PassivHaus Wall Systems

This article is our second in our series discussing Passivhaus 'fabric-first' design principles to improve the energy performance and quality of our buildings.  Focusing on the External Wall, the article highlights the general strategies prevalent today with a focus on Timber Frame Structural solutions, the most popular here in the UK.

Design Principles

The Passivhaus Standard has very stringent requirements with respect to meeting the energy demand of the building when compared to more conventional building standards (Primary Energy Demand of </= 120 kWh/m2.yr and Space Heating Demand </= 15kWh/m2.yr).  In order to meet such demands the thermal performance of the building envelope has to be optimised to achieve the following values;

• U-value of 0.15 W/m2k or less

• Airtightness of 0.6 air changes/ hr @n50 or less

The building envelope therefore must have;

• Very high levels of insulation

• Airtight building fabric

• Windtight building fabric

• Thermal bridge free construction

Within the standard there is no limitation on how these values and criteria can be achieved, allowing creativity in design and multiple building systems to be considered.  By far the most cost effective system for domestic building in Northern Europe however, is Timber Frame Construction.  Within this 'system', the parts themselves are interchangeable, providing the opportunity to choose from a vast array of different elements and components to achieve higher levels of performance and economy.  Such systems are light weight, easy and quick to transport and construct and are robust.  Whilst there are many derivatives of timber frame construction, all are characterised by an internal zone for running services and applied internal wall finishes and an external cladding zone capable of supporting a wide variety of external cladding systems to meet aesthetic and regional requirements.

Broadly speaking, timber frame construction is split into two approaches, open panel construction and closed panel construction.  Open Panel construction relies upon simple open stud framework being erected, then insulated and clad on site.  Closed panel construction optimises the advantages of offsite manufacture, forming modular panels in a factory complete with an enclosed structure, insulation and protective sheathing layers to the internal and external face of the panel.  Open Panel is often slightly cheaper, but Closed Panel construction offers the benefits of improved build quality.

Open Panel Construction

In order to meet the stringent thermal requirements of the Passivhaus Standard,  open panel systems will typically be constructed from 184mm deep timber studs, supplemented by additional thermal layers of insulation placed externally or internally to alleviate thermal bridging through the timber studs impacting upon the overall U-value of the assembly.  This allows a wide use of insulation material to be considered, dependent upon cost, performance and environmental strategy.  A typical assembly comprises the following; 

• External cladding zone

• Windtight, breather membrane

• OSB Sheathing board

• 184mm timber studs at 600mm centres

• Insulation infill

• Airtight and vapour control layer (membrane or board)

• Supplementary thermal insulation layer

• Service void

• Internal wall finish 

Advantages

• Low cost

• Leveraging common construction skills

• Versatile

• Medium speed of construction

• Flexibility in design

Disadvantages

• Highly dependent on site supervision

• Required build quality hard to achieve

• Slow erection times when compared to other methods

Closed Panel Construction

Optimising the advantages of offsite manufacturing techniques, closed panel construction is becoming increasingly popular.  Broadly speaking there are three types of system used.   

Closed Panel System 01

Essentially an offsite version of an open panel system, many suppliers now offer what can be considered an entry level closed panel system.  Typically the panel is supplied with both external and internal sheathing boards fitted with the panel being fully insulated.  In some cases external cladding rails and finishes can also be applied subject to logistical constraints.  These systems can also take advantage of manufactured I-Joists replacing the traditional 184mm stud.  These can provide a deeper wall, increasing insulation and reducing thermal bridging through the stud.

Closed Panel System 02

Another way of increasing the levels of insulation beyond a standard 184mm stud whilst also minimising thermal bridging through the structural timber elements of the wall is via a split-stud or twin stud wall.  In this construction, two lines of studs 90mm wide are placed tied together at intermediate heights of 600mm vertically.  This allows any depth of wall to be constructed to achieve the required thermal performance of the construction.  Other advantages include optimising the integration of intermediate floors and roofs to minimise thermal bridging and improve the thermal performance of the entire structure.  Beyond the structural layer, the system shares the same characteristics of other timber frame systems.

Credit: MBC Timber Frame

Closed Panel System 03

The final closed panel system is a Structural insulated Panels Systems or SIPS for short.  SIPS panels comprise high density EPS or Foam insulation and timber studs sandwiched between two layers of sheathing board, typically OSB board.  The result is a composite panel of high strength and thermal performance capable of large spans.  These panels are able to be used as walls, roofs and even floor cassettes.  Manufactured under factory conditions, the panels are quick and easy to transport and erect on site, achieving wind and watertight status very quickly.  They are best suited to modularised simple forms, their efficiencies quickly compromised when more complicated forms are desired.

Advantages

• Speed of construction on site is high

• Whole façade option

• Improved build quality

• Modular build

• Building performance generally improved

Disadvantages

• Can be costly

• Specialist structural input required

• Supplier choice scarce

• Can be inflexible due to modular build strategy

• Can require specialist erection team

In June of 2019 the UK became the first major economy in the world to commit to producing net zero greenhouse gas emissions by 2050.  Enshrined in Law this means that all industries, including construction, must operate at net zero carbon by 2050.  Indeed in Scotland, the devolved administration has committed to a date of 2045.  Some cities such as Edinburgh and London, have even more aggressive targets of 2030 in mind.  To meet these targets the built environment, in particular our homes, will have a continuing requirement to meet increasing energy efficiency targets.  The net result of this will be a continued drive towards off site manufacturing techniques such as the ones described in this article, to ensure post occupancy energy performance targets are met. 

The above systems are all capable of providing an energy efficient façade and are becoming increasingly popular in the UK, in particular in the Self-Build Home sector.  Choice is available.  All are capable of delivering a building performing to the Passivhaus Standard.  Choice will be dependent upon a combination of factors including cost, speed of construction, technical expertise of your team and complexity of design.

If you have found the above article useful and would like to understand more about the Passivhaus Standard, we would encourage you to read the other articles in our blog available at www.novo-design.co.uk/blog.  Alternatively you can join our mailing list below.