Air tightness

There are five key Certified Passive House design principles;

1

Air Tightness
Elimination of drafts.

2

Super-insulated & thermal bridge-free envelope
Retain heat energy.

3

Heat recovery ventilation (MVHR) Recycles heat energy & provides fresh air.

4

Water tightness
Essential for building durability.

5

High-performance joinery
Building enclosure

1. Air tightness –
elimination of drafts

This is key to maintaining high indoor air quality and energy efficiency in passive houses.

Passive houses are meticulously sealed to prevent air leakage; it is a fundamental principle in passive house design because air leaks lead to drafts and energy loss.

Normal houses don’t have any regard for air tightness; they’ll have the wind blowing through the building walls and the windows, which cools the insulation down, and then it can’t do its job. In the passive house industry, this is what we call a leaky building, but we’re not talking about water; we’re talking about air.

Air barriers, tapes, and sealants are used to ensure that warm or cool air generated by the heating or cooling systems stays inside while preventing drafts and the infiltration of unconditioned outdoor air. Essentially, you’re eliminating draughts – warm air leaking out of the building or cold air leaking into the building. When you’re paying to warm the air inside your building, you lose money when it leaks out. So, the airtight construction capitalises on the money you’ve spent to warm a house up.

Air changes per hour (ACH) is a measurement that quantifies how often air volume within a building is replaced with unconditioned outdoor air through envelope leakage at a pressure differential of 50 Pascals (n50). It’s crucial to understanding a building’s ventilation rate in high-performance design. In a typical house, the rate of ACH ranges from about 4.0 to 8.0. This means the entire air volume is replaced with outdoor air four to eight times in one hour – the warm air within the building leaks through the walls, floor, or ceiling.

A passive house needs to meet a minimum air change target of 0.6 air changes per hour n50 – that’s 0.6 times the air volume of the building, so less than 0.6 times the building air volume is allowed to escape every hour in a Certified Passive House.

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We pressurise and depressurise the building to create a controlled pressure difference between the interior and exterior of the building. This controlled pressure difference assesses the building’s airtightness and how effectively it prevents air leakage. The goal is to measure the air exchange rate at a pressure difference of 50 Pascals (Pa).

A blower door and pressure gauge (manometer) are used to progressively pressurise and depressurise the building over a range of 10 – 70 Pa. Sophisticated software measures the interior and exterior pressure and airflow through the blower door fan, allowing the calculation of air infiltration and exfiltration. The software plots straight line graphs, and the ACH reading at 50Pa is used for the test criteria.

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