per hour). The n50 value and also air infiltration are for a Passive House much lower than that of a typical standard dwelling. The air tightness is extremely important not only due to energy consumption but also in respect to moisture convection and condensation. It is important to prevent humid indoor air to penetrate into a highly insulated structure and to cause condensation and creating potential moisture problem. High quality design and construction have a major role.

The requirement for air tightness affects the architectural design as well. Complicated form, high number of different structural details, service penetrations in structures, missing space for HVAC and electrical installations, and structural details increase the possibility of defects and air leakages. In general the design should support easy to do sealing and tightening techniques.

Thermal mass
Thermal mass helps top utilize passive free energies, such as passive solar energy, heat emitted from occupants or electrical devices. Efficient utilization of thermal mass depends on the amount of thermal mass, surface area of the mass, heat transfer co-efficient of the surfaces and thermal conductivity of coverings, floorings, carpets etc. that also affect on the heat transfer. Thermal mass performs most efficiently if the mass locates on the inner surface of the structure.

However, the amount of thermal mass is not very high, e.g. a massive floor in a lightweight building is sufficient. Efficient utilization of mass requires sliding set points for heating system. Indoor temperature can vary freely inside the sliding scale, and structures can store or supply heat according to the indoor temperature.Thermal mass is only one of the solutions which can be adopted to help maintain comfortable conditions in the summer. Passive cooling and higher ventilation rates may serve as sufficient means to avoid over-heating.

Design tool
The Passive House Planning Package PHPP allows for easy to use and accurate verification of conceptual designs with the Passive House’s performance requirements. It is advisable to check the design versions along the development of the concept using PHPP tool. Passive House certification helps for selecting components that represent the best practice products for various stages of the construction process.

  • Reduced heat losses by high level insulation in walls, roof, floor
  • Use of high performance windows with low U-value but appropriately high solar transmittance
  • Reduced heat loss by extreme air-tightness of the building
  • Draught-free, good indoor air quality by mechanical ventilation system
  • Good thermal comfort by warm interior surfaces
  • Low energy demand by using energy-efficient appliances