Environmental Science

Environmental Science By John Bradley & Dr David Johnston – licensed under the Creative Commons Attribution – Non-Commercial – Share Alike License http://creativecommons.org/licenses/by-nc-sa/2.5/

ENVIRONMENTAL SCIENCE & SERVICES LEVEL 1 Design tools and condensation John Bradley and Dr David Johnston

Identify factors that influence heat loss in buildings Determine fabric and ventilation heat loss Be aware of steady-state and dynamic modelling tools Be aware of energy and environmental assessment methods and standards Be familiar with psychrometric charts Understand the principles of condensation Identify sources of moisture within a building Understand structural and dew point temperature gradients Determine the purpose of a vapour barrier Heat loss from buildings Energy and environmental assessment and standards Condensation Objectives and content Objectives Content

Heat loss from buildings Factors affecting heat loss Calculation of heat loss Steady state methods Dynamic methods

Factors affecting heat loss Total heat loss from buildings is made up of: Fabric heat loss (Qf): Conduction, convection and radiation exchanges Ventilation heat loss (Qv): Exfiltration of indoor air through cracks, gaps and openings Qtotal = Qf +Qv Important factors which affect heat loss are the: • level of insulation contained within the building’s thermal envelope • building’s location • external surface area of the building • temperature difference between the inside and the outside of the building • air change rate of the building.

Calculation of heat loss A number of methods have been devised which can calculate the rate at which heat flows out of a building, and the quantity of heat that is lost within a given time. These methods range in their levels of complexity and in the data input required by the user, from: simple U-value calculations,complex simulation rules-of-thumb techniques Note: All of these methods are approximations, and they can only provide a simplified version of the complex energy interactions that take place within a building. The methods currently available can be split into two types: • Steady-state (non-dynamic) methods • Dynamic methods

Steady-state (non-dynamic) methods These methods offer the simplest approach to modelling the thermal behaviour of a building. They assume that the temperatures inside and outside the building do notchange with time and the various flows of heat from the building occur at constant rates. One of the simplest methods involves determining the building’s fabric and ventilation heat losses. Fabric heat loss (Qf) measured in Watts (W): Qf = Σ (U.A.ΔT) Where: U is the U-value of the building element A is the area of the building element ΔT is the difference between the internal and external temperatures Σ is the sum of the individual values of (U.A.ΔT)

Steady-state (non-dynamic) methods Ventilation heat loss (QV) measured in Watts (W): Qv = cv.N.V.ΔT 3600 Where: cv is the volumetric specific heat capacity of air ( 1200J/m3K) N is the number of air changes of the room/building (ac/h) V is the volume of the room (m3) ΔT is the difference between the inside and the external temperature (°C). The values for the specific heat capacityof air (cV) and the number of seconds in an hour (3600) can be combined together to give the following simplified formula for ventilation heat loss: Qv = 0.33 N.V.ΔT

Steady-state (non-dynamic) methods A number of other steady-state modelling tools are available. These include: • Degree Day Method. • LT4 Method. • BREDEM (Building Research Establishment Domestic Energy Model). In the UK, BREDEM is one of the most widely used non-dynamic approaches to modelling the space heating requirement of dwellings. The core of BREDEM consists of a two-zone model, which reflects the fact that most dwellings are run with the living room temperature several degrees above that of the rest of the house. Typical factors included in the BREDEM calculation are: • Transmission and ventilation losses. • Efficiency and responsiveness of the heating system. • The user's choice of temperatures and heating periods. • Solar gains. • Internal and external temperatures.

Dynamic methods These are more elaborate than steady state methods. They essentially consist of a simplified, mathematical representation of a building's thermal properties and processes. The effects of time are included, allowing these models to more accurately model the dynamic [ever-changing] heat interchanges that occur within the building, as well as those that occur between the building and the external environment. They have increased our ability to understand the complex interactions that occur within buildings, such as those factors that involve thermal mass, solar gain and internal heat gain. A number of such dynamic modelling tools exist: • Simplest - the Admittance Method • More complex methods - computer based dynamic modelling tools such as: ESP; SERI-RES; TRNSYS; and, HTB2. Extensively used to predict the utilisation of solar energy in buildings.

