Self Build - The Process & Considerations

Transcript

We're going to start off today looking at some of the key principles of design for manufacturing and assembly. We'll then consider the ways in which we can use a variety of construction membranes to both align with these principles, and comply with the building regulations.

Following on form this we'll consider how these strategies and materials can be applied to facade systems, both low and high rise, and how they can be incorporated in upgrade works.

Finally we'll take a look at some projects where the principles have been put into effect on sites.

DfMA

Design for manufacture and assembly, or DfMA is an approach traditionally used by other industries such as manufacturing consumer products, which focused on the role of the designer in increasing both the efficiency and quality of the manufacturing process.

The basic principles are based on recommendations going back to the Egan report, "Rethinking Construction" in the late 1990s, which focused on more tightly integrating project processes, adopting techniques and strategies from higher volume industries like automotive to, in Egans words, "do it entirely differently"

How feasible the entirety of the Egan report was or remains is something we could discuss for several lockdowns and still not resolve, but the spirit of the report can be clearly identified in the offsite sector, where factory based manufacturing simplifies the adoption of this approach.

The Key principles can be identified as:

Minimise components, thereby simplifying supply chains, and assembly processes both on and offsite.

Ease of fabrication, focusing on using adhesive bonding rather than mechanical fixings where appropriate, and ensure the design facilitates ease of both offsite manufacture and on-site assembly.

Tolerance and Clarity of parts, ensuring the design performance criteria are within the capabilities of the manufacturing and assembly processes, and minimise the scope for unclear processes to produce substandard results.

Minimise the on-site use of flexible components, such as gaskets and rubber seals which can be more difficult to handle and correctly assemble, particularly under adverse conditions.

Finally, reducing adjustments. By reducing or eliminating the requirements to make intentional fine adjustments, the necessity of unintentional adjustments can also be minimised.

Taken together, these principles aim to ensure the the design performance of the building envelope matches what is built, not simply by demanding strict adherence to design specifications, but by writing those specs to ensure adherence is the simplest and therefore most likely outcome.

MMC

The various approaches to construction derived form these principle are typically referred to as "modern methods of construction" a broad term encompassing everything from timber frame housing to 3d printed concrete elements or cross laminated timber structures.

The common thread of all these types of building is however reducing the scope for variations in design performance arising from the construction process itself by shifting processes offsite into a more controlled environment.

In this context construction membranes have several important roles to play in ensuring these objectives can be met, but in order to maximise their effectiveness, new need to properly understand the range of design considerations applicable.

Heat Air And Moisture Movement

Anyone who's been following our series of webinar recently will hopefully be pretty familiar with this diagram now, as understanding and controlling the heat air and moisture movement is structures is the core principle of a lot of what we do here at the Proctor group.

If the context of our discussion today though we have a broad range of factors influencing this, as these methods of construction can be. and are, applied to every type of structure. Timber frame has long been used in domestic housing, particular up here in Scotland, but it's becoming increasingly common in other sectors.

Likewise volumetric building is increasing and expanding, and systems historically more typical of high rise are finding use on low rise projects. So it's becoming more likely than ever that specifiers will have to learn to work with a huge variety of different detail techniques and materials.

Whatever the situation, 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.

BUILDING REGULATIONS

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 that 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.

We'll also consider the implications of these systems on fire performance, which is covered in Part B for England and Wales, Section 2 in Scotland, Part E in Northern Ireland and Technical Guidance Document B 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. So while we'll try to keep things as broadly applicable as possible, we'll flag up any significant differences where they arise.

DESIGN CONSIDERATIONS

The primary design consideration we're looking at here is limiting unplanned air movement through the building fabric, while reducing the risks associated with condensation and excess moisture.

The most important of these considerations is where in the building element we place both our vapour control layer, and our air barrier layer.

Traditionally these two functions would be performed by the same membrane, as most vapour control layers will also performs the function of an air barrier. Its by no means a given that this method is suitable for all projects however, and this is particular true of facade systems.

For some facade buildups the vapour control function may not even be necessary, and the inclusion of the vapour control layer simply adds an additional process and more complexity for no benefit, just one more thing to go wrong.

Which leads to a third consideration, is using a vapour barrier to control air leakage in fact a liability? While this certainly isn't the case on every project, if our overall design strategy is to simplify and reduce potential points of failure in the design its something worth bearing in mind.

