Simplifying Airtightness In Modular Construction

In todays webinar we’re going to discuss measures to simplify achieving good air tightness in modular and offsite construction applications.
Based on the principal of DfMA - Design for Manufacturing and Assembly - Volumetric or Modular Construction involves the offsite production of “modular units” .
Categorized as a Modern Method of Construction - MMC - they are built to last and deliver an incredible range of different options, that can be tailored to suit every requirement.

Our range of products include unique off-site solutions for the following sectors:

• Private and social/affordable housing
• Purpose built student accommodation
• Self-build projects
• Hotels
• Education and office buildings
• Healthcare including hospitals, health centres and healthcare facilities

As with any type of construction, the basis of good design in modular construction comes from a good understanding of the building regulations themselves. Today we’ll be focusing mainly on the documents relating to energy performance and condensation control. So this would be Approved Documents L and F in England and Wales, Technical Standards 3 and 6 in Scotland, Technical Booklets K and F in Northern Ireland, and Guidance Documents L and F in the Republic of Ireland.
While the specific details in these documents might vary a little, the principles behind the guidance and the recommended approaches are broadly similar.

The first thing we need to consider is the type of building fabric we are dealing with. Concrete, steel and timber based buildings all respond differently to moisture, and contain different amounts of construction moisture which must be allowed to dry out. For example, buildings with a lot of in-situ concrete can take several years to fully dry out, this excess moisture load must be accounted for at the design stage.

Construction moisture can also come from the weather, and this must also be taken into account along with the weather conditions the building will be subjected to once completed. Being weather tight earlier in the construction process and generally being composed of drier materials, offsite construction has an important advantage hygrothermally.

By reducing the initial moisture loading on the building fabric, the design can be more precisely tailored to manage moisture associated with the occupants of the building and the uses the building will be put to, leading in turn to a more efficient fabric envelope and building system.

The influences of these three aspects can then be assessed in terms of the heat, air and moisture movement within the building. This takes into account the heating of the building, as well as the air leakage effects and response of the building fabric to the absorption and desorption of moisture. Factors such as the position and performance of the fabric insulation can also be considered.

To ensure our design adequately manages these complex interactions, we undertake a hygrothermal analysis of the building fabric using a software called WUFI. This breaks the building elements into individual layers and calculates the temperatures, moisture flow and degree of water storage at any point in the building fabric we choose to assess.

Historically, a process known as the Glaser method was used to assess this, however this method makes a few assumptions that limit its accuracy. In contrast WUFI allows us to account for the variable external influence of weather conditions, along with the effects of the fabric’s capacity to store and release moisture. We can also look at how these factors interact over longer time periods.

The steady state analysis used in the Glaser method, while useful, does not always provide a sufficiently detailed picture of the long-term performance of the structure. This more detailed analysis can allow us to consider solutions that may not appear acceptable when assessed under the Glaser method. In our example here, the internal vapour control layer can be removed altogether without creating a condensation risk.
This is made possible by the use of an external vapour permeable air barrier membrane. Our Wraptite membrane self-adheres to the external face of the sheathing, and provides a robust airtight layer without compromising moisture movement through the wall assembly.
This allows vapour control to be assessed independently of the airtight envelope, and optimised to suit the position and type of insulation, and the anticipated moisture loadings.

By removing the vapour control in favour of an external air leakage solution we achieve a few things. Firstly, we remove the associated material and installation costs, but more importantly we remove the need to seal all the service penetrations, meaning we have a more reliable air barrier and can reduce the air leakage rates used at the design stage.

Reducing the air leakage from 3 m3/m2/hr down to 1 can have a significant positive impact on the energy performance of the building, meaning we can reduce the insulation thickness required, or the specification of the insulation boards themselves. This can offer designers more flexibility in insulation types used, for example as regards fire performance, or increasing internal space available to occupants.
We can however only take full advantage of this flexibility if we can depend on achieving the air leakage targets we use in the design process. As we can see here, if we keep the air barrier internal, that means a lot of tape and mastic required to seal all the switches, sockets, pipework and other services that modern buildings require.

A lot of this sealing can be done offsite, but it still takes a lot of time to do correctly and further care is required to ensure the seals are not broken or damaged during transportation and assembly. Even small punctures, broken or misaligned seals or loose tape can significantly increase the air leakage rate when tested, leading to expensive and difficult remediation measures to meet regulation requirements.
This process can be speeded up somewhat by using proprietary seals and components but this can increase costs and complicate supply chains. Using a fully adhered external barrier removes this problem completely, ensuring that the panels arrive in position perfectly sealed and the performance in practice matched the design criteria without requiring any unusual measures or specialist products during the construction process.
In modular systems, incorporating an external air barrier is straightforward and brings several benefits over traditional mechanically fixed membranes.
The membrane can be applied to the panels in any orientation, and being self adhered requires no mechanical fixings. This adhesion reduces the potential for membrane damage both during the module assembly process and while in transit to site.

