International scientific colloquium

Modelling of Saving Resources
Analysis of Heat Losses from Typical Buildings in Riga
A. Jakovics, S. Gendels, H. Trümmann
Abstract
Due to remarkable extent of reconstruction of buildings in Latvia, the analysis of their heat losses should be performed at two stages - the designing and the quality control ofthermal insulation and air thickness. Optimised inspection of the heat insulation properties ofbuilding structures is ensured by combining thermographic express diagnostics, heat transferand air exchange measurements in existing buildings and mathematical modelling of theirheat balance. A special database of information about typical building structures in Latvia hasbeen created. Basing on the obtained data, the analysis of heat losses in typical buildings ispossible with developed software HeatMod before and after various reconstructions. Thisanalysis helps to evaluate the economic efficiency of the reconstruction of buildings.
Introduction
Because of poor insulation of heat and moisture and uncontrollable air exchange in the existing dwelling, public and industrial buildings, there is a sharp increase in their heatingcosts after the transition to market economy. This is caused by abolishing of subventions andstate regulations in the field of energy carrier production, as well as by low efficiency of theirproduction, transportation and consumption. Therefore, the improvement of thermal andmoisture insulation of building structures involves remarkable potential for saving energy andbasing on this, a decreasing of the amount of CO2 emission.
The following scheme is developed for complex monitoring of heat losses and analysis of energy consumption:1. Express methods for detection of insulation defects of heat and humidity in building structures with thermography (EN ISO 6781).
2. Automated computer monitoring of climatic characteristics in buildings (temperature, 3. Automated computer monitoring of energy (heat, electricity, water, etc.) consumption in buildings and the affiliated trend analysis.
4. Measurement exchange intensity of air in building blocks with over/under pressure tests with the so called Blower-Door equipment (EN ISO 9972) 5. Measurement of heat fluxes in building structures (walls, roofs, windows, etc.) and estimation of heat transfer coefficient U (W/m2K) and thermal time-constant τ (h):- standardised (EN ISO 8990) laboratory tests of finished structures (windows, doors, - control-tests in existing old, new and reconstructed buildings in various conditions.
6. Development of databases for U values of typical building structures in Latvia. Their use for designing of the reconstruction of heat insulation and estimation of energyconsumption of buildings 7. Mathematical modelling of heat consumption in buildings for optimal design of reconstruction and estimation of economical efficiency of projects in accordance with EN4108-6.
8. Development of Building-codes (LN-002) and adaption of European standards.
1. Practical Estimation of Heat Losses
1.2. Estimation of Heat Transfer Coefficient of Existing Building Structures
Automated measuring equipment has been developed for measuring of heat transfer coefficient in real exploitation conditions of buildings. The equipment allows to collect thetemperature data inside and outside as well as the heat flux density at the inner surface ofbuilding element during several weeks. One of the possibilities is data collection directly onPC using a highly sensitive (multifunctional) multifunctional measuring card with attachedsensors. Special software is designed for this purpose [1]. An alternative possibility is the useof universal hand-type data collector (e.g., ALMEMO 2290-8). This causes a high portabilityof the measuring equipment and the independence of stationary electrical power supply [2]. Inthis case, the measured data for calculation of heat transfer coefficient are stored and thentransferred to PC by V24 interface after the measurements. The elaborated software UMeasallows to measure U values even in complicated cases (small, unstationary flux or flux withalternating direction), when traditional cumulative approach gives inaccurate results. Thesoftware minimises the difference of heat flux densities in the experiment and inmathematical model of unstationary heat transfer process. The minimisation approachincludes the check of precision of the results. Moreover, it allows to determine the thermaltime-constant [1,2] simultaneously with heat transfer coefficient. This time constantcharacterises the thermal inertia of building constructions.
Tab. 1. Characteristic values of heat transfer coefficient during prolonged exploitation int.
al. influence of moisture.
In many cases, the calculation of U value is impossible at all, because there is lack ofinformation about used materials in building and the structure of outside wall.
The data basis of U values has been developed basing on the results of measurement for various building constructions typical for former USSR. The data basis is necessary formodelling of heat consumption of buildings and for elaboration of restructuring projects.
Incidentally, the measurements demonstrates that the U values of existing buildingconstructions exceed even more than 6 times the recommended values for the elements ofoutside wall in West-Europe - U<0.3.
1.2. Estimation of Thickness of Air Layer for Buildings
A special ventilator with continuously adjustable frequency of rotation is used to detect the degree of airtight-ness of the outer building-wrap in correspondence withinternational standards EN ISO 9972 [5]. The ventilator is combined with measuringinstrument for pressure difference and efficiency of ventilator V′ (m3/h), as well as withframework and cloth for packing system in one of the openings of considered building(window, door, etc.). The measurements are performed creating such a pressure in thebuilding, which exceeds or goes beyond the outside pressure by 50 Pa (see Fig. 1). Theopenings and cracks in outer shell of building can be easily detected creating the over/underpressure by so called Blower Door equipment. This equipment should be used also forthermographic inspection of building [6,7].