Dynamic methods Dynamic thermal models are essential for: • Assessing the importance of thermal mass to the performance of the building • Estimating peak temperatures • Estimating the energy benefits of passive solar energy • Estimating the energy benefits of internal free heat gains • Assessing the performance of any building in which free heat gains cover most or all of the space heating load (e.g. superinsulated buildings) • Assessing the importance of control systems and strategies (optimum start controls, etc.) However, there are a number of problems associated with dynamic thermal models. These include: • All practical implementations are 1 dimensional - buildings are 3 dimensional, and the effects of corners etc. may be important. • All implementations require careful approximation and exercise of judgement • All formations make approximations regarding solar data and ventilation.

Actual versus predicted heat loss However sophisticated the modeling of heat loss it will only be an approximation of the actual heat lost in a building. A co-heating test can show the actual heat lost from a building. A co-heating test is a method of measuring the actual heat loss (both fabric and background ventilation) in W/K attributable to an unoccupied dwelling. It involves heating the inside of a dwelling electrically, using electric resistance point heaters, to an elevated mean internal temperature (typically 25°C) over a specified period of time, typically between 1 to 3 weeks. By measuring the amount of electrical energy that is required to maintain the elevated mean internal temperature each day, the daily heat input (in Watts) to the dwelling can be determined.The greater the energy input required to maintain a particular temperature differential, the greater the heat loss.

Actual versus predicted heat loss Co-heating test equipment Equipment required for a co-heating test includes: • Temperature sensors: to measure internal temperature • Fan heaters: to heat the dwelling. • Circulation fans: to mix the internal air within the dwelling. • Thermostats: to regulate the heat output from the fan heaters. • kWh meters: to measure the electrical energy consumption of fan heaters, circulation fans and datalogger (if mains powered). The kWh meters should have a pulsed output that can be read by the datalogger. • Datalogger: to record the data obtained from inside the dwelling. [Source: Johnston, 2009]

Actual versus predicted heat loss Co-heating tests carried out by Leeds Met suggest that the differences between notional and actual heat loss can be significant: For more details visit: http://www.leedsmet.ac.uk/as/cebe/projects/stamford/index.htm The research at Stamford Brook suggests that one of the routes for the unexplained heat loss is via the party wall cavity into the loft space. This source of heat loss is not accounted for in calculating predicted heat loss using SAP. Actual and predicted power requirement as a function of temperature differential Actual energy required More energy required to maintain the temperature differential than predicted Predicted energy required [Source: Wingfield, Miles-Shenton, Lowe and South, 2007]

Energy and environmental assessment and standards Domestic buildings: assessment methods SAP Domestic buildings: standards Code for Sustainable Homes Non-domestic buildings: assessment methods Non-domestic buildings: standards

Domestic buildings: assessment methods The assessment methods used for most of the UK’s energy standards for domestic buildings are based on BREDEM, the Building Research Establishment Domestic Energy Model. This predicts annual fuel use, fuel costs and CO2 emissions under a standard occupancy pattern (rating refers to the dwelling, not the way it is used by a particular household). BREDEM is the basis of both the National Home Energy Rating and the Standard Assessment Procedure The National Home Energy Rating (NHER) is the leading domestic energy rating scheme in the UK. The NHER rating of a dwelling is based on the estimated total annual fuel use (for space heating, water heating, cooking, lighting and the use of appliances), per square metre of floor space, under standard occupancy. It is expressed on a scale of 0 (very inefficient) to 20 (very low carbon). SAP is the Government's Standard Assessment Procedure for Energy Rating of Dwellings. SAP 2005 is adopted by government as part of the UK national methodology for calculation of the energy performance of buildings. It is used to demonstrate compliance with Building Regulations for dwellings (Part L) and to provide energy ratings for dwellings.

SAP The Government’s Standard Assessment Procedure (SAP) for assessing the energy performance of dwellings provides the following indicators: • The SAP rating is based on the energy costs associated with space heating, water heating, ventilation and lighting, less cost savings from energy generation technologies. It is adjusted for floor area so that it is independent of dwelling size for a given built form. The SAP rating is expressed on a scale of 1 to 100, the higher the number the lower the running costs. • The Environmental Impact rating is based on the annual CO2 emissions associated with space heating, water heating, ventilation and lighting, less the emissions saved by energy generation technologies. It is adjusted for floor area so that it is independent of dwelling size for a given built form. The Environmental Impact rating is expressed on a scale of 1 to 100, the higher the number the better the standard. • The Dwelling CO2 Emission Rate is a similar indicator to the Environmental Impact rating, used for compliance with Building Regulations. It is equal to the annual CO2 emissions per unit floor area for space heating, water heating, ventilation and lighting, less the emissions saved by energy generation technologies, expressed in kg/m2/year. The methodology is compliant with the Energy Performance of Buildings Directive.

