Welcome to the replay for our webinar, Advanced Membranes For Offsite Housing. The presentation lasts for approximately 30 minutes, and is followed by a live Zoom Q & A session with the audience, hosted by our Managing Director, Keira Proctor.
The webinar covers the following topics:
- Offsite Construction Technologies
- Open/Closed Panel Timber Frame, Cross Laminated Timber and Structural Insulated Panel
- Specification of Vapour Control Membranes
- Airtightness & Vapour Permeability
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Today we're discussing Advanced Membrane in Offsite Housing, a topic we know very well here at Proctors, having been involved with the timber frame industry since we launched our Frameshield-100 breather membrane back in the 1980s.
Although timber frame was a remains major part of the construction sector up here in Scotland the past 30 years have seen its use becoming more widespread across the UK, and the technology involved get ever more advanced.
Not only has the traditional timber frame kit of the 1980s evolved to meet increasingly challenging efficiency requirements, but the offsite construction sector has expanded to encompass a variety of innovative types of structure, collectively referred to as Modern Methods of Construction.
So in today webinar we're going to give an overview of these construction types, and the benefits each can bring to the housing industry.
We'll then consider the membrane applications associated with these methods and the roles that can play in optimising building efficiency and streamlining the construction process.
TYPES OF OFFSITE CONSTRUCTION
The common thread between the methods of construction we'll discuss today is the use of offsite manufacturing. Moving processes from a site to a factory environment brings several benefits in a range of areas, from improving manufacturing tolerances to minimising waste.
In previous webinars we've discussed the concept of design for manufacturing and assembly, which applies in varying degrees to these methods.
The key concepts of DfMA revolve around designing out complexity and minimising the potential for errors, and the more controlled environment offered by factory manufacturing makes a lot of this easier to achieve.
As everyone involved in construction knows all too well, building sites are busy environments, with many processes interacting under often less than optimal conditions, which often necessitates a degree of compromise to get anything done.
Weather, unexpected geology, delivery delays affecting critical processes or mechanical breakdowns all have the potential to derail a well planned construction process and affect everything from timescales to budgets.
The more processes that take place under controlled conditions the less potential there is for this kind of disruption.
The flip side to this efficiency is that more offsite processes limit the ability of the process to adapt to changing conditions or unexpected problems. There can also be issues around factory production capacity, access to site for large vehicles or the scalability of processes.
All these systems have strengths and weaknesses and choosing the right structural system to meet project requirements is just as important a specification decision as insulation levels or membrane specifications.
What's most important is adopting a well thought through design process minimising the potential for issues to arise.
OPEN PANEL FRAMES
This is the original type of offsite timber framing, the classic "kit house". These structural panels are partly fabricated in factories, but with a substantial degree of on-site finishing required.
The studs and sheathing boards are assembled in the factory, and the panel are fitted with external breather membranes for weather protection, but the panels are then left "open" meaning that insulation, services and internal finishes are installed onsite.
The studs in this type of panel are the main loadbearing members, transmitting vertical structural loads, which the sheathing boards prevent the studs from twisting and moving.
The wall panels and floor cassettes are transported to site then assembled into the structural framework of the building.
While this part of the process can happen very quickly, giving a nominally wind and watertight shell, time must then be spend fitting external cladding or brickwork, slating or tiling the roof, and fitting services and insulation into the framework.
The big benefit to this type of framing however is the ready available of capacity both in terms of materials and contractors reducing reliance on single sourcing.
CLOSED PANEL FRAMES
This type of panel is similar structurally to open panels, but the insulation is generally factory fitted and enclosed with additional sheathing internally.
This additional sheathing protects the insulation in transit and can also provide additional structural racking.
Because insulation is factory fitted in this type of frame, it can be installed to tighter tolerances with less potential for unwanted air movement around the insulation boards.
A common problem when fitting insulation onsite is that studs are not always at precisely correct spacing's, or vary according to panel dimensions, openings or structural requirements. This can result in insulation boards being cut to fit on site, which cant always be done precisely enough to limit convection around the board.
This convection can lead to reduced performance due to higher that anticipated air leakage, or to condensation problems if the vapour tightness of the insulation is being relied on to provide a vapour control function.
