The links between airtightness, thermal bridging and IEQ
We spend a significant part of our lives inside. Studies have shown that the average adult in Britain spends between 85-92% of their time indoors on a weekly basis.1,2 A poll commissioned in 2016 put these statistics in a different light and described how a typical prison inmate spends more time outside than three-quarters of the children in the UK.
As we accept that our professions and lifestyles are keeping us indoors for longerperiods of time, an awareness of the importance of indoor environmental quality(IEQ) is also growing. The US-based National Institute for Occupational Safety and Health (NIOSH) defines IEQ as “the quality of a building’s environment in relation to the health and well-being of those who occupy space within it”. Key to this definition is the role that IEQ factors like air quality and thermal comfort play in our productivity and enjoyment of life, as well as our physical and psychological health.
Improvements to UK building standards over the years have helped to ensure that an appropriate level of insulation is incorporated into building work. The availability of higher-quality insulation materials and a better understanding of modern building physics work together to improve overall building thermal performance, reduce energy demands and carbon dioxide emissions, and support the thermal comfort of building occupants. However, insulation is only part of the solution to providing excellent IEQ.
Air leakage and thermal bridging
Tests confirm that poor airtightness can be responsible for up to 40% of heat lossfrom buildings because cracks, gaps and holes in the building envelope allows airto bypass the thermal insulation.5 Energy efficient heating systems are also rendered less effective if warmed air simply escapes through a draughty structure while cold air is allowed to flow in.
While adding more insulation should improve thermal performance in theory, thebenefits of thicker insulation may not be fully realised if the envelope’s airtightness is not also addressed – the heat lost through air leakage will simply increase proportionally.
To illustrate this concept, Section 6.2.0 of Building Standards technical handbook2017: domestic buildings (Scotland) describes a typical 1960’s house with poorly fitted windows. In the example, air leakage represents approximately 20% of the total heat lost in the house’s base condition. If the house is then upgraded to modern insulation standards but the leaky windows are not remedied at the same time, the proportion of the total heat lost via air infiltration will increase to over 40%.
Thermal bridges also undermine efforts to insulate properly by providing pathways through the insulation for heat to escape. The most commonly thoughtof thermal bridges are studwork, mechanical fixings and other penetrations through the insulation layer, but thermal bridges can also include larger elements like party cavity walls. As described in Section 3 of Approved Document L1A: Conservation of fuel and power:
“Where outside air flows into the party wall cavity, a cold zone is created which causes heat loss through the wall sections on either side. The air movements involved can be significant and ... the resulting heat losses can be large.”
Unaddressed thermal bridging can result in the formation of condensation within building structures. Interstitial condensation can then cause mold and mildew growth on interior surfaces, compromising indoor air quality and potentially occupants’ health
Clearly, as we continue to spend the majority of our time indoors, the building industry must keep IEQ considerations at the forefront of their designs. Strategies to support IEQ should include appropriate levels of insulation, an uninterrupted airtightness layer, and measures taken to identify and control thermal bridging in order to prevent interstitial condensation.
Wraptherm®: Combining airtightness with insulation
Wraptherm is a composite material comprised of 10mm Spacetherm® aerogel insulation blanket bonded to the face of Wraptite® vapour permeable air barrier. It is ideal for the refurbishment of existing buildings.
Wraptherm is self-adhering and can be applied to practically any substrate without the need for tapes or mechanical fixings. By being installed on the internal face of an existing façade, Wraptherm provides an air barrier on the external side of the ‘services zone’. This means there is less risk for the airtight layer to be accidentally punctured. As well, there is no requirement for expensivespecialist components such as airtight junction boxes or light switches.
Having a nominal thickness of only 11.5mm, Wraptherm also greatly reduces thermal bridging as new framing can be installed directly over it. Additional insulation can then be added within the frame to bring the construction up to therequired U-value.
By combining the airtight and insulating properties of Wraptite and Spacetherm, Wraptherm helps by both improving a building’s thermal performance and supporting IEQ by mitigating the risk of interstitial condensation.
It's Airtight Outside
The Problem - In many constructions, about half of all heat loss is due to air leakage through the building. Given that approximately half of all energy used in the UK is from buildings, it is easy to see that air leakage, or draughts, account for a
considerable amount of energy - and therefore cost.
As thermal insulation requirements have increased over the last few years, the proportion of energy lost through air leakage has become more evident. The ever-increasing thermal insulation required will however be rendered largely ineffective unless
the airtightness of the structure itself is addressed. Tests indicate that air leakage can reduce the effectiveness of thermal insulation by up to 70%; therefore if energy efficiency is to be improved within buildings, this is the most critical area
to focus on.
