WHAT ARE THE FOUR CATEGORIES OF FLOOR CONSTRUCTION DESCRIBED IN BS 5250?

arm-in-steam

BS 5250:2021 Management of moisture in buildings addresses the following four categories of floor construction.

  • Groundbearing floors with damp proof membrane (DPM).
  • Groundbearing floors without DPM.
  • Suspended floors.
  • Basement floors.

For the most part these categories deal with floors at ground level or, in the case of basement floors, below ground. However, the scope of the standard extends to all potential sources of moisture in a building, not just from the ground. ‘Suspended floors’ therefore also deals with intermediate floors, including those over unconditioned spaces.

To find out more about how the latest version of the standard differs from previous versions, read what you need to know about BS 5250:2021.

1. Groundbearing floors with DPM

This type of floor typically consists of a concrete slab that is cast onto, and supported by, a prepared base – usually sand-blinded hardcore. The DPM acts as a barrier to moisture from the ground.

The DPM should be positioned in the floor build-up based on the thermal insulation material used. Many thermal insulation materials are affected by moisture and therefore have to be installed above the DPM, in order to ensure that they deliver their intended performance.

Some materials, including extruded polystyrene (XPS), do not have their performance affected by moisture and can be installed below the DPM. This offers multiple advantages.

On some projects, a lack of understanding about the potential effects of moisture leads to unsuitable insulation materials being installed below the DPM. Specifying a material like XPS guards against the negative impacts of mistakes like this being made.

In addition, where insulation is installed above the DPM, an air and vapour control layer (AVCL) must then be installed over the insulation to limit the risk of interstitial condensation occurring.

Specifying and installing a material like XPS below the DPM means the membrane can serve a dual function by also acting as the AVCL – saving time and cost by only having to install one membrane layer.

The combination of insulation and DPM can be positioned above or below the concrete slab. Figures 3 and 4 of BS 5250:2021 clearly illustrate all of the ways in which this type of floor can be built up. The decision about where to place the insulation layer relative to the concrete slab should be made depending on the intended heating regime of the building.

2. Groundbearing floors without DPM

In modern construction, with the possible exception of some historic buildings, a new floor should not be constructed without a DPM. This type of floor is therefore generally only found in existing buildings, and is very unlikely to have any thermal insulation.

If a traditional floor with no DPM is to be replaced, the addition of thermal insulation and a DPM should be done with care, and with regard to potential knock-on effects. For example, a damp proof course (DPC) will likely be required in the external walls, if one is not already present.

BS 5250:2021 gives detailed guidance on the potential moisture risks that might result from replacing an existing groundbearing floor without DPM.

3. Suspended floors

Suspended floors separating a conditioned space from an unconditioned void or space should feature thermal insulation. At ground level, most new suspended floors are a concrete deck (such as a block and beam system, or a pre-cast slab), with timber joist floors being found in older properties.

The requirement for a DPM depends on the design of the floor and the ground conditions.

Because the thermal insulation is above a ventilated void or space, and not in direct contact with the ground, it is usually not necessary to consider the potential affects of moisture on the material’s properties. Therefore, most types of floor insulation can be considered suitable, with an AVCL typically required over the insulation layer to guard against interstitial condensation.

Again, BS 5250:2021 contains detailed guidance on potential moisture risks and suitable build-ups. 

4. Basement floors

The detailing of basement floors (and walls) depends on the ground conditions on site, whether the basement is new or existing, how the basement will be waterproofed, and the intended use of the basement (including moisture generation inside the room(s)).

Like groundbearing floors with a DPM, if the thermal insulation is to the outside of the basement structure and in contact with the ground, then it needs to be capable of retaining its thermal performance even when exposed to moisture.

Polyfoam XPS supplies floor insulation products suitable for use in groundbearing floors above or below the DPM, and in basements. View our Floorboard Standard and Floorboard Extra products or, for help in specifying the correct type and thickness of insulation in your project, contact us.

HOW DOES THERMAL BRIDGING CONTRIBUTE TO COMPLIANCE WITH PART L 2021?

The introduction of Part L 2021 in England has placed greater emphasis on thermal bridging heat losses and how they are calculated.

