Interior Facade Insulation for Air-Conditioned Buildings in Tropical Climates - Energy Simulation using 'Ladybug Tools'

One of the principles in building physics is to apply thermal insulation always on the 'cold side' of facades. That is why most buildings in cold or temperate climates have exterior thermal insulation. The same principle should apply to air-conditioned buildings in hot climates, in this case with thermal insulation on the inner side of the facades...

To examine the effects of interior thermal insulation, I made a building energy simulation of a simple office building floor located in a hot-humid climate using 'Ladybug Tools' software.

 

Location

As mentioned in one of my previous posts, buildings in hot-humid climates near the equator present a challenge regarding energy-efficient concepts. For this reason, I chose Jakarta which has a hot-humid, more precisely a tropical monsoon climate (Am) according to the Köppen climate classification. Hot-humid climates are characterised by mostly overcast skies, high humidity and frequent rainfall. Air temperature is always high with minimal daily and seasonal fluctuations.

Model

The building model which served as the base for the energy simulation was made in Rhino 3D. I tried to keep the model as simple as possible. The basic parameters of the model are:

• dimensions: plan 6m x 8m, storey height 3,5m, 48m², 168m³, representing one thermal zone
• windows: lowE windows in south and north facade, south window with permanent horizontal shading device, U-value: 1.3 W/(m²K), SHGC: 0.3, VT: 0.64
• materials: 16cm concrete exterior walls at all for sides, concrete slabs connecting to upper and lower storey with identical thermal conditions
• insulation: mineral insulating board, conductivity 0.045 W/(mK), thickness between 20mm and 100mm
• thermal mass: 10m² of 16cm thick concrete wall
•  program: air-conditioned open office use with default schedules and internal loads as provided by EnergyPlus
• no natural ventilation

Screen shot of the Rhino model serving as a base for the energy simulation

Building energy simulation

The building energy simulation was made with 'Ladybug', a plug in for Grasshopper, the graphical algorithm editor for Rhino 3D. Ladybug in turn uses EnergyPlus as calculation engine and some other programs for it's building energy simulations. The good thing about these programs is that they are all for free, except for Rhino 3D which can be used for free during a 90-day trial period. The following illustration shows the Grasshopper canvas (i.e. the graphical algorithm representation) of this simulation.

Grasshopper canvas of the energy simulation

The grasshopper file can be downloaded here.

In this simulation , the annual amount of cooling Energy in kWh/a was calculated. In total there were six simulation runs with different facade constructions:

1. 16mm concrete walls, no thermal insulation
2. 16mm concrete walls with internal insulation, thickness: 20mm
3. 16mm concrete walls with internal insulation, thickness: 40mm
4. 16mm concrete walls with internal insulation, thickness: 60mm
5. 16mm concrete walls with internal insulation, thickness: 80mm
6. 16mm concrete walls with internal insulation, thickness: 100mm  

 

Results

Ladybug provides various tools to graph the simulation results. The following illustration is a 3-dimensional graph of the annual cooling energy for ideal air loads of the air-conditioned building without thermal insulation.

Annual cooling energy - exterior walls without insulation

Due to the climate zone, the cooling loads are similarly high throughout the year with no seasonal differences. The daily cooling is characterised by low loads at night/early morning and high loads during office hours. The hourly cooling loads have their peaks at around 4 PM with up to 5,5 kW.

By adding a layer of thermal insulation at the interior of the facade concrete walls the cooling loads can be significantly reduced. The next graph shows the annual cooling loads of the same building, this time with 40mm interior insulation. In this case, the hourly cooling loads rarely exceed 4kW.

Annual cooling energy - exterior walls with 40mm interior insulation

By combining the results of all six simulations in one line chart, the effect of internal thermal facade insulation of different thicknesses becomes more clear.

Conclusion

As shown in the above line chart, the annual cooling energy can be drastically reduced through the application of internal thermal facade insulation. In the present example, the annual cooling energy could be reduced by more than 20% by adding a 20mm thick thermal insulation layer. With a 40mm thick insulation layer the energy savings are more than 25%. 

