Tag Archives: condensation risk

  • What exactly is a VCL?

    Vapour Control Layer (VCL) Explained

    One of the most commonly used, and widely recognised, acronyms in construction is VCL, which stands for vapour control layer. A VCL is a critical building component designed to protect the building from potential degradation (or poor performance) by managing the passage of water vapour within a building structure. In other words, it is used to manage condensation risk. Condensation is formed when warm moist air condenses into a liquid on contact with a colder surface. A vapour control layer is typically installed on the internal side of the insulation to control the passage of warm moist air (water vapour) entering the structure. However, as a simple acronym, there is a problem because in most applications a specific level or type of performance is required, as a ‘catch all’ acronym VCL is wide open to error.

    Condensation on the internal side of a window, showing that there was plenty of water vapour in the room, which has condensed onto the cold internal window pane. This is a good example of how much water vapour can be available to penetrate and condense within a building structure.

    Unfortunately, and perhaps part of a wider misunderstanding, to many the term VCL is a synonym for polythene sheet, add in only a basic understanding of how condensation forms (as described above) and it is easy to see how the use of the term VCL can cause considerable confusion and anxiety. This is made worse by a large number of alternative terms such as vapour check, vapour permeable membrane, vapour barrier, vapour retarder, ACL, AVCL, vapour diffusion retarder, variable diffusion membrane, monolithic membrane, vapour diffusion barrier, airtight membrane, vapour tight membrane, microporous membrane, breather membrane all used to describe products covered by the acronym VCL …………………frankly it’s no wonder people get confused by all of this. In this article, we will try to bring some clarity to the description and use of internal membranes to help you decide which type to use, where and how. A companion piece is in preparation about external membranes.

    Broadly put there are 3 types of internal membrane.

    Type 1: An impermeable barrier such as polythene, this lets nothing through.  It’s a vapour barrier or a vapour block, has only one function: to stop water, in all forms. An impermeable barrier cannot let water vapour back out of a wall when generated by solar gain - see point below - so this type of membrane has serious limitations in all but a few circumstances. Needless to say, the installation of an impermeable barrier needs to be 100% perfect for it to work.  So, no holes, no gaps at the joints or overlaps, no accidental cuts or nicks and no major scrapes.

    Type 2: A membrane that acts as an impermeable membrane most of the time but has some capacity to allow vapour transfer in certain circumstances.  These are often described as a “vapour retarder” or a “vapour check” and they are designed to work under specific conditions where the inherent properties of the membrane can be relied upon.  Since they only allow the movement of water vapour under these narrow and specific conditions, it is essential to apply them in appropriate situations.

    Type 3: A membrane with variable permeability is often called an intelligent membrane. These are vapour control layers whose ability to allow moisture vapour to pass through depends on circumstances.  Also known as variable diffusion membranes (VDM), most allow vapour movement in both directions, depending upon relative humidity either side of the membrane.

    Some membranes in categories 2 and 3 can be described as breathable, you can read more about breathability here.

    The choice of a membrane can largely be determined by location and build type so a vapour barrier (type 1) can be used under a concrete slab but an intelligent membrane (type 3.) is better to line the inside of a warm roof space.

    The terminology used to describe membranes in buildings is hugely confusing and often ends up being concentrated into the single three letter acronym ‘VCL’ appearing on a drawing, but we know that VCL is a catch-all acronym it means nothing without some context or explanation, for example, we often notice people using the terms VCL and ‘breather membrane’ interchangeably, particularly with regards to pitched roofs. Whilst they have a similar purpose, there are a couple of important differences between the two.

    So why do we need either? Quite simply – the vapour control layer is there to prevent condensation, which can cause a number of problems, including:

    • Structural damage due to rotting timber, whether this be a timber frame, joists or rafters
    • Insulation losing its thermal performance due to having absorbed the moisture
    • Mould, which not only looks unsightly but can also lead to respiratory problems and other health issues

    People generate moisture inside their homes, through breathing, through cooking and particularly by washing themselves and their clothes.  To prevent condensation, we need to eliminate this water vapour from inside the building. We also need to get rid of moisture that is outside the habitable zone but within the building envelope.  This might be water from construction – fresh concrete, for example, takes many months to dry out fully - or perhaps rainwater that seeps through tiled roofs or is wind-driven up under the eaves.

