Comparison of PHPP with SAP 2012 (UK)

Introduction

With ‘nearly zero energy buildings’ (NZEB) due to become a requirement of forthcoming UK Building Regulations[1], there is a larger drive than ever before to find ways of delivering enhanced energy efficiency in new construction. Although there will inevitably be a number of potential ways of meeting the targets, one proposed route is via the adoption of the Passivhaus Standard, which utilises a strong fabric energy efficiency approach to ensure that only relatively small-scale renewable energy sources (RES) are then needed to deliver the remaining energy demand. This approach is being promoted by the EU-funded ‘PassREg’ project across Europe.

Although the Passivhaus standard was developed in Germany, it has been successfully applied on a wide variety of different building types all over the world, including in the UK. In order to achieve certification, it is necessary to model the building’s intended energy performance using the Passivhaus Planning Package (PHPP) design and verification tool. Quality control procedures are then put in place to ensure that the designed performance is realised in practice.

It is often noted by professionals carrying out the Passivhaus design process that it is an unfortunate burden to have to carry out two modelling calculation processes – one using PHPP to verify performance for Passivhaus certification and another using the UK’s National Calculation Methodology (NCM) for Regulatory compliance (the Standard Assessment Procedure, SAP, for domestic buildings and the Simplified Building Energy Model, SBEM, for non-domestic buildings). In other parts of Europe where the relevant NCM calculations and assumptions are more aligned with PHPP, additional functionality has been added to PHPP to produce the necessary EPC output for Regulations (e.g. in Germany and the Brussels region of Belgium).

Queries have been raised by various parties over the comparability of the two measurement methods with a view to identifying whether a standardised approach was feasible. However, it should be remembered that the intention of the SAP and SBEM tools is different to that of PHPP; with the NCM tools being purely to demonstrate compliance against Building Regulation standards and PHPP being more of a design tool to forecast building energy demand.

Previous comparisons of PHPP with SAP

A useful comparative study was carried out on SAP 2005[2] (version 9.81) by members of the Association for Environment Conscious Building (AECB) who were familiar with the workings of both PHPP and SAP . A range of metrics were identified between the two tools that used different assumptions and hence led to different energy use estimates for an equivalent building. With this being the case, it would obviously lead to confusion in the industry, since essentially both tools are based on the same building physics principles. For a number of years the Passivhaus Trust and other advocates have called for the Passivhaus standard (and hence PHPP) to be ‘deemed to comply’ with UK energy Regulations, but with such differences noted it is difficult for policy makers to accept such a request, hence a requirement for a separate SAP calculation to demonstrate compliance with the Regulations still persists.

2012 updates to SAP

Since the AECB study, SAP has been through two relatively major updates[3] – one in 2009 and another in 2012 (further details are given in the annexe). Several changes have been introduced that allow additional data entry refinement to be included in a SAP calculation rather than assuming conservative defaults. While these go some way to closing the apparent modelling gap between SAP and PHPP, the assumptions are still not necessarily the same in all cases. In particular, there are still some ‘utilisation factors’ applied in SAP to reflect the anticipated variability in the quality of construction/ installation in new housing in the UK, e.g. with the installation of MVHR systems. This will serve to increase the energy use forecast compared to PHPP. While Passivhaus designers and developers would argue that this may not be necessary in a Passivhaus due to the quality control procedures put in place during construction, it is reasonable to assume that mainstream ‘regulations-compliant’ construction may benefit from such moderation so energy use estimates are not over-ambitious. After all, for the purposes of Regulatory compliance, the aim is to ensure that a dwelling is no worse than the stated performance – if it is better in reality it will be no detriment to the building or its occupants.

One aspect particularly relevant when considering utilisation factors is for the installation of MVHR systems. Publications by the NHBC Foundation have indicated the likely increase in the use of MVHR systems in the UK, but simultaneously a range of problems associated with product specification and installation quality that may undermine their use[4]. It is therefore reasonable that calculations for Regulatory purposes do not over-reward the performance of such systems. There are calls from the NHBC Foundation and Zero Carbon Hub for the quality of MVHR installations to improve and to ensure appropriate design, installation and commissioning of systems[5]. It is likely that installation standards would need to be more aligned with the expectations of MVHR for Passivhaus certification.

The future of SAP versus PHPP

At present, a dwelling in the UK can achieve Passivhaus certification and achieve Regulatory compliance, although each will need to be calculated in the relevant respective tool. The concern in the short term is that in order to meet the upcoming NZEB requirements a Passivhaus may need to install an increased capacity of renewable energy sources (RES) to compensate for conservative estimates that are made against its quoted fabric performance, thus leading to increased costs.

Any changes to SAP are considered based on data and robust independent researched evidence available from the industry. It therefore seems likely that independent UK studies would be required to help affirm and refine the utilisation factors used in SAP to more closely represent the quality believed to be realised in the execution of a Passivhaus.