Compliance calculations, carried out using the Standard Assessment Procedure (SAP), have accounted for thermal bridging since 2006. The way the Part L 2021 requirements makes it harder than ever before to ignore accurate thermal bridging detailing.

Since 2006, accredited construction details (ACDs) have been available to assist in using accurate psi values (calculations of heat loss through thermal bridges) as part of the SAP calculations. Those ACDs have now been withdrawn, putting more emphasis on using other detail libraries or obtaining project-specific psi value calculations.

Why do national building regulations factor in thermal bridging?

A thermal transmittance, or U-value, is used to express heat loss through building elements such as walls, floors and roofs. Specifically, it is the amount of heat energy transfer through one square metre of the element, per degree of temperature difference between the warm side and the cold side.

Measuring heat losses by U-values alone does not account for all of the heat lost through the building fabric. At junctions between elements – such as where the ground floor meets the external walls, or at the eaves where a roof meets the external walls – the patterns of heat loss are different and the result is a linear thermal bridge.

Traditionally, much of the heat loss at junction details could be attributed to a lack of continuity in the thermal insulation from one element to another. Awareness of the need for a continuous thermal envelope has generally improved, though it is not achieved consistently.

Even where it is achieved, however, thermal bridging heat losses need to be accounted for. Simple changes in geometry of a building act as a thermal bridge too, because of differences between the internal and external surface areas of the building, and the way different construction materials interact with one another.

How have attitudes to thermal bridging changed?

Thermal bridging heat losses were introduced into national building regulations because it was recognised that, as U-values improved, psi values made up a greater proportion of overall heat loss. That trend has continued as subsequent versions of energy efficiency regulations, like Part L in England, have sought better and better building performance.

ACDs were also introduced in 2006, with the aim of showing how continuous insulation and airtightness could be achieved in common junction details. Junctions for different construction types – masonry, timber frame and steel frame – were included.

Each ACD included a checklist of key points for designers and installers so that the intended performance would be delivered on site. The given psi value for the junction could then be included in the SAP calculations.

Despite this, some project teams see thermal bridges and psi value calculations as an imposition or an unwanted extra cost. Others don’t understand the benefit of calculating psi values, even though they have been a feature of mainstream construction for a decade-and-a-half.

The nature of the compliance calculations means it has always been possible to use default psi values in SAP, compensate elsewhere in the specification, and still achieve compliance. Often, this turns out to be more expensive than getting psi value calculations carried out, but it has been an option. With the levels of performance required by Part L 2021, we may finally be seeing an end to any culture of ignoring thermal bridging.

Part L 2021 and the removal of ACDs

Over the course of the last fifteen years, ACDs have not been updated. It’s recognised they’re no longer fit for purpose in terms of the performance we want and need from our buildings, and they have been withdrawn as a result.

Other libraries of junction details have been produced over time, with the intention of supporting designers, specifiers and installers in achieving compliance. Many trade associations and product manufacturers have produced psi values as part of promoting the use of their products.

Where standard details don’t cover a junction as designed, bespoke psi value calculations, carried out by a competent person, remain an option.

Further emphasising the importance of thermal bridging in compliance, Part L 2021 includes more technical guidance on addressing junction details than has previously been the case. Read more about what it says on continuity of insulation at ground floor edges.

Polyfoam XPS has recently released new CAD details showing best practice junction detailing for ground floors, flat roofs and basements. Supporting the launch of our Below DPC insulation board, we have also produced a white paper that includes details and calculated psi values for cavity wall foundation details that meet the moisture resistance requirements of Part L 2021.

Alternatively, contact us to discuss how extruded polystyrene can help you to achieve compliance on your residential or commercial project.

WHAT IS BES 6001 RESPONSIBLE SOURCING CERTIFICATION?

Grasss and factory

BES 6001 is an environmental and sustainability standard, developed by the BRE, which provides a framework for assessing the responsible sourcing of products.

Originally developed specifically for construction products, it has gone on to be adapted for, and used by, other industries. This post focuses on its use in construction.

There can be a perception that it is the manufacturer or organisation who is certified under BES 6001, but in fact it is products that carry the certification. A manufacturer can therefore offer some products that have BES 6001 certification and others that don’t, so clear communication between manufacturers and specifiers is required to avoid confusion.

