SAP and the Importance of U-value Calculations - TIMSA


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About Timsa

Timsa is the thermal insulation manufacturers and suppliers Association. It represents the most well-known insulation manufacturers and distributors in the UK producing most common types of insulation, including:

  • Mineral wool,
  • Rigid polyurethane foam,
  • Expanded polystyrene,
  • Extruded polystyrene,
  • Phenolic foam,
  • Foam glass,
  • Multi foils,
  • Bubble foils.


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Learning Aims

The main purpose of this CPD is to highlight the fundamental role that calculated U-values play in the building regulations concerning energy efficiency for all types of building work and how they are used in the compliance process. It will show that there are currently high levels of inaccuracy in the calculated U-value used in some building regulations admissions and demonstrate the adverse impact of this on both the compliance of the building works and the potential energy efficiency of the final building. It will consider these issues in terms of the UK government's goals for reduction in emissions of CO2 and show how inaccurate calculation of U-values can undermine the benefit that improvements to the energy efficiency of the built environment, are expected to play.


The calculation of U-values is not straight forward making it easy for non-regular practitioners to make mistakes. This CPD will also introduce a new initiative that provides designers, builders and building control officers with assurance that the U-values they use are accurate. This is a scheme which verifies competency in calculating U-values and condensation risk and has been developed by the British Board of Agrément, BBA and Timsa. This CPD is aimed at a range of building professionals.


Real Errors in U-value Calculation

So is there a real need for U-value competency scheme.


U-value Errors

According to a recent study by the energy efficiency partnership for homes and the UK government, which investigated levels of compliance with part L of the building regulations, there is.


The main part of the study concentrated on a sample of SAP 2005 assessments completed by accredited SAP assessors for housing developers. In 68% of the data sets examined errors occurred in interpreting the sad conventions for a number of SAP inputs including incorrect incomplete or contradictory design information being given to the SAP assessor, most commonly associated with U-values. There were further problems due to the incorrect treatment of measurement conventions particularly for flats above ground floor level and heat loss via unheated spaces or room in roof constructions.


Calculation Software

Why are so many inaccurate U-values being included in SAP assessments? The answer of course is not simple, but a significant factor will be the process for the production of U-value calculations. Most U-value calculations are completed on commercially produced software. There are a number of such applications available and each takes a different approach to both the input of data and the range of product data included in the database. There is no regulation of either the accuracy of output or the quality and completeness of product information, included in the software database. All software can be purchased off the shelf by any individual who can then proceed to produce U-value calculations without undertaking any form of training or assessment of competence. In effect there is no regulation of calculation software, no initial assessment of the competence of people wishing to produce U-value calculations, and certainly no audit to confirm ongoing accuracy. The consequence of this is there is no formalised quality control in the production of U-value calculations.


U-value and SAP Calculation in Context

Accurate U-values are important because they underpin the energy performance of buildings and improving energy performance is a major part of the government's strategy to reduce greenhouse gases.


The current UK target is to reduce CO2 emissions by at least 80% below 1990 levels of CO2 by 2050.


The following slides explain the approach in England and Wales to the reduction of CO2 emissions from buildings, similar proposals exist for Scotland and Northern Ireland.


U-values and the Road to Zero Carbon

In December 2006 the government launched its building a greener future consultation proposing a trajectory for all new homes to be zero carbon (Zero CO2 emissions) by 2016 the subsequent policy statement issued in July 2007 confirmed this.


Further in the 2008 budget the government announced its ambitions that all new nondomestic buildings should be zero carbon from 2019. The 2006 revision of the England and Wales building regulations, required that all new buildings both dwellings and non-dwellings, show compliance by meeting a maximum CO2 emissions target. For dwellings this is calculated in SAP and for non-dwellings in SBEM.


These are whole building calculation tools that model the energy required to provide space and water, heating, lighting and ventilation for the building’s standard usage. From the energy demand it is possible to calculate the CO2 emissions. In both SAP and SBEM the thermal efficiency of the building fabric is a major factor in the space heating demand, and this in turn is influenced by the U-values of the various building elements.


