The team from Statewide Roofing arrived promptly at 8:00 am this morning to install our cool roof system. We need to get the flat roof completed before the rains come this weekend. Depending on which forecast one follows, there is a 10% to 40% chance of rain on Saturday and/or Sunday.
We need a roof. And a ‘cool roof’ would be way cool.
Designing Our ‘Cool Roof’
Our roof serves multiple purposes. First, it provides us with shelter (go figure). Beyond this basic function, we have the following requirements:
- Reduce our heating and cooling requirements.
- Allow us to capture 100% of the rainwater from the entire roof.
- Reduce our maintenance requirements.
- Support our solar photovoltaic panels.
The benefits that we will enjoy from our roof design include:
- Reduced energy costs, due to reduced energy consumption and the generation of electricity.
- Reduced maintenance costs, due to lower maintenance requirements and faster and easier maintenance activitivies.
- Increased occupant comfort, due to higher temperatures in the winter and cooler temperatures in the summer.
Roof Surfaces and Defining a ‘Cool Roof’
According to Johns Manville’s handbook on single ply roofing systems,
‘Roof surface temperature is important to reducing air conditioning energy usage and mitigating the Urban Heat Island Effect. Cool roof surfaces minimize heat build-up in the roof membrane caused by solar radiation (the sun) and the transfer of heat from the hot roof into the building. The roof is one of the first lines of defense against the effects of the sun on and in a building. In addition, roof insulation plays a critical role in reducing the heat flow into the building. With proper insulation and the addition of cool roofing products, heat transfer into a building can be reduced significantly.
A cool roof surface is defined by two mechanisms, reflectivity and emissivity. The higher the reflection of the total solar energy spectrum (called the albedo), the less solar energy is absorbed. The second mechanism, emissivity, is the amount of absorbed heat that is re-radiated back into the atmosphere. Thermal emissivity is expressed as a percentage relative to an ideal radiating surface, which is defined as having an emissivity of 100%. Thus, when less energy is absorbed (high solar reflectivity) and more of the absorbed energy is emitted (high thermal emissivity), there is less thermal energy (e.g., heat) in the roof to enter the building. For example, metals have low emissivity, and that is the reason why a highly reflective piece of metal left in the sun still becomes hot compared to a highly reflective, more emissive roof membrane. Reflectance and emittance with proper insulation are critical properties to a properly functioning cool roof.’
What could be cooler than a ‘cool roof’?
Eliminating Water from Ponding on Our Roof
Starting with shelter, we don’t want the roof to leak. On our 12:12 pitch gable roofs, this will not be a problem. However, on our flat roofs (upper-, mid- and lower flat roofs), we may experience ‘ponding’. Allan Courtney, our roofing advisor and roofing contractor, recommended that we design the flat roofs to direct water to the drains. Note that all of the water from our gable roofs, and upper- and mid level flat roofs goes onto our lower flat roof. This can be a lot of water. Al doesn’t want there to be any ‘flat’ areas on our roof for water to pond. Ponded water is bad, as it can be a source of water that could then leaks into our house. We would very much prefer that 100.0% of the water from our roof go into our underground cistern.
In addition, ponded water is heavy, which increases the load on the structure. Increasing the load can cause the roof to deflect, causing more ponding. More ponding can cause more deflection …
So, Al recommended that we have tapered foam crickets from Johns Manville. Each of the tapered foam crickets are manufactured in 4 ft x 4 ft pieces from closed cell polyisocyanurate foam.
ENRGY 3® and ISO 3™ are rigid roof insulation boards composed of a closed cell polyisocyanurate foam core bonded in the manufacturing process to universal fiber glass reinforced facers. ENRGY 3 and ISO 3 utilize an environmentally compliant blowing agent containing pentane hydrocarbon to enhance the thermal performance of the foam insulation. This hydrocarbon has zero ozone depletion potential and conforms to the Montreal Protocol established in 1987. ENRGY 3 and ISO 3 meet the physical property requirements of ASTM C 1289, Type II, Class I, Grade 2 and CAN/ULC S704, Type 2, Class 2. ENRGY 3 and ISO 3 specialty products are also available as tapered panels, precut miters and precut crickets.