Why is BES 6001 responsible sourcing certification needed?

Constructing and maintaining our built environment consumes significant quantities of natural resources. Pressure on those resources is already high, and is only increasing as populations around the globe grow and become more urbanised.

Greater demand for resources makes it even more important to procure them responsibly. Natural resources come from all over the globe, including from communities that are not protected as we would hope or expect, and where people are at risk of exploitation or harm.

For UK construction professionals, having confidence in construction products means going beyond the manufacturer offering a particular product. Responsible sourcing certification shows that the manufacturer understands their supply chain and knows the origin of the materials they’re using, thereby supporting a more sustainable approach to material use.

While BES 6001 certification has typically allowed projects to claim more credits in schemes like BREEAM or LEED, responsible sourcing has taken on a much deeper and more fundamental importance. Transparency is core to ethical business practices, and responsible organisations are actively seeking to measure themselves against available benchmarks.

How does BES 6001 measure responsible sourcing?

Assessment and certification under BES 6001 are carried out by an independent third party, and the standard’s requirements span three different areas. Two of those areas relate to the management practices of the manufacturer, with the third relating to the provenance of the materials used in the product manufacture.

  • Organisational management.
  • Management of sustainable development.
  • Supply chain management.

‘Organisational management’ deals with how the company operates. It starts with having a responsible sourcing policy, and demonstrating legal compliance (locally, nationally and internationally). Management systems must be in place for quality and managing suppliers.

Meanwhile, ‘supply chain management’ looks at material traceability (through ISO 9001 certification, or another full chain of custody scheme), environmental management systems (certified to ISO 14001), and documented health and safety management systems.

What sustainable development criteria does BES 6001 cover?

‘Management of sustainable development’ covers a variety of social, economic and environmental factors. Manufacturers have to demonstrate they have policies and targets in place for each, report results accordingly, and that third parties assess the manufacturer’s performance.

Energy and resource use, waste management and greenhouse gas emissions all fall within this sustainable development area, as do ecotoxicity and water abstraction. Life cycle assessment, transport impacts and business ethics are also covered.

Earlier in this post we talked about the potential international impacts of responsible sourcing (or a lack of it), but BES 6001 also looks at social impact on the local scale. Requirements for managing sustainable development cover employment and skills, and local community engagement.

Specifying construction products that are responsibly sourced therefore has a much wider range of positive impacts than might be first expected.

About Polyfoam XPS and BES 6001

Polyfoam XPS has BES 6001 certification for products in our Floorboard, Roofboard and Laminating Board range. Supporting that certification, we operate the following.

  • ISO 9001-certified quality management system.
  • ISO 14001-certified environmental management system.
  • ISO 45001-certified health and safety management system.
  • ISO 50001-certified energy management system

All of our certificates are available from the technical support page of our website. Alternatively, you can contact us about how our products can help achieve your project’s responsible sourcing goals.

GROUND FLOOR AND FLAT ROOF U-VALUES IN PART L 2021

Specifier looking at site basement

When compared to Part L 2013’s requirements, Part L 2021 features lower notional dwelling U-values and tighter limiting U-values.

The values changed for all thermal elements, including ground floors and flat roofs, which are the two main uses for extruded polystyrene insulation (XPS).

Effective from June 2022, Part L 2021 applies in England and is the focus of this blog post. However, changes to Part L in Wales and Section 6 in Scotland have also introduced improvements, with the similar aim of reducing the energy use and carbon emissions associated with the built environment.

What is the notional specification in Part L 2021?

<p”>The overall aim of Part L 2021 is to deliver a 31% reduction in carbon emissions from dwellings, and a 27% reduction in carbon emissions from buildings other than dwellings, compared to Part L 2013. It is an uplift towards the Future Homes and Future Buildings Standards, both expected in 2025.

As has been the case since 2006, dwellings are assessed against a notional dwelling, which has a specification as defined in Approved Document L1. A range of factors are taken into account alongside building fabric performance, including airtightness, ventilation, heating, hot water and renewable technology. The table below summarises the U-values only.

