RIBA - Passive House & Low Energy Housing Design

Good morning everyone, my name is Keira Proctor, and welcome to our tenth webinar of 2022. 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.

Today we’re running our second RIBA-assessed webinar, this time looking at design and specification criteria for passive and low energy use housing.

This presentation will introduce 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.

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.

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 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. 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 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.

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 watts per metre kelvin while aerogel insulation is far lower at 0.015 watts per metre kelvin.

Some materials, for example blown rigid foam insulation, may have a thermal conductivity that varies over time as blowing agents escape from the foam structure, so it's important to ensure the correct thermal performance figures are used. Third party product certification or UKCA documentation will typically give "aged" performance values if they are relevant to the application.

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 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 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 to 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 is 0.3 watts per metre squared kelvin, 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.

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 simply 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 design is free from 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 those that incorporate aerogel insulation 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.

Ultra-thin internal wall insulation (IWI) systems that use aerogel insulation provide high thermal resistances in minimal thickness, allowing for a more uniform envelope of thermal insulation performance and minimising localised drops in r-values around complex features. The fire rated IWI 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 watts per metre kelvin, while aerogel 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.

Aerogel insulation systems can allow this uniformity to be achieved, and being of comparable permeability to fibre quilt, to be achieved without compromising moisture movement.

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.

When using systems such as aerogels, all product details and installation practices should be reviewed in the context of the Control of Substances Hazardous to Health 2002 Regulations. Manufacturers installation guides and Material Safety Data Sheets should be consulted, as such product may require different installation practices when compared with traditional insulation materials therefore specific data for the specified product should be reviewed.

Likewise the fire performance of the system should be considered. The facing boards used in laminated system, such as Plasterboard or Magnesium Oxide, are typically classified as A1 or A2 under the EN13501-1:2018 standard. This means their reaction to fire will comply with all Approved Document B standards for linings. However some specific situations, such as "relevant buildings" as defined in Regulation 7, may have additional fire rating requirements. Most commonly this is a consideration in building over 18m in height in England and wales, or 11m in Scotland.

In such cases, all materials which become part of an external wall or specified attachment must achieve class A2-s1, d0 or class A1, other than those exempted by regulation 7(3).

While raw silica aerogel is inert and non-combustible, fibrous composite aerogel insulation of the type typically used in construction applications is available with a range of reaction to fire classifications from Class C to Class A1 depending on the fibrous carrier matting used. For context, most mineral fibre insulation products fall into class A1 or A2.

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 metres squared kelvin per watt 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.

It’s 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 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, by pressurising or depressurising the building. The backstop value for this is 10 metres cubed per square metre per hour 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, 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, 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 without access to the source data.

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

S49/50

A building with an air leakage rate roughly meeting the UK regulations limiting value of 10 metres cubed per square metre per hour AP50 will have gaps and cracks equivalent in area to one 20 pence piece per square metre of floor area.

In contrast, a passive house meeting the require 0.6 air changes per hour N50 standard will have gaps equivalent in area to a 5 pence piece every 5 square metres of floor area.

Airtightness & Vapour Control

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.

Most important of these is considering the effects of reduce air leakage on the transfer of moisture around the building. Historically, most buildings had high level of airflow, both intentional and incidental (for example from open chimneys).

Modern buildings where this airflow is not present require more careful, holistic consideration of hygrothermal properties in relation to air movement.

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 or brickwork 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 air leakage performance cannot be considered in isolation, and in parallel it's critical to address moisture control. This is covered by Part C in England and Wales, Section 3 in Scotland and Part F in the republic of Ireland. It's also Part C In Northern Ireland, but this is a different set of regulations to that used in England and Wales.

In all four cases though, specifiers are referred to the British Standard BS5250. This document, the most recent edition of which is BS5250:2021, is the Code of Practice for the Management of Moisture. Prior to the 2021 edition this was simply "Control of Condensation" however this has now been expanded to cover all sources of moisture that can affect buildings.

This reflects an increasing recognition that the effects of heat transfer and the flow of air and moisture around structures are interlinked and should be considered holistically at the design stage.

To this end the standard also defines "connective" and "systemic" effects. Connective effects being the "effects of changing conditions at junctions between different building elements" and systemic being "effects of an action on the whole of the building, or parts not directly connected, due to changes in the hygrothermal condition of the inside of the building".

Both these serve to raise awareness of the interconnected nature of moisture in buildings, although standard does note that assessment methods for the impact of these effects is currently at an early stage.