Domestic buildings: standards There are a number of standards by which domestic energy and environmental performance can be measured: • Building Regulations AD Part L1 • Energy Saving Trust Best Practice Energy Standards • Good Practice: 10% better than Part L1 • Best Practice: 25% improvement on Part L1 • Advanced Practice: 60% improvement on Part L1 • AECB Silver and Gold energy standards • Passivhaus Standard: developed in Germany, supported by EU Commission as pan-European standard for low carbon dwellings • EcoHomes Scheme: assesses buildings against a wide range of environmental criteria and rates them as Pass, Good, Very Good and Excellent. In part, superceded by: • The Code for Sustainable Homes: to be used in the design andconstruction of new homes in England

Code for Sustainable Homes Launched in December 2006 with the publication of ‘Code for Sustainable Homes: A step change in sustainable home building practice”. This introduced a single national standard to be used in the design and construction of new homes in England, based on the BRE’s EcoHomes scheme. The Code is a set of sustainable design principles covering performance in nine areas: • Energy and CO2 • Water • Materials • Surface water run-off • Waste • Pollution • Heath and well being • Management • Ecology In each of these categories, performance targets are proposed in excess of the minimum needed to satisfy Building Regulations, but are considered to be sound best practice, technically feasible, and within the capability of the building industry to supply. The Code uses a rating system of one to six stars

Non-domestic buildings: assessment methods The EU Energy Performance of Buildings Directive (EPBD) requires each member state to create a methodology for the calculation of the energy performance of buildings. The Simplified Building Energy Model (SBEM) has been developed by BRE for government as the default calculation for non-domestic buildings in the UK, and to enable building regulations compliance checks and energy ratings to be carried out on a consistent basis. SBEM is a computer programme that provides an analysis of a building’s energy consumption. It calculates monthly energy use and CO2 emissions of a building given a description of the building geometry, construction, use and HVAC and lighting design. These are then compared with those of a notional building.

Non-domestic buildings: standards There are a number of standards by which non-domestic energy and environmental performance can be measured: • Building Regulations AD L2: sets 5 criteria for new buildings to demonstrate compliance and provides guidance on requirements that apply to existing buildings • BREEAM(Building Research Establishment Environmental Assessment Method) is the main environmental labelling system in the UK for commercial and industrial buildings. The basis of the scheme is the award of a certificate to provide a 'label' for the environmental performance of a building. BREEAM assesses the performance of buildings in 9areas: Management, Health and well-being, Energy, Transport, Water, Materials, Waste Land use and ecology, Pollution. Credits are awarded in each area according to performance. A set of weightings then enables the credits to be added together to produce a single overall score. The building is then rated on a scale of Pass, Good, Very Good, Excellent and Outstanding.

Summary

Condensation Humidity Condensation in buildings Structural temperature gradient Dew point temperature gradient Interstitial condensation risk Minimising condensation risk

Humidity Moisture in the air, or humidity, influences the heat balance and comfort of the human body, the durability of building materials and also causes condensation in buildings Air always contains some water vapour (wv) ie has humidity. The amount of water that can be present in air is related to the ambient temperature. Warmer air can contain more moisture than cooler air. When the upper limit of wv is reached, the air is saturated. The actual amount of wv in the air relative to the maximum saturation level is the relative humidity (RH). The temperature at which air is fully saturated (ie at 100% RH) is the dew point temperature. If the temperature falls below the dew point temperature, the air cannot contain all the wv present and some wv will condense into a liquid (ie condensation will occur). Water vapour is a gas, suspended in air. It exerts a pressure which increases with wv content and/or temperature. The pressure exerted by the molecules of wv contained in the air is the vapour pressure. Pressure and temperature differentials will cause warm, humid air to diffuse to colder areas, colder surfaces and permeate colder structures, leading to cooling of the air and possible condensation.

