All calculations performed by SBEM are explained in detail in the SBEM Technical Manual
Model Inputs
SBEM has no overall concept of the 3D model you have created using the Virtual Environment. This is usually surprising to users and prompts further questions such as...
"I don't understand... I have supplied a 3D model, what does it do with it?"
Answer: Our VE-SBEM interface breaks it down into inputs that SBEM understands
"Well, why have I created it in 3D then?"...
Answer: It is faster to create and also easier to navigate the rooms, and verify that your inputs are correct using the VE interface
"Of course it does, it is approved for Part L, it needs to"
Answer: Looking at how the calculations are performed, you will see this not the case
All of the above are good points... But when you look under the hood (i.e the SBEM Technical Manual), you begin to appreciate what SBEM is doing and why.
SBEM Methodology
SBEM only considers the adjacent condition and not the “Room” that is adjacent to the space. To illustrate this, here is an example…
Two rooms, both heated (conditioned), and connected by a hole.
SBEM can only consider what the adjacent condition is. So for Room 1 above, it knows the adjacent condition for its south wall (with the hole), is a heated/conditioned space. When it calculates heat loss for the space, it will consider this. This is the same for the north wall in Room 2. What it won’t calculate is the heat transfer from Room 1 to Room 2 and vice versa, once the heat has left Room 1, it is not passed across the hole to Room 2. This is because SBEM does not know that Room 1 is directly adjacent to Room 2. All it knows is that the space adjacent to it is a conditioned space.
Viewing Results
Users who also use DSM often comment that they can't view a detailed breakdown of their results when using SBEM e.g...
"What is the heating energy consumption for 10th November in Room 2?"
This is because SBEM only performs 12 calculations for each room, (one for each month in the year), so you would have one result for November, which considers all the inputs for that month; this includes averaged monthly weather conditions (based on CIBSE TRY weather data) as well as various factors and corrections for occupancy and plant operation. Again, the SBEM Technical Manual explains these calculations.
Compare this to our DSM which, at a default time step of 10 minutes, will perform 6 calculations per hour for the entire year (52560 calculations in total, compared to SBEM's 12 independent calculations, each calculation for DSM responding to the conditions in the previous calculation), and can therefore report results for each hour of the simulation (this can be reduced to every 6 minutes via the reporting interval).
SBEM is a compliance tool
SBEM should only be used for the purposes of seeing how a building complies with the building regulations. The developers of SBEM constantly remind users that they should not be experimenting with inputs using SBEM by issuing guidance like below.
“SBEM is not a design tool, and the most appropriate modelling tool for design purposes should be used. Once the valid design configuration has been set up using the appropriate modelling tool, the relevant parameters can then be input into SBEM to assess compliance or generate an EPC. For e.g., the user should not be using SBEM to “experiment” with the performance parameters of systems and selection of different activities in order to make their building pass compliance. Please note that even if a building passes compliance in SBEM, this does not prove that the design will work in practice.”
So basically, you should have a set in stone design before using SBEM. We frequently get queries from users who have tweaked a certain input in VE-SBEM to see the impact is has on the result, which is understandable, but they shouldn't be using SBEM to influence design decisions. If you need to validate your design, or want to see the impact of a switching to a more efficient system, then the best tool to use is ApacheSim (outside of VE Compliance).
Small details
The support team regularly receives models where the user is trying to show compliance using SBEM and they have taken the time to model small details such as brise soleil, trees, and other obstructions that may provide shading. These are fine if you are using DSM (in conjunction with our solar shading calculations), but as SBEM does not consider shading from these features, they are best being left out.
Large Models
The BRE (developers of SBEM), recommend that users only consider using SBEM when a model is between 100 to 150 rooms (see page 28 in the iSBEM User Guide).
From experience, this depends on whether or not you are running a UK Compliance calculation or an EPC. If an EPC, then you will struggle to get a model with over 150 rooms to finish simulating, this is because you have to simulate the Reference building, on top of the Actual & Notional Buildings.
For Compliance, although it goes against the developers guidance, you should be able to get a model with 300 to 350 rooms through without any problems.
If you have a model that is going over this limit then you will need to consider using ApacheSim method (our DSM), which does not have this limitation. If you do choose to go down this path, then you will need to tweak the model so it is set up correctly for DSM. It is not as straightforward as selecting ApacheSim from the method drop down menu and clicking on "Simulate".
Criterion 2 U-value check
As SBEM doesn't have an overall concept of the 3D model, it relies on the "adjacent condition" to figure out where the U-value checks should be carried out. All elements between two internal, heated spaces are ignored. If you have a conditioned and an unconditioned space adjacent to each other, the surfaces on the boundary of the conditioned and unconditioned space will be checked against the limits for the Criterion 2 U-value check, regardless of if the space is internal or external.
In the example above, Room 1, Room 2 & Room 3 are conditioned. Room 4 is unconditioned. The external walls for room 1, 3 will be checked against the limit for an External Wall. Because Room 4 is unconditioned, the internal wall between Room 2 and Room 4 (highlighted red), will also be checked against the limit for an External Wall. So in this example we have a Internal Wall with a U-value of 1.89 being included in the check against the limit for an External Wall.
For information, DSM would treat the internal wall as being internal, and would not group it with the External Walls. (This is assuming either the room type "Heated Or Occupied" or "Internal Void or Warm Roof" is used, and not Other Buffer Space). "Internal Void or Warm Roof" is not a valid room type for SBEM calculations.
Creating a 3D model for VE-SBEM
Following on from the U-value check. The BRE have published specific guidance for modelling certain areas and features in your building in the iSBEM User Guide.
The FAQ linked to below contains some of this guidance, and explains how you should set up a model with voids, indirectly heated rooms or unconditioned spaces for SBEM.
https://www.iesve.com/support/ve/knowle ... value/1685
Some of this guidance is very different to how you would set up your model for a compliance simulation using DSM.
EPC Conventions
We occasionally get questions on why certain guidance we've issued seems to contradict the guidance from the EPC Conventions document. This is usually because the guidance issued by IES is for the users of DSM.
The EPC Conventions is a document produced by a group of Energy Assessors with the aim of achieving a degree of standardisation for inputs to SBEM. It does not have regulatory status. In the main it is concerned with reducing the complexities of real buildings to a form which SBEM can deal with, which if you read the SBEM Technical Manual, is no easy task! In many cases these issues are not relevant to DSMs, in which many of these complexities can be directly modelled, but some of the principles it sets out provide a useful basis for standardisation.
DSM is not SBEM
Occasionally users will say that they are "performing an SBEM" on their building when it turns out that they are actually using DSM for their compliance calculation. It is an easy mistake to make, especially for those who have exclusively used SBEM for years, but as explained above, the two methods are very different and should not be confused.
