Today we're to take a look at how we design housing with low energy use in mind, and how the principles and guidelines of the passive house standards can inform that. While the passive house standards themselves provide an excellent framework for low energy design, the strategies they embody can be applied to benefit all housing regardless of the regulations and assessments applied.

So we'll begin today by taking a look at what those principles are and what requirements and standards apply to meet passive house certification. We'll then move on to consider how these concepts can be applied in a more general context, bringing the benefits of low energy design to all types of projects.

Passive House Background

The Passive House regulatory framework originated in the late 1980s with ideas developed by Bo Adamson or Lund University, Sweden, and Dr Wolfgang Feist of the German Institute for Housing and the Environment.

These research projects led to the construction of several experimental homes in the German city of Darmstadt. Building on these early 90s experiments, Dr Feist founded the Passivehaus Institute in 1996 with the aim of developing and refining this concept, and ultimately promoting its use around the world.

From the first projects outside of Germany in the early 2000s, passive house certified buildings can now be found all over the world.

The actual standard itself however, only codifies a set of principles that have been applied to buildings for a considerably longer time, even back as far as the middle ages, albeit in a far less refined form.

As building regulations have developed, and energy performance become a more important consideration elements from the passive principles have become more and more integrated into modern homes, and are increasing beyond high end self built to be part of mainstream housing practice.

Passive House Principles

There a 5 basic passive principles,

  • Thermal Insulation
  • Thermal Bridge Free Design
  • Airtightness
  • Ventilation with Heat Recovery and Passive House Windows, or more generically: high performance glazing.

The propose 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.

While this approach certainly simplifies the process, and ensure rigorous adherence to good or best practice, it does not always allow designers the necessary flexibility to balance the full range project requirements.

In such case the passive house guidance and principles can be viewed as best practice recommendations for designing buildings assessed by other means such as national building regulations.

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. That said, the verification and certification services provided by the passive house institute can serve to greatly simplify planning and approvals process for such buildings, and reinforced confidence and trust with subsequent buyers and sellers.

So lets now consider how the five basic principles can be applied, and what they mean.

Thermal Insulation

Thermal insulation is what limits the loss of heat from the building envelope, and is the cornerstone of the fabric fist 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.

The fundamental physical property of insulation material is their thermal conductivity, also known as the k-value or lambda value.

This property measure how well heat can flow through the material by conduction. In this mode of heat transfer, the heat energy is transferred between the individual molecules of a material by direct contact.

Because of this reliance on physical contact there's a correlation between the materials density and its thermal conductivity, will less dense materials such as foams and fleeces less able to conduct heat than denser material like bricks or concrete.

Bricks, for example, have a thermal conductivity of 0.77 w/mK (watts per metre kelvin) while our Spacetherm aerogel insulation is far lower at 0.015w/mK.

Thermal conductivity however only looks at the basic material, it doesn't account for the thickness.

To factor in the material thickness, we take the thickness in metres, and divide it by the thermal conductivity to give us the thermal resistance, or r-value in m2K/W (metres squared kelvin per watt).

The next step is to covert that into a value for an entire wall, for which we use the thermal transmittance or u-value, which is the inverse of the sum of the r-values for the different layers in the construction.

U-Values are quoted in w/m2K (watts per metre squared kelvin). Where r-values vary proportionally to thickness of materials, u-value do not, so as we move to lower u-value, we need ot add exponentially more insulation to improve them further.

As an example, the "limiting fabric parameter", more commonly referred to as the "backstop" or worst permissible u-value for a wall given in approved document L for England and Wales is 0.3, and the recommended worst case for passive house is 0.15. The "best practice" passive house value is considerably lower at 0.06.

In practice compliance with either standard is more complex than achieving a simple maximum u-value, but roughly speaking the "worst case" u-value for a passive house would be considered pretty good under national building regulations.

Thermal Bridging

The next principle 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.

If we consider a 140mm thick panel of rigid foam insulation, if we used this in a typical timber frame wall, the u-value when adjusted to account for cold bridging at the timber stud will work out to around 0.22.

