Energy-efficient renovation of houses

InfraMation 2016 Application Paper Submission


Leen Lauriks, and Stijn Verbeke 

University of Antwerp, Faculty of Applied Engineering, Antwerp, Belgium 


Marianne Stranger 

VITO nv, Environmental Risk and Health Unit, Mol, Belgium




Energy consumption in existing buildings can be improved by carrying out energy-efficient renovations  such as placing façade insulation. This paper will determine whether infrared thermography as a non destructive tool can be used to evaluate the effectiveness of interventions on the building envelope. 

Within the RenovAir project (funded by Flemish Government), the quality of the indoor environment after  (energy-efficient) renovations in Belgian houses is studied. Infrared thermography is applied for  qualitatively checking the building envelope’s energy performance. Four renovation scenarios will be  reported in this paper for which measurements before and after renovation were carried out: window  replacement, façade insulation, base floor insulation and a combination of these three. Points of interest  will be identified from the IRT images (e.g. window lintel after placing cavity wall insulation) for which the  images before and after renovation will be compared. The same comparison will be carried out on heat  transport simulations, using the boundary conditions from the houses (e.g. geometrical characteristics)  and the measurements (e.g. weather conditions) as input data. Confrontation between both  methodologies (measurement and simulation) will clarify to which extent thermographic survey can be  used for quantitative assessment of energy-efficient interventions on building envelopes. 

The results show that a qualitative survey is possible, however quantitative analysis is limited due to  variating weather conditions (e.g. clouds influencing the sky radiation on the measured surfaces) between the two measurement moments (before and after renovation). The qualitative assessment was able to  identify whether or not known risks were encountered (e.g. condensation after window replacement). 



The implementation and enforcement of the European Energy Performance of Buildings Directive [1] has  introduced the development of policies and measures to reduce the energy use of buildings. This has  however also led to a number of challenges that need to be addressed, in terms of the impact of high  energy performance on the quality of the indoor climate of buildings, without compromising the comfort,  health and productivity of their occupants. As a consequence, the Flemish government (the Environment,  Nature and Energy Department “LNE” and the Flemish Energy Agency “VEA”) have initiated the Clean Air  Low Energy study in 2012 [2], in which the indoor air quality (IAQ) of energy-efficient, new built houses  and schools was assessed and compared to the more traditional building stock in this region. Even  though the outcomes of this study indicated a neutral, in some cases even positive effect on the indoor  environmental quality (IEQ), a considerable number of IAQ related health complaints in Flanders was  found to be related to renovations and refurbishments of indoor environments [3]. 

The RenovAir project is an explorative study of the indoor air quality in buildings (residences and schools)  before and after energy-efficient renovations running from 2014 until 2016. The Flemish government  funded this project aimed at targetted policy recommendations on energy-efficient renovations and air  filtration. The project consisted of two parts: one investigating the relation between the effectiveness and  the impact on the IAQ of energy-efficient renovations; and a second part studying the effect of air filtration  on the indoor environment in schools. The effectiveness of the renovation in the first part was studied by  using infrared thermography (IRT), wall humidity measurements and blowerdoor tests. The results of the  IRT assessments will be reported in this paper.



The selection of cases 

The impact of energy-efficiënt renovations on IEQ and the potential relation with its effectiveness, were  studied in 17 indoor locations in which 7 different renovation types were executed. All renovation types  selected for RenovAir, are representative energy-efficient measures for the Flemish building stock (e.g.  encouraged by the government by means of financial compensations). The cases studied in this project  

have been selected in such a way that the outcomes, which are obtained on a relatively small scale, can  be extrapolated to the rest of Flanders. 

The studied renovations include: window upgrade, floor insulation, façade insulation, wall treatment against rising damp, the installation of a mechanical ventilation system, air filter replacement as well as  initiatives consisting of several of the individual renovation initiatives. For each renovation type, at least  two cases (i.e. two buildings) were studied, of which at least one case was studied pre and post  renovation. Post renovation assessments took only place 6 months after the renovations. 

This paper will report only on the 11 dwelling renovations that have an influence on the insulation quality  of the building envelope: window upgrade, floor insulation, façade insulation and combined initiatives. 

