A Hedera Green Façade – Energy Performance and Saving Under Different Maritime-Temperate, Winter Weather Conditions

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doi:10.1016/j.buildenv.2015.04.011

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Highlights

•: Replicated treatments were used to investigate thermal properties of green façades during winter.
•: Vegetation significantly reduced energy use in cuboids.
•: Vegetation increased wall insulation properties and surface temperatures.
•: Greatest benefits were associated with more extreme weather.

Abstract

Thermal regulation is a key ecosystem service provided by urban plants. In addition to summer cooling, plants can insulate buildings against heat loss in winter. Research was conducted over two winters using replicated small-scale physical models to simulate heat loss from a built structure and to investigate the insulation properties of plants during cold weather. Brick cuboids were constructed around a water tank maintained at 16^°C and energy use monitored. Covering cuboids with ivy (Hedera helix) reduced mean energy consumption by 21% compared to bare cuboids during the first winter (means of 4.3 and 5.4 kWh per week, respectively). During the second winter, when foliage was more extensive a 37% mean saving was achieved (3.7 compared to 5.9 kWh per week). The presence of Hedera enhanced brick temperatures significantly compared to bare walls. Temperature differences were affected by weather parameters, aspect, diurnal time and canopy density. Largest savings in energy due to vegetation were associated with more extreme weather, such as cold temperatures, strong wind or rain. Under such scenarios green façades could increase energy efficiency by 40-50% and enhance wall surface temperatures by 3^oC. These empirical studies with replicated treatments augment previous research based on urban modelling and data from non-replicated individual buildings in situ. They indicate that planting design requires more attention to ensure the heat saving aspects associated with green façades and shelter belts are optimised. These aspects are discussed within the context of wider urban ecosystem services provided by vegetation, and implications for climate change mitigation.

Keywords

Energy efficiency;
green facade;
green wall;
retrofitting buildings;
thermal performance;
winter energy saving

Nomenclature

ANOVA: Analysis of variance
df: Degrees of freedom
h: Time [hours]
k: Thermal conductivity [W m^-1 K^-1]
kgCO₂^e: Kg carbon dioxide equivalent green-house gas
lbh: Length, breadth, height
LSD: Least significant difference
N: North aspect
n: Number of replicates
P: Probability, lower values represent greater confidence
PC: Planted cuboid
S: South aspect
UC: Un-planted cuboid
U10: Wind speed at 10 m height
v: Versus
w/c: Week commencing

1. Introduction

Energy demand in temperate climates is a key sustainability issue [1]. In developed countries 20-40% of total energy is consumed in buildings [2] and the built environment accounts for >50% of all UK carbon emissions [3] with extensive economic and climate change implications [1]. Green façades/walls and roofs have been the subject of significant attention over recent years partly due to their wider role in urban heat island mitigation 4 and 5, but also their ability to shield buildings from excessive solar gain and cool via evapo-transpiration [6]. This dual cooling can significantly reduce temperatures around the building envelope and hence decrease energy demand for mechanised cooling [7].

Vegetation can also ameliorate winter effects on a building, and in turn reduce heat energy consumption; although this has received comparatively less attention [8]. The premise has been explored over three decades 9, 10 and 11. There remains a lack of research with replicated treatments under field conditions, however, particularly with respect to maritime-temperate climates such as the UK. Most previous studies have been dominated by continental climatic pressures e.g. central/eastern parts of the contiguous USA. Inferences from such research to temperate scenarios are problematic, not least due to typically milder winters, variation in sunlight hours (cloud cover) and solar azimuth angle (hence radiation intensity). Yet, there is an urgent need for innovative and practical options which address the poor energy performance of much of the housing stock in countries such as the UK and Eire. In the UK, 80% of housing was built prior to 1980, with little focus on energy efficiency in construction [12]. Despite being a ‘temperate’ climate, the UK has one of Europe’s highest rates of winter mortality [13] with 23,500 excess deaths in winter 2003/4 [14].

Wind chill and infiltration of cold air (with the associated convective losses) are the most significant factors in the poor energy performance of old housing stock 7, 9 and 15. Infiltration of cold air is undesirable not only due to temperature reduction in the building envelope, but also cold air meeting warm causes water vapor to condense, particularly in cavity spaces. Vegetation covering a building can reduce wind velocity through the surface resistance of the canopy, and thus reduce both cold air infiltration and convective heat loss to a building 7, 9 and 10, and in turn reduce carbon consumed in heating the home or office [16]. These thermal benefits are augmented by a spectrum of well-documented additional benefits within the anthrosphere, not least habitat provision for urban biota [17], intercepting precipitation and reducing run-off rates [18], screening out aerial particulate matter and improving air quality [19], contributing to psychological well-being and improving the aesthetics of the cityscape 20 and 21.

For decades it has been understood that hedges and trees reduce wind-chill to surrounding structures or landforms by providing a wind break; although much of the focus has related to crop or livestock protection within agriculture [e.g. 22]. Some authors have applied these principals to vegetated walls noting a reduction in draughts surrounding apertures, (and hence air flow into/out of a building), together with warmer air retained against the building envelope [23]. Indeed, Dewalle and Heisler [24] suggest that vegetation can reduce cold air infiltration to the building envelope by up to 40%. Subsequently, Heisler [25] predicted through modelling that well-designed shelter-belt planting could result in heat energy savings of 10-25%. Liu and Harris [11] were able to demonstrate that the addition of shelterbelt trees around office buildings in Scotland, UK, reduced convective heat losses, resulting in energy savings of 8%. In addition to the canopy providing aero-dynamic resistance, vegetation can also protect masonry from freeze/thaw, and infiltration of damp following precipitation by forming a physical barrier. Species such as Hedera helix present a multi-layered surface, which aids run off and can stop moisture reaching the wall [26].