If the boards are fitted in the factory this can be minimised, and some closed panels also use blown in expanding foam insulation to further enhance the tightness of this seal.
These panels are transported and erected on site in the same manner as open panel systems, but services are typically installed using internal batten spaces, which can bring further airtightness benefits.
CROSS LAMINATED TIMBER
In a cross laminated timber panel, softwood timber planks, typically 30-40mm thick are glued together to form solid panels. An odd number of layers are glued together at right angles to each other and bonded together under pressure to form solid structural timber panels, similar to large, thick sheets of plywood.
The panels can then be fitted with a vapour permeable membrane to protect them during transit to site and limit the amount of pre-completion moisture in the panels, or left uncovered with membranes added after the layers of insulation later in the construction process.
These panels can in theory be any size and shape but in practice manufacturing and transportation considerations tend to limit the size to around 3m by 16m.
The alternating timber layers give the panels excellent structural properties, and being solid, joints can be made at any point.
Openings for doors and windows can be placed anywhere in the panels, but along with the general layout and structural design of the building, this must be fixed at the point of manufacture and cannot be changed later.
Once the panels are manufactured and delivered, the on-site erection process proceeds extremely quickly, provided the site is correctly prepared and all specifications are correct. However should any changes or alterations be required at this stage, the delays and cost increases can be considerable.
CLT panels only form the structure of the building, all insulation, cladding and services must be added after the panels are erected. While this adds time to the process it does allow a high level of flexibility in terms of varying performance and aesthetics between structurally identical buildings.
Because the panels properties can be varied and optimised to fit more or less any strength or spanning requirements, CLT is commonly used to build high rise structure as well as houses, with the world tallest timber building, a mixed use development in Norway, coming in at 10 storeys, and 85.4m in height.
CLT systems also score very highly as regards sustainability, with sustainably timber panels being fully recyclable and wastage being minimised. This is offset somewhat by the limited UK manufacturing capacity however, with more panels being sourced from mainland Europe and transported.
STRUCTURAL INSULATED PANELS
Structural insulated panel systems are similar in their basic concept to CLT panels, but place insulation within the panels.
A typical insulated panel comprises timber sheets, usually OSB, sandwiching a core of rigid foam insulation.
As with CLT construction, these panels are manufactured in factories before delivery and assembly onsite, so locations of doors and windows and the layout are fixed in place, but because the panels are pre-insulated, there is less work to be done onsite.
Because the panels are not bridged by any less-insulating structural material such as studwork, they can match the thermal performance of traditional timber frame walls with substantially less thickness.
A panel of around 140mm thickness can comfortably meet a u-value of below 0.2 W/m2K (watts per metre squared kelvin), while a typical open panel timber frame with site fitted fibre insulation would only achieve around 0.3 W/m2K in the same thickness.
In common with CLT though, there is scope for problems to arise if project requirements are not fixed and tolerances on site are not achieved.
SIP systems also rely on the use of blown foams and manufactured timber products, so may not always align with project requirements if a more ecologically focused build method is required.
The last option we'll consider today is using volumetric construction methods, where building panels are assembled into completed modules in the factory, then transported to site and assembled into the final building configuration.
In this type of construction, the modules can be pre-fitted with all the services required, and depending on the final cladding choices, can even be pre-clad, requiring only the junctions between modules to be completed on site.
This method fulfils many of the ideals of design for manufacture and assembly by minimising the number of different components involved in the build process, and is therefore ideally suited to buildings such as hotel or student accommodation where a large number of repeating features are required.
That said, it's becoming increasingly common to see these methods used in housing. In these applications it can be easier to achieve enhanced energy performance by reducing air leakage using modular buildings, while the time required on site can be minimised.
This can allow housing developments to be completed onsite more quickly and to a higher standard than could ordinarily be achieved in that time-frame using traditional methods.
As with CLT though, there can be issues with bottlenecks in the supply chain and scalability owing to the relative newness of this type of construction and a correspondingly lower number of suppliers. It's also very difficult to accommodate late changes and adjustments without substantial cost and time penalties.