The IRT measurement methodology 

International standards (e.g. NBN EN 13187) prescribe the boundary conditions for performing infrared  thermographic measurements for qualitative assessment of building envelopes. A constant temperature  and pressure difference across the envelope should be reached. More specifically, the thermographs  should be taken without the influence of solar radiation (which means at least some hours after sunset,  depending on the thermal mass of the structure), without the influence of rain (because humidity can  influence the heat transfer in materials), with a low wind speed (since air movement can influence the  heat exchange at the exposed surfaces) and with a temperature difference of at least 10 degrees Celsius  between indoor and outdoor temperature. In this study, the thermographic assessments were carried out  before or just after sunrise. The house owners were asked to leave the heating on during the night before  the measurement. Although the weather forecast was checked before scheduling an assessment, due to  the unpredictable weather and the large number of weather restrictions, the thermographic assessments  were carried out also when slight deviations from the conditions occurred (e.g. light rain during day before  the measurements). Table 1 gives an overview of the conditions under which the thermographic  assessments took place and Figure 1 and Figure 2 give an example of the results for dwelling no. 5. All measurements were carried out with a FLIR E60bx infrared camera. 

Complementary to the thermographic assessment, a visual inspection of the buildings concerning mould  and humidity problems is performed by using a standardised walk-through survey (based on the validated  survey from the European HITEA study,, which is made available by the project partner  THL, Finland). The resulting check-list contains weather conditions of the day before and during the  measurements, the used construction materials and heating installation of the dwelling and specifics  about the condition of the building (e.g. previous renovation interventions). 

The thermographic images were also used to indicate potentially interesting spots for the humidity  measurements. Building façade materials were checked for humidity problems, with specific checks at  locations of higher risk (e.g. thermal bridges after insulating the façade). The measurements were carried  out with a Testo thermohygrometer 635-2 with material moisture probe (measuring up to 20 weight% of  humidity in building materials).







The IRT measurement results will be compared both quantitatively and qualitatively with the results of  heat-transfer simulations of the known construction components. The static heat transfer simulations are performed using the software packages THERM and WINDOW  [4], [5]. The situation before and after the renovation will then be compared. Due to the different weather  conditions on the two measurement moments, a surface temperature difference between two points on a façade will be compared (e.g. between overall façade and lintel, example in Figure 3 and Figure 4) instead  of the nominal surface temperatures. 

For comparison reasons, the same conditions (interior and exterior temperature and relative humidity) as  during the IR measurements, were used for the simulations. The Belgian legislation was used for all other  boundary conditions (e.g. convection coefficients) [6], [7]. 

For correct simulations, the construction component compositions have to be known, both before and  after the renovation. Of the 5 dwellings that were studied both before and after the renovation, the owners  of three (namely dwelling nos. 1, 3 and 5) could provide this information. The properties of the materials  used in the construction components (e.g. the original brick façade) were sometimes not known, in which  case the reference material parameters from Belgian legislation was used [6].



Based on literature review and technological expertise of the project partners, the studied renovation  types are described with their known risks and benefits. 

Upgrade of windows can be done on different levels. In this study, only cases are studied where both the  glass and the window frame were replaced. In this case, the connection with the wall might be improved  by sealing the joint (e.g. with sprayed polyurethane insulation). 

Window upgrades can have a major influence on the air changing rate in the building when a ventilation  system is not present or foreseen to be installed. When the upgrade of the window is carried out before  the insulation of the walls, two important aspects have to be addressed. First, the risk for condensation  

and mold growth on the walls and/or construction joints increases due to the replacement of single glass  by double glass (or another energy improved glass). Secondly, the position of the window and the  window-sills is important in relation to the future wall insulation, so this should already be taking into  account during the upgrade of the windows. 

The techniques for insulation the floor of a building are as diverse as the floor compositions: a solid floor  on the ground, a wooden beam floor above a cellar, a brick arched floor, etc. The presence of a cellar or  crawl space can offer a solution to insulate the ground floor from below. This solution can also remediate  odour and humidity problems from these underground spaces [8]. A wooden beam floor can be insulated  in between the beams. Solid floors can also be insulated by adding an insulation layer on top of the floor  

(with new floor finishing), which increases the floor thickness (with influence on e.g. stairs). Sprayed  polyurethane insulation (on top of the floor) is often used in this case because of the limited thickness  (good insulation value and possible integration of ducts in insulation layer). This last case was used in  both cases in this study. 

