Good morning everyone, my name is Keira Proctor, and welcome to our third webinar of the year. Our series of webinars has been running now since 2020, and if you’ve missed any you can go back and review them all on-demand right here on our YouTube channel.

You can also catch up on our learning hub at, where you can also book in for follow-ups with our team of experts around the country, access up to date information on our full range of products, or order product samples. In today’s webinar we will be looking at the principles of fabric first design in retrofit construction, and specifically how it works with the EnerPhit standard.

We’ll begin by reviewing the fabric first principles that for the bases of both passive house and EnerPhit, then look at some of the applicable performance criteria and available guidance.

We’ll then consider how this fits into the wider context of moving the build environment towards net-zero carbon, and then look at the specific technical strategies involved and their implications.

Lastly, we’ll take a look at the range of products and systems we can supply.

We’ll then hand over to today special guest, Caitriona Jordan of the Construction Scotland Innovation Centre for an introduction to the work CSIC undertake in the retrofit space, before finishing up with our regular Q&A session where you can put your question to Caitriona and our team of technical experts.

Finally, we’ll finish up with our usual Q&A session, today featuring special guest Caitriona Jordan, Head of Retrofit at the Construction Scotland Innovation Centre


Retrofitting in buildings is the practice of installing additional elements that were not present during construction, and today we’ll be looking at the installation of measures to improve the energy efficiency of the building. The most common of which in existing buildings is usually installing insulation, particularly in easily accessible spaces such as inside of lofts. Upgrading insulation is an example of a fabric first approach to building design, where the elements of the building are designed to reduce reliance on external systems, such as mechanical or electrical heating or cooling systems, as far as possible.

In new builds, it is possible to use the orientation and location of the building to heat the building through solar gain, and to encourage natural ventilation pathways through the construction. In retrofit though, the building is existing, meaning factors like these cannot typically be optimised to the same extent.

Instead, fabric first approaches to Retrofit need to rely mainly on maximising airtightness and insulation.

That said, retrofitting fabric first energy solutions to buildings can also involve improving or making better use of thermal mass.

Thermal mass is the ability of a material to retain heat energy, which is then slowly released as needed. Denser materials, such as stone, have far more thermal mass than less dense ones, and so take longer to heat, but once heated stay warm for much longer. This is the same principle which makes old fashioned storage heaters work.

Buildings built using solid masonry have a much higher thermal mass than framed buildings, another important factor to consider in retrofitting older properties. For example, if a building has solid masonry or concrete floors and walls, it will be easier to maintain a stable temperature within it, while framed buildings need to be heated on demand as they lose heat much more quickly. The thermal mass in the structure helps to smooth out fluctuation in temperature. Fabric first design is a core principle of Passive House construction, which is a set of standards designed to maximise the energy efficiency of a building, and which we have looked at on a number of our other webinars.


Designing to Passive House standards requires a building to meet a number of criteria. It needs to be planned using PHPP, the Passive House Planning Package, which is a calculator used to assess all the elements of a building in terms of their combined thermal efficiency.

The principles of Passive House construction, at least in “heating climates” similar to the UK, also include:

Very high levels of insulation; Extremely high performance windows and doors (including thermally broken and insulated frames); Very low levels of air permeability; Thermal bridge free construction; and a mechanical ventilation and heat recovery system. By combining these factors, it is possible to create buildings which achieve very high levels of energy efficiency.

Passive houses (in a European climate) are certified to have a primary energy demand of less than 120 kWh/m² [Read as KiloWatt Hours per metre squared] each year, including a heating or cooling demand of less than 15. These buildings are also incredibly airtight, with less than 0.6 air changes per hour at 50 pascals.

Unfortunately though, a lot of the measures necessary to reach these high standards are impractical to apply retrospectively, and to this end the Passive House trust also publish a slightly more lenient standard aimed at retrofit constructions.

This is the EnerPhit standard, which was released in 2010, and has more relaxed allowances for space heating and cooling, as well as a reduction in the required airtightness.

An EnerPhit building can use up to 25 kWh/m² each year in space heating or cooling demand, which is not counted as part of the primary energy demand. These buildings can also have an airtightness of up to one air change per hour at fifty pascals. One air change per hour is still a significant target, significantly below UK regulation standards, so extensive testing and leakage detections is an important part of achieving EnerPhit certification.

