There are four key factors which are critical in developing a new timber frame house / system to achieve the Passive house certification standard. These are:

U value

• This is the measure of how quickly or slowly heat transfers through the structure.
• The u value on a standard build is 0.21W/(m2K).
• The target for passive standard is a maximum of 0.15W/(m2K).

Thermal bridge free design

• Thermal bridging is the transfer of heat through the studwork. Studs are timber which transfers the heat quicker than insulation therefore reducing the thermal bridges reduces the heat loss.
• The thermal bridging on a standard build is 0.15W/(mK) or 15%
• The target for passive standard is a maximum of 0.01W/(mK) or 1%

Interior surface temperature

• This is a measure of thermal comfort i.e. If a person is standing near the wall, the temperature difference from the wall surface and the air in the room should not be noticed.
• Standard building systems do not set a standard for this.
• The target for passive standard is to achieve internal surface temperature of the wall of 17 degrees as against an ambient temperature of the room of 20 degrees (i.e. less than 4 degrees temperature difference.

Airtightness

• Airtightness is the sealing of the building from the inside to ensure that no warm air leaks out through the building envelope and that no cold air leaks into the building through the building envelope.
• The airtightness on a standard build is 7 airchanges per hour (ACH).
• The target for passive standard is a maximum of 0.6 ACH.

In every single one of these key factors, the Passive standard target is vastly superior to current standard building practice.