The government have set out the trajectory for reduction of CO2 emissions targets from new build dwellings in building regulations towards 2016. In summary it is proposed to achieve a Zero carbon goal in three steps. The first step is already been taken namely the 2010 revision of part L which requires a 25% improvement in the energy carbon performance or target emission rate TR over the 2006 requirements. The second step will be in 2013 to a 44% improvement. And Thirdly in 2016 to zero carbon. These steps mirror the mandatory improvements indwelling emission rates or der performer name for the actual COscore, required for code for sustainable homes level 3, 4and 6. Although the eventual building regulations definition of zero carbon is likely to be different. No similar trajectory has yet been defined for reduction in emissions from non-dwellings and for work on existing buildings.


The approach is still largely based on reductions to building fabric U-values. Clearly in all cases, there is a fundamental requirement for accurate U-values.


Calculation of CO2 Emissions

SAP is the government standard assessment procedure for energy rating of dwellings. The current version SAP 2009 which is used to produce the DER determines whether a building shows compliance with the building regulations. This principle applies to part L in England and Wales section 6 in Scotland and part F in Northern Ireland.


The DER is based on the energy usage associated with space heating, water heating, ventilation and lighting.


Their savings from energy generation technologies incorporated in the dwelling and is related to the floor area of the building. The energy performance difficult for the dwelling is also produced in the SAP calculation.


Government recognise that the EPC needs to be as accurate as possible to provide a solid foundation for the delivery of the expected savings from new buildings dwellings, and to ensure this every EPC must be produced by an accredited construction domestic energy assessor. The government authorises accreditation schemes which must assess the qualification and examine each prospective candidate. In addition, each scheme will carry out audits on a sample of the SAP calculations and EPCs produced to ensure ongoing quality. Clearly the intention is for a quality control mechanism to apply to SAP calculations. A similar process is in place for non-domestic buildings.


Accuracy of SAP Calculations

As mentioned previously the study by the energy efficiency partnership for homes found that 68% of the data input into SAP calculations had errors inaccurate U-values being the most common. SAP is Complex model of building performance but clearly the accuracy of the output is limited by the quality of the data used.


One of the major factors in the SAP calculation is energy usage for space heating and as restrictions on emissions tighten this is an area where designers can make major savings, by improving the thermal efficiency of the building fabric.


However, if the U-value calculations that act as the basis for these design decisions are inaccurate, the result could be a significant shortfall between the predicted and the actual performance of the building.


There are obvious implications for building regulation compliance, if the error in calculation is picked up in the building control process. However, more worryingly it could result in occupants being unable to heat their homes to a comfortable level. The space heating demand predicted by SAP will reduce as the industry moves towards zero carbon dwellings and consequently any error in U-value calculation will increase the risk of this happening.


So is U-value Calculation Complex?

Is U-value calculation really complex?


Complexity Explained

For most constructions it would not be described as truly complex, but it is detailed and involved. A thorough understanding of the detail is required to accurately complete U-value calculation. At its most basic level of person carrying out a U-value calculation must be able to: select the correct calculation method for the construction proposed; incorporate the right product data; and accurately reflect the construction details, for example the size and frequency of thermal bridging, or the effect of air spaces in the construction.


With this degree of detail, the opportunity for mistakes, easily rises and in extremes presents the opportunity for abuse, whereby a detail can be altered or hidden to make a particular construction fit to a required result.


A benefit of undertaking an accurate U-value analysis is that much of the information required is also used in the condensation risk analysis. It is good practice to carry out such an analysis on all constructions, as it will identify the risk of harmful condensation occurring within the construction.


Similarly, to U-values the calculation of condensation risk also requires the input of the detailed information and there is the potential for serious consequences if errors in this analysis lead to the formation of harmful condensation within the structure.


U-value calculation is at its simplest for a building element which consists solely of flat plane, parallel uniform layers. The heat flow through such an element is directly from inside to outside in a straight line. however virtually all practical construction elements have non uniformities, which severely complicate the heat flow mechanisms.


This needs to be allowed for in the determination of the U-value and can be accommodated by the use of complex computer modelling, that analyses heat flow in 2 and 3 dimensions by various methods.