Further increasing of insulation thickness has only a small effect on cooling energy savings.

Europe's Biggest Energy Consumers - Some More Data Analysis

This time, I will take a closer look at the energy consumption and energy efficiency of European countries. Eurostat Database has a plethora of data on these topics, which can be downloaded for free. I use the programming language Python with Pandas to analyse the data and visualise the results as diagrams.

Primary energy consumption in Europe

Primary energy consumption measures the total energy demand of a country excluding all non-energy use of energy carriers (e.g. natural gas used not for combustion but for producing chemicals) (1). Data was retrieved from the Eurostat database and downloaded in csv format (2).

As you can see in the following line chart showing the EU as a whole, there is a slight decrease in primary energy consumption in the course of the years 2005 to 2015. However, 2015 again shows an increase compared to 2014.

 

The following bar chart shows the primary energy consumption of individual European countries for the most recent year available (2015).

Share of renewable energy in Europe 

Primary energy consumption itself doesn't say anything about the way the consumed energy is produced. To get an idea of how "clean" the consumed energy is I generated some graphs based on the Eurostat data: "Share of renewable energy in gross final energy consumption" (3).

The line chart above shows a shallow linear rise in the share of renewable energy from 2006 to 2015. Interestingly, there is no acceleration in growth of renewable energy share during the given period.

As you can see in the next bar chart, the biggest primary energy consumers in Europe (Germany, France, UK, etc.) have a comparatively small share of renewable energy in gross final energy consumption in 2015.

 

Primary energy consumption per capita in Europe

It's obvious that Europe's biggest primary energy consumers also seem to be countries with rather high productivity and a big population. Therefore, I think it makes sense to put the primary energy consumption in relation to the population and the gross domestic product.

To analyse the primary energy consumption per person, I combined the previously shown data with the Eurostat data on population of European countries (4).

The graph on primary energy consumption per capita in the EU above shows a downward trend very similar to the absolute energy consuption in the EU (see first graph). As you can see, every EU citizen consumed about three tonnes of oil equivalent in 2015.

However, if you take a look at individual countries in 2015 (see below), there is a big difference to the absolute values on primary energy consumption: Europe's biggest primary energy consumers per capita are rather small countries like Iceland or Luxembourg. Countries with a big population and high GDP tend to be found in the mid-range.

Energy intensity in Europe

A second approach would be to relate primary energy consumption to the gross domestic product (GDP). For this purpose I combined the previously shown data on energy consumption with the Eurostat data on gross domestic products of European countries (5). 

Energy consumption in relation to GDP is called energy intensity. According to Wikipedia, "energy intensity is a measure of the energy efficiency of a nation's economy. It's calculated as units of energy per units of GDP." (6)

As you can see in the next bar chart, some of the countries with the highest primary energy consumption in Europe (UK, Italy, Germany) have a comparatively low energy intensity.

Data Visualisation

As in my last post, all raw data was retrieved in the form of csv-files from the respective websites. All graphs were made in Python with the modules Matplotlib, Seaborn and Pandas.

Using Python in combination with pandas is a great way to analyse and edit raw data and combine rdata frames in new ways. With Matplotlib and Seaborn you can visualize the edited data according to your requirements.

If you are interested in how I generated the diagrams in detail, you can view the code that I wrote to analyse energy consumption per capita as a Jupyter notebook here.

 

 References

(1) Primary Energy Consumption, Eurostat Database. Retrieved 21 January 2018.
(2) ibid.

(3) Share of renewable energy in gross final energy consumption by sector, Eurostat Database. Retrieved 21 January 2018.

(4) Population on 1 January by age and sex , Eurostat Database. Retrieved 21 January 2018.

(4) GDP and main components , Eurostat Database. Retrieved 21 January 2018. 

(6) Energy Inensity, Wikipedia. Retrieved 21 January 2018.

