FAQs

Frequently Asked Questions (FAQs)

  • General Questions

    • What is NCM?
      • NCM is the National Calculation Methodology developed for HSE to assess the energy performance of buildings for the purposes of compliance with building regulations and energy certification.
    • Can iSBEM be used in all parts of the British Isles?
      • iSBEM can be used for compliance assessments of non-domestic buildings with Building Regulations Part L in England and Wales, Building Regulations Part F in Northern Ireland, the Scottish Building Regulations Section 6, and BBL11 in the States of Jersey. It can also be used for producing Energy Performance Certificates in England, Wales, Scotland, and Northern Ireland. See the Download page for the appropriate version to use.
    • How do I report a bug in iSBEM?
      • If you are an Energy Assessor, then please direct all your comments and queries to your Accreditation Scheme Provider. If you are not an Energy Assessor and are experiencing problems using the iSBEM software, please see the Feedback page. Note that some apparent anomalies are due to the way the NCM calculations are done, rather than bugs in the software. You may, therefore, find the answer to your query in the NCM Modelling Guide of the devolved administration that is relevant to the calculations you are running (available from the Download page).

  • Building Regulations

    • Where can I find the latest Building Regulations Part L for England and the Non-domestic Building Services Compliance Guide?
      • The current Approved Document Part L2, and other relevant documents for England are available from the government's website at this page. Independent commercial search engines may also help you find these and other relevant documents.
    • Who can answer a query on the Building Regulations and Energy Performance Certificates?

  • SBEM / iSBEM

    • What is SBEM and iSBEM?
      • SBEM stands for Simplified Building Energy Model. It is a computer program which provides an analysis of a non-domestic building’s energy consumption, under standardised operating conditions. SBEM estimates the monthly energy use and carbon emissions of a building when given a description of the building’s geometry, construction, and its building services equipment. iSBEM is the interface to the SBEM calculation engine. Both were developed by BRE for HSE.
    • What is the latest version of iSBEM?
      • The latest version of iSBEM is available from the Download page. Please visit the Latest News page in which we announce the release of any newer versions of the application and documentation as and when they become available. Please ensure you access the Download page of the appropriate country or devolved administration.
    • What are the requirements to run iSBEM?
      • iSBEM runs on Microsoft Access, either a full version or runtime version. For more details, see the installation instructions and also the iSBEM User Guide, which are both available from the Download page.
    • What do the SBEM/iSBEM version numbers mean?
      • Each release of iSBEM/SBEM will have a version number of the form X.Y.z

        'X' represents the Major Release Number and will only change when there is a significant enhancement to the functionality or scope of the calculations, or the data behind them. This should only change very occasionally - a few times only during the life-cycle of a particular version of Part L.

        'Y' is the Minor Release Number, starting at zero, which will change when we make minor improvements, or correct bugs, which have an effect on the calculated results.

        'z' is a letter representing the version number. This will change when we have to correct minor bugs which affect the ease of use of the tool, or the presentation of the output, for example, but have no significant effect on the calculated results.
    • Where can I get iSBEM?
      • iSBEM is freely available to download from the Download page. Please ensure you access the Download page of the appropriate country or devolved administration.
    • How do I install iSBEM?
      • Details on how to install iSBEM can be found in the installation instructions and also in the iSBEM User Guide, which are both available from the Download page.
    • Will iSBEM run on Windows 7?
      • Yes, iSBEM should run on Windows 7 though you will need Microsoft Access, or its runtime, installed for iSBEM to work. Please see the installation instructions on the Download page.
    • Will iSBEM run on Windows 10?
      • Yes, iSBEM should run on Windows 10 though you will need Microsoft Access, or its runtime, installed for iSBEM to work. Please see the installation instructions on the Download page.
    • How can I access the Help feature in iSBEM?
      • The "Help" button (labelled with a "?" mark) can be found at the top right-hand corner of almost every tab in iSBEM. Clicking on this button opens the Help menu. For the definition of a particular parameter, place the curser within that field and press "F1" on your keyboard. This will open the Help page for that specific parameter. Alternatively, you can find information in the iSBEM User Guide which is available from the Download page.
    • iSBEM freezes in my version of Office?
      • See the installation instructions which are available from the Download page.
    • Does iSBEM work on other operating systems apart from Windows operating systems?
      • The approved version of iSBEM has been developed to work on a Microsoft Windows platform only (Windows 7, Windows 10, etc.). Unfortunately, iSBEM is not compatible with Mac or Linux. It is possible for the SBEM calculation engine to run on most platforms, as it is standard C++ and can be compiled in Mac or Linux, but the iSBEM tool itself is a Microsoft Access application which does not run on Linux or Mac.
    • Does iSBEM work on computers with Windows 64-bit operating systems?
      • Yes, see the installation instructions which are available from the Download page and download the appropriate version of the software.
    • Does iSBEM work on computers with Windows 32-bit operating systems?
      • Yes, see the installation instructions which are available from the Download page and download the appropriate version of the software.
    • Is there alternative software to iSBEM/SBEM?
      • Yes. The list of all software approved by DLUHC for regulatory energy calculations in England is available from the Approved Software page. For alternative software approved for calculations in Scotland, Wales, or Northern Ireland, please see the website of the relevant devolved administration.  

