Passive Housing (Leaving Cert Construction Studies): Revision Notes

Passive Housing

Sustainability and Passive Design

What is a Passive House?

• A passive house is not a construction method, but an internationally recognised building standard that focuses on comfort, sustainability, and economy.
• It should not be confused with an energy-efficient house, which simply uses less energy than average.
• A passive house can maintain a comfortable indoor temperature without the use of active heating or cooling systems.

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Key Features

• Passive houses use careful design, insulation, and airtight construction to reduce heat loss and make the most of natural heat from the sun.
• They can reduce purchased energy use by up to 85–90 percent compared to traditional houses.
• In Ireland, passive buildings include one-off houses, schools, healthcare buildings, and social housing developments (such as in Dungannon, Co. Tyrone).
• The Passive House Association of Ireland (PHAI) works to promote and educate people about passive housing.

Criteria for Passive Certification

To be certified as a passive house, a building must meet strict criteria:

• The annual heating demand must be no more than 1.5 litres of heating oil per square metre per year.
• The building must be extremely airtight, with an air change rate of no more than 0.6 per hour at a pressure difference of 50 Pascals.
• It must be super-insulated, with an average U-value of less than 0.15 kilowatt hours per square metre per year.
• There must be no thermal bridges (cold spots where heat can escape).
• The design should be a compact building form with the smallest possible surface area in relation to its volume.
• The building must make passive use of solar energy through correct orientation.
• All glazing and window frames must have a U-value of less than 0.8 kilowatt hours per square metre per year and be triple-glazed.
• Household appliances should be A1-rated for efficiency.
• The total energy use for heating, hot water, ventilation, and household electricity must be no more than 120 kilowatt hours per square metre per year.

Airtightness

• Airtightness prevents heat loss by stopping air leaks through the gaps in the building.
• Common leakage points: wall/floor junctions, windows and doors, roof junctions, service pipe penetrations, chimneys, attic hatches.
• Testing methods:
  • Blower door test – measures air change rate.
  • Thermal imaging – shows cold spots and leakage paths.
  • Smoke tests – detect air movement to show where air is escaping.

Orientation

• Correct orientation allows a building to gain the most heat from the sun.
• Building must be oriented for maximum solar gain:
  • Large windows should face within 20 degrees of south.
  • Large north-facing glazing should be avoided to reduce heat loss.
• Correct orientation works with internal layout so rooms used during the day get most sunlight when needed.
• Triple glazing and low-e coatings can make south-facing windows up to five times more efficient than standard double glazing.

Compact Building Form

• A compact form reduces heat loss by having a smaller surface area compared to its volume.
• For passive certification, the compactness ratio should be 0.7 or less.
• Complex or sprawling building shapes lose more heat.

Passive Foundations

• Passive foundations are highly insulated to prevent heat loss into the ground.
• They stop cold bridging by creating a break between the ground and the inside of the house.

Passive Walls

• Walls must be airtight and have very high levels of insulation.
• A plaster finish without cracks ensures airtightness.
• Example (block wall):
  • External render, external leaf, 250 mm insulation, internal leaf, plaster.
• Example (timber frame):
  • External render, external leaf, cavity, insulation layers, vapour barrier, plasterboard.

Passive Windows and Doors

• 15% of heat lost through doors; 10% through windows.
• Doors and windows must be airtight and highly insulated.
• Heat loss through frames should be minimised by keeping frame sizes small compared to the glass area.
• Triple glazing is essential, and frames must be filled with insulation to prevent cold bridging.

Passive Roofs

• Traditional roofs need additional insulation and airtight membranes to meet passive standards.

Advantages of Passive Design

• Passive houses have very low energy consumption, using 75–90 percent less purchased energy than traditional houses.
• They provide a consistent indoor temperature of around 18–21°C all year.
• They have a lower environmental impact, producing less carbon dioxide because less fuel is burned for heating.

Disadvantages of Passive Design

• A passive house requires a high level of workmanship to meet the precise construction standards.
• Precise detailing for airtightness, insulation, and other features takes a lot of time during the build.
• There is a lack of available expertise in passive construction in some parts of Ireland.
• The initial building cost can be higher than for a traditional home.