Thermal insulation of buildings and cooling demand

Transcription

1 411 Thermal insulation of buildings and cooling demand C. Hamans Rockwool International S.A.,The Netherlands ABSTRACT Thermal insulation in the warm climate can reduce the energy demand for cooling in residential buildings up to 70%. To define the gap in between the actual applied thermal requirements and what would be today s economic optimum, Eurima 1 ordered a study to Ecofys 2 which also quantified the impact of thermal insulation in reducing the cooling demand in residential buildings. A standard row house and multifamily house with relative high thermal mass, average internal gains, external shading, natural ventilation, which already applies reasonable passive cooling strategies, was taken to cover the assumed average situation of a single building in southern Europe. The results show that substantial savings of cooling energy demand can be realised by adding insulation. At the same time it is visible that the total demand for cooling energy is significantly decreasing from very hot climates like Sevilla with 908 cooling degree days to less hot climates like Marseille with 427 cooling degree days. The effect of the single measures of insulating the external walls, roof or ground floor is quantified. Remarkable is the positive effect of roof insulation. This is caused by the especially high temperatures of the roof due to the effect of solar irradiation with according benefits of thermal insulation of this surface. At the same time, the insulation of the floor results in an increase of cooling demand, because the cooling effect of the (cooler) ground is reduced. In a second step a sensitivity analysis was carried out to assess the influence of different situations concerning external shading, internal gains, ventilation strategy and thermal mass on the changing cooling demand if insulation measures are applied. When looking at the total energy demand for cooling, it is in the first place visible that passive cooling strategies like high building mass, external shading, night ventilation and reduction of internal heat loads are effective measures to decrease cooling energy demand. At the same time, adding insulation leads to a reduction of cooling energy demand. This seems to be especially the case for low mass buildings where added insulation 1 Eurima: European Insulation Manufacturers Association, Brussels, 2 Ecofys, Cologne, a leading consultancy in energy studies. can to a large extend replace the thermal inertia of a massive building. It can be stated, that the influence of insulation on cooling demand is relatively constant in different situations with the exception of technical premises like buildings with low mass (leading to significantly larger saving potential) and buildings with no external shading equipment (reducing the savings potential). The study also leads to the conclusion, that the benefit of insulation regarding cooling is quite robust against misbehaviour of tenants who might have higher internal gains or who do not use ventilation strategies such as night ventilation. 1. INTRODUCTION Because the building sector being responsible for about 40% of Europe s total energy consumption, the EPBD 3 is an important step for the European Union to reach in order that it should achieve the level of saving required by the Kyoto Agreement. The EU also is committed to reduce CO 2 emissions by 8 per cent by 2010 relative to the base year of The impact of the EPBD has been quantified in earlier Eurima studies 4 for the potential monetary savings, investments and CO 2 savings. (More recently the EU has targeted to reduce the energy consumption by 20% in 2020.) The calculations of the overall energy performance of buildings, according to the EPBD has to consider an integrated approach, that takes into account the calculation rules given in a suite of CEN standards for all building related energy losses and energy gains. National or regional energy performance requirements are given in national or regional regulations for fully integrated overall energy performance. In many countries additional requirements on the maximum energy transmission for single building components expressed in U- values or R-values are given. However these national U-value requirements for building components (roof, floor, wall, windows, etc.) often describe minimum requirements that do not reflect the economic optimum or specific environmental targets. In many European countries these present required U- 3 EPBD, Energy Performance Buildings Directive 4 Eurima Ecofys studies: see: library/eurima_publications.cfm PALENC Vol 1.indd 411 3/9/2007 1:25:06 µµ