Another particular challenge for architects when using modular construction in housing is to avoid uniformity and repetition in design, careful consideration of cladding choices and building layouts is therefore necessary.
With increases in production capacity however, building this way has potential to not only increase the rate of construction to meet ever increasing demands for housing, and may also go some way to mitigation the challenges posed by social distancing on sites going forward.
By limiting the necessity for concurrent processes, an assembly line approach to construction can better facilitate the need to maintaining physical separation between staff. An important advantage in the short to medium term, and one present to some degree in all forms of offsite construction.
So what role do construction membranes have to play in this way of building?
In order to design structures that are energy efficient and healthy to live and work in, designers have a clear need to balance the heat, air and moisture movement throughout the building envelope.
By either permitting or restricting the passage of air and/or water vapour, construction membranes play a vital role in managing and controlling these interactions. By providing more reliable and consistent hygrothermal performance than, for example, brickwork or timber, construction membranes allow a more precise degree of control over these processes.
This in turn ensures the as built performance of structures meets the design criteria with the minimum of remedial actions required.
MEMBRANES & REGULATIONS
The building regulations typically address condensation moisture in the same sections as damp and moisture ingress from other sources. In the UK and Ireland, that means Part C for England and Wales, Section 3 in Scotland and Part F in the Republic of Ireland. Norther Ireland is also part C, but it's a different document from that applying to England and Wales.
As regards controlling condensation, the relevant clauses in all these regulations refer the specifier to BS5250, the Code of Practice for the Control of Condensation in buildings. This means that wherever in the UK or Ireland a building is, the applicable design principles in terms of minimising the effects of condensation are more or less the same.
In terms of construction membranes, BS5250 gives technical definitions for each type of construction membrane.
The various membranes are defined as follows:
Air/Vapour Control Layer (AVCL):
"Continuous layer of impermeable material"
This simple definition is actually not as helpful as those given in the previous issues of BS5250 which defined air and vapour barrier functions separately. The older definitions are therefore worth considering.
"A layer which prevents the convective movement of air under the normal pressure differences found in buildings and which may also act as a vapour control layer"
It should be noted though that acting as a vapour control layer is not a requirement for airtight layers. Air permeability and Vapour permeability are two independent properties.
Vapour Control Layer
A vapour control layer is defined as a "material of construction that substantially reduces the water vapour transfer through any building element in which it is incorporated by limiting both vapour diffusion and air movement"
The addition of air movement to this definition means that all vapour control layers function as air barriers, however there is an important caveat.
To be fully effective an air barrier has to be fully sealed and continuous, while this is an important consideration regarding vapour transfer too, there's a little more margin for error before a badly sealed VCL will create condensation problems.
Poor air barrier performance however will impact the building energy performance calculations and is subject to pre-completion testing.
"Membrane with a vapour resistance of less than or equal to 0.6 MNs/g"
It's probably more accurate to refer to breather membranes as being "vapour permeable" rather than "breathable" as this helps avoid any confusion associated with the air and vapour permeability of the material.
The BS5250 definition currently concerns only the passage of vapour, such membrane can be either air tight or air open.
This is something that's likely to be addressed in the upcoming revision of BS5250, which as we've discussed before was due this year but may now be delayed due to the pandemic.
We'll keep you updated as soon as anything changes though.
The property referenced in BS5250 is vapour resistance, with a value of less than 0.6 MNs/g (Mega Newton seconds per gram) required to meet the classification criteria. It is however increasingly common to measure membrane performance using Sd-values, in which case a value of 0.12 metres, or less, would be required.
An Sd-value, also referred to as a "vapour diffusion thickness" or cv is simply a measure of the thickness of an air layer that would be required to resist vapour transfer to the same degree.
Type HR and LR Underlays
The term "breather membrane" above is more usually associated with wall applications, membrane used on roofs typically require more vapour permeability. BS5250 defines two classes of roof underlay, Type HR (high resistance) and Type LR (low resistance) with 0.25 MNs/g (or Sd-0.05m) being the cut-off between the two.
