Overview

A passive house is an energy-efficient building with year-round comfort and good indoor environmental conditions without the use of active space heating or cooling systems. The space heat requirement is reduced by means of passive measures to the point at which there is no longer any need for a conventional heating system; the air supply system essentially suffices to distribute the remaining heat requirement. A passive house provides very high level of thermal comfort and provision of whole-house even temperature. The concept is based on minimising heat losses and maximising heat gains, thus enabling the use of simple building services. The appearance of a passive house does not need to differ from a conventional house and living in it does not require any lifestyle changes. Passive houses are light and bright due to large glazed areas designed to optimise solar gains, as well as healthy buildings in which to live and work due to fresh air supply through the ventilation system.

Design of A Passive house

The Passivhaus Standard is a specific construction standard for buildings with good comfort conditions during winter and summer, without traditional space heating systems and without active cooling. Typically this includes optimised insulation levels with minimal thermal bridges, very low air-leakage through the building, utilisation of passive solar and internal gains and good indoor air quality maintained by a mechanical ventilation system with highly efficient heat recovery .Renewable energy sources are used as much as possible to meet the resulting energy demand (PEP, 2006), including that required for the provision of domestic hot water (DHW).

It should be noted that the primary focus in building to the Passivhaus Standard is directed towards creating a thermally efficient envelope which makes the optimum use of free heat gains in order to minimise space heating requirement. While there are also limitations on the amount of primary energy that can be used by a dwelling for such uses as DHW, lighting and household appliances, this will not be the primary focus of these guide lines. That is not intended to imply that such energy uses are insignificant, however. In fact, a passive house will have the same DHW requirements as any typical house in Ireland and given the low energy required for space heating the energy demand for DHW will represent a relatively high proportion of the overall consumption.

The building envelope consists of all elements of the construction which separate the indoor climate from the outdoor climate. The aim of the passive house is to construct a building envelope that will significantly minimise heat loss and optimise solar and internal heat gain to reduce the space heating requirement to 15KWh/(m2year).The following building envelope parameters are fundamental in this process:

1. Well insulated building envelope

2. High energy performing windows and doors

3. Minimised heat loss through thermal bridging

4. Significantly reduced structural air infiltration

5. Optimal use of passive solar and internal heat gains

Building Envelope Insulation

Many building methods can be used in the construction of a passive house ,including masonry, lightweight frames(timber and steel), prefabricated elements, insulated concrete formwork, straw bale and combinations of the above. The prototype house presented in this publication (details in Section 2and 3) illustrates both masonry and timber frame construction as representative of most typically used building methods for dwellings in Ireland.

Continuous insulation of the entire thermal envelope of a building is the most effective measure to reduce heat losses in order to meet the Passivhaus Standard.

Windows & Doors

The recommended approach to the design of a passive house is to have avoid excessive area of north facing glazing and place relatively large windows facing south or due south. This is in order to minimise heat losses through the north facing elevation ,which receives no direct sunlight, while maximising ‘free’ solar heat gains on the south. An advantage of large windows is an increase in interior day light levels which in turn reduces the need for use of electricity for artificial lighting and also ensures a more pleasant natural light filled living environment.

Thermal Bridging

Thermal bridging (i.e. un-insulated joints between walls, floors/walls, ceilings/adjacent walls, windows/walls etc) are weak points of the building envelope and cause unwanted losses of energy which should be eliminated or significantly reduced to a degree that the associated heat losses become negligible. A thermal bridge increases heat loss through the structure, and in some extreme cases this may cause surface condensation or interstitial condensation in the structure. Surface mould growth or wood rot may be the consequences of a thermal bridge. Typical effects of thermal bridges are:

• Significantly increased heat losses.

• Decreased interior surface temperature which may result in high humidity in parts of the construction.

• Mould growth cause by warm internal air condensing on cold surfaces.

All of the above situations can be avoided in houses built to the Passivhaus Standard. The Passivhaus Standard for linear thermal transmittance(ø) should not exceed 0.01W/(mK). This requires the building designer to identify and locate all potential thermal bridging in the construction, careful specification and detailing of those elements providing where possible a continuous layer of insulation, as well as care being taken to execute those elements on site as per design. details.

Structural Air-Tightness and Draught-Proofing

Building an airtight or leak-free structure is imperative to achieving the Passivhaus Standard. If there are gaps in the building structure then uncontrolled amounts of cold external air can infiltrate the building. Achieving a high level of air tightness eliminates cold draughts and associated comfort losses. It also prevents condensation of indoor moist, warm air penetrating the structure, and possible structural damages due to decay, corrosion and frost. Air-tightness is achieved by careful application of appropriate membranes and tapes or wet plastering in concrete construction within the building envelope. A great deal of attention must be paid to detailing and workmanship in order to ensure that the airtight layer is continuous all round the building, especially around junctions between walls and floors, roof, windows, doors, etc. Penetrations of the airtight layer by mechanical and electrical services must be properly sealed.

The air-tightness of a building can be accurately measured by carrying out a blower-door test. The test involves placing a powerful fan suspended in a canvas sheet within a door opening and operating the fan at very high speeds thereby creating either negative or positive pressure within the house. By sucking air out of the house, for example, a negative pressure is created with the result that external air will be sucked in through any gaps or cracks in the building envelope. The pressure used for such tests is 50 Pascal which can be accurately set by the blower door equipment.

Passive Heat Gains

Passive solar gain is optimised by providing an east-west alignment to the building, if possible on the site, resulting in the longest façade facing south, and by placing the majority of the glazing towards the south. Very high quality windows (average U-value