Façade insulation can be placed on the exterior side, the interior side or in the cavity of the wall. For this  paper, only cavity wall insulation cases were studied. 

Cavity wall insulation can be installed on cavity walls that meet some criteria [9] on the width and  orderliness of the cavity, the humidity load on the wall, and the condition and characteristics of bricks and  joints of the outer leaf. The cavity can be injected with an expanding synthetic foam (PUR or UF), with  fibrous flakes (rock wool or glass wool) or with bonded or unbonded granulates (e.g. EPS granules,  perlite, etc). The injection technique is strongly influenced by the injected material. 

Advantages of cavity wall insulation are possible improving air tightness of the wall and avoiding the loss  of space. Points of attention are possible interruptions of the cavity (due to water drains or other ducts,  due to construction joints, due to mortar excess of the joints, etc). 

Combinations of the above described interventions are considered relevant because specific influences  on the indoor air quality might be interacting. For example the increased condensation risk for the walls when upgrading the windows, can be reduced by installing a ventilation system, by insulating the walls or  by a combination of both of them.



In this chapter, a summary of the results will be presented, of both the IRT measurements and the heat  transfer simulations. 


Window upgrades 

The impact of window upgrades on the indoor environment was studied in two houses. Both detached  houses are constructed around 1990, and are situated in a rural environment. Both residences have an  insulated roof and insulated floorings, and have a new heating installation. Whilst house no. 1 has façade  insulation as well as a ventilation system type A (natural ventilation), house no. 2 has no insulated walls  and no mechanical ventilation system or trickle ventilators. Both dwellings upgraded their windows from  older to modern double glazing, thus improving the heat transfer coefficients of the glazing. Although  there was no obligation, both cases comply with the EPB regulations for new buildings (which is a  demand for receiving subsidies for this intervention in Belgium). 

When comparing the IRT measurements with the heat-transfer simulations for house no. 1, there seems to be no logical relation between both of them. The simulation shows a logical pattern: the surface  temperature of the window glazing is lower after than before the renovation, thus leading to a lower  surface temperature difference with the insulated wall. However, the IRT measurements on the window  glazing showed a higher temperature after the renovation than before. This relation cannot be explained  by the interior or exterior air temperatures. However, the sky radiation during the night before the IRT  measurement was not registered, which could have a large influence on the IRT measurements.  Quantitative comparison is therefore not relevant for this case. 

The qualitative assessment of the IRT measurements show that in both houses, the surface temperatures  on windows were found to be low (much lower than indoor temperatures), meaning that the windows are  well insulated. In house no. 2 (where the walls were not insulated) condensation occurred on the interior  side of the upgraded glazing in the rooms that were occupied during the night before the measurement.  Condensation on the walls is a known risk when upgrading windows, however the condensation in house  no. 2 appeared on the window surfaces themselves. No other humidity problems were detected during  the walk-through surveys of both dwellings. 

The risk of creating new thermal bridges was not noticed in these cases, however, it was noticed that  thermal bridges present before the renovation are maintained after the renovation. 


Floor insulation 

Both houses that were studied to assess the impact of floor insulation on the indoor environment, were  constructed between 1950 and 1960. House no. 3 is a terraced dwelling, whilst house no. 4 is a detached  house. In house no. 3, the studied PU foam floor insulation is the first insulating initiative. The renovation  intervention in house no. 3 is not in compliance with the EPB regulations (which was not an obligation),  which is often not feasible due to height restrictions for the floor composition. House no. 4 has insulated  windows, façade- and roof insulation; the installation of PU foam floor insulation is a final step of a  thorough energy-efficient renovation of the house. 

The comparison between IRT measurements and heat-transfer simulations was carried out for house no.  3. For house no. 4, no information on the construction components was available. The comparison for  house no. 3 was carried out on the temperature difference between the surface temperatures of the floor  and a party wall. The IRT measurements and simulations show a good accordance: the floor temperature  is lower than the wall temperature, but the difference decreases after the renovation, although this  decrease is lower when measured with IRT than the simulation prediction. The qualitative assessment of the IRT measurements show that in neither of both houses, thermal  bridges at the junctions with other construction elements (e.g. interior walls) were detected. In house no. 3, a distinct difference in surface temperature of the floor could be detected (from 3°C below indoor air  temperature before renovation to equal to indoor air temperature after renovation).