Key Factors of Global Energy Consumption - Data Analysis with Python

Buildings account for a large portion of global energy consumption. The future development of global energy consumption depends on three key factors: population increase, economic development and energy intensity (1). In this post, i am going to visualise publicly available raw data related to these key factors with diagrams made in python with matplotlib.

 

Global population increase until 2015

World population data was retrieved from the United Nations world population prospects website. The available data covers the years 1950 to 2100. The first chart is a stacked area chart showing the population of the all world regions until 2015.

 

To show the differences in population development of the separate regions more clearly, I plotted the same data as a multiple chart. 

 

Global population increase, future development

The figures from 1950 to 2015 are estimates, the figures after 2015 are projections based on the 'medium fertility variant'. The red dashed line shows the year 2015 representing the separation between estimates and projections.

This multiple chart shows the population separately for each region. Again, the raw data is the same like in the graph above.

 

Global economic growth

The following diagram shows the average annual GDP growth per capita and by country for the last five years (2011 - 2016). The raw data was retrieved from the World Bank Data Bank. Countries with dark purple colours have the highest GDP growth.

This is a screen shot of the original svg-file that can be downloaded here. The svg-file is interactive and shows the GDP growth figures of each separate country.

Global primary energy consumption

The most comprehensive data on primary energy consumption by regions I could find was the BP Statistical Review of World Energy 2016.  It covers the years 1965 to 2015. Unfortunately, the classification by regions is slightly different from the previous graphs. Note that this data includes only commercially-traded fuels (coal, oil, gas), nuclear and modern renewables used in electricity production. As such, it does not include traditional biomass sources. Unfortunately, the classification into regions doesn't correspond exactly to the one used for the data on population growth.

Next, you can see the same data as a multiple chart showing the individual regions.

Data Visualisation

All raw data was retrieved in the form of csv-files from the respective websites. The interactive world map showing the GDP per capita growth 2011 - 2016 was made in Python with the Pygal module. All other graphs were made in Python with the modules Matplotlib, Seaborn and Pandas.

 

References

(1) cf. Daniels, Klaus; Hammann, Ralph E.: Energy Design for Tomorrow, S. 104

U-value optimisation of curtain walls - Part 3

After having dealt with the technical possibilities of thermal optimisation in my last post, this time the focus will be on design aspects: how do changes in facade layout, module size and operable insert units affect the thermal performance of curtain walls?

Reducing operable insert units

The amount of facade profiles (mullions, transoms and frames) affects the Ucw-value in two ways: first, by the U-values of the profiles (which are usually higher than the U-values of the infills/ panels), secondly, by the additional Ψ-values caused by the higher amount of contacting facade components. Therefore, it seems obvious to reduce the amount of operable insert units and replace them with fixed glazing.

The following illustration shows a sequence of our reference curtain wall layout from the previous posts. Below this is the same layout, but this time without operable window units.

As already described in the last post, the U-values of the reference curtain wall layout (i. e. with operable insert units) are as follows (1):

• with standard components:

Ucw = (ΣA×U + ΣΨ×l) / Acw
Ucw = (10.434 W/K + 4.534 W/K) / 12.00 m²
Ucw = 1.2 W/(m²K)  (1.247)

• with thermally optimised components:

Ucw = (ΣA×U + ΣΨ×l) / Acw

Ucw = (5.742W/K + 1.012 W/K) / 12.00 m²

Ucw = 0.6 W/(m²K)  (0.563)

The U-values of the same curtain wall without operable insert units are:

• with standard components:

Ucw = (ΣA×U + ΣΨ×l) / Acw
Ucw = (10.024 W/K + 4.072 W/K) / 12.00 m²
Ucw = 1.2 W/(m²K)  (1.175)

• with thermally optimised components:

Ucw = (ΣA×U + ΣΨ×l) / Acw
Ucw = (5.540 W/K + 0.848 W/K) / 12.00 m²
Ucw = 0.5 W/(m²K)  (0.525)

The use of fixed glazing instead of operable insert units improves the Ucw-value by 6% (standard) and 7% (thermally optimised). By rounding the value to one decimal place in compliance with the standard,  the Ucw-value of the thermally optimised version can be lowered from 0.6 to 0.5 W/(m²K).