    Traditionally we have eliminated moisture by ventilation; for example, by ventilating the space between the insulation and the slate or tile on a pitched roof. However, studies have shown that ventilation directly above an insulation layer can reduce its thermal efficiency, which means more and more people are opting for an unventilated roof.

    Some Definitions may help

    • Airtight layer- prevents the movement of air which may/ may not act as a Vapour Control layer
    • Vapour Control Layer- a material which can limit both movements of vapour by diffusion, and air movement
    • Breather Membrane- defined as a membrane with a vapour resistance less than 0.6 MNs/g situated on the external side of the insulation acts as a weatherproof layer whilst still allowing water vapour to be passed to the outside.

    Understanding your walls, temperatures and condensation

    With a plethora of membranes on the market, each designed to do a different job and behave in a subtly different way it is easy to be confused about which membranes are required to create a dry and airtight building structure.

    Starting with the basics, when insulating walls you create a temperature gradient across them with the warmest being on the inside and coolest on the outside during the winter months. You can imagine a graph of the temperature showing a fairly steady decrease in temperature as you move closer to the outer surface of the wall.

    When you take warm, moist air and cool it (as it will moving through a building structure) you find moisture condenses at a point known as the ‘dew point’ or ‘condensation point’.  This will typically be the intersection of an impermeable or low permeability surface with the temperature falling low enough for water vapour to become liquid.  This is how damp accumulates inside the fabric of your walls or your insulation, to the severe long-term detriment of your building.

    Why you shouldn't use a ‘vapour barrier’ (Type 1. Membrane) in your walls

    Different construction and insulation materials cope differently with condensation. Some materials, such as masonry, can absorb and release it again once the weather warms without too much damage. However, when using vapour impervious insulation in timber frame construction, any condensation forming in the walls tends to be absorbed by the timber, a process that can cause rot. Additionally, during the winter months when this condensation tends to occur, driving rain may also enter the fabric of the building, further increasing moisture levels in walls. It is therefore very important to prevent this condensation process occurring in the first place, for the longevity of the building.

    One further complication to the above process can be found in the summer months. The temperature gradient is often reversed and the higher temperature is found on the outside of the wall and the lower temperature on the inside. This creates a situation where moisture is driven inwards and condensation can form close to the inner face of the wall instead.

    In the UK until relatively recently an impervious vapour barrier was used on the inner face of a timber frame and was thought to prevent condensation formation by simply blocking the flow of moisture-laden air through the wall. However, it has since been found that not only are vapour barriers regularly full of holes which let moisture through during the winter months, they also cause the accumulation of moisture inside the wall during the summer months. This was caused by the barrier preventing moisture from escaping towards the interior of the building.

    Water vapour that has condensed against a VCL, in this instance a polythene sheet (type 1) used as a VCL over mineral wool and behind plasterboard. Damage can be seen on the timber stud. This occurred within 2 years of installation. Photo courtesy of SkamoWall.

    The high humidity levels and warm temperatures found in these walls combined to form perfect conditions for mould and rot to thrive. This was problematic to both the timber structure, as it rotted, but also to the inhabitants of the building as mould spores are well known to cause respiratory problems and ill health.

    Vapour barriers are still useful though. One of the few places above ground level where a complete vapour barrier should be used is in flat roofing when using foil faced PIR insulation. In this case, you need to lay a vapour barrier on top of your flat roof deck before you lay the insulation and your flat roof covering.

    Using a vapour control layer to control condensation

    The answer to keeping timber-framed walls and roofs dry is to use a layer to restrict the flow of moisture but not to try and stop it. In other words a VCL (or a vapour retarder). A VCL is always used as close to the inner face of a wall as possible and reduces the amount of moisture passing through the layer to low levels, ensuring only insignificant amounts of condensation occurring within the structure. Additionally, this will allow moisture that is driven towards the interior in the summer months to slowly pass back inside the building. This prevents the conditions for mould forming and ensures the longevity of the structure.

    Vapour control can be performed very accurately by the many membranes available but it can also be performed at a basic level by OSB, whose vapour resistance (or vapour permeability) is similar to that of some membranes. The benefit of using OSB as a VCL is that it is far more robust than a 0.2mm membrane and does not require the installation of another layer into your timber frame structure if used internally. However, you will need to test the airtightness of the OSB before using it as there is some variation in air permeability. For guaranteed results either use an airtight VCL membrane, such as the ProClima Intello Plus or Constivap or a board such as Unilin Vapour Block or a liquid applied membrane such as Blowerproof. Blowerproof and Intello Plus are both BBA certified.