2 412 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and values for residential buildings may be considered as minimum performance levels. These are no longer entirely based on changed economic conditions resulting from rising energy prices over the past years but reflect an increasing commitment to reduce CO 2 emissions and avoid climate change. With this most recent study Eurima wants to demonstrate that there is room for improvement through the reconsideration of national or regional required or recommended U-values for building components. 2. CRITERIA: COSTS AND CLIMATE Regarding the recommendation of U-values, there are two lines of argument that are reasonable to follow: cost effectiveness and climate targets. 2.1 Cost effectiveness In article 6 Existing buildings the EPBD states that when buildings with a total useful floor area over 1000 m 2 undergo major renovation, their energy performance should be upgraded in order to meet minimum requirements in so far as this is technically, functionally and economically feasible. It is essential to assess which measures are technically, functionally and economically feasible for average local market conditions. 2.2 Climate change In the post Kyoto discussion the EU-25 ministers for the environment set the target for the reduction of greenhouse gas emissions as 70-90% by Taking retrofit cycles of years in the building stock into account, each building which undergo refurbishment in 2010 has to fulfil these targets. To implement the targets on a wide scale in 2010 their feasibility has to be demonstrated now. This raises the question, what does the target of 70-90% reduction actually mean for the maximum energy consumption and the associated minimum insulation standard of retrofitted houses in different European climates? 2.3 Aim, data simplifications The study aims to make recommendations for U-values for the building components wall, roof and (ground)floor for residential buildings (new and existing) on the level of economic optimum. The recommendations are based on: - the climate data in 100 European cities - the economic optimum U-value (heat transmission value in W/m 2 K) in practice representing a certain spread around this theoretical value. - the economic optimum, representing the Best Practice value for a single building component like a wall construction, roof construction or floor construction - a simplified linearity in the investment costs - non-specific prices for insulation materials and auxiliary materials. - the average U-values of non-insulated or existing constructions - energy prices and energy mix per zone.(north, central, south, east) - (social) interest rates of 4% and 6% (west and east respectively) - residential buildings with traditional heating and ventilation systems. (no heat recovery systems, no Passive Houses) Requirements for better U-values driven by the need for higher thermal values when electric heating is applied are not covered. Also requirements for better U-values driven by other building physical conditions like condensation risks or acoustical requirements are not covered. The full study can be downloaded from the Eurima website. Within the context of this paper the heating related part of the study is not further elaborated and not presented here. In the following only the cooling related part of the study and the outcome from this is presented and the city of Seville is taken exemplary in this paper. 3. COOLING DEGREE DAYS In order to meet the comfort conditions quantification of the energy for cooling is either based on the number of corresponding Cooling Degrees (similar to the number of Heating Degrees) or resulting from an iterative numeric calculation (as done in this study with the calculation program TRNSys) driven by maintaining the comfort level in the building at a given comfort temperature. During preparation of this study EUROSTAT was working on a methodology to calculate cooling degree days. Because it had not been finalized, the methodology as applied in the US (ASHRAE) was used. The US switched to a different approach, developed by PNNL a few years ago. The CDD threshold temperature was also changed. Although the climate zoning used in the report information is outdated it doesn t really affect the conclusions for recommendations to regulators to reconsider present relevant requirements in thermal performance of buildings. The use of HDD and CDD for energy modelling is an approximation of reality, but acceptable for the purpose of the report. 4. INSULATION AND COOLING 4.1 Reference buildings In a first step, calculations have been performed for southern Europe for a standard terrace house (attached building) and multifamily house. The geometries have been adopted from the standard houses used in previous PALENC Vol 1.indd 412 3/9/2007 1:25:06 µµ