It should be noted that - despite these improvements in U-values - natural ventilation by means of operable windows generally has a positive effect on thermal comfort and occupant satisfaction (2).

Wider facade module

Another way of reducing the percentage of facade profiles in the total area of the curtain wall is to use a wider facade module.

Changing the facade module from 1.5 m to 2.0 m - as shown above - yields the following results:

• with standard components:

Ucw = (ΣA×U + ΣΨ×l) / Acw
Ucw = (13.655 W/K + 5.224 W/K) / 16.00 m²
Ucw = 1.2 W/(m²K)  (1.180)

• with thermally optimised components:

Ucw = (ΣA×U + ΣΨ×l) / Acw
Ucw = (7.550 W/K + 1.157 W/K) / 16.00 m²
Ucw = 0.5 W/(m²K)  (0.544)

This means that a facade module increased by 33 % like in this case improves the Ucw-value by 6% (standard) or 3% (thermally optimised).

Additional transoms

The horizontal division of a curtain wall can be modified, too. Often, transoms are added for aesthetic reasons.

For example, the rather large insulated aluminium panel could be replaced by a smaller opaque spandrel panel and an additional glazing unit with a transom at parapet height (see above). In this case, the Ucw-values would be as follows:

• with standard components:

Ucw = (ΣA×U + ΣΨ×l) / Acw
Ucw = (13.322 W/K + 3.994 W/K) / 12.00 m²
Ucw = 1.4 W/(m²K)  (1.443)

• with thermally optimised components:

Ucw = (ΣA×U + ΣΨ×l) / Acw
Ucw = (7.111 W/K + 1.248 W/K) / 12.00 m²
Ucw = 0.7 W/(m²K)  (0.697)

The modified curtain wall layout has a Ucw-value worse by 14 % (standard) and 19 % (optimised) than the reference layout. However, this deterioration in Ucw-value results not only from the additional transoms but also from the higher percentage of glazed areas (opaque insulated panels usually have lower U-values than glazed areas).

Thus, the reference curtain wall layout with less transom and glass areas has a Ucw-value that is much better than the modified layout.

More glass

In this example, the percentage of opaque panels was decreased in favour of bigger glazed areas. This leads to the following U-values:

• with standard components:

Ucw = (ΣA×U + ΣΨ×l) / Acw
Ucw = (13.299 W/K + 3.532 W/K) / 12.00 m²
Ucw = 1.4 W/(m²K)  (1.403)

• with thermally optimised components:

Ucw = (ΣA×U + ΣΨ×l) / Acw
Ucw = (7.169 W/K + 1.079 W/K) / 12.00 m²
Ucw = 0.7 W/(m²K)  (0.687)

Although there are no additional transoms like in the previous example, yet the Ucw-values have deteriorated similarly. The main reason for this is the increased frame area (Af) due to the bigger size of the operable window.

Here, too, the reference curtain wall layout has a better thermal performance than the modified layout.

Conclusion

The geometrical optimisation too, has a positive effect on the thermal transmittance of curtain walls. However, the geometrical modifications described here have fewer positive effects on the Ucw-value than the technical optimisation of the individual facade components (as described in the previous post, the Ucw-value could be reduced by half by means of technical optimisation). 

Only two of the four modifications examined here led to an improvement in Ucw value. This suggests that the reference curtain wall layout already has relatively favourable thermal insulation properties due to its simple layout.

In any case, the examined cases allow the conclusion that the following design measures have a positive effect on Ucw-values:

• fewer facade profiles (mullions, transoms, frames)
• larger facade modules
• fewer operable window units
• fewer glazed areas and more opaque insulated panels

The best Ucw-values can be achieved by combining the geometrical optimisation of the facade layout with the technical optimisation of the individual facade components.

