Energy Efficient Insulated Pitched Roof

Our buildings are becoming more and more energy-efficient and may become almost energy neutral in the future. This means that the energy consumption of the buildings during the use phase represents an ever smaller minimal environmental impact. In this post, we focus on the environmental impact of an insulated pitched roof. Of course, through time, with consideration of wear and tear, there is also a need for roof repair regardless of the type of roof.

The CEN TC 350 committee is currently finalizing a harmonized European normative framework for calculating the environmental impact of buildings and building elements. The BBRI already applied the main principles from this standardization for the execution of the life cycle analysis below.

How does efficient flat roof insulation work?

Life cycle analysis

With the help of a life cycle analysis, one can objectively quantify the environmental impact of a product, taking into account its entire lifespan (from cradle to grave). However, there are several methods for carrying out such an analysis, which can produce very different results.

For example, one has to decide which allocation rules to apply in multi-output processes. These are processes that produce more than one product so that their impact must be correctly distributed among the different products.

Also, the choice of the functional unit can make a big difference. This is the reference unit used in determining the environmental impact and serves as the basis for the comparison. This unit is determined on the basis of the specific product requirements. Since the performance of a material mainly depends on its application, construction products are best compared at the building level or at the level of technically equivalent components. Thanks to this approach, the influence of a particular choice of material on the further construction of a component or building is also taken into account.

Finally, if one wants to study the full life cycle of a construction product or building, one must make certain assumptions that will influence the end result. For example, one must try to make a correct estimate of the transport and the lifespan of the materials, the waste processing method, etc.

Analysis at the element level

Below we compare the environmental impact of different insulation materials in a pitched roof. We use 1 m² horizontally projected roof surface as the basis for the comparison. This surface is constructed as a rafter roof (38 x 175 mm with a center-to-center distance of 50 cm), where the cavities were completely filled with soft insulating materials (eg glass wool or cellulose) and the structure was covered with hard insulating plates (eg PUR ) according to the sarking roof method. We also approximated the energy consumption due to heat losses through the roof using the degree days method (with 1200 equivalent degree days).

The bar chart below shows the environmental impact of the various technically equivalent alternatives for an assumed lifetime of 60 years. This shows that the energy consumption due to the heat losses is significant during this period. The degree of insulation of the roof will therefore have a greater influence on the environmental impact than the nature of the insulation material used.

We also note that natural insulation materials, such as sheep’s wool or cork, do not necessarily score better than traditional materials. To illustrate the importance of the aforementioned allocation rules, we calculated two alternatives for sheep wool. Alternative A assumes that the sheep is raised only for its meat (the wool is a waste product and the environmental impact of breeding is fully attributed to the meat), while Alternative B assumes that it is also raised for its wool (distribution of environmental impact according to the economic value: 22.8% for wool and 77.2% for meat).

Finally, we note that the results differ strongly from those reported in foreign publications of, for example, the NIBE (Netherlands Institute for Building Biology and Ecology). This may be due to differences in allocation rules, functional units, waste treatment methods, impact categories and aggregation methods.
3. Decision
The BBRI has a number of analyzes of other building elements in the pipeline, which will be performed again according to the future European normative framework. Since these objective and transparent analyzes (technical annex available on request) will take Belgian construction traditions into account, they can become a reference work on environmental impact for the construction sector.