Humidity: definitions The amount of moisture in a given sample of air can be specified using a number of different variables: • Vapour pressure - The pressure exerted by the molecules of water vapour contained within the air. Units: Pascals (Pa) • Saturated vapour pressure - Vapour pressure which would be given by saturated air at a specific temperature. (Units Pa) • Dew point temperature - The temperature at which a sample of air with a given moisture content becomes saturated. (Units: oC) • Moisture content - The actual quantity of water vapour in a given sample of air. (Units: g/kg of dry air) • Relative humidity - Quantity of water contained within a given sample of air expressed as a percentage of the maximum quantity of water which could be contained within that air at that temperature. (Units: %RH at temp. in oC) CIBSE recommend that for most applications, the relative humidity should be between 40% and 70% for human comfort.

Humidity: sources of If no moisture is added to the internal air of a building, then condensation would not be significant. However, in practice this is not the case. The majority of the moisture within buildings comes from: • Building occupants - sweating and breathing. However, moisture also comes from: • Cooking • Clothes washing • Clothes drying • The use of un-flued heating appliances • Industrial processes Table A10.12 in the CIBSE Guide (CIBSE, 1999) gives estimates of the amount of moisture likely to be produced from a variety of sources. In addition, building materials will absorb moisture from the environment. This moisture will then be released back into the atmosphere when the internal conditions allow it.

Humidity: Psychrometric Chart The variables used to specify the amount of water vapour in the air are interrelated. The relationships can be shown graphically in a Psychrometric Chart. One version of this type of chart is shown below: [Source: Burberry, 1997]

Vertical axis: wv in air exerts a pressure - the vapour pressure (vp), so air containing a large mass of wv has a higher vp than drier air. The amount of water contained in air can be expressed as either vp (in kPa) or the ratio of the mass of the wv to the mass of the air (in g/kg) – the Moisture Content Relative Humidity (RH) curves These isoRH curves show the combinations of temperature and moisture content that produce a given RH% Saturation curve Shows the combination of temperature and moisture content at which the air is saturated ie RH of 100%. The temperature at which saturation occurs is the Dew Point Temperature. The curve is also known as the Dew Point curve.

Examples: What is the vapour pressure of air which is at 20oC and has a relative humidity (RH) of 60% ?  1400 Pa What is the vapour pressure of air which is at 16oC and is saturated ? 1800 Pa  

Example If we have air at 20OC and 50% RH (point A) all the moisture can be held in the air. If more water vapour is introduced into this air and the temperature of the air remains constant, the RH of the air will increase. If the RH is increased to 100% (saturation point) point B, then any additional water vapour that is introduced into this air will be deposited as condensation. If on the other hand, the amount of water vapour in the air remains constant but the temperature of the air falls, the RH of the air will rise because the cooler air can support less moisture. If the air is cooled to around 10OC, the RH will rise to 100% (saturation point) point C and any further cooling will cause the water vapour within the air to condense. B C A

Condensation in buildings There are two types of condensation: • Surface - occurs on surfaces of the building envelope which are at or below the dew point of the air immediately adjacent to them • Interstitial - occurs within or between the layers of the building envelope when the temperature of some part of the structure within the building envelope equals or drops below the dew point temperature. Surface condensation may occur: • On the internal surface of external elements of a building. • On cold pipes and cisterns within a building.

Condensation in buildings Interstitial condensation may occur: • On the surfaces of materials within a structure, particularly on the warm side of relatively vapour resistant layers. • Within the material when the dew point and structural temperatures coincide throughout the material. • On more than one surface in a structure. This is because moisture may evaporate from one surface and re-condense on a colder one. Interstitial condensation [Source: McMullan, 2007]

Condensation in buildings The dampness caused by condensation can result in the following problems: • Lead to mould growth • Damage decorations and fittings • Make insulating materials less effective • Damage important structural materials, i.e. steelwork and timber • In severe conditions it may even lead to structural failure In addition, condensation can cause: • Dimensional changes • Reduction of thermal resistance • Migration of salts and the liberation of chemicals • Electrical failure

