You don't have to experience air conditioning problems to understand the unbearable heat that comes along with the long summer days in Houston.
If you have had the misfortune of having to wait overnight for a repairman to get you back up and running, then you may have experienced the confusing issue of your home actually heating up as the night progressed.
The issue?
That asphalt roof overhead.
An American standard, the shingled roof works wonderfully in northern regions where temperatures dip dramatically overnight, but here in Houston there has to be a different solution.
As a roofing contractor, I can tell you that those asphalt shingle roofs get scorching hot, and the darker the color of the shingle, the hotter it gets. But these asphalt shingles have another problem that make them unsuitable for our hot climate: thermal mass. This means that not only do they soak up the sun's heat like a sponge, but they also hold a tremendous amount of this heat for long after sunset.
Your roof actually absorbs heat all day, and then releases it into the house at night, and all of that insulation doesn't completely block the flow of heat from the attic, it just slows it down.
DESIGNING A BETTER ROOF FOR TEXAS
Our goal was not only to build a roof that would keep not only the house cool, but the attic space cool as well.
Most modern homes in the Houston area have another common design problem; the air conditioning ductwork is located in the attic space.
In order to keep the attic space cool, we need a roof designs that block as much of the sun's heat from entering the roof deck as possible.
In pouring over the research studies for cool roof design, I found three important design factors with proven abilities to reject the sun's heat:
- Metal panels or coatings with a high solar reflectivity and emissivity
- Above-sheathing ventilation
- Above-sheathing radiant barrier
SOLAR REFLECTIVITY, EMISSIVITY, AND SRI
Most "cool roofs" today use metal roofing panels that have been manufactured to reflect a large portion of the sun's radiant heat from their surface and away from the house. Depending on the color of the paint used, these panels can reflect anywhere from 20% (for darker colors) to 75% (white) of the sun's heat. When compared to dark asphalt shingles with a solar reflectivity of only 5%, metal roofing panels can make a real difference in keeping your home cooler.
Below is a chart from a manufacturer of metal roofing panels that lists the solar reflectivity values for their different color choices.
The chart shows the emissivity as well as SRI (Solar Reflectance Index) values for each color.
The term "emissivity" refers to how well a material reradiates or reemits the heat that it has absorbed. SRI is a calculation based on the solar reflectivity and emissivity values, and provides a value for how well a given material (or paint color in this case) rejects the sun's heat.
ABOVE SHEATHING VENTILATION
Above sheathing ventilation (ASV) is simply a vented air space that lies sandwiched between the roof surface (metal roofing panels) and the roof deck, or sheathing.
Most homes utilize ridge vents and soffit vents for ventilating the attic space. ASV also uses ridge and soffit vents, but it ventilates a space above the attic, preventing much of the heat absorbed from the sun from ever entering the attic space.
Ventilation cools by a process called convective heat transfer. As air comes in contact with a hot object, some of the object's heat is transferred to the air, causing it to expand, become less dense, and rise.
There is a very effective method called thermosiphon, which uses the sun's heat to power the removal of heat from a roof with ASV. As the hot air rises and exits out the ridge vent at the top of roof, convection current is created which pulls in cooler air through the soffit vents at the bottom of the roof.
Because the ventilated air space lies just below the roof surface in an ASV roof design (and not in the attic), the air can come in direct contact with the hot metal roof panels. This creates a "supercharged" convective flow of air under the panels. The hotter the metal roof panels get, the faster the air flows and the more heat is removed before it can enter the attic.
Unfortunately, very few "cool roofs" are designed today with above-sheathing ventilation. In fact, many roofing contractors simply attach metal roofing panels directly to the roof sheathing and call it a "cool roof", and some who do create an air space between the panels and the roof deck, place battens horizontally (side to side) across the roof, effectively blocking the flow of air from the soffit vents up to the ridge vent.
Properly designed above-sheathing ventilation is an essential component of a thermally engineered roof system, and forms the second line of defense against the heat of the sun.