Physical and geographical features of the building will also influence efficacy, including orientation, prevailing weather, and thermal characteristics of the masonry, coupled with architectural aspects such as the volume, dimensions, and geometry of the walls and surrounding structures 27 and 28. Such physical characteristics create flux in the microclimate close to a heated wall due to convection and conduction, with factors such as wind-eddy, albedo, humidity, and shade/solar gain creating a dynamic zone of ‘thermal mixing’ adjacent to the wall surface; all of which are influenced by the addition of vegetation [29]. Building occupancy has a significant effect on heat energy consumption altering demand for heating due to variation in the thermal gradient (e.g. care homes require higher temperatures than shops), but also heat loss through factors such use of entry and exit points [30].

In an attempt to minimise the variations encountered in ‘real’ buildings, the work reported here used replicated, heated brick cuboids. The cuboids were constructed with a single layer of brick, analogous to the walls of brick terrace houses typical of inner-city housing stock in UK cities. The ‘cuboids’ were not intended to mimic a ‘real’ house, just provide an experimental basis to evaluate the concept of vegetation used as thermal insulation. Our use of replicated cuboids outdoors were unlikely to fully represent the thermal properties and aero-dynamics around buildings in vivo but a number of the approaches adopted were considered advantageous in attempting to reduce bias associated with individual buildings and associated micro-climates (e.g. uniform, replicated structures located within a small area). Indeed, Hunter et al. [31] have recently criticised studies on green walls due to research design problems; with the small number of experimental studies lacking replication, providing insufficient information about the microclimate parameters measured, and assumptions through modelling studies not always delineated or justified. As such the replicated, empirical-data gathering approach was adopted here.

The research utilised a green façade system rather than a living wall. Green façades comprise of plants in the ground (or in pots), and grown up the side of a building, either attaching themselves directly or trained up a trellis/framework placed against the wall. The green façade was chosen to exploit a simple design that readily translates into practice, and to minimise nutrient, water and energy costs associated with some living wall systems [32]. Hedera helix was selected as it represents a commonly-used garden or landscape plant, often found growing up domestic properties either after intentional planting or self seeding.

The aim of this research was to explore if vegetation can play a role in insulating a wall in a maritime-temperate climate. Through replication, and monitoring heat loss over two UK winters, our objectives were to quantify potential energy and carbon savings; whilst also evaluating the relative effectiveness of vegetation against different winter weather phenomena. The kWh savings and carbon savings are both quantified; however, no attempt has been made to review the embodied carbon in plant provenance, or indirect carbon consumed in plant maintenance in-situ.

The numerous potential benefits for retro-fitting scenarios in older housing stock 33 and 34 validate the importance of this work. Despite climate change increasing global heating, north-west Europe may experience wetter and colder winters due to the weakening of the Atlantic meriodional overturning circulation (AMOC); with severe weather events increasing in both frequency and magnitude [35].

2. Materials and Methods

Brick cuboids were laid out in a matrix design with 12 used in the first (4 Jan. – 31 Mar. 2010) and an additional 8 (i.e. 20 in total) in the second (1 Dec. 2010 – 30 Mar. 2011) experimental phase (Fig. 1). Cuboids were constructed outdoors in a field site at the University of Reading, Reading, UK, using a standard red clay housing brick (classified BSEN 771, Class B, 215 x 103 x 65 mm lbh; thermal properties: k = 1.1 Wm^-1k ^-1, Blockley’s Brick Holdings PLC, Telford, UK). A single skin of bricks was placed on a grey concrete slab footing (682 x 500 x 40 mm lbh) and a ‘damp course’ layer (polypropylene tape 1.05 mm thick) was incorporated above the basal layer ( Fig. 2). The bricks were stacked in a stretcher-bond with a slab ‘roof’; total volume: 0.25 m³ (0.6 x 0.6 x 0.7 m lbh) and each cuboid placed 2 m apart. The bricks were not mortared but were orientated to avoid any obvious air gaps between adjacent bricks. An aluminium foil-coated, plastic air-filled sheet (‘foil bubble-wrap’) was placed on the top and base of each cuboid; hence ‘walls’ were the principal route for heat migration. A sealed 25 l opaque polypropylene container was placed inside, filled with potable water. A calibrated Protx 1020, 75 W thermostatic heater (AquaCare Inc., Gurnee, IIlinois, USA) maintained internal water temperature at 16+/-0.5^oC. Heaters were connected to mains electricity via a Maplin N67HH power consumption monitor (Maplin Electronics, Rotherham, UK); this measured kWh consumed (accurate to 0.5%). Power monitors were checked by recording power consumed over 1 h i.e. 75 W. Equivalent carbon consumed was calculated using the UK Government Defra/DECC conversion factors [36], which correlates 1 kWh to 0.48357 kg carbon dioxide equivalent (kgCO₂^e). This conversion accounts for UK generated, imported energy and grid losses via the UK National Electricity Grid.

Cheryl & ethnovet

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Monday, 11 May 2015