DESIGN GUIDE FOR METAL ROOFING AND CLADDING TO

3.0 Thermal mass Cm valueThe limiting U-value 2.2 W/m2 K for rooflightsquoted in AD-L2A Table 4 is based on the Uvalue having been assessed with the rooflight inthe vertical position. Current versions of iSBEMincluding v1.2.a use a default U-value 1.83 W/m2 K for triple layer rooflights which is adjusted toabout 2.2 U-value in the horizontal plane within thecalculation of CO2 emissions. When entering therooflight U-value in iSBEM it is necessary to checkwhich value has been quoted by the manufacturer(horizontal or vertical) and enter a vertical or worstcase value in iSBEM. Current versions of ISBEMwill incorrectly print a non compliance messageif the permitted limiting 2.2 W/m2 K U-value isentered, this will, we understand, be corrected infuture versions.Roof ventilators including smoke vents are notincluded the CO2 emissions calculation with thecurrent version of iSBEM, although they appearedin earlier versions.The area-weighted average U-value for eachelement is calculated using the following formula Uav (U1 x A1) (U2 x A2) (U3 x A3) .A1 A2 A3 .For metal cladding using built-up systems andinsulated panels the U-value quoted by the systemmanufacturer will be the weighted average includingany spacers, side lap joints and fixings.If the design includes other components (excludingwindows, rooflights and doors which are elementswith their own U-value limits) for example, verticalfeature trims around columns, the weightedaverage for the wall elevation may need to becalculated.When the set air temperature in a building is variedto reflect the hours of use, the thermal mass of thebuilding will influence the energy input for heating.The temperature of the internal walls and floors willbe affected by the internal air temperature.A building with high thermal mass components willrequire more energy to raise the air temperaturebut will remain warm for longer periods if theheating is turned off. A building with low thermalmass will be quicker to heat for the same energybut with little heat storage the air temperature willvary with the heat settings.A European Standard is being written on themethod for calculating effective thermal capacityof an element (Cm value). The method may besummarised as follows:Calculate the contribution of each layer ofconstruction asDensity (kg/m3) thickness (m) specific heatcapacity kJ/(kg K).Add the contribution of each layer together to givethe Cm value for the element according to thefollowing rules Until the total thickness of layers from the internalface exceeds 0.1 m. or until the mid-point of the construction is reached. or until an insulating layer (defined as havinga thermal conductivity of 0.08 W/mK or less isreached).The Cm value may therefore include an averagethermal mass for any steel portal frame andsheeting rails on the heated side of the liner face.Typical metal cladding systems referred to in thispublication will only have a 0.4 to 0.7mm thicknessmetal liner on the internal face of the insulation.Wall systems with the sheeting rail betweenliner and external face or external columns, maytherefore have different Cm values.BRE iSBEM contains a data base of elementconstructions with U-values and Cm values.Versions up to and including v1.2.a, haveassorted Cm values for otherwise similar claddingconstructions in the range 0.7 to 0.25 U-value.MCRMA/EPIC calculations for typical portalframe buildings with metal cladding systems havedetermined values in the range 3 to 10 kJ/m3 Kwith an average of 7 kJ/m3 K. It is recommendedthat a value of 7 kJ/m3 K is used for metal claddingroof and wall systems in iSBEM until the assortedvalues quoted in v1.2.a and earlier versions havebeen amended.4

