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What is a Green Roof and Can You Insulate It?

Last modified: 02.08.2024
Contemporary home with a green roof system, surrounded by open landscape. EWI Store supports eco-friendly building solutions, offering breathable insulation systems and sustainable materials to enhance energy efficiency and environmental performance.

With sustainable living a hot topic, green roofs present an innovative and eco-friendly solution for urban environments. Not only do they offer aesthetic and environmental benefits, but they can also be integrated with insulation to enhance their efficiency. But what exactly is a green roof, and how can you insulate it effectively?

A green roof, also known as a living roof or eco-roof, is a roof that is partially or completely covered with vegetation and a growing medium planted over a waterproof membrane. These roofs are designed to support plant life and offer a range of ecological, economic, and social benefits. Unlike traditional roofs, green roofs integrate nature directly into the building structure, creating a living ecosystem atop the built environment. Green roofs come in two primary types.

  • Depth: A shallow substrate layer typically ranges from 2 to 6 inches.
  • Weight: Lightweight, making them suitable for retrofitting existing buildings with minimal structural enhancements.
  • Vegetation: Support low-maintenance, drought-resistant plants like sedums, mosses, and grasses.
  • Maintenance: Require minimal upkeep and are generally not accessible for regular use.
  • Use Cases: Ideal for buildings with limited load-bearing capacity and for those seeking a low-maintenance green solution.
  • Depth: Have a deeper substrate layer, often exceeding 6 inches, and can be several feet thick.
  • Weight: Heavier, requiring substantial structural support, making them suitable for new constructions or buildings designed to bear additional load.
  • Vegetation: Can support a diverse range of plants, including small trees, shrubs, flowers, and even vegetable gardens.
  • Maintenance: Requires regular maintenance, including irrigation, fertilisation, and weeding, similar to traditional gardens.
  • Use Cases: This material is suitable for buildings with roofs designed to be accessible and used as recreational spaces, such as rooftop gardens or parks.

The construction of a green roof is fairly intensive and requires several layers to function efficiently. The structural deck is the base layer that supports the entire system. A vapour control layer is laid down to prevent moisture ingress. Following this, a thermal insulation layer is installed which reduces heat loss; this is sandwiched between the vapour control layer and a further waterproof membrane. The next layers are equally critical; the root barrier prevents plant roots from damaging the waterproof membrane, the drainage layer removes excess water to prevent waterlogging and ensures proper root aeration, and the filter fabric prevents soil particles from clogging the drainage layer. The growing medium, which varies in composition depending on the type of green roof and the plants used, supports the vegetation layer, which consists of plants selected based on local climate, roof type, and desired aesthetic or functional outcomes.

  • Environmental Impact: They help reduce the urban heat island effect by cooling the air and absorbing pollutants, thus improving air quality. Green roofs also provide birds, insects, and other wildlife habitats, contributing to urban biodiversity.
  • Energy Efficiency: Green roofs act as natural insulators, reducing the need for air conditioning in the summer and heating in the winter. This leads to significant energy savings and lower utility bills.
  • Stormwater Management: By absorbing rainwater, green roofs reduce runoff and alleviate the burden on stormwater drainage systems. This helps mitigate the risk of flooding and decreases the pollution of waterways.
  • Aesthetic Appeal: Green roofs enhance the visual appeal of buildings, offering a natural, green space in the heart of urban areas. They can be designed to be visually pleasing and even used as community spaces for relaxation and recreation.
  • Noise Reduction: The layers of a green roof, particularly the soil and plants, help absorb sound, reducing noise pollution from the outside environment.
  • Extended Roof Lifespan: By protecting the underlying roof materials from ultraviolet radiation and extreme temperature fluctuations, green roofs can extend the lifespan of the roof structure.

Yes, you can insulate a green roof, which can significantly enhance its energy efficiency. Insulating a green roof involves incorporating an additional layer of thermal insulation beneath the waterproof membrane. The insulation tends to be made from rigid insulation boards, which enhance energy efficiency and provide additional stability between the vapour and water control layers. There is no correct choice for insulating material, as EPS, XPS, mineral wool, phenolic boards, and more are all suitable.

EPS is an incredibly lightweight material, making it easy to handle and install without adding excessive load to the roof structure. It is also cost-effective, providing an affordable solution for thermal insulation. EPS boasts good thermal resistance (R-value), which helps reduce heat transfer and enhance energy efficiency. While it may not be as moisture-resistant as XPS, EPS can still handle moisture well when properly protected and maintained. EPS is versatile and can be cut and shaped to fit various roof designs and configurations.

Due to its closed-cell structure, XPS offers superior moisture resistance, protecting against water absorption and infiltration. XPS also has high compressive strength, making it durable and capable of withstanding heavy loads—ideal for roofs with high traffic or additional layers. With its high thermal resistance, XPS maintains its insulating properties even in wet conditions, making it suitable for insulating below the DPC. The material's durability ensures long-term performance without significant degradation over time.

Mineral wool is renowned for its noncombustible fire resistance and excellent safety features.

Its dense structure offers superior sound absorption, which helps reduce noise pollution from external sources. Mineral wool also provides good thermal insulation, maintaining comfortable indoor temperatures. Often made from recycled materials, mineral wool is environmentally friendly and contributes to sustainability goals. Furthermore, its vapour permeability allows moisture to pass through, reducing the risk of condensation and moisture-related issues within the roof structure.

Phenolic boards stand out for their high thermal efficiency, boasting one of the highest R-values per inch among insulation materials.

This superior thermal performance can be achieved with thinner profiles, making phenolic boards a space-efficient option. They also have high fire resistance, adding an extra layer of safety to the building. With a low water absorption rate, phenolic boards ensure their insulating properties remain effective even in damp conditions. They maintain their shape and insulating properties over time, providing consistent performance. Despite their high performance, phenolic boards are relatively lightweight, easing the installation process and reducing structural load.