On the other hand, if we eliminated this cold bridging but using the same thickness of insulation in, for example a structural insulated panel system, the u-value drops to 0.15, purely do to the lack of repeating thermal bridging at the studs.

Thermal bridging at corners and junctions is accounted for using an additional value for linear thermal bridging, knows as a psi value, but in most case these are addressed using standardised pre-assessed details to limit heat loss.

The easiest way to visualise the way thermal bridge free design works in practice is to simple imagine drawing a pencil line round a cross section of the entire heated envelope of the building.

If that can be done without touching anything other than insulation, the deign is free form thermal bridges.

Because of the adverse effect that thermal bridging has on both the u-value of individual elements, and the overall energy performance, whether assessed via PHPP or SAP, reducing cold bridging in low energy building is a matter not just of principle but also of practicality.

Getting u-value anywhere close to Passive House "best practice" performance levels without restricting thermal bridging is virtually impossible, but even in cases where designers are aiming for lower standards, the benefits regarding limiting localised condensation are clear.

Whatever the assessment criteria, high performance insulation systems like the Spacetherm aerogel have a very important role to play in minimising thermal bridging by allowing the thermal resistance to be maximised at any given point.

In locations such as window reveals and door openings, it can be difficult to maintain a similar level of insulation to that found over the main sections of a wall leading to these area being relatively cold.

In extreme cases this can cause the temperature to drop to the point where condensation may occur in these areas, and as fabric insulation levels increase, so does the importance of continuity and uniformity.

Systems such as Spacetherm, which provide high thermal resistances in minimal thickness, allow for a more uniform envelope of thermal insulation performance, with the localised drop in r-values around complex features minimised.

The fire rated Spacetherm SLENTEX A2 systems also allow this continuity to be maintained where reduced combustibility is required, offering significantly higher performance per unit of thickness than conventional fire rated insulation such as mineral fibre.

Fibrous insulation quilts typically have a thermal conductivity in the range of 0.035-044, which Spacetherm is 0.015-019, meaning like for like performance can be achieved in less than half the thickness.

While big thicknesses of insulation are expected and accounted for in passive and low energy projects, it's not always possible to maintain such thickness over every part of the structure. Spacetherm can allow this uniformity to be achieved, and being of comparable permeability to fibre quilt, to be achieved without compromising moisture movement.

Spacetherm can be supplied in a variety of laminated board systems to suit most application, as well as in self adhering cold bridge strips for direct bonding to structural elements such as steelwork.

This need to create uniformity of insulating value and prevent cold spots is also an important aspect to consider when specifying the windows in low energy and passive projects.


Passive and low energy homes must pay particular attention to the specification of windows used, as poorly insulating windows can not only affect the overall heat loss, but as with cold bridges, can also create cold spots and "cold radiation".

This phenomenon occurs where the influence of a cold surface can be felt nearby, often leading to a perception that a space is colder than it really is.

Even by utilising multiple layers and glazing, inert gas fills and thermally reflective coatings though, windows still typically fall a long way short of the performance of the rest of the building envelope, with the passive house specifications calling for windows with a u-value of 0.8 or less, significantly above the opaque elements,

Windows are also important to provide heat input to the building via solar gains, and for this reason passive houses in "heating climates" like the UK tend to position the majority of glazing facing south, in order to maximise this effect.

This requirement to maximise collection of solar heat has to be carefully managed though, as highly insulated and airtight homes can be susceptible to overheating if this is not carefully assessed and managed.

Its a common misconception that passive houses do not have opening windows, but in fact the specifications call for at least one opening window per room in the living spaces.

Opening windows is among the most cost effective and simple means of counteracting overheating, and is useful to prevent poor indoor air quality in exceptional circumstances such as large gatherings.


That leads on to the next concept we need to consider, the airtightness of the structure.

It's often said that airtight buildings can be like "living in a plastic bag" however this is a fundamental misunderstanding of what we mean by airtight buildings.

