The Passive House concept as put forward by the PassivHaus Institute is a very exciting development and is of its time as the fabric elements and the systems involved are now fully developed and widely available.

 

The basic requirements are:

 

Ø      The house will use not more than 15 kWh/m2 per year of delivered energy for space heating

 

Ø      The house will have a Specific Annual Heating Load of less than 10 W/m2. This does take into account the heat gains by way of solar gains through the windows and the internal heat gains due to occupancy. Hence, it should be noted that this is a different figure to the energy required to meet the Specific Heat Loss Characteristic of a house for a given temperature difference

 

Ø      The airtightness of the house will be such that the air infiltration rate will be less than 0.6 Air Changes per Hour at 50 Pa.

 

Ø      Thermal Bridging will be reduced to a minimum

 

Ø      A Heat Recovery Ventilation system is used to supply the necessary air for ventilation. It must have an efficiency of more than 75%. The system will supply at least 0.3 ACH of fresh air

 

Ø      A solar water heating system should be used. Typically, a 3m2 Evacuated Tube Array with a 300 litre domestic hot water cylinder will suffice.

 

Ø      The overall Annual Energy Demand shall be less than xxxxx

 

By these means the necessity for a full primary central heating system are negated. It is generally sufficient to add any extra energy by way of energy delivered from an internal wood pellet stove normally placed in the fireplace of the house or by way of a small heat pump which is extracting energy out of the extracted air which has already gone across the HRV unit. The design of this unit is catered for in the Passive House Planning Package.

 

The Passive House Planning Package is a most useful and comprehensive tool to aid the designer in arriving at a satisfactory solution based on all of the above points. To use this package will require most designers to undertake a certain amount of training. To date courses are being offered by SEI/REIO and by Nicer Training.

 

Typically, to satisfy the above criteria the fabric U-Values must be less than or equal to the following:

 

                                    U-Value (W/m2.K)

Ground Floor                             0.12

External Wall                             0.13

Roof                                            0.14

Windows and Doors               0.80

 

The following points should be given careful consideration:

 

Ø      An air infiltration level of less than 0.6 ACH at 50 Pa is very low indeed. It represents an actual infiltration rate at Normal Pressure of less than 0.03 ACH. It would be our view that with only 0.3 ACH of fresh air being delivered by way of the HRV system that consideration would have to given to the humidity level in the house in winter as it could drop to very low levels indeed – a figure of 35% RH at 20ºC

 

Ø      The Draft Irish Building Regulations call for a fresh air requirement of 0.3 litres of fresh air per second per m2 of floor area. This must be taken into account and supersedes any other requirements as set out in the PHPP.

 

Ø      At this level of airtightness, great care would be required in the fitting and use of an internal wood pellet stove to ensure that there was no depressurisation in the room it is sited. Also, the stove would have to be room sealed i.e. its combustion air would have to come from outside – as it should in all low energy houses.

 

Ø      The difficulty in achieving this level of airtightness should not be underestimated

 

Ø      If the temperature of the house should drop due to non-occupancy in winter then the power required in the heating system would have to be greater than 1200 W in order that the house would heat-up in a short time as power will be required to overcome the Thermal Response / Thermal Mass of the house. Alternatively, homeowners should be told to leave the heating on even when they are away as it will use so little energy in the first place.

 

Ø      To save costs some Passive House designers use a direct acting electric heater in the air duct to provide back-up heating. It should be remembered that when this is taken into account in SEI's DEAP calculation a severe penalty is incurred as the use of electrical energy is multiplied by a factor of 2.7. Very often it can move the rating of the house on the BER scale from being an A2 house to a B2 rating.

 

Ø      Large areas of South facing and indeed East and West facing glazing should be given careful consideration. The possibility of overheating due to solar gains through these windows must be considered and appropriate steps taken.

 

Ø      It is vitally important that the fabric U-Values are achieved in practice. If there is only a 1200 W heater to cater for deficiencies and if the actual heat losses were somewhat higher than the design then it would be disastrous for the comfort levels experienced by the home owner and the running costs would escalate accordingly.

 

There is no doubt that if all of the criteria are adhered to rigidly then a low energy house will result. Comfort levels will be satisfactory and running costs will be very low. However, it is important that all of the above points are taken into consideration by the designer and the builder.

 

Further Information

  Please contact NuTech Renewables Ltd for further Information