LR membranes can be specified without roof ventilation if that application is covered by current third party certification, while HR membrane will typically require ventilation in all circumstances. LR membranes which are also air open will typically have fewer restriction and exemptions applied to non-ventilated use.
To illustrate the way we can use construction membranes to counteract the harmful effects of moisture, we'll consider a typical domestic timber frame wall built using an open panel construction.
This type of wall usually consists of structural timber studwork with a sheathing board of OSB or plywood supplying racking strength. The principles of membrane use apply similarly to all the construction types we've discussed however.
Breather membranes are installed on the cold side of the structure, and today nearly all vapour permeable membranes are manufactured from either polyethylene or polypropylene.
The microscopic structure of these membranes is such that they contain many small and narrow pathways, which as large enough to allow moisture vapour to pass though, but not sufficiently open to allow liquid water through.
Additionally many such membranes contain hydrophobic additives, making the surface of the membrane actively repel liquid water.
Breather Membrane Primary Functions
In the simplest terms, the primary function of a vapour permeable membrane is to provide weather protection to the structure during transportation and construction, then to remain moisture neutral once the building is complete.
Once the timber structure is completed, it may be some time before outer brickwork and roof tiling can be completed. The key performance criteria for such membranes are therefore weather-tightness and vapour permeability.
Test method for vapour permeable membranes are laid out in EN13859 parts 1 and 2. Part 1 covers roofing applications while 2 relates to walls.
Weather-tightness tested to EN 1928 for W1 and EN 13111 for W2 (as per EN 13859-2 walling underlays). W2 is usually sufficient for wall application while for roofing W1 is required.
Water vapour resistance, if greater than or equal to and Sd-value 0.2m should be to EN 1931, if below 0.2m it should be tested to ISO12572.
In this application would typically suggest our Frameshield-100 membrane, which provides simple and cost effective secondary weather protection in applications where no specialist performance characteristics such as airtightness or fire ratings are required.
Vapour Control Layer
Vapour control layers are fitted on the warm side of the construction, usually directly over the studwork behind the internal lining. They are typically made of reinforced polyethylene or a laminate of aluminium and polyethylene.
Because they are designed to be as impermeable as possible, vapour control layers can fulfil the function of limiting air leakage, but if they are to be used in this way, good detailing and installation are very important.
Vapour Control Layer Primary Functions
The primary function of a vapour control layer is to reduce the quantity of moisture vapour entering the building fabric. By ensuring the water vapour remains in the heated spaces of the building, until it can be removed by planned ventilation systems, a well installed VCL can prevent this vapour reaching cold surfaces where it can condense into liquid water and cause damage.
In order to do this effectively the performance of the vapour control layer must be matched to the buildings intended use. Structures like warehouses with large open areas and frequently opened external doors are defined as being low risk and as such require less resistant vapour control layers.
At the other end of the risk scale, swimming pools and gyms typically experience far higher moisture vapour loadings and require correspondingly higher vapour barrier performance.
In the middle of the scale, application such as homes, offices and schools are classed as being medium risk.
In all cases, ensuring the integrity of the vapour control layer is maintained is very important. Any rips and tears or puncture damage to the membrane will provide a path for moisture vapour ingress.
Heavier reinforced membranes are therefore preferable as not only is damage less likely to occur, but it is also less likely to spread across the membrane surface than with unreinforced materials.
Procheck Vapour Control Layer Range
We can supply a range of vapour control layers with performance characteristics to meet almost any project requirement.
The Procheck 125 and 300 membranes provide lightweight, cost effective performance for low risk applications, while the heavier Procheck 500 membrane covers the majority of medium risk cases where no special characteristics are required.
For high risk application the Profoil 861 material uses an aluminium core to boost vapour resistance while a durable polyethylene coating provides corrosion resistance for use in chlorine heavy environments such as swimming pools. The upper surface is also coated blue to reduce glare when working on site.
As the building regulations demand ever increasing energy efficiency from all areas of building design, the use of construction membranes to boost thermal performance has become more prevalent.
By utilising high performance low emissivity coatings to restrict heat flow, reflective construction membranes allow the thermal performance of timber frame walls to be enhanced with little or no increase in thickness.