In addition to improved insulation, energy efficient heating systems will also be ineffective if warm air can escape the building and cold air can seep in. This is reflected in the fact that total space heating costs in an airtight construction may
be as much as 40% less than in a leaky one.
Another issue associated with air leakage in buildings is that warm and often moist air can escape to colder parts of the outer construction where it can cool and condense, leading to a build-up of moisture. This in turn can lead to:-
Decay of organic materials such as timber frames. Saturation of insulating materials, thus reducing their insulative effect (further increasing heat loss). Corrosion of metal components. Frost damage where moisture has accumulated on the cold side of
the insulation.
Air leakage through cracks, holes or gaps causes a significant reduction in the thermal performance and efficiency of a structure. However this can be remedied by careful design and good quality construction practice.
The Solution - Wraptite and Wraptite Tape are effective methods of stopping unwanted air leakage. Wraptite is a self-adhering airtight membrane which consists of a triple layer polypropylene micro-porous film laminate with a proprietary moisture
vapour-permeable adhesive, plus a silicon-coated PET release liner. It can be used in full wall applications, on pitched roofs (under metal, tile, slate, wood shingles and clay tile) and for window flashings or other details.
Wraptite combines the best properties of a vapour-permeable membrane and an airtight membrane in one innovative, affordable self-adhering product. It fully bonds (no primer or mechanical attachments required) to almost any substrate for air tightness
and ease of installation, negating the requirement for sealants or tapes.
Penetrations in sheet material such as pipes, ducts and electrical work also need to be sealed to ensure compliance with air pressure testing. Wraptite Tape is an air tightness tape that has all the benefits of Wraptite, but with the convenience
of a tape. Air tight Wraptite Tape is tear resistant and offers high vapour permeability for internal and external applications. It is also suitable for permanent airtight sealing of membrane overlaps. Detailing is quick and easy, providing an effective,
convenient solution to what can be an expensive problem.
Wraptite: All-in-One - Wraptite products do not require a primer and can be installed in temperatures as low as 0°C.Its wide service temperature range means it can be easily installed in all climates. It is very easy to install, with a tough facer laminate
that resists rips, punctures and tears. Wraptite is fully bonded, hence fasteners are not required at installation; this prevents lateral air movement, further enhancing the building’s thermal performance, and increasing performance in an air leak
test. Being highly water vapour permeable, Wraptite thereby reduces potential for trapped interior moisture and condensation. Wraptite is resistant for up to six weeks of UV and climate exposure.
Wraptite and Wraptite Tape are unsurpassed in performance. Both offering an SD rating of 0.039, Wraptite and Wraptite tape provide a highly vapour-permeable, yet airtight performance for a wide variety of applications and conditions, and offer
an optimal solution for achieving excellent results in an air pressure test. They allow damp sheathing to dry quickly and moisture vapour to escape, ensuring good indoor air quality while reducing incidence of mould, mildew, condensation, timber distortion
and metal corrosion. The unique characteristics of Wraptite allow the moisture vapour to escape the structure easily, while fully maintaining the integrity of the air-tight building envelope.
Not Always Easier From The Inside
Building air tightness is increasing in importance as designers seek to improve their energy efficiency measures without the need for an extra thickness of insulation that loses valuable foot print dimensions - especially in city centre commercial buildings.
The A. Proctor Group always strive to provide construction professionals with alternative, cost effective and easier to install solutions and the introduction of their air barrier systems are exactly that. With over 10 years’ experience of providing
the harshest climates(both hot and cold) in North America, the group is promoting the external application of the air tightness membrane to improve airtightness testing results.
The UK has up until now assumed that sealing the building from the inside was the best route to meeting increasing air tightness test requirements, in the main this has included sealing up the Vapour Control layer or Vapour Barrier as some refer to
it. This can make sense from a building physics point of view and help reduce condensation but the actual application can be fraught with problems when striving for low levels of air tightness in buildings.
From the earliest examples of weather boarded barns to modern skyscrapers, rainscreen construction has been part of the built environment for hundreds of years. Today, with increasing energy performance requirements, and intense competition between
designers to produce unique, aesthetically innovative and eye catching structures, the flexibility offered by rainscreen systems is more important than ever.
In a typical rainscreen curtain wall construction, the insulation is placed continuously over the structural frame and overlayed with a secondary weatherproof layer. The outer cladding is then fixed on rail systems, giving a drained and ventilated cavity
between the insulated structure and the façade. This gives several advantages, not least fully insulating the frame and substantially reducing cold bridging compared to in-frame insulation. This setup also allows for increased design flexibility, as
the outer leaf can be lighter than traditional brick facades, and can therefore adopt more unusual layouts and configurations that would be difficult or impossible to achieve with traditional façade materials. It is also comparatively easy to retrofit
insulated rainscreen systems to existing structures, giving a simple path to improving both appearance and energy efficiency.