Structural temperature gradient Heat will flow through the building fabric from an area of high temperature to one of lower temperature. For homogeneous materials: • The temperature gradient through a construction will change uniformly through each material within the construction. • Temperature difference across a particular material is proportional to the resistance of the material. Therefore: • Materials which have the highest thermal resistances will have the steepest temperature gradients. • Materials with the lowest thermal resistances will have the shallowest temperature gradients. HighRLowR Inside Outside Inside Outside

Structural temperature gradient The boundary temperatures between the individual layers of the building fabric can be determined from the thermal resistances which make up the U-value of that element. The boundary temperature at any point within a building element can be predicted using the following equation: where = temperature difference across a particular layer (oC) = total temperature difference across the structure (oC) = resistance of that layer (m2K/W) = total resistance of the structure (m2K/W) The calculation of the boundary temperatures within a building element enables the temperature gradient throughout the element to be drawn on a scaled diagram.

Structural temperature gradient: example 1. Find the inside surface temperature of an external wall which has a U-value of 1.8 W/m2K, when the internal temperature is 21OC and the external temperature is -1OC. Rsi = 0.123m2K/W Rso = 0.055m2K/W Solution: Total temperature drop across the wall Total resistance of the wall Using: Therefore, temperature drop across internal surface layer = 4.8ºC so inside surface temperature= 21 - 4.8 = 16.2 ºC

Dew point temperature gradient A dew point temperature gradient can be plotted across a wall in a similar manner to a structural temperature gradient. Whereas thermal resistance is important to determine the temperature gradient, vapour resistance is important in determining the dew point gradient. The vapour resistance (RV) describes the ease with which a material will permit the diffusion of water vapour. where = vapour resistance of the material (GNs/kg) = vapour resistivity of the material (GNs/kg m) = thickness of the material (m) The total vapour resistance (RVT) of a multi-layered element is the sum of the vapour resistances of all of the separate components.

Vapour resistivity The vapour resistivity of a number of materials can be seen below. Material Vapour resistivity (GN s/kg m) Aluminium foil >4000 Plywood 150-520 Polythene film 125 Concrete 30-200 Wood 50 Plaster 35-50 Brickwork 25-40 Wood wool 15 Gloss paint 8 Mineral wool 5-6 Building paper 5 [Source: CIBSE, 1988]

Dew point temperature gradient The vapour pressure drop across a building element can be determined in a similar way to the temperature drop across a building element i.e. where = vapour pressure drop across a layer = vapour resistance of that layer = total vapour pressure drop across the structure = total vapour resistance of the structure The vapour pressure changes can be used to produce a vapour pressure gradient. However, it is more useful to obtain the dew point temperature at each boundary layer from the psychrometric chart using the corresponding boundary temperatures and vapour pressures. If the structural and dew point temperature gradients are drawn on the same scaled diagram and compared, the points at which the structuraltemperature falls below the dew point temperature will indicate a risk of interstitial condensation occurring.

Interstitial condensation risk analysis An external wall is constructed of 10mm plasterboard, 22mm of expanded polystyrene board (EPS) and 150mm dense concrete. The thermal resistances of the components, in m2 K/W, are: internal surface resistance 0.123, plasterboard 0.06, EPS 0.75, concrete 0.105, and external surface resistance 0.055. The vapour resistivities of the components, in MN s/g m, are: plasterboard 50, EPS 100, and concrete 30. The inside air is at 20OC and 59% RH; the outside air is at 0OC and saturated. Use a scaled cross section diagram of the wall to plot the structural temperature gradients and the dew point gradients, like this:

Interstitial condensation risk analysis Step 1: Use the thermal resistances to calculate the temperature drop across each layer and the temperature at each boundary. Total temperature drop(θT ) = 20 - 0 = 20OC RT = 1.093 Thermal resistance Temperature drop Boundary temperature Layer R (m2K/W)Δθ(ºC)(ºC) Inside air - - 20 Internal surface 0.123 2.3 - Boundary - - 17.7 Plasterboard 0.06 1.1 - Boundary - - 16.6 EPS 0.75 13.7 - Boundary - - 2.9 Dense concrete 0.105 1.9 - Boundary - - 1.0 External surface 0.055 1.0 - Outside air - - 0.0 RT = 1.093 θT= 20