  • Files and File conversions

    • I have used the conversion tool to convert my project to the current version of iSBEM. Is there anything else I need to do?
      • Yes, after converting a project, it is necessary to click through all the forms and tabs in iSBEM to ensure that all the data is refreshed and updated. You should also ensure that you provide data for any newly introduced input fields which did not exist in the previous version. For further details, please refer to the iSBEM User Guide, which is available from the Download page.
    • Which file do I need to give to my colleagues or my Accreditation Scheme Provider so that they can look at my iSBEM project?
      • In order for someone else to look at your iSBEM project, they will need the ".nct" file for the project. This file has the name which you gave for your project with the ".nct" extension, which by default is found in the "Projects" sub-folder (For e.g., C:\NCM\iSBEM_v5.6.b\Projects\BuildingXX\BuildingXX.nct).

  • Errors

    • The program crashes or produces an error when running the energy calculation for my building model?
      • There is a text file produced by the tool with the extension ".err" which should give you more information about the generated error. This file will report most errors, depending on where in the calculation for the Actual building they occur. There is also an error file for the Notional building ending in "_not.err". If looking at these files does not help you identify the reason for the error and the crash, please send the ".nct" file of the project to your Accreditation Scheme Provider for advice, if you are an Energy Assessor. If you are not an Energy Assessor, please refer to the Feedback page.
    • iSBEM has been working fine, but now when I try to open iSBEM, I get the Microsoft Access error: “Runtime error ‘3048’: Cannot open any more databases”?
      • We suspect that this error is likely caused by a recent Microsoft Windows/Office Update on your computer. We suggest you follow the steps on pages 2-4 of the “installations instructions” document on the Download page in order to add the folder "C:\NCM\" (or wherever you installed iSBEM on your computer if you changed the default location) and its subfolders to "Trusted Locations" in MS Office. This should resolve this particular error.