BS5250 gives detailed prescriptive guidance and example constructions for various wall, roof and floor types, that are, in effect, tried and trusted, and therefore do not require verification by calculation. It does however also provide guidance and method for undertaking such calculations should they be necessary.

We'll move on to disucss these calculations shortly, but firstly lets consider some of the definitions given in BS5250:2021.

When discussing air and vapour control layers, or AVCLs, BS5250:2021 lists these important considerations.

Most obvious is the performance required by the AVCL. This is important as the AVCL is not necessarily a standalone product or system, with BS5250 giving specific examples of glass, metals or plastic floorcoverings as "of themselves constituting all or part of a vapour control plane". The assessment guidance can therefore be used to determine the performance needed to control the risk of condensation, and we'll discuss these methods shortly.

Note 2 in this section discusses the need for additional materials to control air and vapour movement, but it should be stressed here that such dedicated AVCL materials are not a hard-and-fast requirement. If it can be demonstrated the materials forming the element can inherently minimise moisture risks then a specific AVCL can be omitted. This also leaves variety of options for managing these risks open to designers depending on the nature of the building fabric and it's intended use.

The second important consideration is the "practicability of installation" basically how easily the theoretical design can be realised on site.

BS5250:2021 introduces the concepts of ADT Performance, or "as designed, theoretical" and ABIS Performance, "as build, in service". This is an important distinction as these can be very different.

In terms of AVCL performance, this relates mainly to the simplicity of sealing laps and penetrations, and whether or not the need for complex and difficult installation processes can be designed out. It's also important to ensure the design and subsequent installation are robust enough to meet their lifespan requirements.

Lastly, in notes 3 and 4 BS5250:2021 illustrates the many forms air and vapour control layers can take. In essence every layer in a building element plays a role in the management of air a vapour transfer to some extent. Assessing and balancing the performance and practicalities of each part is key to a successful design, and this is particularly true at the higher levels of performance required by passive and low energy buildings.

Note 4 also clarifies that while most specific "AVCL" membranes tend to be impermeable to both air and vapour, it's also possible for membranes to be airtight yet vapour permeable. This again gives designers important flexibility in where they position and how they install the airtight layer, as the "air control" and "vapour control" functions can be separated and performed by a variety of components throughout the element.

We'll come back to this later, but now lets consider the assessment of vapour transfer.

BS5250:2021 references two standards for condensation risk, BS EN ISO 13788:2012 "Hygrothermal performance of building components and building elements. Internal surface temperature to avoid critical surface humidity and interstitial condensation. Calculation methods" and BS EN 15026:2007 "Hygrothermal performance of building components and building elements. Assessment of moisture transfer by numerical simulation".

The simplified "Glaser Method" given in BS EN ISO 13788:2012 is considered by BS5250 to be generally acceptable for most type of constructions, however BS EN 15026:2007 can be used to gain greater detail as to the hygrothermal performance.

In the "Glaser Method" the thermal and vapour transfer properties of the various materials, and simplified environmental conditions are used to model the temperature gradient through the element and also the dew point line. The dew point is the temperature at which the air will because saturated and condensation will occur.

In our example wall here, a cross laminated timber panel with insulation external to the structure, the dew point will generally occur in the outer ventilated cavity, if at all. This is because the insulation is positioned such that it keeps the more vapour resisting inner layers warm enough to prevent condensation. This makes this type of a wall a very robust hygrothermal solution where high levels of insulation are required.

Lets now move on to consider the how the positioning of construction membranes in a wall affect the required performance characteristics.

Firstly we have the "traditional" vapour control layer position on the warm side of the insulation. This is the most common approach historically, and membranes used in this location are usually airtight and vapour impermeable. This approach works by ensuring warm, moisture laden air cannot reach the colder outer parts of the wall construction where it can condense.

More recently, variable vapour resistance membranes have become available that can become more or less vapour permeable in response to environmental conditions. These membranes block the passage of moisture vapour in winter when condensation risks are highest, but allow some degree of moisture transfer in warmer summer conditions.

This can help the building fabric dry out in either direction, helping remove construction moisture and moisture from external sources such as rainfall.

The principle downside to this location of membrane is in the detailing and sealing required during installation. Internally there are substantially more penetrations for services and structural elements. While this can mitigate to a certain extent by using service voids, relocating service runs away from external walls or using a variety of tape sand gaskets, this still introduced additional complexity throughout the process.

If we move the airtight layer to an intermediate position in the construction, such as over a structural insulated panel, or as shown here, over cross laminated timber, we can greatly reduce the number of services that need sealing, and possibly also simplify sealing membrane joints and laps as there will typically be a more solid substrate to fix against. If the membrane is self adhesive, this process becomes easier still.

