Architects' Guide

Passive Architecture

Design

A house has to adapt to varying needs of different users. This demand includes both suitability of a dwelling into desires concerning furnishing and use of the dwelling and a variety of comfort issues. A Passive House’s energy economy makes traditional heating systems obsolete. As radiators are not needed the whole floor surface can be utilized.

A passive house is an extremely energy-efficient residential building with year-round comfortable interior conditions. In a Passive House, energy-efficiency is a route to good indoor conditions. Due to well insulated building envelope and airtight construction the surfaces are warm. There is no risk of cold draft even from window surfaces.

As with traditional housing, a Passive House is suitable for a wide variety of people, and with special consideration can be designed to meet the requirements of special needs groups. Living in a passive house does not require a special life-style, and buildings are not designed for a certain category of people. But it does, like any other house, require an understanding by the occupant regarding it’s operation and maintenance.

Materials

The appearance of a Passive House does not differ from a conventional house. But a Passive House may also have a contemporary look. The concept also allows for experimental and futuristic architecture that can be realized with high energy efficiency and low environmental impacts.

Passive Houses can be built with all the typical building materials for load-bearing frames. The building envelope can compose of wood frames, block or brick structures, and concrete or light-weight concrete structures when adequately insulated. Even modern steel structures can be used building systems in Passive Houses.

Building Site

The way in which a building is sited will have a major effect on its energy efficiency. Orientation plays an important role in optimizing both the heating demand in winter and cooling demand in summer. Optimisation of the design with regard to solar access and protection against cold winds helps to reduce heating load. Passive houses may also require solar protection to reduce the cooling load in summer.

In order to minimise loads on heating or cooling systems in buildings, and to increase the comfort of people both indoors and outdoors, the general design guidelines are

• A low and sunny southern slope where the temperature balance can become positive on sunny winter days, enabling integration of solar energy systems into the development.
• The site allows for good orientation of buildings; style, orientation and pitch of roofs for solar energy collection, optimised distances of buildings, position of broad-leaved trees and planting to reduce over-heating in summer.
• The site locates up from valleys that can gather cold air during nights thus decreasing the mean temperature.

However, a Passive House design emphasizes technologies that do not depend on solar irradiation, such as high level of thermal insulation, excellent airtightness, high quality windows and doors, and heat recovery from ventilation air. Although an optimum is to orientate the house towards south, experiences have shown that Passive Houses perform satisfactorily also on sites facing North.

Dwelling Design

Spatial planning
Compact design decreases a house’s heating energy demand. Energy-saving technologies enable the main focus to be on user and owner requirements and other architectural demands. Trade-offs between different goals may become necessary. However, spatial planning and user requirements should guide the design.

Due to high insulation level of structures and windows and doors, traditional heating systems become unnecessary. Even in cold climate there is no need to place heating systems beneath the windows to prevent cold draft from cool window surfaces. For a dwelling unit design this relieves the whole floor area for active use.

A Passive House can utilize internal heat loads from kitchen equipment, washers, electronics, sauna etc. efficiently for winter time heating. Therefore it is beneficial to locate these heat sources at internal walls as much in the middle of the house as possible. Also heat losses from boilers or heat generators can be utilized in heating.

Installations

A Passive House can be heated with a ventilation heating system. Short and direct routing of air ducts is beneficial for the efficiency of the system. The space required for the ductwork needs to be taken into account in spatial planning. Here the architect should co-operate with HVAC designer, to receive accurate dimensions and space requirements for routing of ductwork.

Suspended ceilings of hallways, wardrobes, bathrooms, and utility rooms are appropriate spaces for routing of ductwork. Also internal floors or spaces above cabinets suit for routing. The ductwork should also locate underneath the air/vapour barrier of the roof. The terminal units may locate on internal walls, leaving the roof free of ventilation installations.

Design Principles
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

The two basic requirements for building envelope structures are high insulation level and good air tightness. Typical U-values of Passive House structures range:

• 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.

A Passive House normally incorporates the following design features:

• 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

Examples

Building envelope
The energy conservation refers to thick insulation layers. A wall thickness including thermal insulation may range from 300 up to 600 mm depending on the load-bearing structure and climate.

Roof insulation thickness may be up to 600 mm. Floor insulation in ventilated floors can be up to 500 mm, and slab-on-ground floors to 250 – 300 mm. In cold climates the frost insulation to prevent frost heave of foundations needs to be designed accordingly, as the floor heat loss does not contribute for defrosting of the ground. In moderate climates the insulation thicknesses are lower.