There may be instances where designers/ developers may be prepared to accept the utilisation factors and assumptions from SAP but find the cost of using multiple calculation tools a barrier. A further option may therefore be to consider the example set in Germany and other areas where additional functionality is added to the standard PHPP tool to provide an output compliant with the UK Regulations. This would require the assumptions used in the Regulations to be applied to data already entered into the PHPP tool so as to produce a ‘normalised’ output for compliance purposes. The remainder of the PHPP worksheets could still then apply for design purposes.

While this sounds like a relatively simple process, there are some fairly significant barriers to this which are procedural and quality related rather than simply limited by software programming. All tools used for Regulatory compliance need to be approved for use (by BRE on behalf of DECC for domestic buildings and DCLG for non-domestic buildings) and have their performance verified to ensure that they give results within acceptable tolerance to all other tools available on the market. It may also be necessary to demonstrate that the behind-the-scenes workings of the PHPP tool, which is essentially a complex spread sheet, could not be manipulated by users in a way that would misrepresent the final results to the compliance body.

A secondary issue, which goes beyond any differences inherent to the calculation methods, is the demonstration of competency of whoever carries out the calculations. In the UK, this has been addressed by the establishment of competent person schemes that sit alongside each of the approved software programmes currently used for compliance. This results in the ‘On Construction’ qualification that allows users to formally lodge ‘as build’ EPCs. If outputs from PHPP were to be considered for Regulatory compliance, an equivalent approved training and competency scheme would inevitably need to be founded to ensure the tool is used correctly and consistently in all cases.

Summary

It seems in the short term at least, there remain a number of metrics that differ between SAP and PHPP that lead to a building’s energy performance being represented differently in each case. For SAP, the utilisation factors applied to MVHR installations in particular could be reconsidered in the future if evidence suggests they are no longer needed due to improvements in installation quality. Meanwhile, the instigation of an appropriate competent person scheme for the validation of PHPP calculations and approval for the tool itself to reliably produce EPC outputs (assuming default values relevant to the UK Regulation assessment benchmarks could be included) would likely bring PHPP closer to being accepted as demonstrating compliance with Part L of the UK Building Regulations.

In reality, those that strive to achieve Passivhaus Certification on projects generally understand and value the high quality construction requirements that are intrinsic to its achievement. They will inevitably have been ‘sold’ on the fact the building will enjoy high levels of comfort and low running costs by following this stringent path, even if it is often more costly from a capital perspective to do so. An additional SAP assessment to demonstrate Regulatory compliance over and above the requirements for Passivhaus and the use of PHPP is unlikely to be an excessive cost burden, just a minor inconvenience.

The real problem arises when Passivhaus is being promoted as a solution for the future delivery of NZEB; if in compliance terms (i.e. using SAP) the energy use forecasts are more conservative, other routes of achieving NZEBs (for instance, potentially relying on a much larger contribution of ‘allowable solutions’ and energy offsetting) may appear to be equally viable or in fact more cost effective than the Passivhaus approach with limited RES. Methods of demonstrating and, to some extent, guaranteeing the performance of installed systems and services with greater levels of accuracy are likely to be key to improving future calculations.

Annexe – changes to SAP compared to PHPP

The attached table gives a summary of some of the key metrics that have previously been identified as using different assumptions between SAP and PHPP, subsequently causing differences in the ultimate energy use predictions. While other differences may exist in the way data is input into each tool, such as different ways of calculating the treated floor area or representing thermal bridging ψ values, these have been excluded here as ultimately equivalent inputs can be made for these parameters in each tool. The metrics presented here therefore reflect main assumptions in the respective tools that will ultimately influence the energy calculations. This is not intended to be a detailed analysis of the effect of the calculation differences, but instead serve to highlight changes that have been introduced in SAP 2012 relative to PHPP. Further viability of including SAP outputs in PHPP have been explored in a study under the PassREg project, available here.

[1] 2016 for domestic buildings, 2020 for other buildings, according to the Recast Environmental Performance of buildings Directive – EPBD; ‘Directive 2010/31/EU of the European Parliament and of the Council, 19/05/10 on the energy performance of buildings (recast)’, L153/13, June 2010.

[2] Reason. L, Clarke. A, ‘Projecting energy use and CO2 emissions from low energy buildings: A comparison of the Passivhaus Planning Package (PHPP) and SAP’, AECB, 2008

[3] PHPP has also been updated in 2013 but not in aspects that would change the key differences identified against SAP in the earlier AECB study.

[4] Dengel. A, Swainson. M, ‘Assessment of MVHR systems and air quality in zero carbon homes’, NF52 – report for the NHBC Foundation, IHS Press, August 2013

[5] ‘Mechanical Ventilation with heat recovery in new homes’ Final report of the Ventilation and Indoor Air Quality task group, Zero Carbon Hub, July 2013