Thermal Performance: Part L1A - Masonryfirst.com

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Thermal Performance: Part L1AMeasuring a dwelling’s overall thermal massSAP calculates a dwelling’s overall thermal mass by multiplying thesurface area for each construction element by its k-value and addingthe results. The total is then divided by the floor area of the dwelling togive the Thermal Mass Parameter (TMP) measured in kJ/m 2 K, where ‘m 2 ’refers to the dwelling’s floor area. Figure 10 shows what is considered inSAP to be low, medium and high levels of thermal mass. Figure 11 showsthe approximate TMP for a typical end terrace house built using a rangeof materials and finishes. It should be noted that the TMP varies to someextent with the size of dwelling.Figure 10: TMP categories in SAP.Thermal mass of dwelling TMP (kJ/m 2 K)Low 100Medium 250High 450Figure 11: Thermal Mass Parameter (TMP) for a typical end terrace house:HouseconstructionTimber ground floorConcrete ground floorStud partitionsBlock partitionsTimber upper floorConcrete upper floorApproximate Thermal Mass Parameter (kJ/m 2 K)50 100 150 200 250 300 350 400 450 500 550 600 650Timber frame 60-80Timber frame 90-120Masonry (600 kg/m 3 blocks) 160-220Masonry (600 kg/m 3 blocks) 190-240Masonry (600 kg/m 3 blocks) 250-370Masonry (1400 kg/m 3 blocks) 270-350Masonry (1400 kg/m 3 blocks) 340-420Masonry (1400 kg/m 3 blocks) 400-540Masonry (2000 kg/m 3 blocks) 340-430Masonry (2000 kg/m 3 blocks) 430-530Masonry (2000 kg/m 3 blocks) 500-650Effect of thermal mass on space heating in SAPDuring the heating season, SAP now takes some account of the wayin which thermal mass can store and later release heat gains from thesun and internal appliances, helping reduce fuel consumption and CO 2emissions.To realise this benefit in SAP, it is important that the:• Level of insulation and air tightness is to a high standard i.e. inaccordance with the FEES or better.• Boiler and heating controls are of a high standard.When these points are optimised in an end terrace house, SAP 2009shows a reduction in annual CO 2 emissions of around 3 to 4% when theTMP is changed from a low value of 100 to a high value of 450. In reality,savings may be more significant as SAP is not intended as a passivedesign tool.Effect of thermal mass on the overheating check in SAPThe overheating check is largely unchanged in the revised versionof SAP. Notable exceptions include the weather data which has beenupdated but does not, as yet, take into account likely future climatechange. Another change is that thermal mass, as represented by the TMP(see above) replaces the former admittance based approach, althoughthis makes little difference to the outcome. Moving from a low to highTMP will reduce the calculated peak internal temperature by about1.5 degrees in the overheating check. In reality, the reduction willoften be greater, particularly where adequate night-time ventilationis possible. Looking ahead, it is likely that the overheating check willbe revisited in line with recommendations contained in a recentZero Carbon Hub report [15] that reviews the modelling tools andassumptions used for new homes. The report calls for an improvedoverheating test that adequately reflects the effects of both passiveand active design features and is based on projected future climaticconditions.12
Thermal Performance: Part L1AAir permeability - concreteand masonry constructionThe average air permeability of new concrete and masonry dwellingshas steadily decreased in recent years as knowledge gained throughpressure testing has informed the design and construction process. Anair permeability rate of around 5 m 3 /h.m 2 is now fairly standard, withbetter values achieved as required.Indeed, results from pressure testing over the last few years show that, ofhousing achieving an air permeability of 3 m 3 /h.m 2 or better, the highestproportion were of masonry construction with a wet plaster finish [16].By wet plastering or parging the blockwork inner leaf, a highly effectiveair barrier can be created which will not degrade appreciably over thebuilding’s life.Insulating concrete formwork (ICF)ICF construction is capable of providing a very low level of air leakagedue to the sealing effect of the concrete and the low permeability ofthe insulating formwork. This combination provides a highly effectiveair barrier capable of achieving an air leakage rate of around 2m 3 /(h.m 2 )or lower [17]. The simplicity and robust nature of ICF helps ensure theintegrity of the air barrier is maintained over the life of the building.Two useful sources of guidance on minimising air permeability include:1. How to achieve good levels of airtightness in masonry homes(available to download at www.concretecentre.com/publications).2. A practical guide to building airtight dwellings (available todownload at www.nhbcfoundation.org).General pressure testingrequirementsAchieving complianceThe performance target has not changed in the 2010 edition of Part Lwhich still imposes a limit of 10 m 3 /h.m 2 to measured air permeabilityand a Design Emissions Rate (DER) that does not exceed the TargetEmission Rate (TER) when calculated using the measuredair permeability.For dwellings which have not been pressure tested, the assessed airpermeability is the average test result obtained for dwellings of thesame type plus a margin of 2 m 3 /(h.m 2 ). The outcome of this changeto the Regulations is that the design air permeability rate should notexceed 8 m 3 /(h.m 2 ) to ensure untested dwellings achieve an assessedair permeability no greater than the backstop value of 10 m 3 /(h.m 2 ). Asa consequence, designs reliant on low air permeability should ideally bepressure tested to avoid this performance penalty.Pressure testing in large developmentsThe basic requirements for pressure testing are as follows:• For each development, a pressure test should be carried out onthree units of each type or half of all the units of that type; whicheveris less.• A block of flats should be treated as a separate developmentirrespective of the number of blocks on site.• The test sample should be selected by the building control bodyin consultation with the pressure tester, so that about half of thescheduled tests for each dwelling type are carried out duringconstruction of the first 25% of that type.• If satisfactory performance is not achieved, remedial measuresshould be carried out and a new test undertaken. This process mayneed to be repeated until compliance is achieved.Pressure testing in small developmentsOn sites of no more than two dwellings, an alternative approach topressure testing would be to demonstrate that during the previous12 months the builder constructed the same dwelling type, which waspressure tested and achieved compliance. Alternatively, a pressure testcan be avoided by assuming a value of 15 m 3 /(h.m 2 ) when calculatingthe DER.ICF construction provides very low air leakage.Pressure testing and Part FThe 2010 edition of Part F requires an enhanced level of ventilationin dwellings with a design air permeability of less than 5 m 3 /(h.m 2 ).However, dwellings which are not pressure tested must include a marginof + 2 m 3 /(h.m 2 ). So, in practice, the need for enhanced ventilationmeasures will actually apply to buildings with an air permeability of3.0 m 3 /(h.m 2 ) or less.This will help avoid a situation whereby dwellings without enhancedventilation measures fall foul of Part F if they are more airtight thanintended when pressure tested i.e. less than 5 m 3 /(h.m 2 ). So, providing theresult is no lower than 3 m 3 /(h.m 2 ), dwellings without enhanced ventilationmeasures are compliant with Part F.13
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