  • Technical questions

    • What is the Notional building?
      • This is defined in the NCM Modelling Guide of the devolved administration that is relevant to the calculations you are running (available from the Download page). In essence, the Notional building is a building of the same size, shape, and use as the Actual building, but with U-values, building services efficiencies, etc. specified in such a way that the energy performance of the Notional building generates the emission and primary energy target rates for the Actual building's compliance with building regulations.
    • I would like to know more about what goes on inside SBEM and how the calculations work.
      • For details about the algorithms used in SBEM's calculations and the referenced standards, please refer to the current version of the SBEM Technical Manual, which is available from the Download page. Other useful documents include the following information paper "A guide to the Simplified Building Energy Model (SBEM), What it does and how it works", by Roger Hitchin, available from: the BRE Bookshop.
    • How do I treat communal areas that accompany units with independent heating systems when using iSBEM to produce an EPC?
    • Why are there domestic-type activities in iSBEM even though the software is for modelling non-domestic buildings?
      • The domestic type activities available under the building type "Residential spaces" in iSBEM are intended to allow the energy calculations for the generation of one EPC for a building which contains residential accommodation above a non-domestic space (e.g., a shop or a pub) provided that the residential space can only be accessed from within the non-domestic space, i.e., the residential part is not designed or altered for use as a separate independent dwelling. In addition to common circulation areas of apartment buildings containing self-contained flats, these are the only cases where iSBEM can be used to model domestic areas. For more information on the appropriate software tools to use for modelling your building, please refer to DLUHC's publication: "Improving the energy efficiency of our buildings - A guide to Energy Performance Certificates for the construction, sale and let of non-dwellings".
    • For a block of self-contained flats, should iSBEM be used for modelling the common areas (corridors and reception)?
    • Which activity should I choose to model a space in my building?
      • Since the building types in the NCM Activity Database closely align with the Town and Country Planning (TCP) Use Classes, you should select the building type which matches the classification of your building from the TCP classes. If in doubt about the classification of your building, please contact the Building Control department of your local council, or the council to which the building is attributed. Then, for each zone in your building, you should select, from under this building type, the activity type which best describes the activities that take place within the space. A brief description of the selected activity from the NCM Activity Database will be visible in the iSBEM interface.
    • Can I have more than one building type in my project?
      • Generally, you should select activities for the spaces in your building from the drop-down menu of activities under the building type appropriate for your building's classification, since the building types in the NCM Activity Database closely align with the Town and Country Planning (TCP) Use Classes. If in doubt about the classification of your building, please contact the Building Control department of your local council. However, if the activity in a particular space in your building in not typical for the building type, it is not a problem in iSBEM if you were to select a different building type when defining a specific zone in order to assign it an appropriate activity.
    • Some of the information in the databases is incorrect?
      • The data in the NCM Databases was set following consultation with stake holders or obtained from established sources. However, some data can become out-of-date. Please send any comments or suggestions you might have to your Accreditation Scheme Provider if you are an Energy Assessor. If you are not an Energy Assessor, then please see the Feedback page.
    • How can I modify the parameters in the NCM Activity Database?
      • The parameters and schedules associated with each activity in the NCM Activity Database are standardised to allow consistent comparisons between buildings and cannot be modified by the user for the purposes of carrying out calculations for compliance assessments and energy certification. Please remember iSBEM/SBEM is not a design tool.