4.0 Thermal bridging valuesA method to assess the additional heat lossthrough repeating thermal bridges was introducedby the England and Wales Part AD-L2: 2002. Theadditional heat loss for junctions for example, roofto wall at the eaves is calculated as a linear heatloss coefficient value (Psi value) W/mK.System manufacturers should provide valuescalculated by competent persons to an approvedmethod for typical junction details. Currently values may be calculated using finite elementcomputer programmes certified to comply withthe method of BS EN ISO 10211. Work is ongoing to refine the method and provide approvedcalculations which will be acceptable to BuildingControl bodies without further justification.Worst case basic junction details are illustrated insections 8 and 9 of this publication which meet theBRE IP01/06 values which can be selected fromthe library in iSBEM for metal cladding. Thesethermal bridge values may be used for insulatedmetal twin skin (section 8) and insulated panelsystems (section 9) provided the junction complieswith the relevant illustration and is built to areasonable standard of workmanship includingcontinuity of insulation and air seals.Alternative recommended junction details areillustrated in both section 8 and 9 which limit energyloss through the junction and will provide savings ofCO2 emissions from the building envelope.Using the recommended MCRMA/EPIC detailsand corresponding values will therefore providea saving of CO2 emissions for the actual buildingBER and resultant target TER. Alternative detailsfor lower and more energy efficient values maybe provided by system manufacturers for most ofthe common junctions.The building plan and elevation dimensions, exceptfor height, are not entered into iSBEM, the programtherefore makes assumptions about the length ofa limited number of junction and height/width ofwindows etc., which may lead to inaccuracies. Inparticular, only one roof-wall value is enteredwhich includes both the eaves and any verge.Roof-wall valueThe equation used by iSBEM to calculate thecombined eaves & verge detail:Roof-wall value 1 3 (2 eaves value 1 verge value)e.g. using BRE IP 01/06 metal cladding library valuesRoof-wall 1 3 (2 0.32 eaves 1 1.15 verge) 0.60 W/mK.A more flexible approach is to sum the actual typesof each roof junction length value divided bythe total perimeter length. For example, four sidedroof of 200m perimeter with parapet and boundarywall gutters at the eavesRoof – wall (120m 0.79. parapet & gutter 80m 0.34 verge & parapet)/200m 0.61 W/mK.If values are used in the iSBEM calculation inplace of the default or library value, then thesewill be identified in the iSBEM compliance report.Where better values are used, Building Controlbodies may request justification and copies ofcalculations to an approved method.Other thermal bridgesOther thermal bridges for example, valley gutterswhich are not included in the standard menu can beentered into iSBEM. The current methodology doesnot include them in the calculation of energy lossfrom the Notional Building, therefore their inclusionwill result in a disproportionate penalty on theactual building BER and TER against the NotionalBuilding benchmark unless the best recommendeddetails are adopted and correctly installed.As a matter of good practice, designers shouldadopt details which minimise thermal bridgingirrespective of the requirements of compliance withthe NCM and Building Regulations. values: regional versions of BuildingRegulationsThe Building Regulations England and Walesand Northern Ireland quote BRE IP01/06 asthe reference document for values. The values shown in this document are calculated inaccordance with the methods quoted in BS EN ISO10211 and BRE IP 01/06.The Regulations (Scotland) quote values fortypical traditional constructions but do not refer toIP 01/06 or other sources of evaluated values forconstructions including metal cladding.In all three regions the values used iniSBEM calculations may need to be justified bydemonstrating compliance with the method given inBS EN ISO 10211.5

