Double Skin Façade Effect on Thermal Comfort and Energy Consumption

in Office Buildings

تأثير الواجهات المزدوجة على الراحة الحرارية وإستهالك الطاقة في المباني اإلدارية

Ahmed Atef Faggal

Department of Architecture – Faculty of Engineering – Ain Shams University

Abstract

This paper is a part of a sequenced simulation analysis which aims at improving the thermal

performance of office buildings through variations of external walls in Egypt. The findings of

this paper presents preliminary thermal analysis done for an air conditioned standard office

building. The study focused on the thermal performance assessment of the building’s local

construction wall materials and the analysis has been carried out using dynamic thermal

simulations using the hourly weather data of Alexandria, Cairo and Aswan. A typical

universal open office space was selected for assessment, wall construction materials were

selected according to survey done for façade materials of office buildings in smart village and

fifth’s settlements in Cairo. The simulations in this paper was carried out in three consecutive

stages, the first stage addressed the air temperature and energy usage on a year round to

define the peak day and the worst orientation among the main four orientations. The second

stage results focused on the peak day occupancy time and the worst orientation performance

of the building using conventional wall. Peak day results were analyzed and compared in the

three studied cities. The third stage results focused on occupancy time and the worst

orientation performance of the building using Double Skin Façade (DSF). It became apparent

that the thermal performance of the conventional wall materials showed that the predicted

percentage of dissatisfied people was (21%) and (27.5%) in Alexandria-Mediterranean

coastal region- and Cairo-semi desert region- respectively. While the higher predicted

percentage of dissatisfied people (34%) was attained in the extreme dry desert region of

Aswan.

Keywords: Double Skin Facades (DSF), Thermal performance, Energy Consumption,

Environmental Control.

Introduction

Building envelopes play an important role in the heat transfer between the exterior and the

interior spaces of the building. Good wall construction parameters contributes to the thermal

comfort conditions inside the building, it leads to minimizing the energy demands of heating

and cooling, hence the overall energy consumption. Building Energy Modeling (BEM) is the

method for predicting the energy consumption of the building through simulation process

which takes into account the building’s numerous thermal characteristics of the building

including envelope materials, orientation, building occupancy, operated and the local climate

data. In this paper the predicted performance of the building has been analyzed using a

dynamic thermal simulation tool called integrated environmental solutions virtual

environment (IES VE) [3]. This software can be used to carry out steady state and dynamic

thermal simulation using hourly weather data and detailed input for building fabric. Several

studies reported that inheriting high thermal massing walls and insulation materials increases

leads to minimizing the overall energy consumption of buildings, Weifeng Ren et al. (2006)

[3] analyzed the applicability of several wall and insulation materials for buildings in hot

summer and cold winter zones from the effect of energy efficiency and reported that at

thickness of insulation 20mm the system could meet the standards and that the usage of

concrete in walls could meet the requirements of standard without the insulation use. From

the related studies, it is apparent that most of them focused on testing the effect of local wall

materials and insulations on energy consumption. Rare studies were carried out to address the

thermal performance through the predicted percentage of dissatisfied people PPD during the

occupancy time, also little research addressed the performance of combined opaque and

transparent walls in extreme dry desert zones like Aswan city.

Objectives

This paper aims at evaluating the thermal performance of an air-conditioned standard office

building using Double Skin Facades (DSF) compared with the performance of existing

conventional wall (combined opaque and transparent walls) as a reference case in the

regional climates of Egypt (Alexandria, Cairo and Aswan).The indoor thermal conditions

defined by space air temperature drop and energy demands of heating and cooling were

analyzed, the predicted mean vote index (PMV) and the predicted percentage of dissatisfied

people (PPD) were defined.

Double-skin Façade Concept

The term Double-skin Façade can be defined as a combination of a traditional single-skin

façade which is doubled on the outside by a second layer, essentially an additional glazed

façade. Each of these layers are commonly referred to as a skin, hence the origin of the

widely used term ‘’Double-skin Façade’’. In addition, a naturally ventilated, sealed or self-

regulating cavity is located between each skin having a width which can range from several

centimeters at the narrowest to several meters for the widest accessible cavities. The glazing

may stretch over an entire structure or over just a portion of it. [1]

The internal layer of glass, typically insulating, serves as part of a conventional structural

wall or a curtain wall, while the additional layer, usually single glazed, is placed in front of

the main glazing and as a result creates the air space.

Energy Use:

Reduction of heating demand during

winter.

Reduction of cooling demand during

summer.