Reducing the air leakage simply means eliminating sources of air infiltration and exfiltration over which we have no control, draughts in other words. A good supply of fresh air is a critical component of healthy indoor environments, and the passive house specification is designed to ensure this is facilitated by controlled and carefully managed ventilation systems with carefully considered impacts on energy performance.

Uncontrolled air movement on the other hand should be reduced as far as possible.

As we've already covered, the thickness of insulation layers increases exponentially with reductions in u-value, and after a certain point this becomes both uneconomical and impractical, even in explicitly low energy buildings.

So it's very important that every other factor affecting energy performance is controlled and limited to minimise energy usage.

In dwellings, there are two main drivers of air leakage: the internal stack pressure caused by the relative buoyancy or warm air and resultant convection, and external forces from the wind.

Because warm air is less dense, it will rise upwards in a building, and can leak out of any air pathways found at higher levels. Correspondingly, as this air rises, colder air must be drawn into the building lower down.

As the warmer air will contain significantly more moisture, this outward air leakage can lead to condensation issues as the leaking air cools and deposits liquid water.

The cooler air entering the structure lower down will lower the internal temperature in the building, necessitating energy input to maintain the temperature of the habitable space.

Externally, the wind forces acting on the building will have similar effects, but instead of the pressure gradient working form bottom to top, it will be aligned to the wind direction. Cold air will be pushed in on the windward side where there is a positive pressure, and warm moist air sucked out on the leeward side where the pressure is negative.


Limiting this air movement is a critical part of low energy design, as even highly insulated building will have poor energy performance if the movement of air is not controlled. Both the building regulations and passive house standard therefore include pressure testing and minimum performance levels, but there are some important differences.

The UK building regulations use a a pressure test result known a AP50, which is the volume of air in cubic metres, that flows through the building fabric area in square metres in 1 hour when a reference pressure of 50 pascals is applied. This pressure can be applied in either direction, buy pressurising or depressurising the building.

The backstop value for this is 10m3/m2/hr in England, Wales and Northern Ireland. In Scotland and the Republic of Ireland, the "preferred" or "reasonable" limit is set at 7, and while it's theoretically possible to comply with a higher value, the necessary trade offs will generally be unacceptable.

Passive House expresses the air leakage differently, using a standard called N50, which is the air changes per hour through the buildings internal heated volume (ACH) at the same 50 pascal reference pressure. In passive house test however the pressure must be applied in both directions, pressurising and depressurising the building, and the two results averaged.

The test equipment and physical procedure is largely the same for both AP50 and N50 test,m so it's perfectly possible to run both simultaneously, but the way the final results are calculated is sufficiently different that conversion between the two is not possible.

The BRE's passive house airtightness "primer" document, highly recommended as follow up reading, gives a useful analogy to illustrate the difference in standards.

A building with an air leakage rate roughly meeting the UK regulations limiting value of 10 m3/m2/hr AP50 will have gaps and cracks equivalent in area to one 20p pence per square metre of floor area. In contrast, a passive house meeting the require 0.6 ACH N50 standard will have gaps equivalent in area to a 5p pence every 5m2 of floor area.


There are numerous ways of making building airtight, and finding the most appropriate method to apply to a given project is usually a matter of balancing a variety of competing requirements. Some strategies are easier than other however.

Some building materials, such a cast concrete, cross laminated timber and structural insulated panels have an inherent advantage when it comes to air leakage as the basic materials involved have low air permeability. Other like blockwork are more permeable so require additional measures to control air leakage.

In all cases though, junctions, services, and the nature of the on site assembly process still pose challenges to achieving a low air leakage rate, even when using the most impermeable base materials.

The conventional solution to this is to seal the inside of the building using a vapour and air barrier membrane. The advantage of this approach is that this membrane fulfils a dual purpose, limiting both air leakage and vapour ingress into the structure, it's also something most contractors are familiar with.

The downside to this approach is that the membrane integrity can be compromised by a variety of components and processes. Being internal, building services such as power, plumbing and network cabling must run through an internal air and vapour control layer.

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.