Reflective Breather Membrane
On the outside of the timber frame, the standard vapour permeable membrane can be upgraded with a reflective outer surface in order to boost the thermal performance of the wall.
Reflective breather membranes retain the characteristics of standard breather membranes, so offer good vapour permeability and weather-tightness, but utilise a property called surface emissivity to reduce the heat flow out of the building.
Low emissivity or Low-E surfaces limit the ability of a material to emit heat via infra-red radiation, similarly to a foil survival blanket. In order to do this, the proportion of heat transfer by radiation should be maximised by introducing an adjacent airspace.
In timber frame walls, this adjacent cavity is already present, so the low emissivity surface can reduce the u-value of the wall with no increase in wall thickness. By simply switching the specification to a reflective membrane, wall u-values can be improved with no other changes to the building specification which makes this a simple change to incorporate into existing designs.
Reflective Vapour Control Layer
The same reflective principle can be applied to vapour control layers internally. When used in this application however, the requirement for an adjacent airspace means that an additional void must be provided to ensure radiative heat transfer is maximised.
In practice this is usually easiest achieved by introducing a service void by adding additional battens over the studs. This void not only ensures the reflective vapour control layer can work effectively, but also allows installation of service runs into the wall without puncturing the air/vapour tight layer. This reduction in penetrations helps improve the effectiveness of this layer and can help simplify the installation of building services.
A. Proctor Group Reflective Membranes
The thermal resistance of a standard airspace of more than 19mm in a wall is 0.18m2K/W, however when using reflective membranes this increases substantially.
The A Proctor has two reflective membranes, the Reflectashield-TF breather membrane and Refectatherm Plus vapour control layer. The thermal performance of both of these membranes have been hotbox tested to ensure an accurate reflection of in service performance.
The Reflectashield-TF increases the thermal resistance of the outer cavity from 0.18 to 0.81 m2K/W, while internally, Reflectatherm Plus has a cavity R-value of 0.78m2K/W.
Both of these values represent significant improvements over non-reflective cavity resistances.
This table shows the some example u-values that are achieved when using our reflective materials. As can be seen in the table, depending on the specific construction used, the use of reflective membranes can mean the difference between compliance with regulations or not.
These values are based on a typical brick clad timber frame wall with either 89, 115 or 140mm depth studs using a 15% timber bridging fraction and either mineral fibre insulation at 040 or 035 thermal conductivity, or 022 rigid foam.
We offer project u-value calculations among our range of technical services, so for specific advice you can get in touch on or by online chat at www.proctorgroup.com.
Air Barrier Membranes
Following many years of increasing insulation requirements, further improvements in that area are now both impractical and economically unrealistic, leaving air leakage reduction as the most effective pathway to future progress.
In buildings, air leakage is the uncontrolled flow of air through gaps and cracks in the building envelope. If this flow is not adequately controlled it can significantly impact the energy performance of a structure and if used correctly, construction membranes can play in important role.
Vapour Permeable Air Barrier
One of the more recent construction membrane developments is the use of external air barrier membranes like Wraptite® and Frametite, which represent a significant departure from accepted traditional practice.
Traditionally the vapour control layer, being inherently air tight, has also performed the function of the airtight layer in the construction. While this solution does work, it places a lot of emphasis on the quality of installation and sealing of the membrane which can be difficult and time consuming.
Moving the air barrier to the outside of the building, away from the 'services zone' means there are far fewer potential penetrations to the air barrier and that there is no requirement for expensive specialist components such as airtight junction boxes, light switches or downlighter hoods.
Air Barrier Terminology
"Airtightness" is an abstract concept referring to the buildings ability to restrict unplanned air movement, but is not a directly measurable property. It's important not to use unclear language such as "low airtightness" as this could be easily misunderstood.
The "Air Leakage Rate" is a measure of the rate that air can escape from a pressurised building envelope. It's expressed in m3/m2/hr (cubic metres of air escaping, per square metre of building floor area, per hour) and is usually measured at a pressure of 50Pa (Pascals). When we talk about "an airtightness of 3", for example, this is the figure we mean, so to avoid confusion it's preferable to refer to "an air leakage rate of 3".