    It is also advisable to try and minimise the amount of moisture that enters your building fabric during construction.  Much of our construction timber, sometimes including expensive windows, roof joist assemblies and even SIPs panels, are commonly stored on site with little or no protection against rain, especially driven rain.   Once wet, they can take a significant amount of time to dry out, contributing to the internal moisture load a new dwelling has to deal with.  This can even delay and degrade the final stages of construction: for example, airtightness tapes on OSB have been known to come off during airtightness testing, not adhering properly because the timber is still too wet.

    The latest type of VCL membrane is the 'intelligent' membrane, such as Proclima's Intello Plus membrane. These are very useful products that remain very vapour tight (low vapour permeability) during the winter months when it’s important to try and prevent moisture from entering your structure from the interior. As temperature and humidity in the walls rises the pores in the membrane open and allow moisture to migrate towards the interior of the building. This gives the best of both worlds and ensures your structures remain as dry as it is possible to be.

    Watch the video to see how an intelligent VCL works

    Using a vapour-check or foil backed plasterboard as a VCL

    Vapour control layers are always required whenever you insulate, irrespective of the insulation used. They should be used to form a continuous airtight layer and so all the joints and any penetrations must always be sealed with the appropriate airtightness tapes. Without good levels of airtightness the VCL does not work and moisture levels cannot be controlled inside the structures. Products such as vapour-check, foil backed or insulated plasterboard tend to act as a vapour barrier but with none of the joints or penetrations sealed. These products should not be used instead of a VCL or where a VCL is used.

    IMPORTANT REMINDER

    Always refer to a qualified designer if in doubt or ask the manufacturer for technical and installation advice, we are always happy to answer any questions about airtightness or vapour control or point you in the right direction.

     

     

    Thanks to Paul Jennings and Chris Brookman for their contributions to this article.

    Paul Jennings has over 30 years’ experience of airtightness testing, in the UK and around the world, and has been extensively involved in the delivery of onerous airtightness specifications in Passivhaus and other low-energy projects. He tested the first UK certified domestic and non-domestic Passivhaus buildings, both in Machynlleth, on the same day more than ten years ago, and recently led the team that used 8 sets of test equipment to carry out the most complex airtightness test carried out in the UK to date, on Agar Grove Phase 1, in London. He trains airtightness testers and pioneered the development and delivery of airtightness champions training courses. He has been instrumental in improving our processes and tools for achieving good airtightness, as well as training sealing operatives and delivering numerous CPDs and conference presentations to a wide range of building professionals on different aspects of airtightness.

    Chris Brookman lives in a Passive house of his own design which he built based on his own life principles of low impact, low energy living and human health. Chris runs Back to Earth, is a recognised expert on green building and passive construction, he has written widely on the subject and is a keen blogger on the practical and technical aspects of delivering sustainable construction; Chris also curated the first online Wood Fibre Insulation course

  • Healthy buildings or toxic buildings?

    The Healthy Home

    In our view, a healthy home is   ‘one that incorporates healthy design elements, non-toxic building materials, and proper construction techniques. It "breathes", emits no toxic gasses, and is resistant to mould and decay.

    Indoor air quality can be worse than outdoor air quality

    Here are our top tips when designing a healthy building.

    • Choose a simple build system
    • Use natural and non-toxic materials
    • Make the best use of natural light
    • Ensure adequate ventilation
    • Ensure that all building elements are compatible
    • Use a breathable vapour open system
    • Make the structure do the work
    • Take a whole-house approach to design
    • Include the end user in the design and build process

     

    The toxicity of construction materials in our homes is a serious issue homes do not have to contain potentially damaging materials.mitigating this should be considered right at the start at the design stage.

    Without a doubt, it is the control of moisture and the ventilation of the building that sits at the root cause of most building decay. We also have a huge issue with applying healthy principles to the biggest issue of all refurbishing existing buildings.  Often in these cases, the prophylactic principle should be applied, where some anticipation of problems such as damp penetration can be mitigated by choosing materials that can hold onto moisture and let it go later (drying out) or at least minimise or contain the problem. The issue with a more synthetic and hermetic approach is that such problems can often remain hidden deep within the building structure for a long time and on discovery lead to costly and extensive repairs.