Such measurements of air exchange coefficient n50=V'/VN (1/h) at 50 Pa difference of pressures, where VN - the volume of building, allows to appreciate the air exchangecoefficient n in natural conditions: n = n50.e, where e is coefficient of wind protection(0.04<e<0.1). The coefficient n is necessary for modelling of heat losses. Additionalstandardised properties of packing of building such as q50=V'/SH ; np50=V'/SF can bedetermined, too, where SH and SF are areas of outer shell and cellar, respectively. Acorrespondig wind load to this difference of pressure (50 Pa) would be caused by a windhitting the house with a speed of about 9 m/s.
V' [m 1400
∆p [Pa]
Fig. 1. Air flux vs. pressure difference for buildings with various degrees of packing.
Planed building normatives of Latvia sets that n50 must not exceed 3.0 for dwelling houses and normal exchange of air without forced ventilation is achieved at n=0.7.
Measurements in various existing slab-buildings of Latvia shows existing results of n > 50 50, because of poor quality of windows and their packing. The value n50 becomes less than 6 only in these buildings, where the residents perform simple packing ofwindows and doors by themselves. To more regrettably manner, the new-built individualdwelling houses before exploitation has n50=3-5. The reason is insufficient packing withisolating material the joint places of building constructions and essential indent fromtechnological requirements building in the windows. Therefore, the heat exchange coefficientfrequently reaches 1-2 in real climatic conditions increasing the heat losses by uncontrollableoutflow of warm air and invasion of cold air.
1.3. Typical Problems
Summing up, the mass of houses built in times of former USSR has very high heat transfer coefficient (see Tab. 1), nearly uncontrollable intensity of air leakage n > and a poor resistance of building constructions to impact of moisture. Whereas, the new-builtand reconstructed houses have defects of building technology. These defects cause that- cold air can get behind the insulating layer by pressure of wind increasing heat losses. Forexample, the measurements of heat transfer coefficient in reconstructed office showedsatisfactory values U=0,3-0,4 W/m2K at lull, but the value increased up to 0.8-1.0 performingrepeated measurements in windy weather. It results from improper infixing of windbreakingfilm and unpacked joint places of insulating materials;- insulating material sags. As a result, the distribution of U values all over the surface ofbuilding construction is irregular. Such situation can happen using the rolls of light mineral-cotton for the walls;- moisture condenses in the layer of heat-insulating material. Consequently, mould developsafter some time. The following situation is found, e.g., on the inner side of heat-insulator ofbuilding construction, where the temperature of dew point has been reached because the heat-insulating material has not been protected by windbreaking film.
The cheapest PVC windows with double packet of ordinary glass are used continually for building and reconstruction in Latvia. Heat transfer coefficient of them reaches 2.8, whilethe windows with U<1.8 costs only 5-10% more. Such action is short-sighted for saving ofenergy and avoiding the risk of damage by humidity.
2. Mathematical Modelling of Heat Losses and Consumption
2.1. Calculation Procedure
The analysis of heat losses of buildings and heat consumption in correspondence with EN 4108-6 [3] is made by mathematical model of the monthly heat balance in building. Themathematical model includes internal and external heat sources, heat losses by convection andconduction through the outer shell of building, and climate conditions in Latvia. The specialelaborated software HeatMod is used for this purpose. The software is easy to use usable inWindows environment for data collection and visualisation of results [4]. 2.2. Example of Analysis for Public Building
measurements and results ofmodelling for the 4-floorbuilding of UL laboratories inZellu street 8. This buildinghas outer brick wall with - V=10192 m3, the area ofouter surface of buildingconstructions - A=3573 m² Fig. 2. Constitution of areas of elements in heated part of the showed that building initially (I) had very high specific heat consumption - QH=49,8kWh/(m3a) (see Fig. 3), where a is year. The constitution of the heated part of outer shell ofbuilding is shown in Fig. 2. The measurements of heat losses showed that the most importantrole belongs to insufficient degree of sealing (especially because of poor windows) which results in - n=1,2 1/h. Analysing the possibilities to reduce the heat losses of buildings, theheat consumption has been calculated for three variants of renovation (II, III, IV). The heattransfer coefficient of respective building elements and degree of sealing would changeessentially during the proceed of renovation (see Tab. 2): Fig. 3. Comparison of normalised heat consumption with respective marginal values ofbuilding normatives in Germany.
Tab. 2. Used data for heat transfer coefficient U and exchange coefficient of air n invarious variants of calculations II. All windows are replaced by packed windows from wood with U=2,4 W/(m2K) - Q=35,3 kWh/(m3a) (Fig. 3).