It should be mentioned that when dealing with very low U-values, rounding the value to one decimal place can lead to excessive generalisations.  For example, results ranging from 0.550 to 0.649 W/(m²K) yield a Ucw-value of 0.6 W/(m²K), although the difference between the two limit values is 15 %.

References

(1) All calculations of thermal transmittance were performed acc. to ISO 12631 (first edition 2012-10-01), Thermal performance of curtain walling - Calculation of thermal transmittance (Reference number: ISO 12631: 2012(E)) 
(2) Cf. Brager, Gail S.; de Dear, Richard: Climate, Comfort and Natural Ventilation, Berkeley 2001, p. 5 et sqq.

U-value optimisation of curtain walls - Part 2

In my last post I have described some basics of the Ucw-value calculation and defined the layout of a reference curtain wall. This time, the U- and Ψ-values ​​of each facade component will be determined in order to calculate the U-value of the complete curtain wall. Then I will try to answer the question of how far the Ucw-value can be reduced by means of thermal optimisation of each facade component.

Many system manufacturers of aluminium curtain walls offer a standard version of their profile systems with lower thermal insulation and a high-insulation version, often called "Passivhaus-tauglich" (i.e. meeting passive house standards) in German-speaking areas. The same can be said for glazing units: one can distinguish between standard double-glazing units and highly insulating triple-glazing units. I will therefore examine the Ucw-value of our reference curtain wall layout with "standard" components and then a version with thermally optimised facade components.

Ucw-value: "standard" version

Table A.1 in ISO 12631:2012 (1) provides a guideline for calculating the thermal transmittance of curtain walling. It describes how to obtain the necessary U-and Ψ-values of the individual facade components for calculating the overall Ucw-value. In most cases, there are two ways to determine these values: they ​​can be found in the corresponding tables of the standard, or they are determined by using calculation or measurement methods set in the standard (usually this is done by the product manufacturer).

The values given in the tables of the standard are usually on the safe side. The U- or Ψ-value of a particular product which has been calculated or measured by the manufacturer is usually lower and therefore better.

Frames

According to the above table A.1, the values ​​for frames, mullions and transoms Uf, Um and Ut can be determined by a numerical calculation method specified in 10077-2:2012, or they are measured according to EN 12412-2:2003. All major manufacturers of curtain wall systems provide the relevant U-values determined according to these standards.

The U-value for a particular mullion or transom depends on the thickness of the filling element and the internal depth of the profile. With a filling element of eg. 28 mm thickness (double-glazing unit) and an internal profile depth of 150 mm, one can assume Ut-and Um-values of about 2.1 W/(m²K) (2). This applies to standard systems without additional thermal insulation measures. The U-values ​​of a standard frame of an operable insert unit also vary depending on the manufacturer and product, with an average value of about 1.8 W/(m²K). Therefore, for our reference curtain wall, we assume the following values:

-> Ut/ Um: 2.1 W/(m²K)

-> Uf: 1.8 W/(m²K)

The values ​​of linear thermal transmittance Ψm,f and Ψt,f can be taken from table B.6, ISO 12631:2012, or they can be calculated according to ISO 10077-2:2012. Table B.6 provides five different junction types and assigns Ψm,f- and Ψt,f-values ​​to these types ranging from 0.05 to 0.11 W/(mK) to. In our case, we assume an average value of 0.07 W/(m²K).

-> Ψm,f; Ψt,f: 0.07 W/(mK)

Glazing

The values for Ug can be taken from ISO 10077-1:2006 or determined by calculation and measurement methods specified in EN 673:2011, EN 674:2011 and EN 675:2011. State of the art for double-glazing units is a Ug-value of about 1.1 W/(m²K).
-> Ug: 1.1 W/(m²K)

The values of linear thermal transmittance Ψf,g, Ψm,g and Ψt,g can be taken from tables B.1, B.2, B.3 and B.4 or calculated according to 10077-2:2012. The standard distinguishes between "normal" and "thermally improved" types of glazing spacer bars. Moreover, the table values ​​depend on the type of neighbouring profiles and of the glazing type. For normal spacer bars with low emissivity glass and aluminium mullions and transoms, Ψm,g- and Ψt,g-values are 0.11 W/(mK). For a metal frame with thermal break, the Ψf,g-value is also 0.11 W/(mK).