    To apply healthy principles to any building project you first need to appreciate that the standards by which most UK construction is governed (and built to) do not account for the ‘health’ of a building in all but the most basic ways. So don’t expect a building that meets Building Regulations to be healthy.

    Damp problems are often first seen as a 'bloom' of household mould often triggered by warm wet air coming into contact with a cold surface, one that is poorly or insufficiently insulated.

    To describe an unhealthy home can be more effective at persuading us to adopt healthy principles. We will all recognise the description of an unhealthy building as one that fails to control the internal environment leading to partial, then increasing, early decay of the building fabric in turn leading to mould growth, rot and a failure of the element(s) to physically perform, the description would further include the use of toxic chemicals in materials and the resulting expulsion into the air of these toxins over time, and it would include the use of materials that contain allergens.

    Now most of us will recognise (and probably have experienced) the symptoms of poor building health but it is surprising how many of the houses built today have this very low on the agenda of considerations. The consequences of damp and unhealthy buildings can mean the aggravation of conditions like asthma, in the UK this is a real problem where 1 in 6 people have asthma a massive increase since the stable base in the 1970s with almost 2000 deaths per annum and 75,000 hospital admissions the cost to the state runs into £billions; most of this is directly linked to dust mite faeces which in turn is directly linked to relative humidity in houses, (as you find in an unhealthy house) other moulds, bacteria and diseases present in the same conditions are also linked to asthma.

    The main contributors to poor building health are the following

    • Water ingress
    • Condensation
    • Failure to control internal moisture
    • Poor build quality
    • The use of toxic materials
    • Poor ventilation
    • Material degradation over time leading to performance failure (e.g. air leaks)
    • Poor design
    A combined use of roof lights to flood a room with daylight and allow natural ventilation

    You can see that it is not only the absence of harmful environmental characteristics but also the presence of beneficial ones that define a healthy building. Designers should begin by avoiding harmful elements and attempt to incorporate supportive beneficial ones. This is why the inclusion of items such as natural light, ventilation and acoustic insulation is as important as layout and functionality in the whole house approach.

    Real progress is only made when the builder and future occupants work closely with the building’s designer to ensure that all these issues are addressed within the context of how the building is intended to be used.

    Thankfully a lot of the approach to building healthy homes is common sense and can be summarised in a few simple principles

    • Choose simpler building systems they are more failsafe
    • Manage moisture by creating a breathable shell to provide a means for managing and buffering variations in moisture
    • Include natural materials in many applications these will outperform synthetic ones.
    • Be involved at every stage

    As highlighted by recent events the toxicity inherent in our building materials can be a lethal problem especially in the case of fire, one of the most important materials used in the construction of a building is insulation, but can your choice of insulation really affect your health?

    A well-insulated house or office will protect your health, comfort and lifestyle but how many of us know and understand how to achieve this?

    Ecomerchant and Steico UK have joined forces to launch a protection campaign. It aims to champion the benefits of using natural insulation products, see www.ecomerchant.co.uk/protexion  where you will find the wheel (illustrated below) which has dynamic segments (links) e.g. health, fire and acoustic which click through to more information on each subject, you can also download wood fibre insulation certifications and find toxicology reports and environmental product declarations, this is the type of clear unambiguous information that allows us to make informed and better design choices.

    The Protexion wheel, each segment links to the relevant role with supporting information, the wheel also links to accreditations, EPD's and toxicology reports. Click the image above to link to the Protexion site.

    How we select insulation needs to be about having a real choice and for specifiers to be equipped with the right knowledge to compare materials on a like-for-like basis.

    To design a well-insulated building, you need to make informed decisions throughout all phases of a construction project to ensure your building performs as you envisage as mentioned above.

    However, selecting the right insulation is about more than just reaching building regulation compliance or ‘keeping in the heat’. It’s about ensuring a building protects its occupants’ entire well-being and comfort in the following ways.

    How well does insulation keep the heat out?

