Good morning, and welcome to today’s webinar. My name is Keira Proctor, and I’m the managing director of the A. Proctor Group, and today’s presentation is the second of 2022. We’ve been running our series of webinars since April of 2020, and if you’ve missed any of them, or are joining us today for the first time, you can go back and catch up with the full series here on our YouTube channel. You can also visit our learning hub at, where you can order product samples, or book in with our team for follow up meetings. Today’s webinar is the first of several this year where we’ll be looking at a specific project, encompassing the products, processes and design considerations involved, and this one is slightly unusual in that it’s also my own house. So we’ll start by looking at the background to the project and some of the planning conditions, then look at the “fabric first” design philosophy and how the principles of passive house design were applied in practice. We’ll then look at some of the more unique design features and discuss the implications of incorporating them. We’ll finish up with our regular Q&A session, today featuring special guest and associate architect David MacFarlane, of Allan Corfield Architects.


My home, Tighetebhu is a bespoke 4-bedroom, 475m2 eco-home near the village of Inchture in Perthshire. Although not passive house certified, the design is based on passive principles, and utilises structural insulated panels, low air leakage rates and heat recovery to deliver outstanding energy performance. The 2-element single and 2-storey design also uses local whinstone, timber, frameless glazing and a strong focus on detail and craftsmanship to complement its surroundings. The plot, directly adjacent to a historic, and listed walled garden posed planning challenges, with the initial application rejected. The revised and successful proposal by AC Architects addressed these issues by creating a direct dialog with the existing historic walls. Although, being listed, these existing walls could not be incorporated into the design, the relationship between the redesigned structure and the surroundings was welcomed by Perth and Kinross planning, as well as Historic Environment Scotland leading to the project being granted approval in late 2016. Beginning on site in August 2017, the project was completed in June 2018, with the combination of offsite panel manufacturing and the high performance Wraptite vapour permeable membrane enabling a simple and fast build process. The SIP system was delivered to site ready to be erected and jointed immediately, and once wrapped in the airtight and vapour permeable membrane, the shell become wind and watertight, allowing internal works to commence immediately. The kit was erected in the November, and despite there being a delay with the windows and doors not being delivered until January 2018, the use of Wraptite around the structural kit and over all window and door opening kept the building entirely wind and watertight over a relatively bad winter. This use of Wraptite to keep the interior warm and dry allowed fitting out to be undertaken in parallel with the final exterior finishing. With the bulk of the build process taking place over the winter months, this temporary weather protection was particularly advantageous.


The design is based around the “fabric first” concept, and uses passive house principles to reduce energy consumption as far as possible. The fabric first approach is based around ensuring the basic building elements are well insulated, airtight and as efficient as possible, rather than relying on mechanical and electrical systems or renewable power.

The five core principles of this passive design are: Thermal Insulation Thermal Bridge Free Design Airtightness Ventilation with Heat Recovery and high performance glazing. The purpose of designing according to these principles is to minimise the energy inputs required to maintain a comfortable indoor temperature. Ideally the space heating and cooling energy requirements can be reduced to the point that passive sources source as solar gain and heat from the building occupants are sufficient for most of the time.

The passive house institute provides software, known as the passive house planning package or PHPP, to calculate these energy loads, and provides prescriptive guidance and details, along with product certification.

The passive house guidance itself is fully open source and freely available, so there’s no need for “official” approval to undertake a passive house project unless you want the project to be certified and accredited. In this case PHPP was used to developed and assess the design, but the final project is not passive house certified. This approach combines the best practice efficiency of Passive Homes with unlimited design flexibility in terms of material and product specification. For self build projects this flexibility allows greater freedom to adjust designs and specification if project scope or priorities change during the build. Whilst that wasn't the case here, its worth bearing in mind if having the “passive house approved” sticker is not considered critical. Tighetebhu is A-rated for both energy efficiency and environmental impact (in terms of CO2), achieving a gold standard under Section 7 of the Scottish Technical Standards. Design CO2 emissions are just 6 kilograms of Carbon Dioxide compared to the Building Standards target mission rate of 17 per square metre per year, . So lets take a look at the five principles in a bit more detail, and how they were implemented.