-> Ψm, g; Ψt, g; Ψf, g: 0.11 W/(mK)

Panels

The thermal transmittance of panels Up can be determined by calculation methods specified in ISO 6946:2007. Since the thickness of the insulating layer in the panel is crucial for the Up-value, we can for now neglect the inner and outer metal cladding as well as other components of the panel. With an insulation thickness of 140 mm and a thermal conductivity λ: 0.035 W/(mK) we reach a U-value of 0.24 W/(m²K).

-> Up: 0.24 W/(m²K)

The Ψp-values can be taken from table B.5 or established by calculations specified in 10077-2:2012. According to table B.5, Ψp-values depend on the thermal conductivity of the spacer, the panel type (Type 1: with air-filled space; Type 2: without air-filled space), as well as on the materials of the inner and outer panel cladding. Thus, panel type 2 with aluminium cladding on both sides and spacers with λ: 0.2 W/(mK) has a Ψp-value of 0.2 W/(mK).

-> Ψp: 0.2 W/(mK)

Determination of Ucw-value

All the necessary data are thus set for the calculation of the reference facade. The following two tables show the subtotals for ΣA×U and for ΣΨ×l.

The Ucw-value for the facade with standard components is thus:

Ucw = (ΣA×U + ΣΨ×l) / Acw

Ucw = (10.434 W/K + 4.534 W/K) / 12.00 m²

Ucw = 1.2 W/(m²K) (1,247)

Ucw-value: thermally optimised version

Frames

High-insulation curtain wall systems usually have an additional insulating body between the profile on the room side and the pressure plate on the external side. With a filling element of eg. 44 mm thickness (triple-glazing unit), Ut- and Um-values can get as low as about 0,8 W/(m²K) (due to the on-going development of all facade components, these and many other values mentioned here might be outdated soon again).

-> Ut; Um: 0.8 W/(m²K)

Additional insulation inserts and glass rebate insulation in frames of an operable insert unit can significantly reduce the Uf-value. Most manufacturers have high-insulation frames with a U-value of 1.0 to 1.2 W/(m²K).

-> Uf: 1.1 W/(m²K)

As described above, Ψm,f and Ψt,f can be taken from table B.6, ISO 12631:2012, or they can be calculated according to ISO 10077-2:2012. Values of about 0.025 W/(mK) are currently possible.

-> Ψm,f; Ψt,f: 0.025 W/(mK)

Glazing

Ug-values for a triple-glazing unit currently range between 0.7 and 0.5 W/(m²K), depending on the glass coatings and the gas used for the space between the glass panes.

Ψf,g, Ψm,g, Ψt,g can also be determined by calculations according to EN ISO 10077-2:2012. In Germany, the working group "Warme Kante" (warm edge) has published data sheets for thermally improved glass edges (spacers) of various manufacturers. With some products, values for a triple-glazing unit used in an operable insert unit can be reduced to 0.030 W/(mK). For our case we use slightly higher values:

-> Ψm,g; Ψt,g; Ψf,g: 0.04 W/(mK)

Panels

The thickness of opaque insulated facade panels is usually limited by the depth of the facade structure. If, for example, the mullion and transom profiles are 150 mm deep, the insulation thickness in the panel usually cannot exceed 150 mm. Another optimisation method would be the use of insulating materials with a lower thermal conductivity. But even here optimisation options are limited.

