Pathway to passive

With the UK’s daunting net zero 2050 target enshrined into law, such efforts to create practical ‘near zero energy’ houses are essential With houses accounting for 40% of energy consumed, achieving the means of constructing new passive houses affordably using solutions such as offsite timber frame and lightweight steel frame systems which can deliver the results is the realistic way forward

Faced with spiralling construction costs, off-site construction has potential to be a cost efficient and sustainable alternative to traditional methods of construction. Its benefits are well documented and while there are still barriers to its adoption, it can provide better working conditions, shorter time on site, and improved environmental performance in the construction process

A recent study from the University of Cambridge and Napier University found that factory made homes can reduce carbon emissions by up to 45% compared to traditional build It is perhaps no surprise that offsite timber and lightweight steel frame systems allow builders to achieve the desired rigorous levels of airtightness required in Passive House construction

The importance of airtightness

Airtightness is a critical feature in Passive House, or any energy efficient home

Airtight membranes are primarily used to form the airtight barrier in passive houses and have several critical roles to play

Firstly, reduce energy demand When a building is not airtight, warm air escapes quickly and is replaced by cold air, which needs to be heated to maintain a comfortable indoor temperature

This is very energy intensive and causes occupant discomfort Secondly, minimise moisture entering the building fabric – airtight membranes are also designed to control vapour movement (i e Air and Vapour Control Layer – AVCL) By creating an airtight system, we are ensuring water vapour cannot freely move from the internal environment into the building fabric

A great deal of moisture is created in our homes from activities such as cooking, cleaning and showering so it is important that this water vapour does not enter our walls and roofs, where it will cool and possibly condense By prioritising the building fabric in this way, the passive house approach to design ensures high levels of thermal performance and superior indoor comfort.

Timber frame design

One way to achieve the Passive House standard is through offsite timber frame construction as it can provide the ideal fabric for buildings to achieve low energy demands and meet the typical U-value requirements, 0 15 W/(m²K) or below in walls for example The factory setting enables a greater control over the quality, detailing and craftmanship needed to meet these high-performance building standards It is also easier to ensure that a structure meets design specifications, and to check it for errors, when standardised and automated processes are used in a more controlled factory environment.

Offsite manufacturers have recently had to respond to the reduction in U-value targets to ensure their system is compliant with new regulations, lower airtightness targets and end user demands for more energy efficient homes In doing so, many timber frame manufacturers have reviewed their wall build ups in pursuit of an optimised solution While increasing insulation thickness is the obvious solution to lowering U-values, this is not always cost effective In addition, it increases the wall thickness which can impact footings, internal floor area and the erection processes

As an alternative, many manufacturers are exploring the use of reflective foil breather membranes and vapour control layers (VCL). When installed adjacent to a cavity, these membranes enhance the building element U-value – they help timber frame manufacturers achieve compliance with Part L without having to increase stud depth Reflective foil membranes are designed to perform their primary function (protect against wind and rain, or control water vapour) as well as improve the heat retention of a building element

When these membranes are installed adjacent to a cavity, they enhance the resistance value (R value) of that cavity –remember, the higher the R value, the better For example, a typical 25mm unventilated air cavity would have an R value of 0 18 m2K/W (equivalent to less than 5mm of PIR insulation) while the same cavity, when adjacent to an Echofoil membrane, has an R value of 0 78 m2K/W (equivalent to more than 15mm of PIR insulation)

Steel frame systems

The fabric build-ups of steel frame systems vary much more than their timber frame counterparts Steel frame manufacturers are less likely to use reflective foil membranes, and instead opt for highspecification AVCL membranes, such as Izoperm Plus or Izoperm Plus FR, and a high-performance breather membrane

Improving the energy efficiency of our homes on our journey to net zero has never been more important Offsite methods of construction are a growing part of the market and are seen as one of the most attractive ways of delivering buildings that are high quality, sustainable and cost effective The development of breather membranes and vapour control layers which offer high performance and can meet new thermal and air permeability targets is set to revolutionise the way timber frame and light gauge steel frame structures are designed and built