What is a Passive House?

 

This term “Passive House” describes a construction standard; it refers to an energy efficient building which provides:

 

  • A high level of thermal comfort
  • Even temperature throughout the entire building/house
  • Eliminates the requirement of conventional space heating or cooling systems

 

The passive house is designed to require little energy for space heating/cooling

 

The building is kept at comfortable indoor temperatures by the use of passive inputs:

 

These passive inputs are provided internally by occupants and appliances and provided externally by passive solar gain through the glazing, and a backup heating system

 

The Passivhaus Standard has been developed by the Passivhaus Institut in Germany.  It was founded by Dr. Wolfgang Feist in 1996 and has been instrumental in the development of the standard throughout Europe.

 

 

Why Build a Passive House?

 

When planning to build a new home, there are a variety of reasons why the Passive House design may be considered:

 

Low energy costs – due to the low requirement for space heating/cooling

 

Healthy living environment – due to the airtight construction and mechanical ventilation

 

Future proofing of the construction – exceed current and imminent Building Regulation changes

o       Target for the Passive House Standard is 2015 for new built houses all over Europe

o       But the European Parliament has called to target 2011

 

 

Considerations of the Passive House Standard:

 

There are some key aspects to be considered with a Passive House Design:

 

·  Air Tightness

·  Thermal Bridging

·  Optimised Insulation levels

·  Mechanical Heat Recovery Ventilation Systems

·  Passive Heat Gains and Orientation

·  Windows

·  Backup Heating System ( Renewable Energy Systems)

 

 

Air Tightness

 

Air-tightness in the passive house structure is essential and this can be a very challenging aspect of the design process.

 

Air-leakage can occur when uncontrolled cold air penetrates into the building.

It can take place through poorly sealed windows/doors, along the joints of the building (walls, floors, roof).

 

In order to measure the air-tightness of a building, a blower-door test can be used.  Positive or negative pressure is created within the house and the pressure used for the test is 50 Pascal.

 

If there are any leaks or draughts these can be located using this test.

 

In order to achieve the Passive House Standard, air-leakage test results must not exceed 0.6 air changes per hour at 50 Pascal (over-pressurisation and under-pressurisation testing)

 

 

Thermal Bridging

A thermal bridge in a building allows heat to be transferred (heat loss) through the structure.  It refers to a material or combination of materials in the building envelope.

 

Care should be taken when designing the junctions between walls, floors, windows and the roof in order to minimise thermal bridging

 

Optimised insulation levels

 

Effective insulation is a key factor to ensure minimal heat loss.

There are four significant areas of building envelope insulation to consider:

Walls, floor, windows and roof.

 

 

Typical insulation materials used in Ireland:

 

  • Mineral
  • Polystyrene
  • Polyurethane
  • Polyisocyanurate
  • Sheepwool
  • Hemp
  • Cellulose

 

It is important to be aware that different insulation materials will be more suitable for different situations / locations.



Mechanical heat recovery ventilation system

 

 

An airtight house will require a mechanical heat recovery ventilation system for good air quality.  The warm air from the “living” areas of the house passes through the heat exchanger and heats the fresh air entering the building.

 

The exhaust air and clean fresh air are kept separate in the heat exchanger.

 

The acceptable rate of air changes in Passive Houses is between 0.3 and 0.4 times the volume of the building per hour.

 

 

Passive Heat Gains and Orientation

 

An ideal position of the building is to have an east-west alignment and the majority of glazing facing south.  The ratio of glass to be used should be determined by the design verification using the PHPP software.

 

Appliances and individuals also contribute towards the internal heat gains.  Therefore to avoid the risk of overheating in the summer, specific devices can be included in the design process (e.g. balcony, overhangs or blinds)

 

Windows

 

It is recommended that a passive house is designed with relatively large windows facing south and to minimise the north facing glazing.

 

The interior daylight levels will be maximised through the large windows and heat loss will be minimized by using small/few windows on the north facing elevation.

 

Average U-value for windows – as part of the Passive House Standard:

0.8 W/(m²K)

  

Typically triple glazed windows units are used in passive houses in Europe.

 

Backup Heating System (Renewable Energy Systems)

 

The space heating requirement in a Passive House is so low that there is no need for a traditional space heating system.

 

The mechanical ventilation system is generally sufficient to transfer the small amount of required heat throughout the house.

 

The temperature of the air can be boosted in a number of ways (post-heating the fresh air after it has been warmed by the MHRV).  For example:

 

Water to Air Heat Exchanger

 

A heating device is placed on the fresh air supply outlet of the MHRV.  A small radiator inside this device is heated by hot water connected to the domestic hot water tank.  The additional heat requirement is determined by a thermostat and the hot water is circulated through the device; hence the title of “water to air heat exchanger”.  The water stops circulating and the air is no longer heated when the house has reached the required temperature.


 

Wood Pellet boiler/stove

 

 

There are a few items to keep in mind when considering a wood stove or boiler:

 

The equipment must be sized appropriately to the heat load of the house (space heating required to maintain comfortable indoor temperatures).

 

Most wood stoves are very efficient – up to 80-90%.  There is very little ash with pellets and a flue will be required to take exhaust emissions away from the house.

 

If the hot water from the DHW tank is to be used to heat the ventilation air, it is essential to have the stove or boiler direct the heat output to the DHW tank.

 

Such units will often be used for both auxiliary water heating and auxiliary space heating

 

A dry storage space for the fuel (pellets, logs etc) is required

 

By using a Wood Pellet/Wood Log boiler as a room heater there is a risk of overheating in the room in which it is located.

 

Solar Thermal Input

 

Solar collectors can be used to provide some of the hot water requirements and typically there are two types of solar collectors used:

 

  • Flat Plate panels
  • Evacuated tubes

 

For the Irish Climate, the amount of hot water produced is very dependant upon the efficiency, orientation and angle of the solar collectors.

 

As a general rule - the area of solar panel per person is approx. 1-2 m²

 

The system should ideally be capable of providing up to 70-100 litres of daily domestic hot water (DHW) needs per person – in season

 

 

Key Target Limits of the Passive House Standard:

 

The following are some key target limits associated with the Passive House Standard:

 

  • Annual demand for space heating should not exceed 15 kWh/(m²a)
  • There is an upper limit of 120kWh/(m²a) for total primary energy demand for space and water heating, ventilation, electricity for household appliances and lighting
  • Air-leakage results must not exceed 0.6 air changes per hour at 50 Pascal (over-pressurisation and under-pressurisation testing)

 

 

 

For more information refer to the SEI publication “Guidelines for the Design and Construction of Passive House Dwellings in Ireland”

Prepared by MosArt Architecture, UCD Energy Research Group and SEI Renewable Energy Information Office