Vacuum insulation panels (VIPs) could be a promising alternative. The use of these panels has not yet become widely accepted. However, one can assume that this insulation method will soon become more popular especially for curtain wall systems. Particular advantages are insulated panels which have the same thickness like glazing units as well as a high degree of prefabrication due to  standardised curtain wall modules.

Vacuum insulation panels which have the same thickness like a triple-glazing unit currently have Up-values of 0,15 to 0,2 W/(m²K) and Ψp-values of about 0,02 W/(mK). Here, too, one can expect further improvements in the near future due to the continuous development of this relatively new panel type.

-> Up: 0.18  W/(m²K)
-> Ψp: 0.02 W/(mK)

Determination of the Ucw-value

Thus, we have all the necessary data to calculate the thermal transmittance of the optimised curtain wall. The following two tables show again the subtotals for ΣA×U and for ΣΨ×l.

The Ucw-value for the facade with standard components is thus:

Ucw = (ΣA×U + ΣΨ×l) / Acw
Ucw = (5.742W/K + 1.012 W/K) / 12.00 m²
Ucw = 0.6 W/(m²K)  (0.563)

Conclusion

Compared to the standard version of our reference curtain wall, the Ucw-value could be reduced by half (from 1.2 to 0.6 W/(m²K)) with the use of thermally optimised components. One can even assume that a Ucw-value of 0.5 W/(m²K) is quite possible by using optimum values for every single facade component.

It is striking that the overall U value of the curtain wall is similar to the U value of the glazing in both cases (standard and optimised). The Ug-value can therefore be taken as an indication for the Ucw-value for typical curtain wall layouts which are similar to the one illustrated here.

In addition, Ψ-values seem to leave more room for improvement than U-values. Comparing the U-values of the standard and the optimised version, values could be cut by half. The Ψ-values, however, could even be reduced to a quarter of the original value.

The individual values with the greatest potential for optimisation are on the one hand the Ut- and Uf-values (improvement from 2.1 to 0.8 W/(m²K)) and on the other hand the Ψp-value (tenfold improvement from 0.2 auf 0.02 W/(mK)).

As indicated above, the thermal optimisation of curtain wall components is only one side of Ucw-value optimisation. In my next post I will therefore be focusing on the other side, the geometrical optimisation of curtain walls.

References

(1) ISO 12631 (first edition 2012-10-01), Thermal performance of curtain walling - Calculation of thermal transmittance (Reference number: ISO 12631: 2012(E))
(2) current aluminium profile systems (February 2015) of brands Schüco, Wicona, Raico and Hueck were used to determine the U- and Ψ-values. Values ​​vary partly depending on manufacturer and product line. In this case, mean values ​​were taken as a basis for calculations.

U-value optimisation of curtain walls - Part 1

Thermally insulated facades are an integral part of energy efficient building concepts. A thermal optimisation of all facade components is needed to meet the ever increasing requirements for facades. In this and the following posts I would like to point out some possibilities of thermal facade optimisation.

 

The specific reason to take on the topic is my current project, which is located in Central Europe. The specifications stipulate a U-value of less than 0.65 W/(m²K) for the curtain walls. Meeting this demand has proved to be quite difficult. In the following posts, I would therefore like to discuss in more detail the possibilities to influence the thermal performance of curtain walling. But first a few basics ...

Ucw-value: Two calculation methods

The decisive physical value of (winter) thermal insulation of facades is the thermal transmittance or U-value. ISO 12631: 2012-10 (1) specifies the procedure for calculating the thermal transmittance of curtain wall structures. The standard provides two calculation methods: the single assessment method and the component assessment method.

The single assessment method is based on detailed computer calculations of the heat transfer through the facade construction and is usually more complex than the component assessment method. In practise, the single assessment method is particularly useful in advanced design stages. It is well suited for special cases such as non-standard facade areas, local penetrations and individual facade component designs. The component assessment method, on the other hand, is very helpful during earlier design stages, because larger geometry or component changes can be incorporated in the calculations with relatively little effort. 