Case Studies


3 Storey Town House

To illustrate the importance of accurate U-value assessments and calculations, we will look at the example of a three storey town house, either semi-detached and terrace or terrace and the impacts that applying incorrect U-values will have.


This house is modelled in SAP 2009 and has a DER of 13.78 kilograms of CO2 per metre squared as a mid-terraced version. And 15.21kg CO2 per metre squared as a semi-detached and terrace. For this dwelling we will illustrate three ways in which a U-value assessment and calculation can go wrong.

  1. Using the wrong factors in a ground floor U-value calculation. A ground floor U-value calculation requires more factors to be considered, than is the case for the wall roof or above ground floor, due to the interaction of the floor with the insulating effect of the ground.
  2. Examine what can go wrong in a U-value calculation of a wall when wrong assumptions are made about the construction of the wall.
  3. Examine the impact of using the wrong product data in a U-value calculation for a roof.


The basic fabric input data used in the SAP analysis is shown on this slide.


Calculating U-values for Ground Floors

Unlike building elements above ground, heat transfer through floors varies over the area of the floor, being greatest at the edge of the floor and least in the middle.


To enable the calculation to model this effect on a solid based ground floor, the ratio of exposed or heat loss perimeter to ground floor area has to be determined. Further information needs to be obtained regarding the thermal conductivity of the ground and the depth, thickness and thermal resistance of the foundation walls.


If the floor is suspended then it is necessary to include the height of the floor above ground, the characteristic wind speed, the area of ventilated openings and the wind shielding factor. All these items have a significant impact on the calculated U-value and as such must be included.


A very simple and common mistake here could be assigning the PA ratio for the mid terrace dwelling to an end terrace dwelling, in the example dwelling and mid terrace has a PA ratio of 0.29 and the end terrace 0.45. If a solid floor construction including an insulation layer within R value of 2.60 was included in the example dwelling, applying the wrong PA ratio would result in the U-value 0.23 being claimed, when an accurate calculation show the value to be 0.26.


If this U-value were carried forward to the SAP modelling, a false DER would be calculated, which was actually 1.2% lower than what was actually achieved.


This is clearly an error it may not be critical if there was a factor of safety in the DER. However, the correctly calculated U-value the ground floor is higher than the maximum allowable back stop U-value for flaws in part L and thus this building is definitely not in compliance.


Other errors can occur and even if the PA ratio is correct and the construction remains the same, using incorrect data for other critical properties such as the thermal conductivity of the ground, all the foundation wall depth and thermal resistance, can give a range of U-values from 0.20 to 0.29 for the end terraced version of the example dwelling.


Calculating U-values for Walls

Calculating U-values for walls may be somewhat simpler than for ground floors because there is obviously no need to include the impact of the ground in the U-value. However, the constructions themselves are often more complex.


Taking timber framed walls as an example BR 443 gives a default value for timber fraction the proportion of the total area of the panel occupied by timber, 15%. There are options to reduce this fraction to 12.5% if certain precise criteria are met. An inexperienced person might work on the assumption that the studs are 38mm thick added 600mm centres, so the timber fraction is 38600 or 6.3%. This error alone can make a difference of 12.5% to the calculated U-value of the wall. In the case of a 140mm timber frame wall with an insulation material of total thermal resistance a 4.35 between the studs, the U-values range from 0. 27, for 15% timber fraction 0.26 for 12.5%, 0.24 for 6. 3%. Using the lowest U-value of 0.24 in the end terrace version of the example building results in a 1.7% apparent decrease in the DER.


The effect of errors in the timber fraction is however very small in comparison with the impact of wrongly allowing for metal frames or large fixings penetrating the insulation layer. At its most extreme where large metal items such as helping hand brackets in a rain screen cladding system, penetrate the insulation layer, then BR443 gives a choice of either carrying out two or three dimensional numerical modelling or applying a correction of plus 0.30 to any U-value calculated using the simplified method.


This is such an extreme correction that numerical modelling is the only viable option but failure to recognise this and ignorance of the conventions, would lead to very substantial errors in the U-value.