Ventilation

In principle, a ventilation heating system is enough for a Passive House. The size of the unit should come from the required air flow rate for better efficiency and noise control.

Ventilation noise is sometimes a problem. The noise from ventilation comes either through ducts or through the device’s envelope. When the ventilation rate is high due to heat supply, the noise level rises accordingly. A separate technical room for the installations and extra noise dampers may be required.

Short air duct routes from the technical room are an optimum. This is not only a requirement for a Passive House but for a good quality building.

As a rule of thumb, to achieve noiseless ventilation the minimum duct dimensions from the unit are

• Small apartments with a floor area less than 50 m2: Ø = 125 mm
• Detached houses or apartments with a floor area less than 125 m2: Ø = 160 mm
• Detached houses apartments with a floor area more than 125 m2: Ø = 200 mm

Terminal units for air distribution to rooms may locate on inner walls.

Heating and cooling

Ventilation heating system can be an integrated system serving for ventilation, heating, hot water, and cooling. In addition, hot water can be supplied partly from solar collectors. In summer solar collectors can supply all the heat for hot water, but obviously in winter extra heat source is needed. Roughly 50% of the total hot water energy demand can be supplied by solar energy. Typically hot water collectors are placed on the roof, and the needed collector area is about 3 - 5 m2 and water tank volume 200 - 500 litres for a house. Additional heat can be supplied in a traditional way, e.g., from electrical, gas or pellet boiler, or ground heat.

Passive Houses

Passive Houses have been built especially in Germany and Austria, where also first whole Passive House housing areas are being planned and developed. The examples here show variations of style and construction tradition from various countries. However, each of the examples fulfils the performance requirements set for a Passive House.

Architect’s checklist

1. Site selection: Appropriate use of passive solar energy in winter.
2. Natural shading for reduced summer time cooling demand
3. Orientation of the building
4. Efficient use of roof for active solar systems by adequate roof pitch
5. Window design for passive solar energy
6. Routing needs and technical room in dwelling unit design
7. Placing of kitchen appliances, heating equipment, water heaters etc. for utilization of internal heat loads
8. Wall, roof, and floor thicknesses with regard to architectural concept
9. Installations inside air barrier for air tightness
10. Space allocation for routing of ventilation duct work
11. Compactness of the house for reduced heat losses
12. Terminal inlets and directness of ventilations duct work in spatial planning
13. Check the conformity of conceptual design variations with Passive House requirements using PHPP tool
14. Verify the conformity of the final design with the Passive House requirements using PHPP tool
15. Choose certified Passive House products

Energy Designers’ Guide

Indoor Conditions

Design aim

A passive house aims at very low energy consumption and low heating power demand compared to contemporary national standards. Thus a Passive House’s CO2 emissions are also low. In a Passive House, energy-efficiency is a route to good indoor conditions. The concept bases on low heat losses of a building, thus enabling simple building services’ systems. For energy design, the basic requirement is adequate dimensioning of the energy systems supplying for the energy demand.

A passive house has a high level of insulation with minimal thermal bridges, low infiltration, and it utilizes passive solar gains and heat recovery to accomplish these performance requirements. Renewable energy sources can be used to meet the resulting energy demand.

A Passive House’s envelope structures are highly insulated and airtight. The house’s heat losses are low. Therefore it does not need traditional heating systems like radiators or floor heating. Due to low heating energy demand, ventilation heating can be the primary heating system. Energy designer’s main task is to set requirements for the building as a whole so that the heating system can operate efficiently in all climate conditions, and serve for good, comfortable, and draught-free indoor conditions.

Air quality

An airtight house requires a well-designed and dimensioned ventilation system for good indoor air quality, and vice versa: any ventilation system (natural, mechanical or hybrid) requires good airtightness to make ventilation flows controllable. Energy-efficiency can not be considered as a ground to reduce the ventilation rate. Building code requirement should be considered as minimum acceptable ventilation.

Different rooms may have different temperatures due to solar gains, occupation, and internal heat loads. Room based control or passive measures for temperature differentiation may be necessary if high comfort requirements are set for individual rooms. In centralized ventilation heating system supply air temperature is the same for the whole supply. In a room based control system the supply air can be heated at or close to air inlet terminals. The system allows for a wide range of fast and accurate control of the inlet air temperature.

Over-heating is possible due to solar gains in summer. Energy designer needs to make early-on scenarios on the over-heating probability, and suggest ways and means to avoid it. Passive ways and means such as solar shades, night cooling, and increased ventilation are preferred. Mechanical cooling systems should be avoided. Ground air duct or water systems for air pre-heating or cooling can be used.

