Simplifying airtightness in modular & offsite construction

The A. Proctor Group has, for over 50 years, been serving the construction industry with an extensive portfolio of technically advanced thermal insulation, specialist membranes and vapour control layers, providing an extensive range of superior high-performance products suitable for modular and off-site construction.

The basis of best practice in modular construction comes from an understanding of the relevant building regulations and a holistic approach to the building design. In doing so we consider six core aspects in the process related to the balance of Heat Air Moisture Management (HAMM):

• Building
• Weather
• Occupants
• Heat
• Air
• Moisture

Building

The first area to consider is the type of building fabric involved. 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.

Weather

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 hygrothermal advantage.

Occupants

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 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. This detailed analysis allows us to consider solutions that may mean that 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. The 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.

By removing the vapour control in favour of an external air leakage solution we remove the associated material and installation costs, and 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 applied at the design stage.

In modular systems, incorporating an external air barrier is simple and brings several benefits over traditional mechanically fixed membranes.

Wraptite 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 the site.

The panels are then assembled and the joints taped using Wraptite Tape, ensuring no adhesion issues or tape compatibility problems, and the panel assembly is now resistant to air leakage. The wall, roof and floor panels can then be assembled into modules, and Wraptite split-liner tape used to complete the airtight seal between adjacent assemblies. The completed modules can then be transported to site with full protection from the elements.

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