5.0 Condensation risk6.0 Air permeabilityMinimising condensation risk should be a designcriteria for all buildings for which Building Regulationsrefer to the methods of BS 5250. Especially withhigh humidity environments such as swimming pools,designers have to take measures to minimise therisk of surface condensation which may form nearthermal bridges. Condensation risk analysis isnot included in the compliance requirements ofNCM, but is directly related to best practice fordetails with the minimum thermal bridge.BS 5250: 2002 establishes five classes of internalhumidity by the intended use of the building asshown in Table 2.Insulated metal systems which comply with theU-value for Building Regulations will not normallybe at risk of surface condensation unless there is asignificant thermal bridge.A surface temperature factor fmin value is calculatedby the approved software programs used todetermine values for thermal bridging. Table2 shows typical building use with the expectedhumidity class and minimum f value necessary tominimise the risk of condensation forming nearthermal bridges.Humidity Building type/useclass1Storage areas2Offices, shopsMinimumf value0.300.503Dwellings with low occupancy0.654Dwellings with highoccupancy, sports halls,kitchens, canteens;Buildings heated with unflued gas heaters.0.805Special buildings for example,laundry, brewery, swimmingpools0.90Table 2: Internal humidity classes defined byBS 5250 and the minimum temperature factorrecommended by building useThe junction details illustrated in sections 8 and 9of this publication have f values suitable for use inhumidity class 1 to 3.The recommended details are suitable formore humid conditions, but designers shouldconsult the system manufacturer for full productrecommendations in the more severe conditions forexample, swimming pools.6Air permeability testing of completed buildingsis mandatory in almost all cases of new buildin England and Wales from April 2006 and inNorthern Ireland from November 2006. TheScottish Regulations include the same airpermeability limit for use in compliance with theNCM, although testing buildings at completion isonly mandatory if a lower limit is selected.The England and Wales Building Regulationsrequire that the completed building envelope istested for air permeability and achieves a standardof no more than 10 m3/(h.m2) at 50 Pa pressure.Better, that is, lower design values of airpermeability may be used for iSBEM compliancecalculations, however the completed buildingenvelope will have to achieve the stated lower ratewhen tested.The details illustrated in sections 8 and 9 of thispublication are intended with a reasonable standardof workmanship to achieve the limit of 10 m3/(h.m2) at 50 Pa. for metal cladding sections of theenvelope. Air permeability is measured for thewhole envelope and therefore poor quality airsealing of other materials and components formingthe air barrier may cause greater air loss thanachieved through the metal cladding.Metal cladding manufacturers will be able to providejunction details for their systems which achievelower air permeability values than the BuildingRegulations limit. Where these enhanced detailsare used with limited areas of other materials alsodesigned to minimise air leakage, designs mayconsider adopting lower air permeability values.As building size increases the length of junctionsin metal cladding which may leak, air reduces andthe area of almost impermeable metal claddingincreases. As a result the air permeability of largebuildings should reduce as floor area increases.The values in Table 3 are suggested as achievablewith metal clad buildings built to a reasonablestandard of workmanship.Plan area of metal cladbuilding0 to 2,500 m22Air permeability10.0 m3/(h.m2)2,501 to 10,000 m7.5 m3/(h.m2)over 10,000 m25 to 6 m3/(h.m2)Table 3: Guidance on air permeability vs building size

7.0 Junction details introduction8.0 Junction details - built-upmetal claddingThe common types of metal cladding junction whichhave to be entered in iSBEM calculations areillustrated in section 8 for built-up metal claddingand section 9 for insulated panel cladding. In eachcase a basic design is illustrated which will at leastequal the corresponding worst case value asquoted in table 4 of BRE IP 01/06. These valuesare available from the default menu in iSBEM.Recommended details with better values are alsoillustrated for most junctions which if adopted willaid compliance with the building’s target emissions.Specific systems may include improved designswhich should be used to achieve best practice.The pages may be copied and used as a checklistfor the design against SBEM calculations.BRE IP 01/06 gives advice that where the valueof a particular junction detail is unknown, but wherethe junction detail is as recommended in MCRMA/EPIC Technical Paper 17 for metal claddingconstructions, then the value of can be taken tobe the value for the equivalent junction detail or itcan be taken to be the default value from Table 4 ofIP 01/06 as appropriate.8.1 The value quoted in Table 4 of BRE IP 01/06metal cladding values are included in the menuof iSBEM and may be selected as junctionsincluding metal cladding as IP 01/06.8.2 Constructed with adequate site workmanshipthese basic details comply with therequirements of the Building Regulations forinsulation as a minimum standard which canand should be improved in the actual design.8.3 MCRMA and EPIC members have producedgeneric details with improved values shownin each case as figure b; these designs willreduce CO2 emissions by reducing energyloss through the junction. This can provide areduction on the total building CO2 emissions.8.4 System manufacturers should be able to provideother details using their products with values,calculated by competent persons, which canbe substituted for those illustrated in this guide.These alternative values can be manuallyentered in iSBEM. Building Control bodies mayrequire proof that any such improved valuesare correct and have been used in the actualconstruction.The BRE IP 01/06 values for junctions includingmetal cladding are included as the library defaultsin iSBEM, with the exception of roof-wall asexplained in section 4.JunctiondetailRoof –wall(combineseaves &verge)Wall groundfloorWall-wallcornerWall floornot groundLintel abovewindow ordoorSill belowwindowJamb atwindow ordoorLibrary value Detail No. for detailforto IP 01/06 criteriacorrespondingMetalIP 01/06 details Built-upmetalpanelsin 71.27818Table 4: Summary of values found in iSBEM7