Notional specification U-values (W/m²K) for main thermal elements of dwellings
Thermal elementPart L 2013Part L 2021
Ground floor0.130.13
External wall0.180.18
Flat roof0.130.11
Pitched roof0.130.11

While it might appear that relatively little has changed in terms of thermal element U-values, it’s worth bearing in mind that the actual U-values that a project will need to achieve depends on the specification of all those other factors we described above.

Ground floors often represent an excellent opportunity to include more insulation without significantly impacting the structural design of the building, so balanced and economical Part L 2021 specifications may feature ground floor U-values lower than 0.13 W/m²K, with roofs likely to need to be at or around the 0.11 W/m²K in the notional specification.

For buildings other than dwellings, whole building assessments are still required. However, the notional building specification does not feature in Part L 2021. Instead, Approved Document L2 lists limiting values that thermal elements must achieve.

U-values (W/m²K) for main thermal elements in buildings other than dwellings
Thermal elementPart L 2013 notional specificationPart L 2021 limiting values
Ground floor0.220.18
External wall0.260.26
Flat roof0.180.18
Pitched roof0.180.16

What are limiting U-values in Part L 2021?

The notional dwelling/building approach offers designers and specifiers flexibility in achieving a compliant specification. However, to ensure that building fabric performance is not compromised as part of that flexibility, the regulations include limiting or ‘worst-case’ U-values that thermal elements cannot exceed.

Limiting U-values (W/m²K) for main thermal elements of dwellings

Thermal element

Part L 2013

Part L 2021

Ground floor

0.25

0.18

External wall

0.30

0.26

Flat roof

0.20

0.16

Pitched roof

0.20

0.16

As we have seen, in Part L 2021 the limiting values for buildings other than dwellings now serve as the main guide to thermal element performance, as well as being the worst-case values that should be achieved.

Limiting U-values (W/m²K) for main thermal elements of buildings other than dwellings

Thermal element

Part L 2013

Part L 2021

Ground floor

0.25

0.18

External wall

0.35

0.26

Flat roof

0.25

0.18

Pitched roof

0.25

0.16

As part of our extensive technical services, Polyfoam XPS provides U-value calculations for any thermal element featuring our insulation products, including ground floors and flat roofs. Contact us with details of your build-ups, or to discuss how extruded polystyrene can help you to achieve your project U-values.

POLYFOAM XPS LAUNCHES NEW CAD DETAILS TO SHOWCASE BEST PRACTICE

New CAD details from Polyfoam XPS will help designers, specifiers and installers to get the best results from extruded polystyrene (XPS) insulation.

The details, which are available in both PDF and DWG format, reflect current best practice in traditional XPS applications like ground floors, basements, and inverted flat roofs. They also include a new application, showcasing how XPS can be used in masonry cavity walls below DPC.

“We’ve put a lot of work into ensuring these new details really help people get the best performance in common details,” said Rob Firman, Technical and Specification Manager at Polyfoam XPS.

“New energy efficiency requirements in the Building Regulations continue to emphasise the importance of good junction detailing and minimising thermal bridging,” Rob continued. “Accredited Construction Details have been discontinued so it’s important that, as an insulation manufacturer, we give people the tools to help them deliver high performance buildings.”

Traditionally, XPS insulation is associated with ground floor and inverted flat roof constructions. But its moisture resistant properties also make it ideally suited to being used at the base of external walls, below DPC level.

Rob Firman added: “We recently published a white paper outlining how XPS insulation can be used to prevent issues with moisture below DPC in external wall build-ups. Making these new details available enhances the support we’re offering to people looking to get this crucial area of the building right.”

Visit the technical support section of the Polyfoam XPS website to download the new CAD details. For support with using extruded polystyrene insulation in a ground floor, basement, flat roof, or below DPC in a cavity wall, contact us.

WHAT CREDITS CAN BE CLAIMED FOR EPDS IN CERTIFICATION SCHEMES?

Certification schemes such as BREEAM and LEED make credits available if the specified construction products have environmental product declarations (EPDs).

The number of credits varies depending on the type of EPD and whether the EPD has been externally verified. The uses of EPDs also extend beyond certification schemes.

EPDs report a variety of environmental impacts, including global warming potential (GWP) and ozone depletion potential (ODP). Anyone who has specified thermal insulation will know these terms and be familiar with seeing them on product literature, and EPDs formalise the declaration of these impacts and allow them to be compared between different manufacturers and products.

