i nternational g reen w all c onference - 4-5 th september 2014 - the green wall centre,...

INTERNATIONAL GREEN WALL CONFERENCE - 4-5th September 2014 - The Green Wall Centre, Staffordshire University, Stoke-on-Trent, UK

Green Facades – How does plant choice affect wall cooling and wall

insulation?

Ross CameronUniversity of Sheffield, UKJane Taylor, Martin Emmett

[email protected]


The Context

• UK Victorian / Edwardian building stock• Brick dominated• Difficult to ‘retrofit’• Domestic properties – owned or rented• Replicated real houses???• ‘Down-sized’ – replicated ‘mini-buildings’ and

walls• Controlled environment studies


3 Experimental protocols

• ‘Mini-city’ – cuboids• ‘Stand-alone’ wall sections • Controlled environment cabinets


Designing the ‘mini-city’!

• 20 Brick cuboids • 10 vegetated• 10 blank• Water tank in centre

– Winter studies

• Temp sensors 5cm from cuboid wall surface (and behind vegetation)


How do weather scenarios affect temperature profiles of air next to cuboids? (Mean data - Southern aspect)


How do cooling profiles vary with diurnal cycle?

• Wall system• Prunus v wet pot v blank

wall• Temp sensors for

air adjacent to wall


Mean hourly air temperature for walls flanked by Prunus (South = Δ; North = ), ▲ Pot+media (South = □; North = ) or blank ■ Control walls (South = ○; North = ●). Data restricted to days with ≥ 5 h irradiance > 120 Wm2, August to September 2009. Bars = LSD (P = 0.05) blocked by

date. Residual d.f. = 30 each time. Ambient temperature – dashed line.

Prunus south aspect


Wall surface temperatures

At discrete times thermal images showed a 10oC difference between vegetated walls and blanks• E.g.Wall behind foliage = 24.0oC, Control wall = 33.9oC Pot+media wall = 33.2oC; LSD = 0.81 (P = 0.05) d.f. = 12.


What are the implications to the cavity space within a wall?

• Phaseolus vulgaris


Air temp – at wall exterior


Cavity Air temp.


What types of plant are best at cooling?

Wall ShrubsClimbers

‘Functional’ plants


Fuchsia

Stachys

Jasminum

Hedera

Prunus

Lonicera


Controlled environment studies• Vegetated v non-vegetated walls• Warming programme with heater (300 W) within cabinet• Air and wall surface temperatures• Some programme runs with normal plants and some with leaves

covered with poly (1-acetyloxiethylene) to block stomata - inhibited 96-98% of normal ET (shade temps)

• Some runs with wet media only (evaporation)• Plant / pot temps stabilised at 18oC before placement in cabinets• Wall temps stabilised at 26.5oC and monitored change as plants

introduced. • Plants etc. weighed before and after to determine water loss


Attained temp and water loss data for

• a = transpiring plant and media, • b = non-transpiring plant and media • c = media alone. • This allowed calculation of the cooling effect on the wall (oC) due to total cooling of

– planted trough – plant evapo-transpiration – shade and – media evaporation

• Plants destructively harvested and data for:– leaf number, total leaf biomass (dry weight), mean leaf dry weight, mean specific leaf

area, mean leaf thickness (individual leaf areas/leaf biomass) and total stem biomass (dry weight).

• Wall leaf area index (WLAI) was calculated as a ratio of total leaf area compared to exposed wall area and used to estimate the density of the foliage covering the wall.


Temp. reduction on wall surface


Temp reduction – per unit of leaf area


Summer Cooling - Conclusions

• Vegetation can cool the air around a wall by 3-5oC in UK context

• Wall surfaces as much as 10oC different• Reduces cavity temp by 5oC in mid afternoon• Different plant species have different cooling properties

– Extent of cover – Depth of foliage – Some better at Shading– Some better at ETp cooling

• Water supply important in ensuring optimum performance• Green facades appear a viable retrofit option - UK


Shrubs and Climbers Their role in thermal performance of built

structures in winter


What about reducing energy consumption in winter

– brick mini-building again?

• Replicated ‘mini-buildings’ with heated water tank within brick enclosure

• Water tank - Temp maintained at 16oC throughout• Energy use monitored over 2 consecutive winters


Fig. 1. Energy use in Control (Blank) and Vegetated (Hedera) cuboids, during winter scenarios. Part-cover = foliage covering approx. 80%, and Full-cover = foliage covering 100% of cuboid. Bars = LSD values, df=79.


Results

• Mean winter energy reduction = 38%– varied between 17% and 45% saving per week,

based on weather scenarios.• At 03.00 and 09.00 am cuboid wall

temperatures behind plants significantly warmer than ambient (mean 3.6oC)– not the case for blank walls.


How does species choice affect Winter insulation?


Replicated ‘Cavity’ Walls

• ‘Dense’ Prunus v ‘Open’ Cotoneaster franchetti v Control (Blank)

• Late winter data• Wall surface and cavity temperatures


Fig. 4. Cold, Non-Snow scenarios. Mean air temperature recorded at wall exterior (blue) or in wall cavity (shaded), as affected by species. Bars = LSD values Left = comparisons for exterior temperature, df=154 and Right = comparisons for cavity temperatures, df=107.


Results

• Both the Prunus and Cotoneaster warmed the walls at night - Interior and Exterior

• Denser Prunus was more effective at warming the exterior.

• During sunny periods, however, plants particularly the Prunus inhibited solar gain to the wall.


Winter - Conclusions

• Plants can insulate walls and improve temperatures / energy conservation in winter.

• The benefits are greatest under colder / more adverse weather conditions.

• Overall - – Plant species choice needs more consideration when

recommending wall screening plants.– Replicated ‘artificial’ buildings provided a high degree of statistical

robustness – need to scale up.

Thanks to Horticultural Trades Association, UK. for funding this research.


Questions ?


i nternational g reen w all c onference - 4-5 th september 2014 - the green wall centre,...

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