The panels are then assembled and the joints taped using the same material in tape form, ensuring no adhesion issues or tape compatibility problems. Once the joints are taped, the panel assembly is resistant to air leakage, and any service installations such as wiring or pipework can be undertaken independently, without risking damage to the airtight layer.
These wall, roof and floor panels can then be assembled into modules, and the Wraptite split-liner tape used to complete the airtight seal between adjacent assemblies. This tape simplifies making such joints by splitting the release liner in half, allowing the two surfaces to be stuck independently, making the joint faster and more accurate.
The completed modules can then be transported to site.
During transportation the self-adhered membrane ensures watertightness, protecting internal components and minimising on-site remediation work.

The building can then be assembled on site and completed as normal. Many systems are accredited within BOPAS – Building Property Assurance Scheme – which offers assurance of structural integrity for modern methods of construction. To date, Wraptite is currently included within two systems.
As well as ensuring the integrity of the air barrier is maintained, the self-adhered Wraptite membrane brings important benefits in terms of fire resistance.
Since December 2018, membranes used in external walls over 18m in height have required a minimum fire rating of B-s3,d0. While there are some materials with A1 or A2 ratings that can be used to provide weather protection, they typically don’t offer sufficient vapour permeability to ensure a healthy building envelope is maintained - by allowing construction moisture in particular to dry out.
Class B membranes are far better in permeability terms, and don’t usually contribute enough calorific value to have a significant effect on the development of a fire.

The BR135 guidance document uses the following model to illustrate the development of a fire. A rapid fire spread occurs when an initial fire develops and flashes over, and is then spread to all areas simultaneously by the outer cladding layers. This then starts fires across all the areas of the building.
In cases of restricted fire spread, the initial fire develops and flashes over, but is then either contained, or will ignite a single secondary fire directly adjacent. The fire will only develop further if this secondary fire also develops. This is a much slower process.
Let us consider solely how the membrane in facade construction reacts to fire - if the membrane is mechanically fixed and taped, there is the potential for fire to spread on both surfaces of the material as oxygen can feed into the fire on either side. This additional oxygen supply can also allow the fire to develop more rapidly.

With a fully adhered membrane, only the outer surface can contribute to the development of the fire, as there are no gaps between the membrane and the substrate. This inhibits the supply of oxygen to the fire and slows down the spread of fire across the membrane.
So, we’ve looked at how the Wraptite membrane reacts to fire, and how it complies with the relevant technical standards, but if we want to go beyond fire resistance and introduce a degree of fire protection, then another option is to use the Fireshield membrane.
Fireshield is a vapour permeable membrane with a unique surface composition which actively reacts to prevent fire taking hold. Fireshield can be used over insulation boards to provide an additional layer of fire protection to the internal structure of the building.

In this comparative test, based on the EN13823 Single Burning Item test, the Fireshield on the left, reacts substantially less than the other Class B fire resisting membrane on the right. The surface of the Fireshield acts similarly to an intumescent coating, “frothing up” to protect the insulation boards and structure.
In this test the unique coating protects the sheathing long enough for the external fuel source to be exhausted and the fire to self extinguish. The standard class B membrane, although it shrinks away, and does not contribute to the development of the fire, does nothing to protect the structure or delay the progression of the fire.
Used in this way, Fireshield can help prevent fire from outside the building causing damage both during construction and in service.
During the construction process, Fireshield can also be used on the external cavity face to improve the fire robustness of closed panel assemblies when installed over external sheathing alongside suitable non-combustible internal linings.
Fireshield is the first fire resistant vapour permeable membrane approved for inclusion in the structural timber association tested product listing for fire robustness during construction. As part of such a construction, Fireshield will be part of a system to limit the spread of fire, which can allow for reduced spacing to adjacent properties.

The Proctor Groups range of advanced construction membranes are supported by our range of technical services to assist in integrating our systems into whatever construction methods and practices are in use by our diverse range of customers.
Perhaps the most important aspect of this is providing hands on assistance with detailing of membranes and the production of drawings to allow correct installations to take place.
Alongside this, we can also undertake WUFI assessments to ensure the specified construction is able to deal with the moisture loads placed on it, and to optimise the material specification to ensure the most efficient envelope is achieved.
We also provide a full library of BIM data and template constructions for all our products, available in both Revit and IFC formats for compatibility with a wide range of software and systems.

Finally our on-site toolbox talks ensure the entire project team is confident working with our materials and has a full understanding of how they work and the benefits they bring to the projects.