HYGROTHERMAL ASSESSMENT

If we're proposing removing the vapour control layer, we need to be sure that the element is capable of controlling any risk of condensation. As regular viewers of our webinar will know by now, there are a couple of different ways we can look at assessing these risks.

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 the newer WUFI method 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 for quick and simple checks, 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 construction here, the internal vapour control layer can be removed altogether without creating a condensation risk.

MANAGING AIR LEAKAGE

If we remove the vapour control layer, we still need to manage our air leakage, and this can be done simply and effectively using an external air barrier membrane like our Wraptite.

This simplified and more effective process allows designers to target more ambition air leakage rates at the design stage, taking advantage of the improvements in energy performance this brings.

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.

If implemented in the design at an early stage, this enhanced airtightness need not lead to building requiring complex mechanical ventilation systems as other options such as passive ventilation strategies can be implemented.

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 volumetric 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.

Wraptite On Roofs

The airtightness advantages of Wraptite can also be realised in warm roof applications, such as we can see here on this local authority childrens home in Hampshire.

In this case the wraptite membrane is used both on the timber frame walls and the roof, allowing the air tight layer to remain fully continuous around the entire structure.

The Wraptite membrane is fully BBA certified for warm roof applications making this type of construction as simple and effective route to good air barrier detailing on site as the membrane on offsite panels can be jointed simply with Wraptite tape.

Alternatively if the roof membrane is installed on site, it can simply be wrapped around the eaves on lapped onto the wall panels without leaving any gaps for air leakage or water ingress during construction.

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.

FIRE PERFORMANCE

Since December 2018, membranes used in external walls over 18m in height have required a minimum fire rating of B-s3,d0. In Scotland this is reduced to 11m in height.

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.

TYPE OF FIRE SPREAD

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 like Wraptite, 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 TESTS

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, or can be used directly on the sheathing boards of a timber frame structure.

In this comparative test conducted by the Univeristy of Edinburgh, 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 a appropriately tested construction, Fireshield can be used as part of a system to limit the spread of fire, which can allow for reduced spacing to adjacent properties. This must however be done in accordance with the Structural Timber Associations procedures and test methods.

Example Projects

Edgeware Rd.

The first of our example projects is a new build apartment development where the original design called for an air leakage rate of 5, and used rigid PIR foam insulation over a concrete frame.

After the tragic events of Grenfell in 2017, a priority for this project was to replaced the originally specifies PIR foam insulation board with incombustible mineral fibre.

This project was still on the drawing board when the tragic events of Grenfell took place in 2017, so before the project began onsite options were explore to remove the PIR in favour of mineral fibre insulation.

This introduced a substantial shortfall in thermal performance, which the designers were not keen to offset via increased thickness.

Specifying Wraptite as their air barrier membrane allowed the design air leakage to be reduced form 5 to 3, going some way to offsetting this increase in thickness.

As well the obvious reduction in building footprint form the reduced insulation thickness, there were several additional benefits.

A detailed WUFI condensation risk analysis on the use of Wraptite allowed the removed of the internal vapour control layer, and it's associated installation costs.

The use of Wraptite also allowed a reduction in the specification of the sheathing boards, owing to the weather tightness of the membrane externally, and reduced the requirmeent for EPDM membrane over the structural frame, infills and at flashings around windows.

Taken together, the cost savings from these changes more than offset the increase in membrane cost from the use of Wraptite, allowing the specific project goals of improved fire performance to be delivered alongside a cost saving.

Finchley Road

Our second example is a re cladding project, were an existing residential high rise with a facade of ACM panels is being refurbished and upgraded to incorporate material with enhanced reaction to fire performance.

In this project the existing ACM panels and rigid foam insulation boards we completely removed from the facades and replaced mineral fibre insulation and incombustible cladding panels.

As part of these works the vapour permeable Fireshield membrane was specified to provide additional protection not only from weather but also from fire spread within the cladding system.

As part of a project specifically focused on reducing the combustibility of the entire facade construction, the intumescent surface of the fireshield membrane made it the obvious choice of membrane to provide temporary weather protection without compromising the overall project objectives.

So whether the primary aspects of a protect are airtightness, weathertightness or fire performance construction membrane have an important role to play in facade systems and offsite construction, for both new build and refurbishment projects.