In England, new buildings can currently be built to an air permeability of 10 m³/m²h@50Pa [Read as metres cubed per metre squared per hour at fifty pascals], though this will drop to 8 in June 2022 when the part L regulation is updated.

It is important to note that these are two different measurements, and are not directly comparable. The requirement given in the regulations is based on the surface area of the building, while the number of air changes per hour as used in EnerPhit, is based on the heated volume. Scotland allows an air permeability of up to 7 m³/m²h@50Pa, while Northern Ireland has a requirement to design to 8, though 10 can be accepted as built. In the Republic of Ireland, the backstop value for all new buildings is as low as 5m³/m²/h@50Pa.

These values are just for new build though, the national regulations currently do not require any particular standard for air leakage to be met in retrofit projects. Passive House and Enerphit also have more stringent targets for u values than building regulations do. While building regulations specify a blanket regulation across the country, The Passive House Trust specifies a u value across climate zones.

The UK has three Passive House climate zones. Warm temperate climate, which we have through Devon, Cornwall, and London, have u value targets within the EnerPhit system of 0.3 W/m²K [Read as Watts per metre squared Kelvin] with exterior insulation and 0.5 W/m²K with interior insulation.

The difference in requirements across the two types of construction is due to the fact that internal insulation reduces the usable area of a building, and so an allowance is made for lesser performance. In cool climate areas, which constitute most of the UK, the Passive House Trust sets a target of 0.15 with external insulation and 0.35 W/m²K internally in EnerPhit projects.

Finally, cold climate zones, in the North of Scotland and parts of the borders, have targets of 0.12 and 0.3 W/m²K respectively.

Building regulations in England and Wales set a u value target for the refurbishment of external walls in residential buildings of 0.3 W/m²K, or 0.55 if insulation can only be applied inside of the cavity. It also allows existing walls with u values below the threshold of 0.7 W/m²K to remain unimproved.

Scotland requires a similar improvement to u values in the conversion or renovation of heated buildings, with all external walls to achieve 0.3 W/m²K. The Republic of Ireland declares a target u value of 0.55 for a conversion or renovation of a cavity wall, and 0.35 W/m²K on all other walls.

This means that, in many cases, insulating to the maximum requirement for building regulations is often sufficient to achieve the u value for EnerPhit as well, especially if the insulation is applied internally. Some constructions in certain locations do require a lot more insulation to achieve the standard, however. For example, an externally insulated building in the North of Scotland would need to meet 0.12 W/m²K. Roofs must meet more stringent u-value requirements the building regulations. Part L of the building regulations requires renovated warm pitched roofs and flat roofs to achieve a u value of 0.18 W/m²K, and 0.16 in a cold pitched roof (where the insulation follows the line of the flat ceiling). The building regulations in Northern Ireland use these same u-values, but “conservation of fuel and power” is covered by part F instead of Part L.

Part L will however be updated in June 2022, so all retrofitted roofs in England and Wales will require a u value of 0.16 W/m²K, unless the existing roof is already below the threshold of 0.35.

In Scotland, all roofs in retrofit constructions need to achieve 0.25 W/m²K. The building regulations for the Republic of Ireland specifies that cold pitched roofs need to achieve 0.16, while flat roofs and warm pitched roofs need to be renovated to only 0.25W/m²K.

The EnerPhit standard, in contrast, doesn’t differentiate between roofs and walls, and simply has a single u-value requirement for the whole external envelope. However, EnerPhit does not make the same allowances for internal insulation in roofs as it does for walls, as the insulation would not be restricting the floor area.

This means that for most of the UK, a roof renovated to the EnerPhit standard would need to achieve 0.15 W/m²K. For a typical warm pitched roof with rigid insulation between and under the rafters, this would likely mean having 150mm between and 30mm below, assuming a standard PIR insulation.


Newly constructed buildings are able to be more efficient than renovated ones, as is reflected in the better requirements from both Building regulations and the Passive House Trust. Research from the International Passive House Association states that modern buildings built to the Passive House standard are up to 90% more efficient than existing building stock.

The UK government has set a target of reaching net zero by 2050. Since the adoption of the Paris Agreement and the release of the IPCC Special Report on Global Warming of 1.5°C, a growing number of countries have committed to net zero emissions targets.

As of June 2021, 31 countries and the European Union have set such a target, either in law or in a policy document, and more than 100 countries have proposed - or are considering net zero targets.