On the other hand, moving the membrane to this position does mean the vapour permeability must be considered more carefully. In our CLT example here, the membrane can be either vapour permeable or vapour tight, as the majority of the insulation will be further outside. The precise level of vapour resistance needed will depend how vapour permeable the CLT panel. If the panel has sufficient vapour resistance to limit condensation risks, then a vapour tight membrane may not be required at all.

If there is insulation present on the warm of this membrane though, such as in a SIP wall or timber frame, then the membrane should be vapour permeable to prevent trapping moisture within the insulation layers. This is particularly true if additional layers of insulation are specified in a "warm frame" construction.

Positioning the a vapour permeable airtight layer in this location, with insulation to either side, makes the correct balance of vapour permeability throughout the construction very important, and project specific condensation risk analysis calculations are recommended.

If additional detail or analysis over a multi-year time period is required, the EN15026:2007 method can be used, but it should be recognised that these calculations require a higher degree of experience and expertise to undertake than the simpler "glaser method" analysis.

The final possibility for the location of the membrane is outside the heated envelope in the cavity behind the outer cladding or rainscreen.

Being on the cold side of the insulation, membranes in this location must be vapour permeable to prevent trapping moisture, and weather resistant to provide temporary protection during construction.

Providing this temporary weather protection means there will often be a membrane in this location regardless of air and vapour control requirements, so if the "airtight" layer is positioned elsewhere in the construction, a membrane in this location need not necessarily be airtight, although it may help reduce convective heat loss around the insulation if it is.

Membranes used in wall construction may also be subject to fire performance requirements depending on the project, and this is especially true of membranes used in this location.

Although Regulation 7(2) requires the main materials which become part of an external wall of ‘relevant buildings’ to have a fire classification of A2-s1,d0 or A1 in accordance with BS EN 13501-1:2018, there are several exemptions granted in Section 12.16 of Approved Document B, Volume 2 Section B4.

From the approved document, a “relevant building” means a building with a storey (not including roof-top plant areas or any storey consisting exclusively of plant rooms) at least 18 metres above ground level and which:

• 1.contains one or more dwellings
• 2.contains an institution
• 3.contains a room for residential purposes (excluding any room in a hostel, hotel or boarding house)

In this definition “above ground level” in relation to a storey means above ground level when measured from the lowest ground level adjoining the outside of a building to the top of the floor surface of the storey.

Of relevance to our discussion today is 12.16 (a), which states:

Membranes used as part of the external wall construction above ground level should achieve a minimum of class B-s3, d0

This exemption is partly in recognition that some materials such as vapour permeable membranes may not be able to achieve this fire class while maintaining sufficient vapour permeability.

A similar exemption is provided in Scottish Technical Standard 2, but the cut-off point for a construction to be considered as “high rise” is lower in Scotland, at 11m as opposed to 18.

These exemptions allow designers a little more flexibility to balance fire performance against other project requirements, but it is especially important the fire classifications and testing are up to date and based on relevant test criteria. Fire testing information and certification for materials should be available from system suppliers.

The placement of the air and vapour control layer, or layers, within a building should be considered in the wider context not just of the transfer of heat, air and moisture, but also with regard to installation practicalities and long term robustness.

As performance levels in terms of both fabric insulation and envelope airtightness increase to the level required by the Passive House standard, minor defects in construction can have a far greater impact on the long term as-built performance.

Because of this, designers increasingly must consider now just how to design structures which offer the best theoretical performance, but also which can be constructed in the simplest and most durable manner.

The last of the passive principles is the use of ventilation with heat recovery, in which heat from extracted airflow is 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 the passive house certification. Without heat recovery ventilation systems it can be difficult to meet the space heating energy requirements.

When using this kind of system it's important to ensure end users are made aware of maintenance requirements. Most MVHR systems will require regular filter changes and other servicing to ensure they are operating correctly. These requirements and interval will vary between systems however, so specific advice should be sought form the supplier of the specified system. 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, it is 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 is important here is ensuring the building as built meets what is specified.

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 7 metres cubed per square metre per hour and actually achieve 3, then ventilation may not be able to supply sufficient fresh air to provide acceptable air quality. What is important is ensuring the building is constructed within its design specification, thus allowing all the building systems, active or passive, to perform as intended.

Heat Pumps 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 then 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 is 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.

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 than 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.

That brings us to the end of todays presentation, and we’ll now move on to the Q&A session.