Image 1: Different layers of a roof (Source: Berkeley Lab)
A roof can have a major impact on the temperature, comfort and overall energy efficiency of a building, especially for large buildings that have a significantly greater roof to wall area ratio. The shape, color, and material of a roof collectively play pivotal roles in determining its performance. An adequately insulated roof is indispensable for preventing heat loss in winter and heat gain during summer. Opting for light-colored or cool roofs enhances sunlight reflection, resulting in lower solar energy absorption and a subsequent reduction in the building's temperature—especially advantageous in hot climates. Implementing such enhancements can not only mitigate space heating and cooling loads but can also contribute to decreased utility bills and carbon emissions.
Recommendations provided:
Asset Score Report Recommendation:
1. Add Roof Insulation
Cost: $ to $$ based on whether it is possible to add blow-in insulation or if re-roofing is required.
The roof insulation is one of the key factors impacting the thermal envelope of a building. The applicability of roof insulation upgrades depends on roof construction type and presence of existing insulation. Basic recommendations based on the type of roof and ceiling construction are given below.
Detailed recommendations on the quantity of insulation suggested for different building types and climate zones are available in the ASHRAE Advanced Energy Design Guides:
https://www.energy.gov/eere/buildings/advanced-energy-design-guides.
Recommendations based on roof types:
Roof type: Pitched Roof/Shingles/Shakes
Attic Ceiling: Based on your current insulation level and space availability, add sufficient insulation to obtain the minimum insulation level for your climate location. A higher insulation R-value improves the thermal performance of your roof, keeping the interior space cooler in summer and warmer in winter. In addition, adding higher insulation levels beyond the minimum codes requirements may increase the cost-effectiveness of the upgrade; however, there will be diminishing returns, and therefore cost should be weighed carefully against performance. If there is no existing insulation, consider a high insulation level such as R-30 or R-38. Roof insulation should extend to the exterior of the walls to minimize edge effects.
Suspended Ceiling: If there is a suspended ceiling, add insulation on top of the suspended ceiling. When suspended ceilings with removable ceiling tiles are used, the insulation performance is best when installed at the roof line.
Roof type: Flat, Built-up Roof
Add at least 1 inch (e.g., at least R-5 or R-10) of roof insulation and re-roof. A higher insulation R-value improves the thermal performance of your roof, but the cost-effectiveness of additional insulation should be evaluated. Insulation above the deck should be continuous rigid boards because no framing members are present that would introduce thermal bridges or short circuits to bypass the insulation. If two layers are used, the board edges should be staggered to reduce the potential for convection losses or thermal bridging. If an inverted or protected membrane roof system is used, at least one layer of insulation should be placed above the membrane and a maximum of one layer placed beneath the membrane.
Roof type: Metal Surfacing
Add insulation to interior surface, such as 2 to 4 inches of fiberglass or 1 to 2 inches of foam. Thermal blocks cannot be used when through-fastened in a roof that is screwed directly to the purlins because the blocks diminish the structural load-carrying capacity by “softening” the connection and restraint provided to the purlin by the roof. For Climate Zones 1-3, recommended construction is a filled cavity with the first insulation layer perpendicular to and over the top of the purlins and the second layer of insulation parallel to and between the purlins. For Climate Zones 4-7, recommended construction is a linear system with the first layer of insulation parallel to and between the purlins and the second layer of insulation perpendicular to and over the top of the purlins. For Climate Zone 8, recommended construction is a linear system with the first and second layers of insulation parallel to and between the purlins and the third layer of insulation perpendicular to and over the top of the purlins.
2. Install a Cool Roof
Cost: $ to $$ based on the ease of adding an additional reflecting layer to the existing roof.
Cool roofs are constructed with materials that reflect sunlight and emit thermal energy, resulting in a roof surface that remains cooler than conventional counterparts. This reduces the heat transfer significantly diminishing the demand for mechanical cooling within the building. It is similar to wearing light-colored clothing that keeps a person cool on a sunny day.
The coolness of a roof is primarily determined by two key characteristics: solar reflectance or SR (how much sunlight it reflects) and thermal emittance or TE (how much heat it releases). These attributes are quantified on a scale ranging from 0 to 1, with 1 indicating 100% reflectivity or emissivity. SR and TE are further used to calculate the Solar Reflectance Index of a roof that enables comparisons of different roofing products (https://www.coolroofs.org/).
This measure involves replacing the existing roof membrane with a cool roof membrane. It is recommended only for climate zones 1-3 since implementing a cool roof tends to yield energy savings in the warmer climates.
Benefits of a cool roof (as mentioned by U.S. Department of Energy - https://www.energy.gov/energysaver/cool-roofs):
A cool roof can enhance a building and the well-being of its occupants by:
- Lowering energy expenses through a reduction in air conditioning requirements.
- Assisting older, inefficient, or undersized air conditioners in delivering adequate cooling for increasingly warmer summers.
- Qualifying for cost-saving rebates offered by utilities or government programs.
- Enabling the possibility of downsizing new or replacement air conditioning systems, resulting in cost savings and potential efficiency gains.
- Enhancing indoor comfort and safety in spaces without air conditioning.
- Decreasing roof temperature, potentially extending the service life of the roof.
Resources:
For an in-depth understanding of cool roofs and their technical specifications, consider exploring the resources below:
- https://www.energy.gov/energysaver/cool-roofs
- Cool Roof Rating Council - https://www.coolroofs.org/
- The U.S. Department of Energy’s Guidelines for Selecting Cool Roofs (DOE 2010)
- LBNL Heat Island Group - https://heatisland.lbl.gov/coolscience/cool-roofs
- https://www.epa.gov/heatislands/using-cool-roofs-reduce-heat-islands