Reduction of peak heating/cooling loads.

Use of natural daylight instead of artificial

as much as possible. [10]

Figure (1) Typical mid floor plan and section

The structure of a Double Skin Façade System

The layers of the façade are described below as shown in Figure (1):

• Exterior Glazing: Usually it is a hardened single glazing. This exterior façade can be fully

glazed. This additional skin reduces sound levels at particularly loud locations, such as

airports or high traffic urban areas.

• Interior glazing: Insulating double glazing unit (clear, low E coating, solar control glazing,

etc. can be used). Almost always this layer is not completely glazed. The interior window can

be opened by the user, even in the case of tall buildings subject to wind pressures. This may

allow natural ventilation of the offices. [12]

• Air cavity (also called an air corridor or intermediate space) situated between the skins is

naturally, or mechanically ventilated. The air cavity ventilation strategy may vary with the

time. its width ranging from 20 cm to several meters, this width influence the way that the

façade is maintained. The air change between the environment and the cavity is dependent on

the wind pressure conditions on the building’s skin, the stack effect and the discharge

coefficient of the openings. These vents can either be left open all the time (passive systems),

or opened by hand or by machine (active system). Active systems are very complicated and

therefore expensive in terms of construction and maintenance. [10]

Manually or automatically controlled solar shading: Such as venetian blinds and louvers, is integrated between the two skins in order to improve

the indoor climate with active or passive techniques. Its location between the two skins

protects them from the influences of the weather and air pollution so that these shading

devices are less expensive than systems mounted on the exterior. During cooling conditions,

the Venetian blinds (or roller shades) cover the full height of the façade and are tilted to block

direct sun. Absorbed solar radiation is either convected within the intermediate space or re-

radiated to the interior and exterior. Low-emittance coatings on the interior glass façade

reduce radiative heat gains to the interior. [14]

Double Skin Façades Types

The types are described below as shown in Figure (2):

Figure (2) Double Skin Façades Types

A. Box window type

In this case horizontal and vertical partitioning divide the façade in smaller and independent

boxes

The box façade is one of the oldest forms of

double-skin façade configuration.

It is comprised of modular single storey

double-skin façade box units which are

divided by structural bay widths or on a room-

by-room basis.

The exterior single glazed skin contains

openings in order to allow the ingress of fresh

air and the egress of stale air. Resulting in the

ability of both the intermediate space and

internal rooms to be naturally ventilated. [7] Figure (3) Typical Box window type section

Box Façade configuration is most commonly used in situations where consideration is

given due to high external noise levels and when there are special requirements

regarding the transmittance of sound between adjoining rooms.

B. Shaft box type

In this case a set of box window elements are placed in the façade. These elements

are connected via vertical shafts situated in the façade. These shafts ensure an

increased stack effect.

The Shaft-Box Façade is a unique variation of a Box Façade configuration with a

combination of both a Double-Skin Façade with a Multi-Storey Cavity and one with a

single-storey cavity.

The vertical height of the shaft creates strong uplift forces due to the increased stack

effect and draws the air from the box façade elements up to the top of the shaft where

it is exhausted.

Shaft-Box Façade configuration is typically used in low-rise buildings. [6]

Figure (4) Typical Shaft box type section

C. Corridor façade

Horizontal partitioning is realized for acoustical, fire security or ventilation reasons.

Figure (5) Typical Corridor façade section

It does not contain any vertical divisions except those that are required at the corner

of the building or elsewhere due to structural, acoustic or fire protection reasons.

The exterior single glazed skin contains openings that are usually positioned in a

staggered format from bay to bay in order to prevent stale air extracted on one floor

entering the cavity space of the floor immediately above.

A Corridor Façade configuration is typically used in the situation of high-rise

buildings. [7]

D. Multi storey Double Skin Façade

In this case no horizontal or vertical partitioning exists between the two

skins. The air cavity ventilation is realized via large openings near the

floor and the roof of the building.

The air cavity ventilation is realized via large openings near the

floor and the roof of the building.

A Multi-Storey Façade configuration is most suited where

external noise levels are high and acoustic insulation is a key

design requirement. [6]

Figure (6) Typical Multi storey Double Skin Façade section

E. The Louvers Facades

With this kind of façade, the exterior skin is

composed of motorized transparent rotating

louvers.