In most rainscreen systems, the outer cladding panels are not fully wind and watertight, making the performance of the secondary weather protection membrane more critical than with brick or blockwork outer leaves. While most such membranes have adequate
air barrier performance, they are heavily dependent on quality of installation to ensure the overall air leakage rate targets are met. Failure to meet the specified targets can cause substantial increases in the energy performance of the completed
building, and this “performance gap” has been identified in many studies of predicted vs actual building energy use. Positioning an air barrier on the outside of the insulation rather than internally simplifies this process considerably, as there are
less building service and structural penetrations to be sealed, however careful consideration of the specification and installation of this barrier remains critical during design and construction. While such construction is not widely used in the UK,
the A. Proctor Group’s Wraptite external air barrier system provides a simple and robust solution that can be easily incorporated into existing site practices.
Wraptite, fully vapour permeable self adhesive air barrier, is available either as a membrane or as a fully permeable tape, with a unique vapour permeable adhesive backing. This allows a simple and fast installation procedure, with the fully adhered
membrane sealing to all common substrate materials. Its microporous film core ensures a low air permeance of 0.01m3m-2hr-1 while maintaining a high permeable to water vapour, with an sdvalue of 0.039m. This versatile, high performance material can
be used in full wall applications, at floor cassette or corner junctions or as an adaptable site-formed airtight flashing around door and window penetrations, providing an airtight seal without compromising moisture movement. In tape form, Wraptite
can be used to seal panel joints in airtight substrates such as OSB, or used to seal penetrations from services or structural elements. Its excellent adhesion characteristics also make it an effective method of jointing conventional air barrier membranes,
thus maintaining a high vapour permeability envelope across the entire building.
For nearly 30 years, the A. Proctor Group have been at the forefront of construction membrane technology. From the revolutionary air/vapour permeable Roofshield underlay to the low emissivity Reflectashield timber frame breather membrane, the A. Proctor
Group continually work with suppliers and research organisations to ensure our products meet the most advanced technical requirements while providing exceptional value for our customers.
Airtightness from a Contractors Perspective
“Adam Taylor, business development manager (building envelope) at A. Proctor Group, talks to RCI about the role of contractors in installing effective air barrier systems as a means to controlling and reducing air leaks”
Air leakage through cracks, gaps, holes and improperly sealed elements such as doors and windows can cause a significant reduction in the performance of even thermally insulated building envelopes. Contractors have a key role to play in the installation of effective air barrier systems which have become essential in achieving the most effective means of controlling and reducing air leaks.
As thermal insulation requirements have increased over the last few years, the proportion of energy lost through air leakage has become more evident. The ever-increasing thermal insulation required will, however, be rendered largely ineffective unless the airtightness of the structure itself is addressed. Air leakage greatly reduces the effect of thermal insulation; therefore if energy efficiency is to be improved within buildings, this is the most critical area to focus on.
The two main ways to achieve airtightness in the building envelope are internally or externally, or in other terms, “inside of the services zone’ or ‘outside of the services zone’. For the contractor, the use of traditional internal air barriers can be more complex and costly to install, due to the need to accommodate building services such as electrical, lighting, heating and drainage systems. An internal air barrier is only as good as it’s installation. If all the service penetrations are not adequately sealed, performance will be compromised.
For many years, external air barriers have been commonly used in North American building design and construction. By moving the air barrier to the external side of the structural frame, external air barrier systems such as Wraptite® from A. Proctor Group allow for an almost penetration-free airtight layer, which can be installed faster and more robustly. This offers an effective but simple system comprising a self-adhesive vapour permeable air barrier membrane, plus vapour permeable sealing tape, Wraptite Corners and Wraptite Liquid Flashing, and provides effective secondary weather protection while preventing trapped moisture and air leakage. Far simpler than internal options an external air barrier system like Wraptite will maintain the envelope’s integrity, with less building services and structural penetrations to be sealed, and less room for error.
The Wraptite air barrier system from the A. Proctor Group is the only self-adhering vapour permeable air barrier cer tified by the BBA and combines the important properties of vapour permeability and airtightness in one self-adhering membrane. This approach saves contractors costs on both the labour and materials required to achieve the demands of energy efficiency in buildings.
An example in use is the administrative headquarters building of the Royal College of Pathologists in London where the Wraptite system was installed as a solution for airtightness, weather protection and breathability.
Cladding contractor Windell installed the Wraptite System as an external air barrier and alternative to a traditional standard breather membrane. The use of a standard membrane would have required mechanical fixing and provided some challenges given the concrete structure of the building. As an alternative, the Wraptite self-adhesive membrane was applied, quickly and easily to the external envelope in continuous pieces.