Air supply

Supply air can be delivered through air inlet terminal on inner walls. Supply through roof inlets close to the exterior wall may give a distribution to the whole zone, but this is a question of dimensioning and quality of inlet terminals. The advantage of placing inlets on internal walls is the lower need for suspended ceilings or embedding of ducts into structures. That is an architectural advantage and at the same visual comfort in the house.

A Passive House utilizes free energy such as passive solar energy and heat from the occupants and equipment. Adequate thermal mass is recommended for efficient utilization these energy sources. A massive exposed floor or ceiling is enough to provide adequate mass in Central and Northern Europe. Also night ventilation with cool air might also be necessary. On the system design point of view the air temperature needs to have gliding set points, between which the room temperature may fluctuate. High thermal insulation level together with adequate mass helps for reducing cooling in summer.

Thermal comfort

In ventilation heating system supply air is distributed to all rooms, hall or vestibule, and separate kitchens. However, floor heating is in cold climate typical for comfort purposes in wet rooms for comfort and moisture control. The floor temperature should be designed to be lower than in conventional floor heating systems. Suitable level is, e.g., 2 - 4oC above room temperature. Higher temperature difference may cause over-heating.

Thermal comfort and indoor air quality require good mixing of supply air into the room air. The mixing minimizes temperature gradient in the room. Good air mixing is especially challenging in winter as the supply air is warm (near 50oC). If the inlet terminals are placed high, air velocity needs to be high enough to achieve good mixing rate. On the other hand the velocity should not exceed 0.15-0.20 m/s in the occupied zone since high velocities may cause comfort problems.

Vertical temperature difference should not exceed 2oC between heights of 0.1 m and 1.1 m, referring to ankle and neck level of a person sitting. Thermal comfort may become a problem in high rooms (2-storey high) due to poor air mixing if the inlet terminals are placed high. In a Passive House the building envelope and windows have such a high thermal quality that enable to maintain low vertical temperature differences. The air distribution from floors underneath windows gives a rather good air mixture, but dust from floors might be carried by the air flow.

Building Envelope

Design requirements

A passive House is defined with performance requirements for heating energy consumption, heating power demand, and total primary energy consumption. To accomplish the power demand requirement, energy designer needs to be sure of the building envelope characteristics. On the other hand, in the concept design phase, the energy designer may need to set requirements for building envelope characteristics, for to be able to limit the energy and power demand.

The heating power demand of a Passive House is only 10W/m2 or less. Therefore the peak demand calculation for each room needs to be accurate. Oversized heating system makes the control more difficult and thus they consume more energy. They are also an extra cost.

Design co-operation between architectural, energy, and structural design is required. Energy designer can guide architecture not to place windows into difficult structural locations where air circulation may be limited. As there are no heat sources underneath the windows, condensation may occur even with high-performance windows.

Envelope properties

U-values and thus the heat losses of a passive house are low. Typical U-value ranges used in Passive houses are according to climate, including thermal bridges:

• Wall 0,1 – 0,15 W/m2K
• Floor 0,09 – 0,15 W/m2K
• Roof 0,08 – 0,15 W/m2K
• Window < 0,8 - 1,0 W/m2K
• Door < 0,4 – 0,8 W/m2K

Good air tightness of a Passive house refers to n50 < 0,6 air changes per hour at 50 Pa pressure difference. This also means that heat losses through infiltration are low. The Passive House certification scheme requires air tightness tests to be carried out for a finished building, increasing thus the reliability of building envelope characteristics.

It is essential to design simple and easy to use systems. Also easy use of control systems is important. It should be kept in mind that no special attitude is needed to live in a passive house. However, energy performance and systems of a Passive House should be made clear to the occupants.

Energy Calculation

There are two kinds of energy calculation typically needed for a Passive House. Correspondence to national energy requirements can be done by following the guidelines of these standards. These calculations typically use monthly based climate data.

The Passive House Planning Package (PHPP) enables a possibility to easy-to-use and accurate analysis of concept design. PHPP calculation can be used for assessing the performance of the house with regard to Passive House definition. However, energy designer must make sure that the tool is accepted for calculations for building permission. Different countries have their own calculation methods to assess the compliance with building regulations.

The performance requirements of the Passive House may require dynamic calculations at least if a house concept is new and not much experience is gained. Indoor climate and thermal environment calculations require dynamic simulation. Dynamic simulation tools like IDA-ICE, TRNSYS, TASE, Energy+, Capsol etc. can be applied also in solving complicated building envelope/system combinations.

Performance

Simple systems

The aim of a passive House is to gain high energy-efficiency and comfort by high quality building envelope and simple building service’s system. The extra investment into building envelope can be mostly gained back by simplifying the system. Solutions that serve for multiple purposes are thus advantageous.