In closed position, these louvers constitute a

relatively airtight façade. In open position,

they allow an increased ventilation of the air

cavity. [7]

Figure (7) Typical Louvers Facades section

Thermal Performance of Double Skin Façades: This section discusses how DSFs perform in two different climates (winter and summer):

A. During the winter, the external additional skin provides improved insulation and the

results will improve if the intermediate space (cavity) is closed (partially or completely)

during the heating period. Because the reduced air flow speed and increased temperature

inside the cavity lowers the heat transfer rate on the surface of the glass which leads to a

reduction of heat loss. This has the effect of maintaining higher temperatures on the inside

part of the interior pane. [13]

B. During the summer, once radiation passes into the building, it is absorbed by the building

fabric and re-radiated as long-wave infrared energy that does not pass back through the glass.

As a result, the air in the cavity will be heated via convection. The hot air flow in the cavity

can pass through the glazing outside and inside the space via conduction. As the cavity

warms up the stack effect is improved respectively, as well as the blinds that located between

the two skins reduce the solar heat gain.

Figure 1

Figure (9) Perspective for the office building constructed in the IES VE simulation program & Typical

mid floor plan and section

The Figure (8) shows the effect of various

factors on heat transfer through the

building‘s envelope and illustrates the

impact of solar radiation, conduction, and

convection on the airflow through the DSF

cavity. A DSF system results in less heat

transferred from outside to inside, and less

energy required to cool the space. [4]

Figure (8) Heat transfer through a DSF on a summer

day(Haase, 2006)

Energy Performance of Double Skin Façades: The building industry makes up a considerable fraction of world’s energy consumption. The

adverse effects of a growing energy demand such as depletion in fossil fuel reserves and

natural resources hassled the building industry to a search for new technologies that result in

less energy consumption together with the maximum utilization of natural resources. Energy-

and ecology-conscious European countries incorporated the well-being of occupants while

conducting research on innovative technologies.

In view of the fact that double-skin façades offer a healthy and comfortable milieu for the

occupants and use natural resources hence consume less energy they became a promising

invention for all concerns. The analysis of the performance of the double-skin façades and

energy consumption is inconclusive at this time. However, based upon thermal performance

analysis have been done so far, a double-skin façade perform better and provide some energy

reduction, particularly on the heating side cycle, from a standard double glazed unit wall. [4]

Case Study Methodology

A typical air-conditioned standard office building was selected for assessment based on a

survey of the office buildings constructed in smart village and the fifth’s settlement in Cairo.

Building comprises four floors with 4m height for each floor. For saving simulation time a

typical mid floor (second floor) is chosen for the thermal simulations. Plan is rectangular

(2000m2), core is centralized and the office space is open area (Figure 1, Figure 2). Core

details has been simplified and set to be thermally adiabatic.

Construction materials were chosen based on the common façade materials, transparent

materials were double glazed curtain walls with argon gap fill, opaque materials were

concrete hollow blocks with artificial stone plaster (Figure 10, Table 1).

Building occupancy template was set to five day working, Fridays and Saturdays are

weekends, building to be occupied from 9:00 to 17:00, lunch break is set from 12:00 to

13:00. Table 2 summarizes the building activity template set in this study.

Simulations were carried out in three consecutive stages for the chosen cities (Alexandria,

Cairo and Aswan) representing the main regional climates of Egypt (Mediterranean coastal

region, semi-desert region and the extreme dry desert region) as follows:

Figure (10) Details of opaque and transparent walls used in the study.

Stage one addressed the air temperature and energy usage on a year round to define the

peak day for air temperature and the worst orientation among the main four orientations

in each city (Figure 4) indexed the four orientations as Case A,B,C and D.

Stage two focused on evaluating the thermal performance of the office building using a

conventional wall as a reference case on the peak day occupancy time and the worst

orientation performance of the building determined in stage one. The predicted mean vote

(PMV) and the percentage of dissatisfied people (PDD) were defined and compared with

office building standards (5-15%). Figure (10)

Stage three focused evaluating the thermal performance of the office building using DSF

compared with the results of the reference case in stage two. In this stage, all the double

glazed curtain walls were substituted with a double skin façade (Corridor type with 1.3 m

wide cavity between the two layers). A grilled corridor were added in each floor used for

maintenance. It acts as a sun breaker to reduce solar heat gain inside the office space.

(Figure 6 shows the double skin façade location -marked with red - on the reference case

plan).

Figure (11) the four orientation scenarios to be simulated in stage one.

Figure (12) Application of double skin façade (1.3 m wide) over the glazing of the reference case.

Results: Stage one: The whole year round results

Table 3 summarized the whole year results for the four scenarios of the building orientations.