  • Zoning

    • What is an indirectly conditioned space?
      • For spaces such as corridors or access areas, which are not directly served by an HVAC system (i.e., have no direct supply of heating or cooling) but are likely to be "indirectly" conditioned by the surrounding areas due to the high level of interaction with those spaces (allowing the heated air to move freely from the directly conditioned spaces to the indirectly conditioned ones), they should be considered heated or conditioned "indirectly" by the same HVAC system that supplies the most prevalent surrounding area. In this case, you should assign the HVAC system of the main adjacent space to that indirectly conditioned zone also (although the space is not directly conditioned, the energy to overcome any losses from or gains to it is still required via the conditioned zone and, therefore, has to be included in the calculation). An example of this would be an open corridor (to heated offices) or a stairwell next and open to offices, i.e., which might have a few envelope elements but is mostly open to the surrounding conditioned areas and which is not directly conditioned but is conditioned through the movement of air (and heat) from the adjacent offices into the corridor. Furthermore, envelope elements between a "directly" conditioned space and an "indirectly" conditioned space should be defined in the iSBEM model as adjacent to a "conditioned adjoining space" and not to an "unheated adjoining space". On the other hand, if a zone is unheated and totally enclosed thus heated air cannot freely move from a heated zone into it, such as a plant room, a store room, or a toilet, you are advised to define it in iSBEM as "Zones without HVAC". For further guidance, please refer to the iSBEM User Guide, which is available from the Download page.
    • I have heard that daylight zoning in no longer required. Is this true?
      • Sub-dividing zones according to daylight is part of the NCM zoning rules and is still applicable (please refer to the NCM Modelling Guide, which is available from the Download page). However, the current version of iSBEM contains a feature which gives the user the option to either: do the sub-division themselves, as with older iSBEM versions, or let the tool do it instead, i.e., the user would enter the zones divided as per the other zoning rules but without applying the daylight sub-division and then select to let iSBEM automatically sub-divide the zones into daylighting sub-zones, where needed, following the zoning rules for zones with access to daylight from windows and/or rooflights. If you select not to let iSBEM do the sub-division automatically, then you need to specify the percentage area of each zone where the lighting is controlled so as to respond to daylight. If that percentage is entered as 100%, this means that the whole area of the zone has lighting controls which respond to daylight, and iSBEM will perform no further sub-divisions for this zone (this is the case if you have already done the daylighting sub-division yourself, as with older versions of the tool). If the percentage value that you enter is less than 100%, for e.g., 70%, then iSBEM will sub-divide the zone into two daylight zones whose areas are 70% and 30% of the total area of the zone, respectively. iSBEM will then consider that the 70% daylight area will have lighting controls responding to daylight while the lighting in the remaining 30% daylight area will not be responding to daylight. NB: If your zone has an atypical shape or layout of windows and/or rooflights, and you are worried that iSBEM's basic automatic sub-division might not correctly reflect the access to daylight in the zone, it is recommended that you carry out the daylighting sub-division yourself, as with previous iSBEM versions.
    • How do I zone my project in iSBEM when it involves an office located inside a warehouse?
      • In order to zone this scenario, we recommend the following approach:
        If the top of the office is an accessible (for e.g., via a ladder) and usable space (for e.g., as a storage area), then:
        - Define the office area as a zone with its appropriate floor area as the zone area, appropriate zone height, and define all the envelopes which enclose it as usual, with the appropriate adjacencies.
        - Define the space above the office as a zone whose area is the area of the office’s ceiling and enter its appropriate zone height. Define also its envelopes where applicable, such as its floor envelope, its roof envelope represented by warehouse’s roof area that is directly above it, and any areas of any applicable walls (for e.g., in the corner of the warehouse above the office), along with the appropriate adjacencies.
        - Define the warehouse space as a zone whose area is that usable on the ground, i.e., the zone area should not include the area of the office’s ceiling that has already been defined as a zone in its own right. Define the full zone height appropriate for the warehouse zone. Define all the envelopes enclosing the warehouse, excluding any envelope areas already associated with the "storage" zone above the office. For e.g., the roof envelope of the warehouse zone should be net of the area of the roof already assigned as the roof envelope to the "storage" zone above the office.
        In this case, SBEM will calculate the total usable floor area of the building as the area of the office zone + area of the storage zone above the office + area of warehouse zone.
        On the other hand, if the top of the office is an inaccessible and un-usable space, i.e., it is simply an unoccupied void above the office and cannot be defined as a zone, then:
        - Define the office area as a zone with its appropriate floor area as the zone area, appropriate zone height, and define all the envelopes which enclose it as usual, with the appropriate adjacencies.
        - Define the warehouse space as a zone whose area is that usable on the ground, i.e., the warehouse zone area should not include the area of the office’s ceiling. Define the full zone height appropriate for the warehouse zone. Define all the envelopes enclosing the warehouse including that at the top of the office as a ceiling envelope, with the appropriate adjacencies. The roof envelope of the warehouse zone should include all the area of the warehouse roof including that covering the "void" above the office and, for e.g., any walls forming a corner above the office.
        In this case, SBEM will calculate the total usable floor area of the building as the area of the office zone + area of warehouse zone. The void space above the office will not be counted towards the useable floor area of the building.
    • How should risers and other small unconditioned spaces be treated in iSBEM?
      • Small unconditioned areas such as small risers and store cupboards can be absorbed into adjacent conditioned spaces. In iSBEM, this would involve adding their floor area to the adjacent conditioned space. For larger areas, these areas should be treated as indirectly conditioned spaces (see FAQ on this subject above).
    • How can I define borrowed light where daylight is transferred to an internal zone (for e.g., through glass partitions between façade zones and internal zones)?
      • If this is a key feature in the design of your building, and the impact of daylight is significant, then you will need to use one of the DLUHC-approved dynamic simulation models (DSMs), instead of iSBEM/SBEM, in order to model your building. Otherwise, i.e., if this is not a significant feature in the building, then ignore it in your iSBEM model.

  • Default Values

    • Some of the default values in iSBEM do not comply with Building Regulations.
      • The default values in iSBEM are neither recommended nor best practice settings. They are conservative values, more representative of existing rather than new buildings, and are intended to be over-written by the user, based on their knowledge as an accredited Energy Assessor and the available building data. Software defaults should be used only if the particular data is not available for the existing building to be modelled.