Ventilation heating system is an efficient heat distribution system that serves for both ventilation and space heating. If an exhaust air heat pump is included into the scheme, the whole system serves for heating of hot water as well. Ventilation heating system is by far the most economical heating system for an energy-efficient house. A Passive House requires heat recovery from ventilation, and as such ventilation system serves both for air exchange and space heating.

The efficiency of the heat recovery unit should be as high as possible. Mechanical exhaust ventilation, natural ventilation, or hybrid ventilation systems are not an alternative. The heat recovery unit needs to have a by-pass possibility due to the risk of over-heating. De-frosting is needed at least in cold climates. De-frosting by heat or cyclic use of heat recovery for de-frosting reduces the efficiency of heat recovery. Thus other ways and means should be applied as far as possible. Ground duct pre-heating of supply air may reduce or eliminate the de-frosting demand.

Ventilation

The yearly energy efficiency depends also on the size of the unit and diameters of the ductwork. Over-sized machines have lower specific power demand over the whole range of required air flow rates. Rotating (regenerative) wheel heat recovery units have in general higher efficiency rates than flat plate (recuperative) heat exchangers. In some countries rotating wheel heat recovery units are not recommended.

In Passive House ventilation heating systems are preferred. Since the ventilation rate and air volumes are most of the time similar to conventional housing the noise control is not different to standard residential buildings. However, according to recent research results, noise from ventilation devices is usually experienced disturbing even though it does not exceed the building code requirements (28 dB(A)).

People prefer noise levels of 22-25 dB(A) in bedrooms. They also tend to control ventilation according to disturbing noise level especially if the background noise level is low, e.g., in the single family housing areas. In addition to conventional system’s damper an extra damper may be required for the supply air duct. The sound dampers should be designed in a way that the ventilation can be near the maximum and still on acceptable noise level.

Design Checklist

1. Assessment of reliability of building characteristics
2. Routings together with architect and structural designer
3. Draught control and window size
4. Solar control
5. Thermal mass assessment
6. Minimum ventilation rate and control
7. Inlet terminal placing place and mixing strategy
8. Over heating and simulations
9. Wet room heating
10. Noise control
11. Energy calculation

Structural Designers’ Guide

Technical Characteristics

A Passive House requires reliable solutions for components of the building envelope and building services systems. The Passive House certification scheme allows the design team to choose products that are accepted as Passive House Components. In addition, European Product approval schemes may be utilized in looking for components with adequate and tested properties.

The heating energy demand of a house can be significantly reduced by improving the thermal insulation level of structures. Heat losses through the structures are the major source of the total heat loss. Ventilation refers to about one third of the total heat loss. A Passive House is an entity where the main benefit of the high thermal insulation level can be achieved only by adequate dimensioning of heating and ventilation systems.

Comfort

Both the appearance of the house and thermal comfort requirements for a modern building are high. Good thermal insulation and air tight structures contribute to draught-free indoor climate and desired indoor temperature. Thermal insulation layer need to be protected both from the inside by continuous air barrier and from the outside by continuous wind barrier. Both air and wind barrier perform adequately only if all discontinuities of the layer are sealed.

A Passive House has high inner surface temperatures due to high insulation level. Thermal bridges may cause cooling of a surface locally. Both draught and cold surfaces increase the need for comfort heating even though the room air temperature is adequate.

A proper and controlled ventilation quarantines good quality of indoor air. Fresh air preheating, e.g., in ground ducts, and heat recovery from exhaust air increase the temperature of supply air. If ventilation heating system is used, the ventilation does not cause any feeling of draught.

The heating demand of a Passive House is very low, and thus cold air leaks are experienced more easily as discomfort as, e.g., in a normal house with traditional radiator or floor heating system. Air leaks at the wall to floor connection are especially harmful, as there are no heat sources to compensate the cold draft on the floor level.

Building System

U-values

A Passive House does not restrict the selection of building system. Also, materials for load-bearing materials can be chosen quite freely. The high performance requirements in terms of heating energy consumption and heating power demand require low building envelope heat losses. The following guidelines for thermal properties of the building envelope can be used as guidelines for system selection:

• U–values of building components according to climate, including thermal bridges:
• Wall 0,09 – 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
• Air tightness n50 < 0,6 air changes per hour at 50 Pa pressure difference
• Window solar transmittance g > 50%

The lower end U-values concern detached houses in Northern climates and the high end values Central European row or apartment houses.