The UK was among the first to legislate for this, doing so in 2019, and since 1990, the UK has reduced our Greenhouse Gas emissions by 48%. There’s a lot of progress to be made though.

It is estimated that around 80% of the buildings we will be using by the net zero target date of 2050 have already been constructed, so there is a critical need to decarbonise our existing stock to make this target achievable. The UK’s housing stock is also among the oldest in Europe, with over 20% of homes being more than one hundred years old.

In government-owned social housing, there is an additional target set by the EESSH to consider, which is the Energy Efficiency Standard for Social Housing. This states that all of these residences must achieve an EPC rating of B by 2032, which means that there is a large additional requirement to retrofit these buildings with energy efficient solutions. The Scottish government has also set its own targets regarding heating in buildings, and among these aim to install one million heat pumps by 2030. Gas central heating is currently a major barrier to a carbon net zero economy, and the heating demand currently accounts for 50.7% of Scotland’s total energy consumption.

To date though, fewer than 50,000 air-source heat pumps are estimated to have been installed in the UK, and 85% of the country is still heated by gas boilers. In 2020, the UK government committed to a goal of installing up to 600,000 heat pumps per year up to 2028.


SOLID WALLS Improving the energy efficiency of a building is not always without its challenges. A lot buildings that require retrofitted energy efficiency have solid masonry walls, and these types of walls typically have the most to gain in terms of efficiency. For these, a good approach in terms of improving insulation is to apply it externally.

This can significantly increase the thermal performance of the wall, and depending on the insulation used, can also be the optimal strategy in terms of the hygrothermal performance of the building. It allows the masonry to be fully within the insulation envelope, which means that it is not cut off from the heat of the building, and so does not gain excessive moisture as a result.

If the insulation and external cladding or render are vapour permeable, the entire construction allows the passage of moisture, and therefore any water vapour from the room is able to permeate through to the outside with minimal difficulty. It is also an excellent use of thermal massing, as the is able to act as a heat sink within the insulated envelope. The difficulty with this approach is that buildings in need of retrofitted insulation are, of course, already existing.

Many of these buildings are historically listed, or in conservation areas. This means that it may not be possible or desirable to change the external appearance of the building. The building could also be in an area where it is not possible to change the building’s footprint, for example in terraced housing (where external insulation would sit out of line with the rest of the row) or buildings built at the edge of the property boundary.

External insulation, while arguably the best solution, is not always a viable option.


Cavity wall construction, which became popular in the early 20th century, means that some buildings have the option of insulating within the cavity. Typically, this involves blowing insulation into the cavity.

While this is an effective way to use what would otherwise be just an empty void, it needs to be done carefully to ensure that the insulation is applied evenly. Insulation in this cavity can also create a bridge across which moisture can track, which must be avoided. This means that the type of insulation used must be carefully considered in regards to the requirements of the specific project and how it deals with wind driven rain and other factors.

Also, filling the cavity often does not allow enough insulation to be added to bring buildings to their required u values, particularly if the renovation is being done to the EnerPhit standard or to other similarly high levels of insulation.

A typical cavity wall using brick and block might have a u value of 1.7 W/m²K, without accounting for any internal finishes. Blowing insulation into this cavity might decrease this to around 0.6 with a typical polystyrene bead insulation, which would not meet the u value requirements in Scotland or from the EnerPhit standard.

In England and Wales, which have specifically relaxed requirements for cavity wall retrofit solutions, this would almost achieve regulations, so as long as the internal lining brought it down to 0.55, it would be acceptable to building control.

Buildings with cavity walls will therefore normally require additional insulation either internally or externally to reach a high standard of fabric insulation.

FRAMED WALLS Framed wall constructions are easier to insulate, as they include a large cavity the size of the structural member within the wall. If the internal linings can be replaced, filling these studs, or updating the insulation within them, is an excellent way to improve the thermal efficiency of the building.

A vapour control layer can also typically be added to the inside of these frames, which reduces the risk of condensation and provides airtightness to the building, and we will discuss our solutions to this and other issues later in this webinar.

Many roofs are of similar constructions to framed walls, and can usually be insulated between their rafters and have a VCL applied to manage the risk of condensation. Flat roof constructions require more care, as they can be more prone to condensation.


If there’s no cavities, and external systems can’t be used, the only way to insulate an existing wall is to do it internally, which can cause an array of potential issues if not considered carefully. For this reason walls of this type are sometime referred to as “hard to treat”.