  • Overheating

    • Where did the overheating calculation, which existed in older versions of iSBEM, go?
      • The overheating calculation implemented in older iSBEM versions up to v3.5.b followed the methodology described in DLUHC's 2008 NCM Modelling Guide in order to provide an approximate estimate of overheating in the building, mainly to inform the triggering of the appropriate recommendation for the EPC. The overheating calculation was replaced, since the release of ADL2A 2010, in iSBEM_v4.1.e onwards, with the solar gains calculation for criterion 3 of Part L. The methodology used is specified in the current version of the NCM Modelling Guide, which is available from the Download page.

  • Project Database

    • Why are the effective thermal capacity (Km) values different in the NCM Constructions Database for some constructions in Northern Ireland from those for the rest of the UK?
      • In Northern Ireland, builders tended to use wet plaster rather than plasterboard, and this could have led to higher effective thermal capacity (Km) values for some types of constructions compared to those in England and Wales.
    • Changing the U-values of my constructions does not have much effect on the results in iSBEM.
      • In some non-domestic buildings, it might be the case that heating is responsible for only a small proportion of the total carbon dioxide emissions, compared to, for e.g., lighting. Further insulation, in this case, would have a limited effect on the total emissions, and may, in fact, increase the cooling loads, if applicable, and consequently increase the calculated emissions.
    • How is the effective thermal capacity (Km) calculated for a construction?
      • The Km values are calculated as follows: For each construction element, calculate the contribution of each layer of construction as: density (kg/m3) x thickness (m) x specific heat capacity (kJ/(kgK)). Starting from the layer of the construction closest to the space, add these values together until any one of the following conditions is satisfied:
        - the total thickness of layers exceeds 0.1 m
        - you have reached the mid-point of the construction
        - you have reached an insulating layer
    • What is the definition of metal cladding?
      • Constructions involving metal cladding are roof or wall systems where metal forms an integral part of the construction, such as, metal twin-skin systems where the insulation is located between the metal skins and where the metal skins are typically 0.4 mm to 1.2 mm thick. Metal cladding systems are divided into two broad categories: (a) built-up metal cladding systems involving rail and bracket or z-spacer systems with insulation within the panels, and (b) composite-panel metal cladding systems with insulation inside the panels. If the metal is simply used as an external shield against the weather, such as a rainscreen, this is not, for the purposes of iSBEM calculations, considered as "metal cladding".
    • Why do I need to specify whether my construction involves metal cladding if I am manually entering the thermal properties, rather than selecting a construction from the NCM Database?
      • This information is needed for the calculation of the heat losses via thermal bridges as the software needs to know whether to use the set of psi values for junctions involving metal cladding or the set of psi values for junctions not involving metal cladding.
    • What do I do if I am using construction materials with different properties from those in the NCM Construction Database?
      • You can define your own constructions by manually inputting their thermal properties into iSBEM. You will need to input the U-value and effective thermal capacity (Km) value. Note that supporting documentation for the values you enter may be requested by Building Control, in case of compliance assessments, and by your Accreditation Scheme Provider for auditing purposes, in case of EPC calculations.

  • Geometry

    • When entering the area of a wall with openings, do I deduct the area of the openings from the total area of the wall before inputting it into iSBEM?
      • No, the area of any envelope element you input into iSBEM should be gross, i.e., inclusive of all openings such as windows, roof windows, rooflights, and doors. The software will later deduct the areas of openings, as appropriate, in order to calculate the net areas of the envelopes.
    • What is the surface area ratio for a window or a rooflight?
      • In iSBEM, the surface area ratio is the "developed area to projected area" ratio for the window or rooflight. The developed area is the total area of the glass plus the frame, and the projected area is the area of the opening in the wall/roof. Therefore, for domed or conical rooflights, for example, this ratio would be larger than 1, and for typical windows and flat rooflights, the value is 1. It cannot have a value which is less than 1.
    • What are psi values, and why are they needed?
      • The psi value is the linear thermal transmittance, in W/mK, of junctions. Psi values are needed for calculating the heat losses through thermal bridges at junctions in the building. For more details, please see the iSBEM User Guide, which is available from the Download page.