George Marcantonio, the Site Manager of Windell, commented: “The application of the self-adhesive Wraptite System has proven really easy to use, and quick to apply, with no requirement to return for additional fixing or accessories. We will certainly be using the system for future projects and recommend it without hesitation.”
By reducing the likelihood of potential failures to meet designed airtightness levels, the Wraptite System helps contractors to ensure compliance with building regulations, achieving an effective airtight barrier, whilst saving time and cost on site.
The best way to meet airtightness requirements is by using an internal VCL?
In this series, the A. Proctor Group examines some commonly held myths in the construction industry concerning thermal and acoustic insulation, condensation control, fire protection, and ground gas protection. In this article, we ask the question: Is it true? The best way to meet airtightness requirements is by using an internal VCL?
A VCL, or vapour control layer, is a layer within a building construction that prevents water vapour passage. The VCL is positioned on the side of a construction element that would have the highest vapour pressure and therefore protects the insulation and structure from the ingress of water vapour.
In the UK, vapour drive flows from inside to outside, so a vapour control layer should be applied to the warm side of the insulation. VCLs are typically made of polyethylene, or foil, which are impermeable to both water vapour and to air. Therefore, they are often called Air and Vapour Control Layers, or AVCLs. As these AVCLs are air impermeable, many constructions incorporate them as the primary airtightness line for the building, though this does present some challenges that must be overcome.
Being positioned on the internal face of a construction means that the AVCL interfaces with many building services, which could damage it. For example, services such as plumbing or electrical fixtures in the external walls will need to puncture the airtight line. Typically, these require specialist airtight backing boxes or other seals which need to be taped to the AVCL to ensure the continuity of the airtight line. Any hole in the airtight line that has been improperly sealed is detrimental to the airtightness of the construction. Therefore, the VCL and plasterboard fixings need to be carefully considered during the design phase.
Additionally, the internal lining of the building is subject to punctures created by the building user. Things like picture hooks and television brackets cause unexpected penetrations in the airtight membrane, which can lead to air leakage that has not been accounted for in the original design of the building.
Internal airtight membranes also require careful and accurate detailing around floors, ceilings, and internal walls. The level of airtightness that an internal membrane can provide to a building depends on how well it is detailed, with small holes potentially creating significant issues.
An alternative and more effective option is to make the building airtight from the outside. Using an airtight vapour permeable membrane means the entire structure can be wrapped externally. This external membrane means that a homogenous airtight line is formed around the entire construction, which results in interaction with far fewer building services.
Using an external airtight membrane provides the benefit that it only needs to interact with the fixing brackets and possibly external insulation. This results in a more robust construction, which maintains its effective airtightness even after completion.
A commonly held belief is that all airtight membranes are vapour control layers, which would cause condensation if used outside of the insulation. As part of its extensive high-performance range of construction membranes, the A Proctor Group offers Wraptite®, a unique, fully self-adhered airtight and vapour permeable membrane. The Wraptite membrane is typically adhered to the sheathing board. The membrane can span across concrete frames and columns, allowing for complete encapsulation of the building with detailing that is far easier. Because Wraptite is self-adhered, it doesn’t require fixings, and in exceptional circumstances that do require fixings (e.g. for a rainscreen façade), the adhesive will provide a measure of airtightness.
Depending on the type and placement of the insulation in the construction, the need for a vapour control layer can be negated entirely. However, it is typically best to include a VCL as good practice. If the airtightness is handled externally and the construction works from a hygrothermal perspective, any work internally would only improve the performance. Often, a vapour control layer, in these cases, is only needed for moisture management, which simplifies the detailing required.
Part L of the Building Regulations has recently been updated to ensure that new constructions in the UK achieve no more than 7 m³/m²/h@50Pa. This revision to the regulations means that for each square metre of a building element, no more than 7 cubic metres of air should be able to permeate through it in an hour when the building is pressurised to 50 pascals. Building Regulations also require that 50% of all homes built are tested for air leakage to meet this target. Many contractors choose to take both a pre-test and a final test, with the pre-tests typically being conducted when the internal lining (and, therefore, the typical airtight line) has not been installed. This often results in buildings that allow more than 10 m³/m²/h@50Pa of air to permeate, though this would be expected to be reduced when the internal linings are installed. If the airtight line is outside the construction, the end result would be far closer to the final outcome.
As building design becomes more and more energy focused, the allowable air permeability of a construction reduces. Many buildings are designed to achieve below 3 m³/m²/h@50Pa, while energy-efficient standards such as Passivhaus are increasing in popularity, resulting in an ever-increasing need for robust airtightness. Traditionally, this has been provided at the internal lining through an AVCL, but as this article has demonstrated, there are alternative, more effective methods. The most airtight building will use both internal and external airtight membranes, resulting in buildings with almost no uncontrolled air leakage or draughts at all.