How are EPDs treated in BREEAM and LEED?

Since the launch of its updated new construction standard in 2018, BREEAM has shifted focus away from ratings given by the Green Guide to Specification. Instead, credits are awarded for the availability of EPDs. As older versions of the standard fall out of use, the Green Guide ratings will eventually become completely redundant.

BREEAM requires EPDs to be verified by a third-party. For the Mat 02 category, it awards points based on whether EPDs are generic (0.5 points), manufacturer-specific (0.75 points) or product-specific (1.5 points). However, if an EPD is not externally verified to EN 15804 then it cannot contribute to claiming points.

Like BREEAM, the LEED certification scheme recognises the importance of externally verified EPDs, and then places different values on different EPD types. It awards 0.25 points for generic EPDs, up to a full point for product-specific EPDs.

Wider adoption of EPDs beyond certification schemes

The value of externally verified EPDs, and the accuracy and specificity of product-specific EPDs in particular, is increasingly being seen in procurement and specification. Regardless of whether a voluntary certification is being sought, EPDs are being requested to support carbon emissions reductions and net zero carbon targets.

Declarations of GWP are starting to become a requirement of centrally-funded government projects, which is driving further interest in EPDs. Like other mandates that have come before, such as BIM, once these things become the norm on public projects, a trickle-down effect tends to occur as different parties get used to asking for, seeing and sharing the information.

There can be no doubt that EPDs are going to become an ever-present part of construction product specification. Because they report the environmental impact of a product’s life cycle, including the raw material sourcing and manufacture, they also contribute to evidence of responsible sourcing, alongside other certification like BES 6001.

Polyfoam XPS provides externally verified EPDs, produced by BRE Global, that report on the environmental impact of its Standard and Extra range floor and roof insulation products. Download them from our technical support page, along with our BES 6001 responsible sourcing certification.

Stay up to date with all our new blog posts as they’re published by subscribing to our email newsletter, The Build-Up. You can also contact us to discuss how our EPDs can help with your current project.

WHAT DOES PART L 2021 SAY ABOUT CONTINUITY OF INSULATION AT GROUND FLOOR EDGES?

Hard hat on plans

Ground floor edges, at the abutment with external walls, are one of the most common areas for breaks in continuity of a building’s thermal envelope to occur.

However, as we seek to further reduce carbon emissions from buildings, and as our understanding of moisture behaviour improves, regulations like Part L 2021 are starting to provide more specific guidance on addressing them.

Floor edges are difficult to detail because the loadbearing external wall has to continue to foundation level. This usually prevents a connection between the ground floor insulation and the chosen external wall insulation, unless very specific, high-performance solutions are selected.

How does Part L 2021 address continuity of insulation?

Solutions to lengthen the path of the thermal bridge at the floor edge have been established for some time, because eliminating the thermal bridge entirely is difficult in typical construction methods – but it’s important to be aware of how that detailing needs to adapt.

Compared to Part L 2013, Part L 2021 sets out more detailed technical guidance on how continuity of insulation must be addressed.

Approved Document L1A 2013 said that the building fabric should be constructed “to a reasonable standard so that the insulation is reasonably continuous over the whole building envelope.”

Approved Document L1 2021 – which, depending when you are reading this, takes or took effect in June 2022 – maintains the “reasonably continuous” wording, but goes on to provide specific guidance in a number of new areas.

Designers, specifiers and installers need to be aware that the insulation layer should be identified on drawings, and that a review of drawings should take place to ensure the layer is “continuous, buildable and robust”. An on-site audit should then take place, before work is concealed, to assess that the designed details have been constructed.

What new floor and foundation requirements are in Part L 2021?

These requirements are not specific to ground floor edges – but there are also a couple of new requirements relating to floors and foundations. These aim to address the thermal bridge at the floor edge, and to further lengthen it, over and above existing measures.

The first point specifies that the perimeter upstand insulation be a minimum thickness of 25mm. Judging by the details we see, this is already commonplace in detailing. Nevertheless, it remained possible that people were specifying and/or installing less than 25mm, or even none at all.

The second point says, “moisture-resistant insulation should be fitted below damp-proof course level and extend to the foundation block/structure.”