  • Building Services

    • My HVAC system is not included in the options within iSBEM. What can I do?
      • If your system is not included within iSBEM's system options, you might be able to use one of the DLUHC-approved DSMs, instead of iSBEM, to model your building. Alternatively, it could be acceptable to use a 'workaround' by selecting a substitute system, in order to produce approximate results using the closest system type to yours out of the systems in iSBEM. However, decisions made in this process must be justifiable to Building Control, in case of compliance calculations, and to your Accreditation Scheme Provider for auditing purposes, in case of EPC calculations. When using a substitute system in iSBEM, if your system provides heating and cooling, a heating and cooling system should be chosen, etc. It is also advisable, if possible, to manually input heating and cooling generator efficiencies and SFP values that are appropriate for your system in order to over-write the software's defaults for the selected system substitute. You should also select the same fuel type used by your system.
    • I have an activity that requires tight humidity control. How can this be entered into iSBEM?
      • iSBEM does not currently consider energy for humidification and dehumidification in the energy calculations. This may be included in future versions. You might be able to use one of the DLUHC-approved DSMs, instead of iSBEM/SBEM, to model your building.
    • Do I need to specify boiler size in iSBEM?
      • No, you do not. For further information, please refer to DLUHC's Approved Document Part L2, which is available from the government's website at this page.
    • The fuel type used in my building is not listed in iSBEM. What do I do?
      • The fuel options in iSBEM are those listed in HSE's NCM Modelling Guide, which is available from the Download page. If the fuel used by the systems in your building is not included within these fuel options, it would be acceptable to use a 'workaround' by selecting a substitute fuel, in order to produce approximate results using the fuel type which has the CO2 emission factor and primary energy factor closest to those of your fuel. However, decisions made in this process must be justifiable to Building Control, in case of compliance calculations, and to your Accreditation Scheme Provider for auditing purposes, in case of EPC calculations.
    • What is auxiliary energy?
      • The auxiliary energy covers the energy used by fans, pumps, and controls. Its calculation depends on the HVAC system type selected, as well as, other information provided by the user regarding the SFP, duct and AHU leakage, and control provision. It is applied to the total floor area served by a particular HVAC system and depends on the duration of occupation and operation in the zones served (corresponding to the schedules for the activities assigned to the zones from the NCM Activity Database). Any energy for a secondary hot water circulation, de-stratification fans, and forced circulation for solar water heating systems, if applicable, etc., is also included in the auxiliary energy calculated for a building in iSBEM.
    • How should I define the specific fan power (SPF) in iSBEM if my HVAC system is 'fan coil units' or 'indoor packaged cabinet'?
      • If the HVAC system selected is 'Fan coil systems' or 'Indoor packaged cabinet (VAV)', enter the SFP for the central plant in Building Services form > HVAC Systems tab > System Adjustment sub-tab, and then enter the SFP for the terminal units in the Building Services form > Zones tab > Ventilation (cont.) sub-tab for all the zones served by this HVAC system.
    • What are the enhanced management and control features needed in order to claim the adjustment factor offered in the Approved Documents?
      • The adjustment factor is available for "automatic monitoring and targeting with alarms for out of range values". According to DLUHC's ADL2, this means a complete installation that measures, records, transmits, analyses, reports, and communicates meaningful energy management information to enable the operator to manage the energy it uses. For guidance on how to define this feature in iSBEM, please refer to the iSBEM User Guide which is available from the Download page.
    • Why does the HWS energy increase so much when I define the system as having storage but not enter any storage vessel data?
      • In iSBEM, the user can enter either (a) the storage volume and vessel insulation type and thickness, or (b) the monthly storage losses. If none of these values are input, i.e., the boxes are left blank, default values are used by iSBEM for the calculation. These default values are fairly conservative and assume, for example, that the storage tank is poorly insulated. We recommend that users enter the actual storage data for the building. For further details on how to define this in iSBEM, please refer to the iSBEM User Guide which is available from the Download page.
    • Why does the HWS need to be assigned to all zones?
      • A hot water system (HWS) should be assigned to all zones in the building model, including those spaces where no hot water is actually drawn. For example, the HW demand in an office building's toilets is generated by the occupants of the nearby occupied areas in the building, such as offices, meetings rooms, reception, etc., rather than the toilet area itself. For further details on how to define this in iSBEM, please refer to the iSBEM User Guide which is available from the Download page.

  • Reports

    • Are there more detailed output reports available?
      • There are various reports output from iSBEM which you may find useful when analysing how energy is used in your modelled building, and how the different fuels used contribute to the CO2 emissions. Details of how to access these reports can be found in the iSBEM User Guide, which is available from the Download page.