Introduction: Green Roof Thermodynamic Model

The purpose of this project is to analyze thermodynamic properties when comparing a traditional, industrial concrete roof to a modern green roof. Heat will transfer differently through the different roof layers. This scale model will allow for the measurement of the temperature differences between layers, and ultimately the heat transfer coefficient can be calculated for both roof types. This build will take approximately 6 days: one day for roof layer housing, two days for casting concrete, one day for roof layer and utility installation, one day for Arduino coding and testing, and one day for data collection.

Tools Used: Drill, table saw, band saw, 3D printer, computer

Materials Used:

  • 3 x 8-ft, 2"x4" stud
  • 2 x 3/4", 4'x8' plywood sheet
  • 1 x 2'x2' foam board insulation
  • 1 x 1-lb box wood screws
  • 1 x 14oz, plumber's putty
  • 1 x 14oz, Flex Seal
  • 1 x 50-lb bag, concrete mix
  • 1 x 1-lb bag, aquarium gravel
  • 1 x 5"x50' roll, waterproof membrane
  • 1 x 8-qt bag, potting mix
  • 1 x 3-lb bag, grass seed
  • 6 x 1/2" dia, 90 deg PVC elbow
  • 1 x 1/2" dia, 10' PVC pipe
  • 1 x water reservoir
  • 5 x 1W solar panel
  • 1 x battery charge controller
  • 1 x 12V lead acid battery
  • 4 x TMP006 IR temperature sensor
  • 10 x DS18B20 thermocouple
  • 1 x Arduino Uno
  • 1 x aquatic in-line pump
  • 1 x adhesive epoxy
  • 2 x 4" metal handles
  • 1 x 2'x2' acrylic sheet

Step 1: Build the Base

The 2"x4" stud is first cut into two 18.5" pieces and two 12" pieces. These pieces are then fastened as shown with screws. The 3/4" plywood is then cut into a 22.5"x12" piece that is screwed into place on top of the studs to form the base.

Step 2: Build the Frame

The longer outside walls are then cut using the 3/4" plywood into two 22.5"x12" pieces and fastened as shown using screws. The end walls are cut using the same plywood into two 12"x12" pieces and fastened to the base and wall using screws as well. *Note: One 22.5" wall was not fully fastened so that it could be removed later for accessibility within the box.

The inner walls are cut using the plywood into two 10.5"x8" pieces. These pieces are fastened 9.75" from the inside of the outside walls to form a 1.5" gap between each other. *Note: Again, one wall is not fully fastened to the inner walls to make accessibility easier later.

Step 3: Build the Shelves

The shelves that will hold the roof layers are built next. From the previous step's notes, you will see one wall has been removed for accessibility.

Four 10.5" long pieces are cut from the 2"x4" stud to form the shelf supports. Two pieces are fastened 3.75" from the top of the walls to the top of the studs on one side as shown. The other two pieces are fastened 2.25" from the top of the walls to the top of the studs.

Two 10.5"x9.75" pieces are cut from the 3/4" plywood to form the shelves. These pieces are simply placed on top of the shelf supports and do not require fastening.

Step 4: Cast the Concrete

Before the concrete can be cast, all necessary components must be sealed to avoid concrete dripping into undesirable areas. The drain for the green roof layer (the lower of the two sides) must first be installed so the concrete can be cast around this drain. A 0.75" diameter hole was drilled using a hole saw 1.5" from the end wall and 1" from the adjacent wall on the shelf. Another 0.75" diameter hole was drilled 1.5" from the end wall and 6" from the top of what will now be the back wall. Connect two 6" pieces of PVC pipe through these two holes using an elbow.

The top of the shelf will now be sealed so concrete does not seep through the cracks of the shelf and water from the wet concrete does not damage the wood. FlexSeal was first sprayed on each shelf as shown in white. Next, plumber's putty was used to seal around the pipe and corners where the shelf met the walls and where the walls met each other on each side.

One 50-lb bag of concrete was mixed with water in a 5-gal bucket according to the bag specifications using a mixing attachment for the drill. The concrete was poured onto each shelf forming a 1.5" thick slab. A putty knife was found useful to smooth out the concrete. The concrete was left for 2 days to set.

Step 5: Install Insulation

The inside volume of each side needed to be equal to ensure comparable heat transfer properties. For this to occur, two 10.5"x9.75" pieces of 3/4" plywood were placed inside the concrete roof side to attain an equal volume as the green roof side.

Insulation was then installed as shown. Insulation serves to reduce heat transfer through the outside walls. Since the point is to test heat transfer through the roofs, heat transfer through the other surfaces should be reduced as much as practical. The insulation was cut appropriately to ensure the inside walls of the two sides were adequately insulated.

Step 6: Install Thermocouples and IR Temperature Sensors

1" holes were then drilled through the back wall and insulation of each side to allow the thermocouple (TC) and IR temp sensor wires to pass through. 4 TC's and 1 IR sensor were used on the interior of each side. TC's were hot glued to the insulation in the center of each of the four walls. The IR sensors were mounted facing the underside of the roof with about 3" standoff (per the sensor's instructions).

Two more TC's were used on the exterior of the green roof side. One was placed between the concrete and stone layers, the other between the stone and grass layers. These will be used to assess the individual effectiveness of each layer.

The two remaining IR sensors were placed facing the top surface of each roof, again with about a 3" standoff.

The interior TC measurements will be read periodically by the Arduino, averaged, and used to calculate the rate of heat transfer into the box. The IR sensors will read the temperatures of the roof surfaces themselves. From these temperatures, the outer surface temperatures, and the rate of heat transfer, the overall heat transfer coefficients of the two roofs can be estimated. Please see the operating instructions for more information.

Step 7: Install Handles

As the box became increasingly heavy, handles were added to make transportation easier. Simple metal handles, as shown, were fastened with screws to each side of the box.

Step 8: Add Acrylic

To make the layers visible, the front plywood sheet was cut 9.75"x3" on the green roof side and 9.75"x1.5" on the concrete roof side. A thin bracket was glued in each cut that holds the acrylic. The acrylic sheet was then cut into a 9.5"x2.75" piece and a 9.5"x1.25" piece to fit into the bracket.

Step 9: Solar Panel Mount

The mount for the solar panels was created using the wood stud. Two 8-in 2"x4" pieces were cut from the stud and fastened to the end walls using screws as shown. A 24-in piece was then cut and fastened using screws between the 8-in end pieces. Custom 3-D printed solar panel brackets were used to hold the solar panels. These brackets were adhered to the wood using an epoxy adhesive. The solar panels were then inserted into these brackets.

Step 10: Add Irrigation System

A simple irrigation system was created to provide moisture to the grass on the green roof. An open tank catches drainage water (see Step 4) from the green roof where it can then be pumped back to the grass when needed. This is meant to simulate a rain-water catchment type irrigation system that may be found on a real green roof building.

The sprinkler system was made by cutting the 1/2" PVC into lengths as follows: 2 x 12"; 6 x 5"; 1 x 15". 1/8" holes were drilled along the bottoms on those sections overhanging the grass to serve as nozzles. Tees and elbows were used to construct the piping network as shown in the photo.

The pump was placed in the bottom of the tank and wired to the battery through a switch. A band strap secures the riser pipe to the back of the box to hold the assembly in place.

Step 11: Complete Green Roof Layer

The completed green roof layer is comprised of concrete, gravel, soil, and grass. A 1/2" layer of aquatic gravel was placed on top of the cast concrete. A layer of sod will be added on top just before testing begins (so the grass doesn't die in storage).