Façade insulation 

Whilst dwelling no. 5 is located in a woody environment, dwelling no. 6 is located in a more rural area. In  both cases, window, floor and roof insulations were yet applied before the measurements were  performed. Both cases insulated their façades by placing cavity insulation, thus improving the heat  transfer coefficients of the façades. Both cavities were filled with 5 cm of insulation (the minimum  thickness for technical feasibility). Although there was no obligation, both cases comply with the EPB  regulations for post-insulation of cavity walls (and with the requirements of maximum thermal conductivity  of the applied insulation material for receiving subsidies in Belgium). 

When comparing the IRT measurements with the heat-transfer simulations for house no. 5 (Figure 1Figure 2, Figure 3, Figure 4), the window lintels are consequently warmer than the overall front façade,  although the effect is more apparent at the IRT measurements than on the simulations. The difference  increases after the renovation, thus making the thermal bridges at the window lintels even more  prominent. 

Thermal bridges appearing at the junctions in the façades (e.g. window lintels) are known risks when  placing façade insulation. The qualitative assessment of the IRT measurements revealed this thermal  bridges at the window lintels in dwelling no. 5 but not in dwelling no. 6. In dwelling no. 6, heat leakages  around the doors in the façades are present, probably caused by air leakages. 


Combined renovations 

In total 5 different cases of thorough renovations were studied in RenovAir, of which one was studied  before and after these renovations. The other 4 cases were only studied after the renovation. The studied  cases contain houses constructed between 1933 and 1972, including 2 semi-detached houses, 2  detached houses and one terraced house (the latter house was studied before and after the renovation  took place, house no. 11). All cases included window upgrades and insulation of the façade, roof as well  as floor. House no. 7 and 8 have been renovated in phases, spread over a time frame of 3 years  maximum. Houses 9 until 11 have been renovated in a shorter period of time, with renovation durations of  less than 1 year. 

The heat transfer coefficients of all parts of the thorough renovation cases, were drastically improved.  None of the studied cases were obliged to be in compliance with the EPB regulations. The included  window and insulated roof upgrades however were for all cases in compliance, while the façade  insulation was only in some cases in compliance. The upgraded floor insulation was for all cases not in  compliance (similar to effectiveness of separate floor insulation intervention). 

Due to missing information on the construction component composition, no heat transfer simulations of  these cases was performed. The IRT measurements were thus qualitatively analysed. 

In house no. 11, thermal bridges were detected (e.g. ring girder at the base of the sloped roof in front and  rear façade) both before and after the renovation (Figure 8 and Figure 9). These thermal bridges became  more pronounced after the renovation (although part of that phenomenon is caused by higher air  temperature difference between interior and exterior). 

Heat leakages around the joinery parts in the façades are present in most of the studied cases (garage in  house no. 8, front door in house no. 7, 9, and 11), probably caused by air leakages (Figure 6). Thermal  bridges (window lintels in house no. 7 and 8, and ring girder in house no. 11) are often visible in the thermograms (Figure 7). There seems to be no relation between the phasing of the renovations and the  presence of these thermal bridges.






The effectiveness of the energy-efficient renovations was checked both quantitatively and qualitatively. 

The quantitative assessment (by comparing IRT measurements with heat-transfer simulations) was  performed for 3 cases. Two of them showed good accordance, although still with some nominative  differences. One case showed no relation between IRT and simulations. Large as well as smaller  deviations are probably caused by unknown factors (e.g. sky radiation during night before the IRT  measurements). More monitoring is thus necessary to assess the effectiveness of the renovations  quantitatively. 

On a qualitative level, the effectiveness can be clearly checked. For example, the identified risks for each  renovation type (e.g. thermal bridges when upgrading windows) could be assessed by using IRT  measurements, combined with a walk-through survey. 