First and most obviously, internal insulation takes up floor space within the property, which is especially a factor in places where floor space is at a premium. Many smaller rooms simply cannot accommodate the depth of insulation that would be required to bring a solid wall down to the necessary u value requirements of 0.3 W/m²K.

For example, if we look at a standard 215mm brick wall, it would take over 100mm of a standard mineral wool insulation to meet this target, more likely be up to 140mm when timber bridging from supporting studwork is accounted for.

To prevent damp tracking through the wall, this studwork would typically be offset from the wall with a cavity, meaning that such a system requires over 200mm of space from each external wall of a building. Aside from the loss of floor space with such a solution, this would cover any internal listed features, such as cornicing. These may need to be moved to bring them forwards onto the insulation, which requires specialist contractors and bears the risk of breaking the plasterwork. Electrical sockets and fittings, along with any other services on the external wall, also need to be relocated.

While more modern thin internal wall solutions such as our Spacetherm can significantly reduce this build-up, achieving very low u-value in solid walls will always necessitate a degree of complexity and compromise, not least where moisture is concerned. HYGROTHERMAL CONSIDERATIONS Applying internal insulation to a solid external wall should not be done without consideration of the risk to its hygrothermal performance. Uninsulated solid masonry walls have had decades, or perhaps hundreds of years of having the masonry at the temperature of the heated space inside.

Cutting the masonry off from that heat could be detrimental to the construction. As these walls are exposed externally, they are subject to driving precipitation, which can be driven deeper into the masonry by solar radiation.

It may have been the case that the heat from inside the building was either pushing this water back out, or allowing it to evaporate into the room, where it would be dispersed by natural ventilation, especially if the building was draughty, as many older properties are.

When internal insulation is applied to the wall, the masonry becomes colder, and therefore less able to lose moisture. This could create a risk of water ingress between the insulation and the masonry, or within the masonry itself.

Although our Spacetherm system is vapour permeable, many thin internal lining boards are not, which can increase condensation risks, as water in the masonry is be unable to permeate through the insulation into the room.

Additionally, as retrofitted buildings, particularly those which achieve the EnerPhit standard, are much more airtight than the existing building was, there is less natural ventilation in the room to remove this moisture. A vapour permeable insulation solution can help mitigate this risk. When retrofitting a building, it is therefore important to consider the balance of heat, air and moisture movement as a cohesive and unified strategy. Overly insulating a building without considering the effects on ventilation or the risk of condensation can result in disaster. For solid masonry walls, the risk from mismanaged moisture can be particularly significant.

BS 5250, the British Standard of the code of practice for the management of moisture in buildings, acknowledges this, and states that the industry standard calculation method for the assessment of condensation in most constructions, is not sufficient to assess the risk involved with internally insulated solid masonry walls.

This standard method, the “Glaser method” following the procedures of BS EN ISO 13788, uses a simplified steady state analysis that does not account for many of the factors that lead to elevated condensation risk specific to sold walls.

In solid walls, BS5250 now recommends dynamic hygrothermal analysis using the method given in EN 15026. This method, while a lot more complex and specialised, properly accounts for these additional factors such as driving rain, longer periods of moisture storage, and capillary action.


In common with Passive House, the EnerPhit approach requires a mechanical ventilation and heat recovery unit (or, MVHR) to be installed, which ensures that there is sufficient air movement, without wasting heat energy.

While the MVHR system ensures sufficient fresh air is delivered to maintain indoor air quality, to work effectively and efficiently, such systems need uncontrolled air leakage through the building fabric to be minimised.

In this instance, the passive fabric first approach, low air leakage, allows an external active system to work effectively, so it’s important these two approaches are considered holistically. If the passive air leakage rate and active ventilation systems are not well matched this can lead to problems with indoor air quality. It’s also especially important that the performance “as build” matches the design values used at the specification stage. While focussing primarily on fabric first, the Passive House trust does advocate for active systems to boost energy efficiency where appropriate. So in addition to MVHR units, it is advised where possible to make use of renewable energy systems such as solar panels and ground or air source heat pumps, but ONLY after the fabric of the building has been considered.

The Passive House Trust even provides a database of certified components which have been tested to be usable in Passive House and EnerPhit applications.


WRAPTITE At the A Proctor Group, we have a number of solutions which improve the energy efficiency of the fabric of the building. Some of our products, such as our Wraptite, have even been certified as Passive House components for their use in airtight constructions.