To further complicate the issue, "Air Permeability" is the figure used to express air barrier performance.
In most cases for an air barrier membrane this will be zero, or as close to zero as to not matter, so this should not be taken to imply any effect on the air leakage performance of the building as a whole.
Wraptite Air Barrier
The Wraptite air barrier membrane is designed to adhere to the external face of timber frame panels to provide an effective and robust barrier to the movement of air. By moving the airtight layer externally Wraptite greatly simplifies the installation of building services internally as less account must be taken of penetrations.
The mechanical durability of Wraptite, further enhanced by being fully bonded to the substrate, ensures the air barrier layer can be factory installed and transported to site while maintaining its integrity.
Being highly weather resistant with a W1 rating, the membrane provides effective temporary weather protection as well as ensuring the building meets its designed air leakage rate targets, reducing the likelihood of expensive and time consuming remediation.
Variable Vapour Control Layer
Although moving the air barrier layer externally represents a more dependable solution to minimising air leakage, control of vapour movement by diffusion is still important and must be carefully considered and managed if necessary.
At the same time it should be ensured that structures are able to dry out effectively in all conditions, and in such cases variable vapour control layers such as our Procheck Adapt have an important role to play in highly airtight constructions.
Procheck® Adapt VCL
As homes become increasingly better insulated, more airtight, and more energy efficient, effective management of moisture becomes ever more critical.
An important factor in efficiently managing the year round flow of moisture in buildings is ensuring that systems and fabric can modify their behaviour to account for changing circumstances.
While active systems such as ventilation can ramp performance up or down as required, making the building fabric itself adapt to hygrothermal changes poses more of a challenge.
Procheck Adapt varies its vapour resistance in response to environmental conditions, meaning its resistance to the passage of water vapour increases in winter to block the ingress of moisture into structures when internal vapour pressure is at its highest.
Correspondingly its resistance lowers over the drier summer months allowing the warmer temperatures and lower moisture loadings to dry the structure out, ensuring a healthy environment is maintained year round.
This means the building fabric is protected from damaging moisture levels during cold, wet months of the year and it will allow the fabric to dry out effectively in warmer, drier months.
Procheck Adapts' translucent structure eases fixing to structural frames and in conjunction with its integral tape allows for a fast installation time.
Combining the external air performance of the Wraptite membrane with the flexible internal resistance of Procheck Adapt provides a double layer of air resistance to reduce air leakage rates, while ensuring an optimum transfer of construction moisture in any direction.
Airtightness Vs Insulation
While fabric insulation is effective in reducing heat loss from the building envelope, as insulation levels increase its effectiveness per unit thickness begins to drop off. This is because the thermal transmittance or u-value of a construction is the inverse of its thermal resistance.
What this means in practice is that boosting insulation levels is only practical and cost effective up to a point. Beyond that point we need to consider alternative measures to restrict heat loss from buildings.
Minimising air leakage is the most effective additional measure that can be taken to boost thermal performance.
The holistic energy performance of dwellings is assessed via SAP Calculations, which incorporate the effects of all building fabric and system on its overall carbon dioxide emissions.
In this type of assessment, lowering the air leakage rate form 7m3/m2/hr to 1 m3/m2/hr, can give a similar uplift to energy performance to increasing the fabric insulation from 130mm to over 350mm.
While this reduction in air leakage sounds dramatic, by using the Wraptite air barrier membrane, adhered to the external face of the sheathing and taking care to ensure it is well installed around windows, doors and sealed to any other penetrations, this is not difficult to achieve in practice.
Air Barrier Sealing
The primary advantage of Wraptite isn't necessarily improving the airtightness RESULT, but making arriving at that result significantly easier, faster, and less affected by the actions/skills of installers.
Modern homes contain a huge variety of different services such as switches, sockets, network points and TV connections, all of which require cable runs and therefore penetrations through an internal airtight layer.
This means that in addition to taking care to ensure these installations are scheduled and detailed such that penetrations through the airtight layer are minimised, specialised proprietary switch boxes and other hardware may be required.