Thermal bridges

Minimum amount of thermal bridges is crucial for the performance of the building. A thermal bridge is typically a building part that penetrates thermal insulation and that has substantially higher thermal conductivity than the thermal insulation. The window or door to wall, wall to floor, and wall to roof connections may have thermal bridges, and these details should be considered carefully. A thermal bridge increases heat loss through the structure, and in some extreme cases this may cause surface condensation or interstitial condensation into the structure. Surface mold growth or wood rot may be the consequences of a thermal bridge.

Linear thermal conductance in these connections should not exceed 0.01 W/mK. The relative effect of a thermal bridge increases according to increasing insulation level.

The design team should consider window dimensions and frame structure distances. If the window dimensions do not fit the dimensions and the distances of the frame system, extra studs are required for window support and inner boarding installations in wooden structures. Architect and structural designer should together decide the window dimensions to find effective solutions. Also, in a wooden building envelope a number of frame studs are unnecessary, e.g. in corner details, and window and door connections. Especially in corner details an easy solution is to add frame members for fastening of inner and outer boards and other structures. These extra frames are thermal bridges that should be avoided.

Air tightness

The requirement for air tightness of a passive House is n50 = 0.6 ach (air changes per hour at 50 Pa pressure difference). The air tightness is extremely important not only due to energy consumption and comfort but also with respect to moisture convection and condensation in the structures. For the design of an air barrier following rules can be applied:

• A material layer that has an air permeability of 1 x 10-6 m3/m2 s Pa maximum, including all joints perform as air barriers, e.g., plastic film vapour barrier, building paper, concrete element structures with sealed joints, or fair-faced inner brick walls with plaster. Air barrier needs to be continuous over the whole building envelope.
• Electrical installations should be surface installations, or in any case inside the air barrier. To help for electrical installations the air barrier may locate not more than 50 mm inside the insulation layer. In general the use of conduit spaces in front of the air barrier can be recommended (either insulated or not).
• Window or door frame to wall connection should be filled with insulation, and sealed from both sides. Use of positioning systems help for installation of heavy windows and to provide good connection with the structure, or to integrate the windows in a prefabricated wall system.
• Ventilation ductwork installation should be inside the air barrier. Only fresh air and exhaust air ducts need to penetrate the air barrier
• The HVAC and sanitary installations and service penetrations of electricity, water, gas, and etc. systems should be sealed using flanges or other means of tightening

Wind tightness

Wind barrier refers typically to a layer outside a frame structure protecting the thermal insulation. The wind barrier should be continuous over the building envelope. Connections, joints, and other details need to be sealed using sealants, sealing compounds, flanges, and high quality of workmanship. Basically all insulation structures need to have an air barrier, e.g.:

• A material layer that has a air permeability of 3 10-6 m3/m2 s Pa maximum, including all the joints
• Wood fibre, gypsum or other board with sealed joints on top of the framing
• Exterior insulation composite system with rendering
• Wind proof mineral wool or EPS insulation with sealed joints
• Fair-faced brick wall without air gaps between insulation and brick wall

Performance

The energy design of a Passive House requires adequate information on the building envelope characteristics. A typical problem in building design is over-dimensioning of heating systems due to poor reliability of thermal characteristics of envelope structures, anticipated air leakages, anticipated thermal bridging, and poor knowledge of window and door properties. Over-sized heating systems cost more, their control is more difficult, and thus they consume more energy.

Therefore the U-values should include the effects of at least major thermal bridges. To accomplish this requirement, the U-value calculation should be carried out using 2- or 3-dimensional numerical methods, as hand calculations are not sufficient for, e.g., frame structures of wood or perforated steel members.

The goal in structural design should be in simple solutions that can be repeated in various projects. Thermal properties of structures can then be assessed adequately by using ISO EN 6946 or ISO EN 10112 standards.

The Passive House Planning Package (PHPP) allows for comparison of different building systems and insulation levels with regard to heat losses. Some structures, such as perforated steel members require 3-dimensional tools for even for simple U-value calculations.

Checklist

1. Design co-operation to find and solve design problems that affect other designers outcome
2. Aim for as simple as possible solutions to increase the reliability of solutions
3. Consider space requirements for HVAC installations
4. Minimize thermal bridging or use exterior insulation systems to reduce their effects
5. Consider the need of all structural components; they may influence on thermal properties and cost efficiency
6. Use modular dimensions, e.g., 600 mm for frame walls and windows
7. Design for air barrier
8. Consider ways and means to seal all components leading through a structure
9. Design for wind barrier
10. Consider the order of site work already in design
11. Consider moisture dry-out from thick structures; avoid double vapour barriers in wet rooms