How extruded polystyrene (XPS) insulation can help to meet Part L 2021’s requirements

Even before publication of Approved Documents L1 and L2 in December 2021, at Polyfoam XPS we were seeing increased demand for insulation solutions in the base of masonry cavity walls.

Specifiers wanted a moisture-resistant insulation that could be used to improve thermal performance below the DPC, without any risk that the thermal performance at the junction would be compromised if the insulation came into contact with moisture. Compared to other lightweight, rigid foam insulation boards, XPS has the lowest moisture uptake.

Unlike expanded polystyrene (EPS) and polyisocyanurate (PIR) products, for example, XPS can be used as the ground floor insulation layer below the damp proof membrane (DPM). Installing it in the base of cavity walls to reduce thermal bridging is therefore a natural extension of the product’s capabilities.

To support the increased use of XPS in wall constructions below DPC, Polyfoam XPS has published a white paper detailing how extruded polystyrene can be specified and installed, and providing psi values for common ground/floor external wall details to demonstrate the thermal benefits that can be achieved.

Read more about, and download, our below DPC white paper. Alternatively, contact us to discuss how XPS can help your project to achieve better floor edge detailing.

SPECIFIER NEWSLETTER – ISSUE 14 – MARCH 2022

We continue our look at environmental product declarations in 2022 by focusing this month on the similarities and differences between EPDs from different manufacturers.

To make sustainable product choices for construction projects, it’s necessary to compare environmental product declarations – but you need to be sure you’re comparing like for like.

Elsewhere, you can download our new white paper about below DPC insulation for external walls.

As always, if you’ve got any questions about anything that we cover in the newsletter, contact us and we’ll address them in future issues.

A picture containing outdoor, grass, sky, tree

Focus on comparing EPDs

Comparing EPDs offered by different manufacturers

On its own, an EPD is not a statement of whether a product is ‘sustainable’ or not. It is a tool for reporting the environmental impact of that product over its life cycle. Only through comparing the EPDs of different materials and products can an assessment can be made as to whether one will help to meet a project’s sustainability goals better than another.

When different manufacturers make EPDs available for their products, it’s important to look for similarities and differences in the reporting so that an appropriate assessment can be made.

Generic EPDs and manufacturer-specific EPDs

A first step in understanding commonality between EPDs is having an awareness of whether a product is being represented by a generic EPD or a manufacturer-specific EPD.

Generic EPDs use average data for similar products produced by a range of manufacturers, and report environmental impact accordingly. They might be offered by a trade association who has gathered data from its member companies, for example.

You could, therefore, find yourself requesting an EPD from two different manufacturers, and being provided with identical documents.

A generic EPD could be broadly representative of the environmental impact your product specification will have. However, there will always be a question as to how accurate it is, especially if a project is unique in a way that is unlikely to have been captured by ‘average’ data. More preferable is to obtain a manufacturer-specific EPD or, even better, a product-specific EPD.

A manufacturer-specific EPD can apply to more than one product (within a specific category of products) produced by a single manufacturer. A product-specific EPD applies to a single product from a single manufacturer.

In seeking to be transparent about the environmental impact of construction projects, the more specific the data the better.

What is functional equivalence in an EPD?

The environmental impact of a construction product is reported for a ‘unit size’ of that product. The EPDs that Polyfoam XPS makes available, for example, are based on one cubic metre of our extruded polystyrene. In the EPD document, this unit size is called the ‘functional equivalence’.

When comparing two EPDs from different sources, it’s important to check whether the functional equivalence is the same or different. For example, while our EPD uses one cubic metre as the basis for its reporting, there are EPDs for other types of lightweight rigid foam insulation that use one square metre of a specific product thickness.

One square metre of a 100mm thick insulation board has one-tenth the volume of one cubic metre of another insulation type. Even if the two insulation products had a broadly similar environmental impact, there would be a substantial difference in the figures reported by the EPD.

Manufacturers select the unit based on their production processes, so none of this is to say that one way is correct and another is wrong. It is simply something that specifiers have to check for when taking the EPD reporting at face value.

