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1. Why Airtightness?

Air leakage causes hot (heated) air to leave external air to draught inwards. This has the effect of increasing your heating bill and reducing comfort levels. Air leakage can detract from the thermal performance of a building, by providing a ventilation path within the construction for heat loss, thereby “short-circuiting” the thermal insulation, and reducing the U-value of the building elements. A building with an N50 of 7.0 has an air change rate per hour of 0.35 under normal atmospheric pressure, this means 35% of the warmed air is lost to the outside every hour.


2. How is Airtightness Measured?

Blower Door Test is where a fan is mounted on an external door and air is pumped out the house at a controlled rate. All the planned vents are closed and services sealed for the test. The fan speeds adjusted to create a building pressure of -50 Pascals. The air flow (V50) being expelled to maintain -50 Pa is recorded as m³/hr. Air re-enters the house through the leakage points, this volume of air entering at -50 PA tells us how sealed the house is.


3. Airtightness Test Terminology?

The Building Envelope is the area of the ground floor, highest ceilings and inside of the external walls. See diagram. Air Permeability is the air leakage parameter used in the EN ISO 13829, the Part L Building Regulations and is utilised in the DEAP (Ireland) and SAP (UK) energy performance tools. A building’s specification normally refers to this parameter. Air Changes Per hour is useful for quantifying the impact of air leakage and is used in the design of mechanical ventilations systems. Our example house has 3.57 air changes per hour (ACH) at -50 pascals. Under normal atmospheric condition the house has 0.17 ACH (3.57/20). That translates to 17% of the buildings air being changed every hour unnecessarily through leakage paths.


4. How Do You Achieve Airtightness?

Airtightness means cutting out unwanted draughts. Draughts can be so slight as to be imperceptible, but even slight droughts increase heat loss, sometimes dramatically. The way to good airtightness is a continuous air-resistant layer all around the inside of the building. Before a project starts you should understand what level of airtightness you require, and what it is likely to cost you. The BER may have a target permeability. Building control requires a max permeability of 7m³/(hr*m²). If Heat Recovery Ventilation (HRV) is installed there should be a max permeability of 2.5m³/(hr*m²).

Dwelling Parameters:
- Floor Area: 98m²,
- Surface (Envelope) Area 249m²,
- Volume 261m³.

Test Results:
- V50 (Airflow at -50Pa) 831m³/hr,
- n50 (Air changes 1/hr) 3.19,
- q50 (Permeability) 3.34m³/(hr*m²),
- ELA (Eq leakage Area at 4pa) 195.3cm²,
- Air changes at normal pressure 0.16.