Thermal insulation is what limits the loss of heat from the building envelope, and is the cornerstone of the fabric first approach to building design. This approach seeks to maximise the performance of the basic components of the building, rather than relying on mechanical or electrical systems.

There are three main metrics used to measure insulation performance. Thermal conductivity or Lambda value, thermal resistance or R-value, and thermal transmittance or U-value. The first of these, thermal conductivity, is a property of the particular material, regardless of it’s thickness. So in the case of the panels used at Tighetebhu, the core of each panel is a rigid polyurethane foam, with a thermal conductivity of 0.024 watts per metre kelvin. Thermal conductivity is a good way to compare the relative performance of different materials, but in design terms we need to also account for how much material is used. For this we use the thermal resistance, or R-Value.

The panels used here are 172mm thick, which comprises two skins of 15mm OSB with 142mm of the PU foam between them. To derive the thermal resistance we take the thickness is metres, 0.142, and divide by the thermal conductivity of 0.024, giving us an R-value of 5.91 metres squared kelvin per watt. The R-value still doesn't tell us the whole story though, as this is the insulation in isolation. The next step is to calculate the thermal transmittance or U-Value, which is the rate at which heat flows through the entire wall assembly. In this case the wall also uses the Reflectatherm Plus insulating vapour control layer with a service void and plasterboard internally. Externally, our Wraptite vapour permeable air barrier was applied across the whole kit and finished in either Stone, Timber or Render. To determine the u-value of the whole wall element, we take the sum of thermal resistances for each layer of the wall, and take the inverse. This is the u-value, which is measured using watts per metre squared kelvin, and the lower the value the better.

The building regulations in Scotland recommend u-value of 0.11 for roofs, 1.4 for windows, 0.17 for walls, and 0.15 for floors, which are the values used in the “notional building” used in calculations to determine compliance with building regulation. Backstop, or worst permissible, values are higher at 0.15, 1.6, 0.22 and 0.18 respectively.

Tighetebhu’s u-values are 0.12 for the roof, 1.0 for the windows, 0.14 for the walls and 0.12 for the floor. Being below the notional values, these u-values make a net gain to the energy performance, which could allow trade offs to be made in other areas like glazing if necessary.


The second principle of passive house is thermal bridge free design.

A thermal bridge occurs either where a material of higher thermal conductivity intersects a less conductive material, or where building elements join, such as at floor zones or corners. Thermal bridges have two principle negative effects. Firstly, they increase the overall heat loss by providing an easier pathway for heat to escape, and secondly they create cold spots internally. Condensation and mould growth can occur in these areas, which has a detrimental effect on the indoor environment.

The first of these, typically where structural elements like studs in a timber frame wall intersect the insulation is accounted for via an adjustment in the u-value calculation. This takes the thermal conductivity and proportional area of the bridges, and adjust the u-value accordingly.

To see how much difference this makes to the u-value, lets take a look at the 0.14 of the SIP panel wall. The system uses a much lower quantity of structural timber compared to a conventional timber frame wall. So when we calculate the u-value for a SIP wall, convention is to use a bridging fraction of just 4%. In a conventional timber frame, this rises to 15%, so a much greater correction is needed and our wall u-value of 0.14 becomes 0.17 instead. The second situation thermal bridging can occur is at junctions such as floor zones and corners, is accounted for by using an additional value for linear thermal bridging, knows as a psi value, but in most cases these are addressed using standardised pre-assessed details to limit heat loss rather than being unique to a project. SIP systems have a further advantage here in that the more standardised nature of such constructions means less deviate from these standard linear bridging details is needed, simplifying compliance considerable. A further benefit of a SIP system at junctions is that the quantity of timber required at these joints is much reduced. Let's consider typical junction details for an external corner and an internal wall. At the panel junctions of Tighetebhu’s SIP wall and roof elements, while there is a small area where insulation is reduced, compared to the equivalent in a conventional frame this is minimal. While it’s a simple matter to add additional layers of insulation to a frame to counteract this, avoiding the problem in the first place simplifies things further.