Continue reading

polyfoam xps floor

Polyfoam resource

We get a lot of questions about insulating cavity walls below DPC level. Designers want to reduce thermal bridging heat losses at the floor/wall junction, but are concerned about the potential for insulation products to take up moisture.

To address these questions and concerns, we’ve published a white paper that looks at detailing below DPC insulation in detail. Read more and download the DPC insulation white paper.

What’s caught our eye this month

Updates to Approved Documents – architecturaltechnology.com

The Government has recently published updates to the following Approved Documents, due to a previous publishing error. These take effect on 15 June 2022………Continue reading

VAT abolished on energy-saving materials and solar panels – architectsjournal.co.uk

Chancellor Rishi Sunak has announced he is abolishing VAT on energy-saving materials, heat pumps and solar panels to help homeowners keep energy costs down……….Continue reading

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COMPARING EPDS OFFERED BY DIFFERENT MANUFACTURERS

green buildings illustration

To make sustainable product choices for construction projects, it’s necessary to compare environmental product declarations (EPDs). When different manufacturers make EPDs available for their products, it’s important to look for similarities and differences in the reporting so that appropriate judgements can be made.

On its own, an EPD is not a statement of whether a product is ‘sustainable’ or not. It is a tool for reporting the environmental impact of that product over its life cycle. Only through comparing the EPDs of different materials and products can an assessment can be made as to whether one will help to meet the project’s sustainability goals better than another.

Generic EPDs and manufacturer-specific EPDs

A first step in understanding commonality between EPDs is having an awareness of whether a product is being represented by a generic EPD or a manufacturer-specific EPD.

Generic EPDs use average data for similar products produced by a range of manufacturers, and report environmental impact accordingly. They might be offered by a trade association who has gathered data from its member companies, for example.

You could, therefore, find yourself requesting an EPD from two different manufacturers, and being provided with identical documents.

A generic EPD could be broadly representative of the environmental impact your product specification will have. However, there will always be a question as to how accurate it is, especially if a project is unique in a way that is unlikely to have been captured by ‘average’ data. More preferable is to obtain a manufacturer-specific EPD or, even better, a product-specific EPD.

A manufacturer-specific EPD can apply to more than one product (within a specific category of products) produced by a single manufacturer. A product-specific EPD applies to a single product from a single manufacturer.

In seeking to be transparent about the environmental impact of construction projects, the more specific the data the better.

What is functional equivalence in an EPD?

The environmental impact of a construction product is reported for a ‘unit size’ of that product. The EPDs that Polyfoam XPS makes available, for example, are based on one cubic metre of our extruded polystyrene. In the EPD document, this unit size is called the ‘functional equivalence’.

When comparing two EPDs from different sources, it’s important to check whether the functional equivalence is the same or different. For example, while our EPD uses one cubic metre as the basis for its reporting, there are EPDs for other types of lightweight rigid foam insulation that use one square metre of a specific product thickness.

One square metre of a 100mm thick insulation board has one-tenth the volume of one cubic metre of another insulation type. Even if the two insulation products had a broadly similar environmental impact, there would be a substantial difference in the figures reported by the EPD.

Manufacturers select the unit based on their production processes, so none of this is to say that one way is correct and another is wrong. It is simply something that specifiers have to check for when taking the EPD reporting at face value.

The scope of life cycle reporting in EPDs

In a previous blog post we described how EPDs report environmental impact across a series of stages and modules. The structure of these stages is designed to reflect the distinct stages of how a construction product is manufactured, delivered to site, used on site, and dealt with at the end of the building’s useful life.

‘Cradle to gate’ refers to the processes involved with manufacturing a product and it leaving the factory, and is covered by modules A1 to A3 of life cycle assessment. ‘Cradle to practical completion’ also deals with the installation of the product on site, being covered by modules A1 to A5.

‘Cradle to grave’ spans the complete life cycle of a product, including its use and what happens to it at the end of life – covered by modules A, B and C of life cycle assessment. Module D can also be factored in.

EPDs can be either a ‘cradle to gate’ type or a ‘cradle to grave’ type. There is a third option of ‘cradle to gate with options’, which means that relevant parts of module C can be accounted for alongside modules A1 to A3. This is useful for a product like insulation, which incurs minimal use stage (module B) impacts, but where the manufacturer wants to report a fuller scope than just ‘cradle to gate’.