Unless contractors of all disciplines are aware of the importance of this layer, suitably trained and equipped to avoid damaging it, and afforded sufficient time and space to undertake works in a manner that avoids puncturing this membrane, the scope for problems is limitless.

Features like service voids, flexible pipe and cable gaskets, and sealed light fittings can all go some way to mitigating these risks, but these all increase costs and time demands, and aren't necessarily familiar to all contractors.

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.

It can also take far longer on site to ensure every joint and penetration is adequately taped and sealed, often requiring several different types of tape for different circumstances. Acquiring all these tapes and sealants can also pose challenges if needed at short notice.

Although this is changing gradually, with more contractors gaming passive house certification, it's perhaps prudent for specifiers of low energy buildings in the UK to look at alternatives methods to mitigate these risks at the design stage.


Moving the air barrier layer to the outside of the building, well away from any services and following trades, allows this to happen.

Our Wraptite product is a self adhered, vapour permeable but airtight membrane suitable for use in both wall and roof applications. Its also certified by both the passive house institute and the BBA, so can be specified with confidence as an effective air barrier layer.

On the outside of a building there are far fewer penetrations to seal, and those that are present tend to be larger and simpler to seal. Because the membrane is adhered fully to the substrate it's also better able to resist damage.

The self adhesive backing also makes sealing laps and joints simple and fast, and the membrane itself can be fitted offsite, allowing it to provide protection during transportation as well as during construction.

In an offsite installation, the joints between adjacent panels or modules can be simply jointed onsite using our split-liner tape, which ensures fast and effective seals can be made in the air barrier.

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.

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 primary benefit of the Wraptite system is not so much that it reduces the overall air leakage rate, more that it make achieving design air leakage rates far easier and more reliable. This in turn allows the benefits of airtight construction to be fully accounted for and integrated into a holistic design without fear of expensive and awkward remediation if targets are not met.

The vapour permeability or wraptite also allows flexibility in insulation placement. It's perfectly possible, for example, to wrap a timber frame building with insulation between studwork in wraptite, then fit additional external insulation layers to limit cold bridging without causing any moisture problems.


That's not so say we don't need to consider moisture control in this type of construction. An internal vapour and air barrier will also serve to limit the ingress of moisture vapour into the construction, so if we move the air barrier externally we need to ensure that moisture is adequately controlled to prevent any condensation problems from arising.

This can be achieved either by using insulation with a lower permeability to moisture vapour, such as most types of rigid foams, or by using a warm frame construction with insulation placed externally to limit temperature drops and ensure the dew point does not occur internally.

Another options is to use a variable resistance membrane such as our Procheck Adapt.

This type of specialist membrane has been used in passive house constructions across Europe for many years, and when combined with an external air barrier membrane provides a comprehensive moisture and air movement control solution.

The Procheck Adapt varies it's resistance to moisture vapour according to environmental conditions, becoming more resistant to vapour ingress in winter when condensation risks are highest, then less permeable over summer to facilitate drying out and allowing a more breathable construction.

Used alongside Wraptite, Procheck Adapt provides an additional level of protection against both air leakage and moisture issues. Any moisture that does make it into the construction as a result of damage during installation can easily dry out in either direction, which the movement of air is reliably restricted by the external air barrier.

We also have the capability to run WUFI based condensation risk calculations covering a wide range of factors to ensure buildings are able to managed to interactions between heat, air and moisture movement.

We've covered this in more detail already in our Membranes Explained and Intro to Building Physics webinars, and you can go back and review those on our website or youtube channel if you'd like a bit more detail on that, or you can book a virtual meeting with one of our team if you'd prefer that.


The last of the passive principles is the use of ventilation with heat recovery, in which heat from extracted airflow i recovered through a heat exchanged 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, and such systems are generally required as part of a the passive house certification. Without heat recovery ventilation systems it can be difficult to meet the space heating energy requirements.

This requirement in the passive house specification often leads to an incorrect assumption that all airtight buildings require mechanical ventilation. If designers are not working to the passive house certification specification, then various other options for both ventilation, and heat recovery are available.