ABOVE-SHEATHING RADIANT BARRIER
Most people are familiar with the use of a radiant barrier in an attic space, which involves stapling sheets of radiant foil to the underside of the roof rafters. This type of installation can help to keep your home cooler but the heat has already entered the attic and is being conducted through the house's structure before it is reflected by the radiant barrier. Also, it can be difficult to install a continuous barrier in an attic space, and some roofs may not have an attic space under all parts of the roof.
The most important design consideration in a radiant barrier installation is creating an air space between the barrier and the source of the radiant heat. Without this air space, the radiant barrier cannot work because the heat flow will be conductive in nature rather that radiative.
Above-sheathing radiant barriers can reflect 97% of the radiant heat away from your home. Installing a radiant barrier below the metal roof panels, and within the above-sheathing ventilation space has two huge benefits:
- Radiant heat being reemitted by the metal roofing panel is blocked from entering the home and reflected back towards the panel causes the temperature of the metal roofing panel to rise even higher, providing more "fuel" to power the thermosiphon ventilation under the panel.
- A radiant barrier under a metal roof panel means that the color of the panel doesn't necessarily have to be white for the roof to have good solar reflectivity. Darker color roof panels will absorb more of the sun's radiant heat, but an above-sheathing radiant barrier will reject a large amount of this absorbed heat before it can enter the home.
A radiant barrier placed above the roof deck is the final line of defense against the hot Texas sun, and can significantly enhance the performance of the metal roof panels and above-sheathing ventilation components of the cool roof.
TYING IT ALL TOGETHER: THE BATTEN AND COUNTER BATTEN ROOF STRUCTURE
Integrating the high SRI metal roofing panels, the above-sheathing ventilation, and the above-sheathing radiant barrier is a unique double-layer roof batten structure called a batten and counter batten system.
A crisscross pattern of roof battens under the roof deck provides four important things:
- Large, unrestricted channel for the above-sheathing ventilation under the metal roof panels
- Needed airspace for the radiant barrier to function properly
- A path for any dripping condensation that might form on the underside of the metal roofing panel an unimpeded path to drain to exit the roof (Metal roofs that are attached with just horizontal battens can have water damage to the battens over time)
- Minimized conduction of heat from the metal roofing panels to the roof deck by limiting the surface contact between the two surfaces.
A COOL ROOF THAT PERFORMS WELL IN THE WINTER ?
A major drawback to a traditional cool roof is their poor performance during the cold winter months. While cooling costs can be dramatically reduced with a cool roof, the cost to heat the home in winter will increase because the sun cannot supply heat to the home through the roof.
The good news is that a cool roof with above-sheathing ventilation actually performs much better during the winter than a typical cool roof. In testing done by the Oak Ridge National Laboratory in 2006, it was found that above-sheathing ventilation negated the heating penalty associated with cool roof designs.
The air space underneath the roof acts as a conduction block for heat flowing between the metal roof panels and roof deck in both directions, so the home with a ventilated cool roof doesn't gain heat like a home with an asphalt shingle roof on winter days, but it also doesn't lose radiant heat to the sky on winter nights!
THE TEXAS SMART ROOF IS BORN
So there you have it, a cool roof designed to keep your attic and home cool, and take a huge load off of your air conditioner. We call it the Texas Smart Roof because it just makes good sense in so many ways:
Minimizes how much of the sun's heat is absorbed by the roof and transmitted into your home
Harnesses the sun's heat to passively power the removal of absorbed heat from the roof - the hotter it gets, the greater the cooling from ventilation
Uses a radiant barrier in the most effective way, providing an uninterrupted blanket of shade over the entire home
Negates the extra heating expense associated with typical cool roofs in winter
Has at least three times the lifespan of an ordinary asphalt shingle roof, making it the last roof that you will probably ever buy for your home
Pays for itself in utility bill savings alone
Metal roofing panels are 100% recyclable, making it a smart choice for the environment
