How are Building Energy Ratings (BER's) Calculated?
The question on how building energy ratings (BER’s) are calculated is a common one. There are many posts and articles which attempt to explain how building energy ratings are calculated, however most are simply an overview of the process. It is a very important question, something which can be difficult to explain and often misunderstood. This post is an assessment of the Dwelling Energy Assessment Procedure (DEAP) Model and its associated methodology. I hope you find it interesting and it gives you a better understanding of how BERs are calculated.
Fundamentally DEAP is a calculation tool surrounded by a set methodology. The purpose of the DEAP tool is for demonstrating compliance with building regulations and establishing the energy rating of dwellings. It can be used as a design guidance tool when designing buildings to achieve a particular rating target.
It carries out an assessment of the asset rather than the actual operation of the dwelling by the occupants. The complexity is therefore limited to a point where it achieves its aim and can be implemented in a way that is cost effective.
Data is entered in a specific manner in line with the conjunctions set out in the DEAP manual. The quality of the information available to input will have a bearing on the outcome. Defaults are available when actual data cannot be sourced. The method seeks only to assess the fixed assets so does not include all the energy used and simplifies the actual behaviour of the occupants. It calculates standard occupancy rates with associated assumptions for occupant behaviours. It excludes many everyday energy using appliances such as for cooking, television, computers & washing. This reduces the amount of data to collect and input, in the interest of achieving a balance between the complexity of the method and the value of the outputs.
DEAP utilises several key considerations to accumulate the data for input to the calculator. The following paragraphs will provide some insight into each.
The size and geometry of the dwelling is established. The overall floor and living area are calculated. The method includes a standard way of establishing the living area by means of selecting the largest public room. This may not always reflect the actual room in the dwelling, used and heated by the occupants as their main living area. The overall floor area is the basis for many of the assumptions used by the model.
The heat loss associated with ventilation is calculated. This takes account of infiltration due to openings, air tightness of the structure, exposure of the dwelling and the type of controlled ventilation present. Default values are used to calculate the air changes per hour unless an actual air tightness test is carried out. Passive vents are generally only counted if they are not controllable. This may not reflect their actual use are ability to prevent air infiltration if closed. Intermittent fans for cooking and extract from wet areas are counted in terms of ventilation heat loss. The heat loss due to ventilation is added to the fabric heat loss to give the total heat loss value.
Fabric Heat Loss
The thermal insulation of the different elements of the building fabric is calculated. This includes the doors, walls, floors, and roofs. The DEAP methodology assigns default U-values based on the age of the element. A U-value can be calculated if enough information is available. The heat loss through a planer element is complex in nature and therefore the U-value is an attempt to standardise the way in which heat loss is calculated but may not represent the actual heat loss through the element. For the purposes of DEAP it is a satisfactory method to utilise.
Similarly heat loss due to thermal bridging is also complex by nature. If information is not available, the DEAP method assigns a default value based on a multiplier factor over the total area of elements. This approach is unlikely to accurately reflect the actual heat loss associated with thermal bridges and is generally an overly conservative value. A more accurate approach is to use thermal modelling of the junctions to obtain a value for heat loss which can be used over the length of the thermal bridge. Like the U-value this is an attempt to standardise the method of calculating the heat loss associated with thermal bridging. It relies heavily on verifying the level of workmanship which took place during construction. It is not realistic to assume all areas can be verified as completed exactly in accordance with the design details and this is a factor in why it may not represent the actual heat loss through the junction. It is however satisfactory for the purposes of DEAP.
Windows & Other Glazed Elements
The windows are assessed for both heat loss and solar gains. The direction and over shading of the window will determine the amount of solar gain. DEAP assigns values based on the global radiation on surfaces. DEAP assigns a default U-value and solar transmittance value for the glazing based on the year of installation. The actual values can be input if available from certified data and linked to the dwelling address. The heat loss due to windows is added to the fabric heat loss.