This is our fully self-adhered, airtight and vapour permeable membrane. It is primarily used on the sheathing board of framed constructions, but can provide some utility in retrofits as well. When applied to a solid masonry wall, it provides a robust airtight layer, and unlike a vapour control layer, won’t impair the moisture vapour transfer through the wall.

This “moisture neutral” characteristic, means the membrane can be located anywhere in the construction without increasing the risk of condensation. So the airtight line can be positioned wherever it is easiest to maintain continuity, irrespective of the the location of the insulation

If the air barrier is applied outside of the insulation, this will require fewer penetrations for things like services than if it was at the internal lining, but this is not always easy to achieve in retrofit constructions. Having the flexibility to position the membrane anywhere in the build-up does make maintaining continuity easier though.


A different solution for airtightness which is sometimes more suited to retrofit construction is our Procheck Adapt. This is a vapour variable vapour control layer, and is also certified as a Passive House component for airtightness. Vapour variable means that it reacts to temperature and humidity, becoming more permeable in summer months, which allows the masonry to lose moisture as needed.

In winter months when it is more humid, Procheck Adapt is a robust vapour control layer, providing a vapour resistance of 94m sd. This is generally more than enough to prevent condensation in typical wall constructions.

As the temperature rises and the relative humidity falls, however, the pores of the membrane open up, becoming much more vapour permeable – down to as low as 0.4m sd.

During the summer, when the membrane is vapour open, the UK sees brief spikes of reverse vapour drive, during which a vapour control layer can actually be detrimental to the construction.

The vapour variability allows the building to become much more vapour permeable when it needs to be, while preventing condensation during the worst months. SPACETHERM AEROGEL Another product we can offer is our Spacetherm insulation. This is our high performance aerogel insulation, which we can provide bonded to a variety of boards to suit various applications.

On its own, Spacetherm Blanket is a versatile and flexible material, which is both vapour permeable and hydrophobic. This can be used as a standalone insulation blanket in a variety of applications, or can be specified as a laminated panel.

One of the laminated systems, our Spacetherm Multi, which can be as thin as 16mm, comprises aerogel insulation and a 6mm Magnesium Oxide facing board. This thin profile makes it ideally suited to limiting cold spots, for example at window reveals, where space to accommodate traditional insulation can be limited.

Thicker profiles using conventional plasterboard are also available, which can be fixed either on timber battens, or with optional plywood reinforcement, directly to masonry walls. With additional layers of insulation, these systems can allow walls to meet lower u-value that woudl ordinarily be possible in the available space.

For example, 45 – 50mm of insulation would get a typical solid brick wall down to the 0.3 W/m²K required for building regulations and EnerPhit certification.

Another solution for solid wall applications is our Spacetherm Wall Liner, a thin thermal laminate roughly the same thickness as a standard layer of plasterboard. This incorporates 10mm of Spacetherm insulation and a 3mm Magnesium Oxide board, and is adhered to the wall to provide a significant thermal improvement compared to the uninsulated wall.

Typically, it would bring a solid stone wall (which might have a starting u-value of around 2.3 W/m²K) down to a u value of between 0.7 and 0.9. While some way short of the values given in the regulations, this nonetheless represents a noticeable drop in energy lost through the wall.

in projects where decorative features such as cornicing and plasterwork may rule out any more invasive solutions, products like this can deliver worthwhile improvements.

Spacetherm can also be used in the form of self adhesive cold bridge strips, which are easy to apply to minimise thermal bridging in a construction. Reducing thermal bridging is an important part of fabric first design, but in retrofit applications is also one of the most difficult to address.

Used in this way, the aerogel strips can help ensure performance is maximised within the constraints imposed by the existing structure.

Lastly, we can provide our Wraptherm, comprising aerogel insulation bonded to our Wraptite air barrier. This is designed to combine thermal and airtightness benefits into one easy to apply system, which can both reduce air leakage and limit thermal bridging around complex structural geometry, without compromising the movement of moisture vapour.

Finally, we’ll finish up with our usual Q&A session, today featuring special guest Caitriona Jordan, Head of Retrofit at the Construction Scotland Innovation Centre

We will now have a brief introduction the low carbon learning program at the Construction Scotland Innovation Centre, before moving on to our Q&A session.

This Webinar Includes
  • Fabric first methodology
  • Importance of thermal performance
  • Importance of air tightness
  • Assessment methodologies