In fact its perfectly possible to have good indoor air quality at low leakage levels with a fully passive ventilation system provided this is designed into the building early enough. What important here is ensuring the building as built meets what is specified, and here is where systems like Wraptite are particularly useful.

If you design the building heating and ventilation systems to operate with an air leakage rate of 3m3/m2/hr, but when tested the building only achieves 7, then problems can arise as highly optimised heating systems may not be able to compensate for this shortfall.

Conversely if you design to 7m3/m2/hr and actually achieve 3, then ventilation may not be able to supply sufficient fresh air to provide acceptable air quality.

Whats important is ensuring the building is constructed within its design specification, thus allowing all the building systems, active or passive, to perform as intended.


A further possibility for the recovery of heat to lower the energy use in a home is to use a heat pump in the heating system. There are two primary types of heat pump, air source and ground source.

Air sourced heat pumps or ASHPs use heat exchanging fluids and compressors to extract heat from the ambient external air and supply it to the interior of the home, either via a more or less conventional central heating or underfloor system in an air-to-water system or via warm air circulation in an air-to-air system.

Ground sourced heat pumps recover heat from collector loops buried underground. Fluid is circulated through these loops and picks up heat from the ground, which is them fed through a compressor and heat exchanger and used to heat the home.

Both of these systems commonly use a large water tank to act as a heat store, using the thermal mass of the water to retain heat energy until it it required by the heating and hot water systems in the house.

Ensuring this heat store is well insulated is a critical part of ensuring these systems operate efficiently, and we have supplied the Spacetherm insulation in blanket form for use insulating a huge variety for tanks and vessels.

Both ground and air sourced heat pumps are designed to operate at lower temperatures than fossil fuel or electric heating systems, and this lower operating temperature works particularly well with underfloor heating systems.

These lower temperatures, and well as the lower running costs, mean these systems are typically designed to run continuously or almost continuously, and have less scope that traditional heating systems to boost their output to meet additional demand.

The better the energy performance of the dwelling they are fitted to the more effective they will be. This makes these kinds of systems a good fit for low energy homes.

This relative lack of "spare" heating output also reinforces the importance of predictable and reliable energy performance at the specification stage, as the only practical solution to an underspecced underfloor heating system is often the use of auxiliary electrical or solid fuel heating which may not be a good fit with an energy and ecology focused project.


Although its not passive house certified, my own house used many of these principles to deliver an award winning low energy design by Scottish practice Alan Corfield Architects.

Completed 2 years ago, it's a structural insulated panel home, adopts the passive principles of thermal bridge free design and highly airtight construction, and also used a mechanical ventilation system with heat recovery to minimise energy requirements.

If you've been following our webinar series, we discussed structural insulated panels last time, and this particular system is the Kingspans TEK panel system, supplied and erected by SIP Scotland.

These panels use a rigid urethane core with OSB facings, which limits cold bridging by having no structural penetrations through the insulation layers. OSB also delivers good air leakage performance, and this is boosted here by using both wraptite externally, and our Reflectatherm Premier reflective vapour control layer on the inside.

This reflective system not only lowers the u-value of the wall with its 0.78 thermal resistance, but the adjacent service void helps minimise penetration through the VCL. This reduces the potential for moisture vapour ingress and and further limits air movement.

The Reflectatherm premier, like the Procheck adapt, features an integrated lap joint tape, helping facilitate a good quality installation.

The use of systems and materials which simplify installation, along with experience and expertise of specialist contractors familiar with the use of SIP systems and delivery of low energy focussed projects was key to delivering good results.

So hopefully we've covered a lot of the basics today and given you a good overview of where to start with passive and low energy designs. There really is a wealth of amazing resources out there from BRE documentation to the passive house institute guidance, and we've barely scratched the surface of the factors that go into creating and delivering a successful low energy home.

We do have our team of technical experts across the country though, so anyone involved in these kinds of projects, simply get in touch and we can assist right through the process.

This Webinar Includes
  • Passive and low energy design principles
  • Efficient construction technologies
  • Managing Moisture and indoor air quality
  • Fabric first energy performance