For window upgrades, the risk for condensation was determined in one of both cases. No new thermal  bridges were encountered; however, the existing ones were preserved. For floor insulation, the risk of  creating new thermal bridges at connection with other construction elements, was not encountered in  either of both houses. The risk of creating thermal bridges after placing cavity wall insulation was however  determined in one of both studied dwellings. Finally, heat leakages around the joinery parts in the façades  are present in most of the studied combined renovation cases. Thermal bridges are often visible although  there seems to be no relation with the phasing of the renovations.


Based on the qualitative assessment of the effectiveness of the energy-efficient renovations, the following  recommendations for policy makers were formulated:


1. According to the RenovAir data, thermal bridges present before the renovation are in some cases  found to be more pronounced post renovation. The air tightness around the joinery in the façades  was a point of particular interest in the combined renovation cases. Older front doors and garages  clearly showed heat losses around the frames. The awareness for these phenomena could be  raised (e.g. by incorporating it in subsidies or by mandatory follow-up of renovation planning by  an expert). 

A similar finding was reported in the Finland ‘Mould and Moisture programme’ [10]. In a new  Decree (REF) set by the Social Affairs and Health Ministry in Finland, the government offers the  possibility for a professional house inspection (person with qualifications according to  requirements set by the government) in houses with health hazards, who formulates  recommendations for a suitable renovation of the private dwelling. 

2. Based on known risks out of the literature review from the RenovAir project, the risks associated  with combined renovations can be limited by incorporating future renovation phases in the  technical execution of earlier renovation. For example, for wall insulation and replacing windows,  this principle is already integrated in the Belgian subsidies policy (by providing larger subsidies  when both interventions are executed within 12 months from each other). An extension of this  principle (e.g. combination of insulation measures with installing a ventilation system) as well as  more elaborate technical requirements for receiving subsidies (e.g. which is already in place for  cavity insulation) could be interesting to increase the quality of future renovation projects.



1. EPBD, “Energy performance of buildings directive 2010/31/EU (recast),” Off. J. Eur. Union, pp.  13–35, 2010. 

  1. Stranger, S. Verbeke, J. Laverge, D. Wuyts, J. Lauwers, K. De Brouwere, L. Verbeke, D.  Poelmans, F. Boonen, A. Janssens, and B. Ingelaere, “Clean air, low energy,” 2012. [3] M. Stranger, F. Geyskens, L. Verbeke, R. Swinnen, D. Poelmans, W. Swaans, and E. Goelen,  “Surveillance van klachtenvrije woningen,” 2012. 

2. LBNL, “THERM.” Lawrence Berkeley National Laboratory - Environmental Energy Technologies  Division - Building Technologies Department, Berkeley, 2015. 

3. LBNL, “WINDOW.” Lawrence Berkeley National Laboratory - Environmental Energy Technologies  Division - Building Technologies Department, Berkeley, 2016.

4. Transmissie referentie document. Belgium, 2010, pp. 74848–74936. 

5. NBN, “NBN EN 12831 ANB: Heating systems in buildings - Method for calculation of the design  heat load - National annex,” Brussel, 2015. 

6. E. Mlecnik, L. Vandaele, and A. De Herde, “Low energy housing retrofit (LEHR), Final report,”  Brussels, 2010. 

7. A. Janssens and J. Wijnants, “TVN 246: Na-isolatie van spouwmuren door het opvullen van de  luchtspouw,” 2012. 8. Ministry of the Environment, “Moisture and mould programme,” 2013. [Online]. Available: [Accessed: 31-Jul-2016].



The author wishes to first of all thank the Flemish Government and more specifically the Environment,  Nature and Energy Department “LNE” and the Flanders Environment Agency “VMM”, for funding this  project. The author also shows gratitude to all the project partners: The Flemish Institute for  Technological Research “VITO”, THL Finland (the Environmental Microbiology Unit, National Institute for  Health and Welfare), Ghent University (Architecture and Urban Planning department), PROVIKMO vzw (Study and documentation service) and NAV (the Flemish Architects organization). 



Leen Lauriks got her PhD in Engineering Sciences in 2012. She is a professor at the University of  Antwerp and part of the Green Building Engineering section of the EMIB Research Group. Her research  focuses on in-situ measurement techniques and protocols (e.g. IR thermography) on existing buildings  and energy retrofitting. She teaches in construction techniques and materials.

Related Articles