This difference is even more apparent when we look at an internal wall junction, where in a SIP wall only fixings penetrate the insulation layer. The timber frame wall in contrast needs additional timber to provide a fixing, increasing the thermal bridging at the joint. While each of these areas makes a fairly minimal contribution to the overall heat loss rate, taken together they all add up, especially for large and complex projects with complex geometry. A good example of this on this project is the change of level between the kitchen and living area, where there is substantial step in the ground floor slab. To minimise the cold bridge in this area, without increasing the depth of vertical elements, 10mm Spacetherm aerogel blanket was applied directly to the internal face of the foundation. This flexible high performance insulation matting is ideal for use in situation such as this, where even a relatively small R-value is sufficient to prevent the formation of a cold spot internally. This approach is also useful in areas where space may be limited, such as in window reveals, or where larger sections of structural steelwork pass through insulation layers.


At the A. Proctor Group, we have long recognised the importance of a holistic approach to managing heat, air and moisture in buildings, and achieving good air leakage performance is key to this. By “air tightness” we are referring to limiting the uncontrolled movement of air though gaps, cracks, and porous material in the heated envelope. This air movement causes not just infiltration of cold air from outside, and corresponding exfiltration of heated air, but can also pull and push moisture laden air into the building fabric. If this moisture laden air cools down too much, it can condense into liquid water, leading to damp and mould growth.

Air infiltration is measured by either pressurising or depressurising the building to a level of 50 Pascals, and the results are given in cubic metres of air that leak in or out, per hour, per square metre of floor area. The Scottish building standard don't set a maximum or backstop air leakage rate, but recommend a maximum of value of 10, with 15 being used as the default value for assessment if no specifc rate is stated. To calculate the target emissions for the building, a value of 7 is used, so above 10, the energy performance standard becomes incredibly difficult to meet. By far exceeding this requirement, Tighetebhu’s air leakage rate of 0.64 makes it a net gain in terms of building regulations compliance. If energy efficiency was not the primary concern, this net gain could be used to trade off other area such as thermal insulation or glazing area. Here however, it plays a critical element in reducing heat loss and corresponding space heating energy use. To visualise these values, buildings with an air leakage rate of 10 m3/m2/hr will have gaps and cracks equivalent in area to one 20p pence per square metre of floor area. In contrast, buildings like Tighetebhu, which meet the more stringent passive house standards will have gaps equivalent in area to a 5p pence every 5m2 of floor area.


There are several factors that contribute to this very low air leakage.

Firstly the structural panels themselves are inherently airtight, with both the foam core and the OSB facings offering very limited pathways for air movement. The panel joints are made using a smaller infill panel, which similar properties, and as both these infills and the main panels themselves are made offsite in factories, the improved dimensional tolerance ensure extremely tight joints can be made reliably. Next the SIP structure is wrapped in our Wraptite vapour permeable air barrier membrane. This self-adhesive membrane further seals the panel joints, and the junctions between the walls and the roof.

This facilitates lower air leakage in two ways.

Firstly, it provides a robust airtight layer which, being fully adhered to a rigid substrate, is able to resist damage from the weather. The self-adhesive material also makes it far simpler to achieve a high quality on-site installation, with far less possibility of poorly sealed junctions and membrane laps reducing the effectiveness of the air barrier. Secondly, because it is vapour permeable, it can be positioned anywhere in the construction without affective the movement of moisture. This means the airtight layer can be located where it easiest to both keep continuous, and avoid penetrations. If, as is traditional, the internal vapour control layer is used as the air barrier, this require all service runs and penetration to be designed and installed to minimise puncturing the membrane. While this need not necessarily pose a problem, the nature of sites and trades, particularity in the UK, is such that VCL integrity cannot necessarily be depended on. In, for example, Scandinavian countries where low air leakage rates have long been standard practice, every plumber and electrician will be used to working carefully around air barriers, however in the UK its not uncommon for holes to be cut and drilled and work undertaken with no thought given to any knock on effect on air movement.

Wraptite moves the air barrier to the outside, well clear of most services. This also facilitates straightforward sealing of complex curves and forms, and means features such as floor zones and roof/wall junctions can be easily dealt with by simply wrapping the whole building in a single product.