Results were simplified to show the maximum recorded air temperature, and the maximum

recorded energy used. For Alexandria and Cairo, the peak day determined was 22nd August,

maximum air temperatures recorded 31.87 C, 33.97 C and the maximum cooling plant

sensible loads were 388.02 KW, 472.25KW respectively. Aswan showed different day which

was 15th August with maximum air temperature 37.25 C, and the maximum cooling plant

sensible loads was 598.48 KW. In all studied cities the worst orientation performance was

case D. (worst orientation was chosen as a part of sequenced simulations to improve the

thermal performance of office buildings through applying advanced integrated walls, hence

the worst performance is chosen for the assessment of the improvement).

Table (1) Maximum air temperature/energy use for all scenarios in the studied cities.

Stage two: The peak day results using conventional wall type of reference case

Tables 4, 5, 6 illustrated the air temperature, energy use and PPD results for the three studied

cities in the peak day determined in the first stage of simulations. The demand of cooling

loads to drop air temperature to the thermal comfort level (23 ° C) was defined in this stage.

1. Alexandria

Maximum cooling loads required 388.02 KW to drop temperature from 31.87 ° C to 23 ° C.

Predicted people dissatisfied 21% is slightly higher than office building standards.

2. Cairo

Maximum cooling loads required 472.25 KW to drop temperature from 33.97 ° C to 23 ° C.

Predicted people dissatisfied 27.5 %.

3. Aswan

Maximum cooling loads required 598.48 KW to drop temperature from 37.25° C to 23 ° C.

Predicted people dissatisfied 34% is extremely higher than standards for office building.

Stage three: The peak day results using DSF compared with the reference case

Tables 7, 8, 6 illustrated the air temperature, energy use and PPD results for the three studied

cities in the peak day determined in the first stage of simulations. In this stage, the thermal

simulation results of the double skin façade strategy for the chosen three cities (Alexandria,

Cairo and Aswan), results of the simulation were compared with the reference case.

Table (2) Peak day simulation results using DSF for Alexandria.

Table (3) Peak day simulation results using DSF for Cairo.

Table (4) Peak day simulation results using DSF for Aswan.

Summary

This paper investigated the thermal performance of an office building using Double Skin

Façade (DSF) compared with a reference case used conventional wall materials in the

regional climates of Egypt, it aimed at assessing the thermal performance of the external

walls by addressing the effect on users through predicted percentage of dissatisfied people

PPD. For the whole year round simulations the peak day determined for Alexandria and

Aswan was 22nd August, while Aswan showed different day 15th August. Orientation of Case

D recorded the highest energy use in all studied cities. Hence the worst thermal performance.

Simulation results showed that using the conventional double glazed curtain wall and the

concrete hollow blocks with artificial stone plaster in the three studied cities required a must

use of mechanical cooling to achieve thermal comfort in the studied space, this was shown

during the lunch break when all mechanical cooling are switched of that PPD was 83%, 94%

and 98% for Alexandria, Cairo and Aswan respectively, which is a complete dissatisfaction

of people using the space. While using the mechanical cooling to maintain space temperature

at comfort levels resulted in PPD were 21%, 27.5% and 34%. This showed that for all studied

cities the predicted percentage of dissatisfied people are exceeding office space standards

according to ASHRAE standards (min5%-max 15%).This opened the door for further

research to improve the thermal performance of this space by adopting advanced integrated

facades instead of conventional facades.

Conclusions and Recommendations

Double Skin Façades for office buildings were developed mostly in Europe in order to arrive

at increased transparency combining acceptable indoor environment with reduced energy use.

The main disadvantage of this system is that in countries with high solar gains the air

temperatures inside the cavity are increased during periods with warm weather, leading to

overheating problems. Double Skin Façade has to be designed for a certain building location

and façade orientation otherwise the performance of the system will not be satisfactory.

The design parameters that have to be studied in order to improve the façade performance

and ensure reduced energy use and good indoor environment are:

Design and type of the façade

Structural design of the façade

Geometry of the cavity

Use of the air inside the cavity – type of cavity ventilation – HVAC strategy

Opening principles of the cavity, the interior and the exterior façade

Type of glazing, shading and lighting devices

Material choice for the panes and the shading devices

Positioning of shading devices

The experiences of Double skin façade systems in hot area are best resulting when the

orientation of buildings are towards north and south, with horizontal shading systems and

automatically opening windows, along with the width of cavity space equal to 30cm, which

provides better results in terms of shading system, natural ventilation, sustainability and

energy efficiency.

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