To direct the water to the drains, the crickets need to be engineered so that, in every instance, there is a downward slope that leads to a drain. Johns Manville provided us with this design service as it is included in the cost of their product. Here is the design of our tapered insulation, which Leo Richardson drafted.
Although we don’t need to have additional R-value for our roof, installing tapered insulation as crickets will eliminate the structural thermal bridges on our flat roofs.
Reducing Energy Consumption
Our roof design started with reducing our energy consumption, which is one of the reasons for using SIPs (structural insulated panels). Our SIP roof provides thermal resistance of R-47. This high thermal resistance does not prevent solar radiation from reaching the inside of the house and heating the house. To reduce energy consumption for cooling the house, we need to reflect that heat back into the sky. Reflecting solar radiation can be accomplished with a roof surface with high solar reflectance and high thermal emittance. This is the value of a light colored (e.g., white) roof.
The R-47 thermal resistance does not include the structural elements inside the SIPs that create thermal bridges. For example, we have a 5.25×16 SCL (structural composite lumber) in the center of our upper flat roof that is not insulated. Without the tapered insulation, we would have thermal bridges where heat (energy) can escape, or enter, our house. The tapered insulation will reduce the thermal bridges as it will add, on average, R-7.5 to our thermal envelope on our flat roofs.
Components of Our Roofing System
In evaluating the roof surface over the SIPs, Al recommended a white, 60 mil TPO Membrane from Johns Manville. This product has an extremely high initial solar reflectance of 0.77 and an extremely high initial thermal emittance of 87%. To put these values into context, meeting the EPA’s Energy Star standards requires a solar reflectance of 0.65 and meeting the higher standards of California’s Title 24 requirements for commercial buildings requires solar reflectance of 0.70 and thermal emittance of 75%.
Al recommended TPO Membrane as it is manufactured from thermoplastic polyolefin (TPO). Single ply roof membranes are manufactured using EPDM, PVC or TPO. Al prefers TPO because it is easier to keep clean, which lowers our maintance requirements and, consequently, maintenance costs. Dirt will lower the solar reflectance and lower the thermal emittance.
JM TPO is one of the latest single ply, flat roofing materials on the market. The current membrane formulations are reinforced with a polyester fabric and manufactured using an ultraviolet-resistant thermoplastic polyolefin formulation. TPO from JM comes in several thicknesses. It is designed for use in mechanically fastened and adhered roofing applications in new, re-roof and re-cover roof constructions. It is fire and chemical resistant and contains UV inhibitors for added longevity.
Products with a longer useful and functional life contribute to the durability of the building envelope, which can result in a lower overall total cost of ownership for our house.
Regarding the attachment system, Al recommended that we go with mechanical fasteners. The alternative is a fully adhered roof membrane. Given his experience, Al recommended that we use Carlisle’s HP-x fasteners with 3 inch plates for the tapered insulation and 2-3/8 inch Piranha plates to mechanically fasten the membrane.
Over the tapered foam insulation and under the single ply membrane, Al recommended FR-10 slip sheet. The slip sheet serves two purposes – allows the membrane to move as it expands and contracts with heat and cold, and adds protection against flame spread and flame penetration through the roof system.
Installing Our Cool Roof
The six-person team from Statewide Roofing brought a portable diesel generator on our site and moved their tools onto the roof. Then, the roofing materials arrived from Ford Wholesale, and they were placed on our roof with a rear pivot steer Telehandler.
The team got to work quickly, cleaning the debris from the roof and distributing the 4×4 sections of tapered insulation. Then, they attached the insulation with screws to the SIPs and put down the slip sheets. The membrane went down next, fastened with the screws and Piranha plates.
Then, the detail work started, which is welding the seams of the membrane. it was a sunny, hot day, and the new white membrane reflected the solar heat, making it even hotter for the team working on the roof.
All of the tapered foam insulation that was secured to the roof was covered before the team left for the day.
Way cool …