Calculating U-values for Roofs

Over the last few years there has been one area of particular controversy, namely the thermal performance claimed for some thin multiple layer reflective products or multi foils as they are more commonly known. Currently there are no harmonised European norms for these products although there is a CU AP common understanding of assessment procedure, and part L refers to BR443, which gives guidelines for testing the thermal performance of these products using international standards organisation, ISO test methods.


Some multi foil manufacturers have followed the BR443 guidelines, tested their products and gained BBA certificates. Other manufacturers have used non ISO methods to test their products. The ISO and non ISO test methods give widely divergent values for the thermal performance of products when all intents and purposes, the physical characteristics of the product themselves are very similar. To examine the effect of these different values on thermal performance of the example dwelling, it is first necessary to establish the range of claims.


When measured according to BR443, a 30mm think multi foil installed in a sloping roof with 225mm unventilated air layers either side as an R value of approximately 1.7 to 1.9. Values in this range are measured by thermal testing experts, such as the national physical laboratory and the BBA and are also reflected in the result obtained from tests carried out according to the CUAP. The alternative non ISO test methods applied to a similar thickness product, are used to justify claimed R values in the range of 5.0 to 6.0. Claiming this level of performance would appear to be in conflict with the laws of physics. If the multi foil was the only insulation included using an R value of 5.0 results in a roof U-value approximately 0.20 and an R value of 1.9 in 0.46. This is a 230% difference and obviously very significant. When applied to our example dwelling increasing the DER by 8.5% would result in the emission of an extra 164 kilograms of CO2 per annuum for lifetime of the building or if you prefer one unnecessary car journey per year from Land's End to Aberdeen. Multi foil insulation with correct thermal claims and backed up with additional insulation satisfies the building regulations and is a perfectly valid way to insulate a building. Using a multi foil insulation alone based on the higher claimed R values is likely to result in significantly higher energy usage and carbon emissions than predicted.


Case Studies Conclusion

So, in summary we have seen how misapplying details in the construction such as timber bridging or the conditions for a ground floor, while still using the correct U-value calculation method, can make significant differences to the output of the SAP calculation and a greater difference to an individual U-value. These errors could lead to a building actually being non-compliant with building regulations when it appears to be compliant.


These small errors occurred when the correct U-value calculation method was used with correct product data and input in the calculation software. Only the details of the construction were in error. If the wrong calculation method or wrong product data is used and the errors can become far more significant.


The goal of zero carbon buildings has been established by government with a deadline of 2016 for dwellings and 2019 for other buildings. This will naturally lead to a drive for lower U-values to reduce space heating demand. Ensuring that the calculations which support these U-values are accurate is a fundamental step towards this goal.


U-value Accreditation Scheme

Fortunately, there is now way the accuracy of U-value can be assured Timsa with the BBA have created a scheme for assessing the competency of companies or organisations to complete U-value calculations in line with all the standards and conventions. Membership of the scheme is available to any company and is administered by the BBA. Each company that is a member of the scheme must nominate at least one suitable person who is thoroughly assessed as being competent on a nominated piece or pieces of software for all types of calculation.


This has the benefit of assessing the competence of the individual to obtain the correct answers with each piece of software used and is effectively testing the software as well. A competent person within the company scheme then assesses the ability of other individuals in their company and can also authorise them to carry out U-value calculations. Every company member of the scheme has to create an archive for all the U-value calculations conducted and also set up a sampling procedure to obtain calculations for checking. This creates an audit trail and ensures quality assurance. The scheme requires the internal auditing by the competent person of sample calculations on a regular and frequent basis and Secondly requires annual external auditing by the BBA of the administration of the company scheme. All calculations issued under the scheme will carry the logo of the BBA Timsa scheme for calculation competency. This will be a guarantee that the calculation has been produced by an individual who is competent to do so, on software that can provide accurate results. Any calculation issued under the scheme will be supported by full documentation.



So, in summary we have seen how complex accurate U-value calculation can be. We've seen how it is crucial in calculating the reduction in greenhouse gases and CO2 emissions. We've seen how important it is in meeting the changing requirements in building regulations. And we've seen how the BBA Timsa scheme for calculation competency provides an accurate and regulated standard for U-value assessment.

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