As well as potentially increasing costs, the requirement to use such hardware may lead to delays if it not readily available for purchase. A worse outcome may also be the substitution of less well sealed components with corresponding reduction in performance.
External air barriers, like Wraptite, offer excellent reliability & consistency. Window, door & pipe penetrations are made airtight in a straightforward manner with our Wraptite Tape or liquid flashing.
This simplicity and reliability ensures the air leakage rate is more dependable and repeatable across buildings, which as we've seen, gives significantly improved design flexibility in other areas such as thermal insulation.
Unplanned/uncontrolled air movement, or "draughts" as we have referred to it for centuries, removes the ability to control and manage the environment. The desired outcome is not the absolute elimination of all air movement and transfer, just of that over which we do not have control.
This means design and specification of ventilation systems is important to consider holistically with the fabric insulation and air barrier systems. Any type of ventilation system, from fully passive to a fully active system, with heat recovery can be made to work effectively at any design air leakage provided it is design in early enough.
Buildings are never "too airtight" or "too leaky" they are "out with their design specification". Any air leakage rate (within regulatory backstop) can be accommodated depending on what other trade-offs or compromises are made.
Systems like Wraptite help ensure low design air leakage rates can be dependably achieved on site reducing the need to either undertake remedial action to "fix" leaky building fabric or introduce additional ventilation to compensate for as built performance not meeting design criteria.
Fire Resistant Membranes
Another important role for construction membranes in today's building is limiting the development and spread of fire within buildings and in cavities. Recent tragic events have highlighted the importance of this.
FR Membrane Primary Functions
On the outside of the heated envelope, fire resistant vapour permeable membranes can play an important part in limiting fire spread in facades and also in restricting the spread of flames between adjacent buildings. They can also provide important temporary fire protection during construction periods, particularly important in large scale timber frame construction.
The key property in fire resistance is the EN13501 fire classification, also known as "Euroclass". In external applications this minimum acceptable classification of fire performance is required is Euroclass B, s1-d0, where B is the main reaction to fire classification and the s and d components related to the production of smoke and flaming droplets.
This classification is primarily required by building regulations when used at over 18m in height, or 11m in Scotland, reflecting the particular importance of controlling fire spread in high rise applications. It is however becoming more common to specify fire resistant breather membranes in low-rise applications going beyond the regulation requirements.
In addition to offering good vapour permeability, weather resistance and air barrier performance, the Fireshield membrane utilises a unique intumescent surface composition to actively restrict the development of fires.
It achieves a Class B, s1-d0 fire rating; however in practice its performance in response to fire development is dramatically different.
As can be seen here in this comparative test, the class B membrane on the right shrinks away and does not contribute to the fire's development, however the Fireshield on the left remains in place, with its coating protecting the substrate from ignition.
This test was conducted by the University of Edinburgh and is based on the EN13823: Single Burning Item Test and was conducted as part of the process for inclusion in the Structural Timber Association product papers for their FR Build specification.
Fire Resistant Vapour Control Layers
Internally, fire resistant vapour control layers play an important role in preventing the development of fire in internal spaces, and also in limiting transfer of those fires to the outer areas of facades.
This is in addition to preventing the ingress of moisture vapour, and reducing air leakage.
Procheck Fire Resistant VCLs
Procheck® A2 is a laminated glass fibre and foil composition protected by a clear lacquer. This gives Procheck® A2 a unique additional benefit of a low emissivity surface which when used with a service cavity can enhance the overall thermal performance of the construction as well as providing high fire performance with its A2-s1,d0 fire classification.
Procheck FR200 is used as a fire retardant vapour control layer in roof and wall structures in new build and renovation projects. Procheck FR200 has a Reaction to Fire classification of B-s1, d0 which provides assurance of fire performance for the structure. Procheck FR200 air and vapour tight membrane improves energy efficiency and reduces the condensation risk.
Used together these fire resistant membranes can provide an easily applied solution to boosting fire protection of the building fabric and limiting dangerous fire spread across all areas of the structure, from inside to outside.
Particularly in high rise or flatted developments, ensuring fire spread is as restricted as much as is practical, and the protective measures are reliable and durable, is a critical part of the overall design strategy.