Rhea ThompsonPhD Candidate

Marine Estuarine & Environmental Science ProgramUniversity of Maryland, College Park

Retention and Thermal Performance of a Thin Sloped Green Roof on a Sustainable Home

Location: Rockville, MD Manufacturer: Live RoofArea: 312 ft2

Depth: 2.5”Slope: 13o

Roof Membrane: R-Value 50Soil Components : • 84.4 % Shale• 4.4 % Sand• 11.1% Compost

Stormwater

Management

Air Quality

Biodiversity

Energy Savings

Aeshetics

Urban Heat

Island

Evapotranspiration links water and energy

Learning Objectives1. Research the effect of storm

characteristics (size, frequency, intensity) and soil moisture on retention

2. Determine the trade-offs of soil moisture on thermal performance and green roof cooling

3. Research the effect of soil moisture on evapotranspiration (ET) rates

Image Credit: Scott Tjaden

Calculating Retention & ET

±ΔS =P-RO-ET

Retention increases with event size

0 10 20 30 40 50 60 70 80 90 10005

101520

f(x) = 0.203940750803771 x − 0.0374074408015517R² = 0.524725985012699

f(x) = 0.13037413874999 x + 0.934963415301213R² = 0.614330734942987

July-Nov

Precipitation (mm)

Rete

ntio

n (m

m)

P<0.0001

0 5 10 15 20 25 30 35 40 45 50020406080

f(x) = 2.53381212643459 ln(x) + 10.5350877319616R² = 0.00482214401161447

f(x) = NaN ln(x) NaNR² = NaN

Precipitation (mm)

Rete

ntio

n (%

)Larger storm events produced less retention as a percent of precipitation in July-November

P<0.0001

Low antecedent water content improved retention in July-November

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.1605

101520

f(x) = − 29.6301845392849 x + 5.89798756445569R² = 0.104686168954636

f(x) = − 22.2020693910455 x + 6.05682953745975R² = 0.255163150048322

July-Nov Linear (July-Nov)

Pre event VWC (m3/m3)

Rete

ntio

n (m

m)

P<0.05

Summary: Stormwater Retention•Green Roof retention increased with event size and low soil moisture•Green roof was less efficient at retaining water as storm size increased• Storm intensity and frequency had no effect

𝑅−𝑉𝑎𝑙𝑢𝑒𝐺𝑅=∆𝑇 /𝑄

𝟏𝟎𝟎𝐎𝐅

𝟕𝟓𝐎𝐅

¿¿

Ambient Temperature

Soil Heat Flux

Under Tray Temperature

Measuring ThermalPerformance

Image Credit: Scott Tjaden

Image Credit: Scott Tjaden

Reflected Radiation

Incident Solar Radiation

Evapotranspiration

Measuring ThermalPerformance

Mean Green Roof R-Value (2.40 Btu/hr · ft2 · F)

Jun Aug Oct Nov Jan Mar0.010.020.030.040.050.060.0

Green Roof WaterShedMaryland Standard

R Va

lue

(Btu

/hr ·

ft2

· F)

The potential for energy savings is greatest for poorly insulated buildings

Existing Roof Membrane

R-Value (hr · ft2 · F/Btu)

Roof Membrane + Green Roof

R-Value (hr · ft2 · F/Btu)

Percent Increase

(%)

50 52.4 4.8130 32.4 8.0125 27.4 9.6120 22.4 12.0215 17.4 16.02

0 0.02 0.04 0.06 0.08 0.1 0.12 0.1401234567

f(x) = − 24.118305956841 x + 3.95471855648063R² = 0.610449732617138

July-September

Average Daily Soil Moisture (m3/m3)

R Va

lue

(hr ·

ft2

· F/

Btu)

P<0.0001

Thermal Performance decreases with water content

0 0.02 0.04 0.06 0.08 0.1 0.12 0.1402468

10

f(x) = − 38.8752122769247 x + 5.22253190485296R² = 0.476150584854126

July-Septe...

Average Daily VWC (m3/m3)

Cool

ing

(oC)

Green Roof provides cooling benefit up to 9.3o C

P<0.0001

Summary: Thermal Performance• The green roof (R-Value 2.40 hr · ft2 · F/Btu) on WaterShed

provides little thermal relief in comparison to the home’s insulated membrane (R-Value 50 hr · ft2 · F F/Btu) • The potential for energy savings is greatest for poorly

insulated buildings • Thermal performance decreased with water content• Cooling benefit up to 9.3o C with low water content

• Assigning a green roof an R-Value is an imperfect method of assessing its thermal performance, as evapotranspiration is key to cooling

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14012345

f(x) = 22.4900534510692 x − 0.00490467998602218R² = 0.721985550161787

July-Sept Linear (July-Sept)

Average Daily VWC (m3/m3)

ET (m

m/d

ay)

P<0.0001

ET increases with water content, and may be key to improving retention and thermal performance

Conclusion: To Irrigate or Not?• The strong links between plant evapotranspiration rates, stormwater retention, thermal performance and soil moisture indicate:• Plants may be the key to improved retention and thermal performance

Conclusion: To Irrigate or Not?• The strong links between plant evapotranspiration rates, stormwater retention, thermal performance and soil moisture indicate:• Plants may be the key to improved retention and thermal performance• Irrigating may be important to improving plant growth, which in turn may improve retention and thermal performance due to increased evapotranspiration

Conclusion: To Irrigate or Not? ET

ET• The strong links between plant evapotranspiration rates, stormwater retention, thermal performance and soil moisture indicate:• Plants may be the key to improved retention and thermal performance• Irrigating may be important to improving plant growth, which in turn may improve retention and thermal performance due to increased evapotranspiration

Conclusion: To Irrigate or Not? ET

ET• However, there are trade-offs to be considered for irrigating green roofs• Too much water will impair

retention and thermal performance• A delicate balance between

the above factors must determined

• Findings important as greater rainfall and more frequent and intense storms are expected under climate change, which could further impair retention and thermal performance