DEAP calculates a volume of hot water necessary to meet the needs of the occupants of the dwelling. It determines the occupancy based on the floor area. For many reasons, this calculation of occupancy may not reflect the actual number of people using hot water in the dwelling. Examples of factors which could impact on the actual usage of water are as follows:
Age profile – A family compared to a single person or retired pensioners etc
Occupation – Working from home compared to working away.
Visitors – Visiting or short stays by young adults or frequent visitors.
The calculation does not round the number of people, baths, or showers to a whole number. For example the DEAP calculation results in the number of showers taken as 1.97 and the number of baths taken as 0.57. Since there is no such thing as taking half a bath or a percentage of a person, this shows DEAP is not attempting to reflect the actual usage by occupants. Instead, it is treating water usage as proportionate to the asset.
DEAP converts the water usage into the amount of energy required to heat the quantity of water calculated. The losses associated with distribution and storage of the hot water in the dwelling are calculated based on inputs related to insulation, controls, and volumes. These are added to provide a total energy output from the main water heater. Depending on the controls in the dwelling the DEAP method determines if a portion of the water heated will be carried out by a supplementary heater in the summer months when the main heater may not be utilised. If adequate controls to separate the main water heater from heating the space are not in place, then a portion is assigned to the supplementary (electric) hot water heater. The assumptions taken in this method does attempt to reflect the actual usage of the dwelling as well as how energy will be lost from the hot water system.
DEAP takes account of calculated energy required for lighting. The information is sourced from either a lighting design (wattage and efficiency) or site surveys (number of each lamp type). If data is not available for the type of lamp, the method assigns default values in lumen / watt. A correction factor is calculated to take account of daylighting by utilising the glazing ratio. The method assumes a portion of the light is provided by portable lighting. An actual count on the number of portable lights in a dwelling is possible but is not carried out by an assessor. This would not be something which would take a significant increase in resources to achieve. It reflects the need for the methodology to account for portable lighting as it is a realistic source of energy use in most dwellings, however it shows the method is focused on measuring fixed assets and not items which have a high probability of changing over time.
The method calculates a monthly energy balance for space heating taking account of gains and losses with reference to external standardised data for temperature and radiation. It aggregates these figures over a heating season spanning from October to May inclusive. It calculates a required mean temperature during the heating hours. This is based on assumptions of heating the living area to 21 degrees Celsius and the remainder of the dwelling to 18 degrees Celsius. The living area fraction is therefore an important influence. DEAP has a standard method of deciding which area of the dwelling is the living area fraction and therefore heated to a higher temperature. It does not attempt to determine what area the occupants use as their living area.
DEAP accounts for a heating period of 8 hours per day. The actual heating period may be less depending on how the occupants operate the dwelling. All areas of the dwelling may not be heated to the assumed temperatures. The occupants may attempt to conserve fuel where possible and may not heat parts of the dwelling which are not frequently used. In the interest of financial savings, they may be willing to sacrifice comfort levels associated with constant temperatures. The DEAP methodology does not attempt to take account of the actual heating patterns and again focuses on the asset.
The space heating system energy use takes account of the system controls and responsiveness. DEAP applies penalties to the mean temperatures required to account for inadequate controls of space heating. This is calculated as an additional heating requirement and added to the total. A lack of controls will have a large impact on the quantity of energy required to heat the dwelling. If compared to lighting controls the design of heating controls can sometimes be inadequate. Eliminating the need for supplementary hot water in the summer will represent a significant reduction in energy use.
DEAP takes account of the heating system efficiency and fuel characteristics. The efficiency of the heating sources is established from either certified data or a default value in accordance with the methodology. This is applied to the calculation and gives the total energy required. The actual efficiency of the heating system may be different to this value. If a default is used due to a lack of information, it may be a conservative value. The operation of the system may have an impact. For example, a poorly serviced boiler not operating at the expected efficiency as set out by the manufacturer, tested under more ideal conditions. The method applies a hierarchy when selecting the efficiency with the preference towards using the manufacturers declared efficiencies form certified test data over default values. This demonstrates the method does place a high level of importance on inputting accurate data regarding efficiencies but does not attempt to consider the operation of the system by the homeowner. Its focus is on the fixed assets only.