The U-value of curtain walling systems (short: Ucw) according to the component assessment method is calculated using the following complicated-looking but ultimately simple formula:

Ucw = (ΣAg×Ug + ΣAp×Up + ΣAf×Uf + ΣAm×Um + ΣAt×Ut + Σlf,g×Ψf,g + Σlm,g×Ψm,g + Σlt,g×Ψt,g + Σlp×Ψp + Σlm,f×Ψm,f + Σlt,f×Ψt,f) / Acw

where

• A is the area [m²]
• U is the thermal transmittance [W/(m²K)]
• l is the length [m]
• Ψ is the linear thermal transmittance due to the combined thermal effects [W/(mK)]

the subscripts mean:

• cw : curtain wall
• g : glazing
• p : panel
• f : frame
• m : mullion
• t : transom

The following figure illustrates the meaning of the different U- and Ψ-values:

U- and Ψ-values: schematic section through a curtain wall

According to the above formula, the U-values ​​of the individual components are multiplied by the respective facade surfaces and the Ψ-values ​​are multiplied by the corresponding lengths. Both products are then divided by the total facade area. The above formula can therefore be summarised as follows:

Ucw = (ΣA×U + ΣΨ×l) / Acw

Ultimately, the thermal transmittance values of the components are weighted according to their area percentage, with the Ψ-value additionally taking into account the thermal interaction between contacting components.

Essentially, there are therefore two groups of factors in the U-value calculation: physical factors and geometrical factors. In other words, there are technical aspects on the one hand and design aspects on the other hand (here, the interface between design and technology shows up once more, reflecting the title of this blog).

What is a "typical" curtain wall?

If the aim is to come to general conclusions about the possibilities to influence the thermal performance of curtain walls, it makes sense to choose a facade layout which is as general as possible. But what does such a facade layout look like?

Office buildings are a classical field of application for curtain walling. According to Eugene Kohn and Paul Katz, a typical planning module in the US is 1,5m (5 ft.), while in Europe and Asia it is 1,2 m (3' 11'') and 1,5 m (5' 0''). The typical floor to floor height of high rise office buildings in the US is between 13' 0'' and 13' 6'' (4.0 and 4.2 m), in Europe (Germany and France) it is 3.75 m (12' 4'') (2).

A general curtain wall layout - so to speak, the lowest common denominator of a global facade layout - could therefore look like this:

The horizontal division is also kept as simple as possible: there is an opaque area (here: 1.6 m for installation space + floor slab + parapet) and a glazed area (here: 2.4 m high). A transom and mullion width of 50 mm is assumed. Added to this is an operable insert unit with a frame width of 80 mm.

As seen above, the U-value of the curtain wall is determined in large parts by the surface area A of each facade component and the length l for contacting components. Therefore, it seems appropriate to have a closer look at these values ​​and their percentage of the total facade area. The following figure shows the areas of the each component of our reference facade in different colours.

The surface areas A and the percentage of the total facade are:

As expected, the glazing and panel areas Ag and Ap account with almost 90 % for the majority of the facade area. However, the facade profiles (Af, Am, At), despite the relatively slim profile widths, reach ​​almost 1.3 m² per facade field.

The following figure shows once more the reference facade. This time, the lengths l for contacting facade components are highlighted.

The lengths l and their percentage are:

It is remarkable that the lengths l total approximately 34 m despite the relatively simple division of the reference facade module with only few profiles. The area around the insert unit accounts for a large percentage of these lengths, since there are not only the contact areas between the frame and glass (lf, g) but also between mullion or transom and frame (lm, f and lt, f).

After the geometry of our reference facade has been defined, we need the U- and Ψ-values ​​of the individual facade components in order to calculate the Ucw-Value. More on this in the next post ...

References

(1) ISO 12631: 2012-10, Thermal performance of curtain walling - Calculation of thermal transmittance (ISO 12631: 2012)
(2) Cf. Kohn, A. Eugene; Katz, Paul: Building Type Basics for Office Buildings, New York 2002, p. 35 et sqq.