Again, this is not necessarily a judgement on what is the ‘correct’ way to report impacts. As EPDs continue to mature then it will be desirable that reporting is done consistently across all modules to give the fullest possible picture of environmental impact.

For the purposes of this blog post, the important message is that the scope of reporting for similar types of products might be different. That difference should be taken into account when making an assessment of the reported impacts.

Polyfoam XPS has ‘cradle to gate with options’ EPDs that report on the environmental impact of its flooring and roofing products. The EPDs are independently verified and produced by BRE Global. For more information about EPDs, subscribe to our newsletter, The Build-Up. You can also download our EPDs from our technical support page, or contact us to discuss your current project.

HOW DO POINT THERMAL TRANSMITTANCES MEASURE HEAT LOSS FROM FLAT ROOFS?

How does rainfall affect inverted roof U-value calculations?

A point thermal transmittance measures heat loss through an isolated thermal bridge, and is expressed as a chi value. A point thermal bridge can occur anywhere in the building fabric, but for this blog post we’ll refer to common examples found in flat roof constructions.

A chi value is a counterpart to a psi value, which measures linear thermal bridging heat losses. Like psi values, chi values can only be calculated using numerical modelling (or 3D modelling) techniques. It is not possible to calculate a chi value using software designed to calculate U-values by the combined method.

What are examples of point thermal bridges in flat roofing?

A common point thermal bridge we’re asked about is when a concrete plinth forms part of the structural roof deck. The plinth, or a series of plinths, is usually required to support rooftop plant whose weight is too much for the roof’s thermal insulation layer to bear.

The plinth(s) must obviously extend high enough above the roof’s finished surface to provide a sufficient upstand, and they therefore cause a break in the continuity of the insulation layer.

When a situation like this occurs in a roof design, our technical helpdesk is often asked if one of the following two solutions can be adopted.

  • Carry out a U-value calculation for the flat roof as normal, but include a bridging percentage in the insulation that equates to the area of the roof covered by the plinth(s).
  • Calculate the U-value for the insulated roof as normal, and a separate U-value for the roof build-up through the plinth. The idea is to then do an area-weighted U-value calculation to adjust the performance of the roof as a whole.

What are the issues with addressing point thermal bridges like this?

The first option effectively treats the concrete as a repeating thermal bridge, even though it doesn’t repeat regularly. (Typical repeating thermal bridges in building fabric include mortar joints in masonry, or timber joists in a roof.)

Not only is it an incorrect application of the combined method, the calculation method is also likely to fail because of a discrepancy between the upper and lower limit values used to work out the U-value.

The second option is problematic because the combined method is designed to calculate heat loss through whole construction elements, and not small areas of unique construction. It might provide a representative heat loss for a small proportion of the (already relatively small) plinth detail, but it doesn’t account for the interaction between the edge of the plinth and the insulated roof build-up.

This interaction needs to be thermally modelled, which is why an appropriately qualified person should be engaged to produce chi value calculations.

What are the advantages of calculating point thermal bridging chi values?

The primary benefit of obtaining numerically modelled chi values is that it provides accurate point thermal transmittance values, in contrast to the two options discussed above.

A follow-on benefit is that, like with linear thermal bridging, numerical modelling also produces surface temperature factors for the detail in question. That means it’s possible to assess any condensation risk at the points of increased heat loss.

The U-value of a flat roof shouldn’t exceed 0.35 W/m2K at any point, in order to avoid the risk of surface condensation inside the building. Sometimes, despite the best design intentions, it’s impossible to avoid a scenario where that threshold isn’t met across the entire roof – for example, if structural requirements, like the plinths we’ve been talking about, need to take precedence.

In such unavoidable situations, the point thermal bridge, and the number of them, should be minimised as far as possible.

Because point thermal bridges are undesirable, it’s even more important to get accurate thermal modelling carried out in order to help assess the heat loss, and any potential consequences of it, at the detail in question.

The U-value of the roof as a whole, meanwhile, can be adjusted by multiplying the chi value by the number of point thermal bridges per square metre, and adding the result to the U-value.

For U-values calculated in accordance with the combined method, and advice on dealing with tricky detailing issues, contact us to discuss your current project.

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