III. Additionally, heat insulation of outer wall is improved up to U=0,3 W/(m2K), and heat insulation of attic is improved, too, and gabled roof has been built - Q=20,2 kWh/m3a.
This renovation project has been carried out.
IV. The windows are replaced by windows with heat insulating packets (U=2,0 W/(m2K)), "ideal" heat insulation of outside walls, ceiling of attic, and heated cellar (U=0,25W/(m2K)). In this case, the heat consumption for heating period decreases down to Q=14,3kWh/m3a, which would be much better than actual existing building-normatives for saving ofheat in Germany for such a house - Q=19,9 kWh/(m3a) (comparison see in Fig. 3).
The constitution of the windows in outer shell of the building is relatively small - 13 %.
Nevertheless, the change of them would decrease the necessary heat consumption by 30 %. Itcan be explained not only by decreasing of heat transfer coefficient from U=3,0 down toU=2,4, but principally due to increasing of the degree of the sealing of building from n=1,2down to n=0,7 - 9 (Fig. 3-5). The additional heat insulation of outside walls and building ofroof decreases the energy consumption to 40 % of the initial (Fig. 3,4).
Figures 4 and 5 demonstrate that part of the losses are covered internal and solar heat sources. In case of good heat insulation (IV), they cover up to half the heat losses of building.
The thermal inertia of building changes. It slightly influences the duration of heating season in correspondence with the calculation of marginal heating temperature (Fig. 6). Initially, thebuilding should be heated also in May and September to maintain the optimal thermalconditions (T=20ºC). The improving of heat insulation (in correspondence with calculationIV) decreases the heating season, i.e., heating for "ideal" improvement of heat insulation isnecessary only from October to March in climatic conditions of Latvia. Despite of the smallheat gain through outside walls and windows, they do not worsen substantially make notworse the heat consumption of building (Fig. 7). Gable roof not only decreases the heatlosses (Fig. 3 and 4), but also principally protects the building construction from action ofmoisture and creates an additional usable space. Therefore, the reconstruction efficiency cannot be valued only from aspect of saving heat energy. It must be stressed that reconstructionof ventilation system and integration in of forced ventilation (if absent) for such kind ofmunicipal building. Moreover, other factors should be included planning the investments:especially designing and maintaining of the longevity of building constructions, and alsosocial and human aspects.
Fig. 4. Normalised annual heat consumption of building in initial state (I)and various variants of renovation (II, III, IV) Fig. 5. Annual constitution of heat losses and heat sources Fig. 6. Monthly heat consumption in heating season Fig. 7. Annual constitution of heat losses through the elements of theouter shell of building Conclusions
Optimal way to check heat and moisture insulation of building structures is combination of thermographic express diagnostics and measurements of heat transfer andcharacteristic values air thickness. The obtained values represent an essential part of the data,which are collected in data base for modelling of heat losses and heat consumption ofbuildings basing on monthly heat balance [4]. Public and industrial buildings in Latviaapproved the approach of making measurements and calculations for multiform dwelling. Thescheme is used for the evaluation of economic efficiency to reconstruct the buildings at thephase of planing. The scheme can also be applied in other countries in Central and EasternEurope.
References
[1] Jakovics, A., Jekabsons, N., Mühlbauer, A., Trümmann, H.: Bestimmung des effektiven
Wärmedurchgangskoeffizienten von Bauelementen unter praxisnahen Bedingungen. Elektrowärmeinternational. - 1997, No A2, pp. 77- 83.
[2] Jakovics, A., Gendelis, S., Krievans, Z.: Determination of the heat transfer coefficient of construction elements for existing buildings. Progress in Engineering Heat Transfer - Gdansk, 1999, pp. 145-152.
[3] DIN V 4108-6. Wärmeschutz im Hochbau. Berechnung des Jahresheizwärmebedarfs von Gebäuden. 1995, [4] Jakovics, A., Gendelis, S., Krievans, Z.: Numerical modelling of heat losses and heat requirements for buildings. Latvian journal of physics and technical sciences. - 2000, No. 3, pp. 3-18.
[5] EN ISO 9972: Thermal insulation. Determination of building airtightness. Fan pressurization method. [6] ISO 6781: Thermal insulation. Qualitative detection of thermal irregularities in building envelopes. [7] Jakovics, A., Trümmann, H., Banga, A., Mühlbauer, A.: Einsatz der Gebäudethermografie zur Diagnostik der Wärmedämmung an osteuropäischen Hochbauten. Elektrowärme international. - 1998, No. A3, S. 93-100.
Authors
Dr.-Phys. Jakovics, Andris

Source: http://www.modlab.lv/publications/msr2001/pdf/190-197.pdf