The Wraptite is fully BBA certified for use on walls and in warm roofs, so in this case the entire superstructure of the building was wrapped. Once fitted, the building shell is then wind and watertight so progress can be made on fitting out, and air pressure testing can be undertaken earlier in the build schedule, giving more flexibility on timings, and potentially easier fixes to any problems to do arise.

The final point to consider with air leakage is simple attention to detail. Designing, specifying and scheduling with airtight construction and energy in mind greatly simplifies the process. At Tighetebhu this attention to detail even extends to a specialised airtight cat flap, complete with collars for my two Bengals Thomas and Miles which allows only them to enter and leave the house at their leisure. Also very good for locking them in or out from our phone app remotely and keeping an eye on them!


When unplanned air leakage is reduced to such low levels, a well planned and executed ventilation system becomes more important. In common with most passive homes, Tighetebhu uses a mechanical ventilation system with heat recovery to maintain a healthy indoor environment while minimising heat loss associated with ventilation. In MVHR systems heat from extracted airflow is recovered through a heat exchanger and fed back into the building via the incoming supply air. This improves the efficiency of the building by reducing heat wasted by the extract airflow, which in turn reduces the space heating requirements. The system is also fitted with a summer bypass, so fresh air can be brought in unheated when necessary. The MVHR can be turned up or down depending on the time of year and requirements of the occupants. The gas-fired heating system runs underfloor heating in the downstairs areas. The upstairs rooms, while fitted with conventional radiators, in practice require very little heating due to the overall efficiency of the external fabric envelope. As well as the high efficiency heating and heat recovery, the house is designed to take advantage of solar gains to provide additional space heating.


Glazing performance and orientation are important considerations in any low energy building, and the widows used here are frameless triple glazed units, achieving u-values below 1, and most of the glazed elements are south facing. The windows supplied by LUMI are unique in terms of their aesthetics and offer a unique look to the overall project. As part of the initial design of the house, a solar path analysis was conducted to model how sunlight would fall on the building.

This data can then be fed into the energy modelling software and any design tweaks necessary can be made. This allows the position of glazing and layout of rooms to be optimised to take advantage of solar gains, with shading elements added to reduce potential overheating. This holistic view of energy use, heat gain and how the building is used is key to delivering a successful low energy design. To this end the accommodation areas where typically lower temperatures are designable are separated from the kitchen and living/entertaining space, where temperatures would typically be higher. The living area is also more heavily glazed to maximising solar heat gain and natural lighting. This separation of functions also helps maximise the effectiveness of the zoned heating controls, ensuring space heating distributed as efficiently as possible.


From the outset of the project, a strong focus on high quality design, finishes and craftmanship was a principal consideration, as was making a fully accessible home. The central glazed atrium and entrance hallway highlights this, where a handcrafted walnut staircase and matching walnut flooring sit next to a glass fronted lift. These handcrafted details continue into the entertainment space with led-lit bespoke cornicing, and Georgian detailing contrasting traditional elegance internally with the more modern aesthetic found externally. This approach to detailing continues into the cinema and games rooms, where traditional wall paneling and cornicing contrasts with constellation ceiling lighting and modern audio visual equipment. The upstairs balcony is home to a seating area out side the nursery, with the landing spanning the whole upstairs area. A learning curve for us was the area outside the upstairs lift, where we hadn’t planned for the doors opening outwards, resulting in a retrospective Juliet balcony. An afterthought but we have come to like it! The master bedroom suite features a large balcony and sunken en-suite bath with views over the historic walled garden, and the garden to the front is home to a recently added garden office and outdoor kitchen designed and constructed Sonder Limited. The building follows similar principals to the main house, with underfloor heating, a Structural panel envelope and use of Wraptite, making it a comfortable and energy efficient multi-functional work and entertainment space. Sonder did such a fantastic job with the design and delivery of the project, it feels like an extension of the house rather than a garden office.

This Webinar Includes
  • Case Study of Award Winning Eco-House
  • Use of SIP Systems to Minimise Thermal Bridging
  • Design & Build for Low Air Leakage Rates
  • Balancing Heat, Air and Moisture in Construction