    In the UK, thermal insulation to protect from the cold is essential, particularly given ever-increasing energy costs. However, as demand for usable square footage of buildings increases, basement and loft conversions are the routes many now take. However, these parts of a home or office, are the spaces most prone to extremes in temperature. They, therefore, need more thought – i.e. how do you keep a space warm in winter but, for a loft, how to keep it cool come summer.

    Compared with synthetic insulation materials, wood fibre insulation has a much higher density. This higher density means that natural insulation makes for a better heat buffer as the high midday temperature will only reach the internal side and be lost at night when the temperature is already cooler outside.

    How a building’s breathability is hurting our health

    A breathable structure is one that allows the passage of moisture.

    With 90 percent of all building construction problems associated with water in some way, breathability is essential in measuring a building’s performance and preventing the accumulation of harmful water within the building’s fabric.  These are fundamental in reducing health risks from mould, mites that those suffering from respiratory illnesses such as asthma and chronic obstructive pulmonary disease (COPD) are particularly susceptible to.

    For effective breathability, there are four essential components that need to be considered:

    • a moisture pathway
    • a driving force
    • a sorptive fabric
    • vapour control.

    Natural fibre insulation is most effective as it suppresses potentially harmful water by binding and releasing moisture which helps regulate humidity levels as the moisture moves.

    Easy-to-fit insulation

    A well-designed building takes into consideration how a material performs throughout the building’s entire life cycle. This includes ease of installation. Steico’s wood fibre insulation is simple and easy to fit (either packed or friction-fitted), eliminating installer error, keeping construction programmes, tight and costs, low.

    How sustainability will save you time and money

    While all insulation is helping the environment by limiting energy being burnt for heat, natural fibre insulation materials are comparatively more robust. This means that when it comes to disposal, they can be composted – i.e. no specialist waste facilities or landfill. Throughout their lifecycle, they will additionally have a much lower, and often, negative carbon footprint.

    More than just protecting your home from fire

    All insulations will meet fire safety standards, but this is a minimum rating. The key differentiator between natural and synthetic is that natural insulations will prevent the spread of fire and if burnt, will not give off toxic fumes such as cyanide as polyisocyanurates (PIR) might. See article link below to Alliance for Sustainable Building Products (ASBP) Healthy Buildings or Toxic Buildings?

    Will the house be standing in 100 years?

    Condensation is one of the costliest risks to buildings causing huge maintenance repairs and structural damage. Natural materials are better able to absorb and release water whilst remaining dry meaning it is better able to protect from and buffer moisture thereby becoming a key part of healthy living.

    Comfort for occupants

    When selecting insulation for a building, there are implications for the health of the occupants, the structure of the building, its impact on the environment, its acoustic properties, durability and carbon footprint.

    Cancelling out the noise for a peaceful night’s sleep

    The higher density of natural insulations - such as wood fibre - makes them better at reducing noise. Sounds external to the building, such as traffic or music, as well as those from within the building, through walls and ceilings are attenuated better by wood fibre than synthetic equivalents. In providing better protection from acoustic pollutants, occupants often report a building as being more restful and relaxing thereby encouraging better mental health.

    When a building is well-designed and well-built, occupants should be at their peak comfort. With the average person spending approximately 80% of their lives in enclosed rooms, an occupant’s well-being is imperative. Therefore, the products used to achieve this should cover all the issues affecting a building’s construction, its impact on both its occupants and nature.

    Further reading

    ASBP Healthy Buildings Conference summary of key points, https://asbp.org.uk/asbp-news/healthy-buildings-or-toxic-buildings

    Read the expert’s view on healthy buildings including Professor Stephen Holgate CBE, Clinical Professor of Immunopharmacology at the University of Southampton and co-author of The Royal College of Physicians ‘Every breath we take‘ report, who explains why poor quality air is a lethal problem that affects us all, Consultant, Clinical Psychologist at UCL, Dr Sarah Mackenzie Ross who looks at the rapid rise in new chemical entities in our day-to-day environments and the consequences on our health, CIBSE’s Head of Sustainability Development Julie Godefroy  who questions the role of Building Regulations in delivering healthy buildings and Professor Anna Stec, fire toxicity expert from University of Central Lancashire who looks at the potential fatal effects when plastics in the home burn.

    Visit

    www.asbp.org.uk for more on sustainable building products

    www.ecomerchant.co.uk/protexion to see how insulation can provide so much more than keeping the heat in

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