3 413 Eurima studies, being a single family house, terraced with 120 m² usable floor area. (SFH), and a multi family house (MFH) being a building block with m² usable floor area. The reference buildings have internal gains of 3 W/m², whereas there is an external shading of windows at 75% and natural ventilation. The average daytime air-exchange rate is taken at 0,65 (infiltration and ventilation via windows), and the air exchange rate over night is 2,5. There is assumed to be an active cooling via an air-conditioning system with a maximum comfort temperature to keep at 25 C. Initial starting point: no insulation is applied. 4.2 Improvement packages. In order to study the effect of insulation, internal heat gains and shadowing on the cooling demand to keep the comfort conditions in the reference buildings, the following standard package of measures was applied: - wall: U-value reduced from 1,7 to 0,6 W/m²K - roof: U-value reduced from 2,25 to 0,5 W/m²K - floor: U-value reduced from 1,0 to 0,5 W/m²K In a second run an improvement package considering the optimum insulation thickness for these building components was defined. 5. STANDARD PACKAGE 5.1 Impact building components When looking at the single measures wall-, roof- or floor insulation on their own, it is apparent that they contribute in different ways to the savings of the combined insulation measures. The insulation of a wall of the reference single family house in Seville reduces the cooling energy demand by 4 kwh/m² and year when applying the standard package. See Figure 1. Surprisingly the effect of roof insulation is very positive. This is due to the particularly high temperatures of the roof caused by solar radiation which leads to higher surface temperatures and a consequential thermal insulation benefit. The insulation of the ground-floor results in an increase of cooling demand in hot climates. This is caused by the reduction of the cooling effect because of the relative cool temperatures of the ground in the summer situation. On the other hand during the winter season the insulation of the ground floor results in heating-energy savings. Therefore the recommended U-values in this study take into account the effect of insulation on heating- and cooling demand. However, regarding floor insulation further restrictions could be given to meet demands such as the acoustic comfort (contact noise), building physics (level of surface temperature given the humidity conditions in order to avoid condensation or desired fast response-time of floor heating) that might require more insulation (a lower U-value) for floors. An analogous effect of insulation on cooling demand, yet at a lower level, can be found for a single family house in Marseille and Lyon. 6. SENSITIVITY ANALYSIS A sensitivity analysis was carried out to assess the impact of different situations concerning external shading, internal heat gains, ventilation strategy and thermal mass on the cooling demand, in relation to the degree of insulation applied. The following scenarios have been simulated: - No external shading of windows (reference situation: 75% shading) - Higher internal gains of 5 W/m² (reference situation: 3 W/m²) - No night ventilation (reference situation: air exchange rate night: 2,5) - Low mass wood-frame-buildings (reference: bricks and concrete) 6.1 Results for Seville The results of the sensitivity analysis for the reference single family house (SFH) and the reference multifamily house (MFH) for the city of Seville can be taken from Figure 2 and Figure 3 Figure 1: Energy savings from insulation improvement in a SFH and MFH in Seville PALENC Vol 1.indd 413 3/9/2007 1:25:06 µµ

4 414 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and 6.2 Insulation is robust against behaviour Focusing on the cooling energy savings resulting from added insulation (difference between U values for situations with and without insulation, shown in Figure 4), it can be concluded, that the influence of insulation on cooling demand is relatively constant in the different situations with the exception of (technical) premises like buildings with low mass (leading to significantly larger saving potential) and buildings with no external shading equipment (reducing the savings potential). Figure 2 Sensitivity analysis cooling demand, SFH, Seville Figure 4 Sensitivity analysis savings in energy for cooling in Seville. Figure 3 Sensitivity analysis cooling demand, MFH, Seville When looking at the total energy demand for cooling, it is immediately apparent that traditional passive cooling strategies like, external shading, reduction of internal heat loads, night ventilation and high building mass (as achieved in the reference situation) are effective measures to decrease cooling energy demand. This is the case for both single - and multifamily houses. Beyond that, increased insulation levels lead in all the cases described (reference-situation and in the combined measures of the sensitivity analysis) to a further reduction of cooling energy demand in the summer situation. This means that the same insulation material that reduces heat losses of the building during winter also reduces cooling energy demand in the summer situation. This can be somewhat