Interstitial condensation risk analysis Step 2: Use vapour resistances to calculate the vapour pressure drops across each of the layers then, using the psychrometric chart, find the dew point temperature at each boundary. Inside vapour pressure = 1400 Pa Outside vapour pressure = 600 Pa Total vapour pressure drop = 1400 - 600 = 800 Pa Thickness Vapour Vapour VP drop VP at Dew point at Layer (m) resistivity resistance (Pa) boundary (Pa) boundary (ºC) L rv RV =L.rv ΔP Internal surface - - negligible - - - Boundary - - - - 1400 12 Plasterboard 0.010 50 0.553 - - Boundary - - - - 134711.5 EPS 0.025 100 2.5267 - - Boundary - - - - 10807.4 Dense concrete 0.150 30 4.5 480 - - Boundary - - - - 600 0 External surface - - negligible RVT = 7.5 ΔP = 800

Interstitial condensation risk analysis Step 3: Plot the boundary temperatures on a scaled section of the wall and join the points to produce a temperature gradient. Step 4: Plot the dew point temperatures on the scaled, section diagram and produce a dew point gradient. Scaled section diagram [Source: McMullan, 2007]

Minimising condensation risk: Vapour barriers In walls constructed from permeable materials, water vapour can pass through the structure, and evaporate from the outside surface. However, if a wall has an impermeable outer face, evaporation will be prevented, and the material may become saturated. This may lead to deterioration of the building fabric and structure and mould growth. The risk of interstitial condensation in building elements can be reduced if water vapour is prevented from permeating through the construction. Although no material is a perfect barrier to the transfer of water vapour some materials do offer an acceptably high resistance. Such materials when used in this way are called vapour barriers or a vapour check e.g., aluminium foil and polythene sheeting - all of which have high vapour resistances. Vapour barriers should be fitted to the warm side of the structure to minimise vapour penetration.

Minimising condensation risk: Design There are a number of ways in which good design can prevent the occurrence of condensation within buildings. The factors that should be considered during the design process are: Factor Means of reducing condensation risk Moisture input Moisture production from occupants cannot be controlled. However, the moisture input from other sources should be either controlled by extracting the moisture at source, or reduced by changing processes. Ventilation Increasing the ventilation rate will help to remove the moist air within the building. Ventilation will be most effective if it is near the source of moisture generation. Care should be taken to avoid excessive ventilation as this will have an energy penalty. Heating Heating a building will raise the internal surface temperatures, keeping them above the dew point temperature of the air inside the building. Care must be taken to prevent excessive heating as this will not only incur an energy penalty, but may also raise temperatures above comfort conditions. If heating is intermittent, more continuous heating will raise internal surface temperatures and structural temperatures, thus reducing the condensation risk. Insulation Insulating the building envelopewill help to raise the internal surface temperatures of the building.

References BSI (2005) BS 5250: Code of Practice for Control of Condensation in Dwellings. London: British Standards Institution. BURBERRY, P. (1997) Environmental Services 8th Edition. Mitchell’s Building Construction Series CIBSE (1999) Volume A Design Data, 5th Edition. London: Chartered Institution of Building Services Engineers CIBSE (2004) Guide to Energy Efficiency, London: Chartered Institution of Building Services Engineers CIBSE (2006) Energy Assessment and Reporting Methodology, London: Chartered Institution of Building Services Engineers CIBSE (2007) , Environmental Design Guide A, London: Chartered Institution of Building Services Engineers CIBSE (2007 a) SustainabilityGuide, London: Chartered Institution of Building Services Engineers DEFRA (2005) The Government’s Standard Assessment Procedure for Energy Rating of Dwellings 2005 Edition, Watford: BRE JOHNSTON, D. (2009) GHA monitoring programme – Phase 1. Report Number 1 – Draft Co-heating Testing Proposal Leeds: Leeds Metropolitan University McMULLAN, R. (2007) Environmental Science in Buildings. 6th Edition, London: MacMillan RIBA (2007) Low Carbon Standards and Assessment Methods London: Royal Institute of British Architects UK Green Building Council (2007) Report on Carbon Reductions in Non-Domestic Buildings London: DCLG WINGFIELD, J. BELL, M. MILES-SHENTON, D. LOWE, B. and SOUTH, T. (2007) Interim Report Number 7 – Co-heating tests and Investigation of party Wall Thermal Bypass. Partners in Innovation Project: CI 39/3/663. Leeds: Leeds Metropolitan University.

Environmental Science