Detail 1 eaves with gutter for built-up systemsGutter support armTypical roof sheetsupported eavesgutterVented fillersRoof ConstructionU 0.25 W/m2KLiner fillersEavesflashingEaves beam withholes and joints sealedWall ConstructionU 0.35 W/m2KFig 1a Basic eaves and gutterBuilt-up metal roof construction of 0.25 U-value and 0.35 U-value wall. Roof liner extends to almost touchthe outer sheet or trim of wall, air gap between wall and roof. value 0.25 W/mKfmin 0.76Gutter support armTypical roof sheetsupported eavesgutterVented fillersRoof ConstructionU 0.25 W/m2KLiner fillersEavesflashingEaves beam withholes and joints sealedWall ConstructionU 0.35 W/m2KFig 1b Recommended design for eavesas fig 1a above but the liner sheets do not cross the insulation which is continuous from wall to roof. value 0.02 W/mKfmin 0.95Key features Roof supported gutter avoids thermal bridge issues. Gutters with brackets fixed to walls might requireadditional supports, and create thermal bridges. Seal all holes and joints in eaves beam to avoid bypassing liner seals. All fixings and seals must be to manufacturer’s recommendations.iSBEM menu provides a single roof-wall value, not the IP 01/06 eaves value.(see section 4 calculation method for combined roof-wall value)MCRMA & EPIC Technical Paper 17- detail No. 1 – built-up metal system8

Detail 2 verge for built-up metal systemRoof U 0.25 W/m2KVerge flashingWall U 0.35 W/m2KTaped lapsLiner fillerCleader angle withsealed jointsFig 2a Basic verge for built-up metal roof & wallBuilt-up system with either roof or wall liner sheet bridging across insulation. value 0.28 W/mKfmin 0.79Roof U 0.25 W/m2KVerge flashingWall U 0.35 W/m2KTaped lapsLiner fillerCleader angle withsealed jointsFig 2b Recommended design for vergeAs fig 2a above but with liner sheets that do not cross the insulation which is continuous from wall to roof. value 0.02 W/mKfmin 0.95Key features Cleader angle required to provide structural support and air sealing between purlins. Joints must bestructural and not interfere with seals. Fasteners securing cleader angles to purlins must not interfere with seals. Seal all holes and joints in cleader angles to avoid bypassing liner seals. All fixings and seals must be to manufacturer’s recommendations.iSBEM menu provides a single roof-wall value, not the IP 01/06 verge value.(see section 4 calculation method for combined roof-wall value)MCRMA & EPIC Technical Paper 17- detail No. 2 – built-up metal system9

Detail 3 drip sill (junction at base of cladding wall)Wall - U 0.35 W/m2KLiner fillerSill flashingSealConcrete flooror masonry wallFloor/wall sealFig 3a Basic drip sill below wallWall to floor junction with little or no wall insulation below floor level. value 1.15 W/mKfmin 0.48Wall - U 0.35 W/m2KLiner fillerInternal trim (sealed joints)Closure flashingto contain insulationSealSill/dripflashingConcrete flooror masonry wallFig 3b Recommended design for drip sillWall insulation extends down side of floor slab and drip fixed to outer sheet so that it does not bridgeinsulation. Floor slab k 1.0 W/mK. value 0.75 W/mKfmin 0.71Key features The same principles of construction apply to vertical or horizontal profiled external metal cladding. Thermal bridging is minimised in the metal construction, provided that the insulation covers at least100mm of wall/floor. The value is largely controlled by the wall/floor materials, light weight blocks for example will improvethe value. Floor assumed 250mm thick and k 1.0 W/mK. Internal trim should be typically 1.5mm thick, with sealed joints. The seal at wall/floor junction must be substantial to cater for inevitable variation in concrete/masonry. Ensure internal trim fixing heads do not compromise air seals. Use rivets, or remove temporary hexhead types, as work progresses. All fixings and seals must be to manufacturer’s recommendations.Basic IP 01/06 metal cladding 1.15 W/mK quoted in iSBEM menu.MCRMA & EPIC Technical Paper 17- detai

7.0 Junction details - introduction 7 8.0 Junction details – built-up metal cladding 7 9.0 Junction details – insulated metal panels 18 10.0 Checklist for recommended data entry 29 11.0 Case study – portal frame metal clad building 30 APPEN