different from the feeling that people might have when thinking of wearing for example a pullover during summer. But in this case the high internal gains of a body are the reason for the resulting discomfort. The same applies to buildings: high internal heat gains and insufficient ventilation might cause the discomfort. The situation within an insulated house can better be compared to a thermos bottle, which keeps beverages hot in winter and cool in summer by reducing the heat transfer from hot to cold temperatures - regardless its direction. This leads to the conclusion, that the benefit of insulation regarding cooling is quite robust against the different behaviour of tenants who might have higher internal gains from e.g. electric applications 5 or who do not use ventilation strategies such as night ventilation. This is an important conclusion when examining the figures in the following, which show the effect of insulation on cooling demand resulting from calculations of optimal U-values based on cost efficiency. The above also is valid for the milder climate conditions in Marseille and Lyon, which were taken as indicative examples. From this it was concluded that regardless of the climate conditions (climate zones) a sound package of insulation measures reduces the energy demand for cooling (in residential buildings) 7. OPTIMUM U-VALUE PACKAGE If not a standard package of insulation measures is applied (bringing down the thermal transmission losses down to U=0,5 W/m 2 K) but a calculated U-value for the specific situation on the basis of their economic optimum, the energy savings for the cooling demand are even bigger. The optimum insulation thickness for roof, for wall and for the floor was calculated, on the basis 5 Concerning internal gains the situation can differ for non-residential buildings (esp. office-buildings) where very high internal gains from computers, lighting etc. occur which might reduce, or even reverse, the positive effect of insulation on cooling demand. PALENC Vol 1.indd 414 3/9/2007 1:25:06 µµ

5 415 of 2 energy price scenarios. In the following only the results for the PEAK-price scenario are given 6. With the climate conditions (in HDD and CDD) the optimum U-value was calculated taking into account the energy savings costs on nett present value for a 30 years period, based on the local energy mix, guided by the PEAK price for oil (70 US$ per barrel) and the investment costs for the insulation measures. In the last column of the Table 1 the results of this calculation of the optimum U value s are given. The table also shows the starting reference and the standard package as specified in chapter 5. Table 1 U-value scenarios for SevilleS U-value [W/m 2 K] Reference Standard Package Peak price scenario Wall 1,70 0,60 0,32 Roof 2,25 0,50 0,27 Floor 1,00 0,50 1,06 The optimum U-values for roof, wall and floor may seem rather unusual compared to the existing situation, but prove that considerable contributions in the reduction of the cooling demand can be realised in buildings.. Applying the optimum insulation thicknesses for a single family house in Seville in wall, roof and floor is able to reduce the energy demand for cooling from 31 kwh/ m 2 per year down to 9 kwh/m 2 per year. For a MFH this is respectively from 18 down to 8 kwh/m 2. In percentages: a reduction of the cooling demand of approx. 70% and 55%. See Figure 5 there is room for improvement. Part of the study also learned that particularly in the warm climate zones in Europe, considerable improvements can be achieved for residential buildings in reducing the energy demand for cooling. This of course applies to new build houses, but the study demonstrated that the results also apply to the existing housing stock: well insulated components contribute considerably to the reduction of energy demand for cooling. More in particular it leads to the conclusions: - Insulation reduces the energy demand for cooling for new and existing (residential) buildings. - Roof-insulation is most effective, followed by insulation of exterior walls. Floor insulation has to be applied in balance with its contribution in both winter and summer conditions - Even under less favourable conditions (no shading, higher internal gains or a sub-optimal ventilation regime) insulation reduces the energy demand. - The positive effect of insulation seems to be especially the case for low mass buildings, where added insulation can to a large extend replace the thermal inertia of a massive building. Figure 5 Energy demand for cooling in Seville 8. CONCLUSIONS The complete study, with the aim to stimulate regulators to review the existing thermal performance demands on component level, learned that almost all over Europe 6 Two scenarios were taken in the Eurima-Ecofys report: the IEA s World Energy Outlook (WEO) 2006 scenario and the Peakprice scenario. PALENC Vol 1.indd 415 3/9/2007 1:25:06 µµ