Environmental impact trade-offs in building envelope retrofit strategies

Purpose

The use of high levels of thermal insulation is a common practice towards reducing the energy consumption of the existing building stock; however, the embodied burdens associated with the additional insulation material are usually not taken into account and questions regarding the risks of over-specifying the insulation levels have been emerging, particularly for mild climate regions. This article addresses the issue presenting an integrated approach that combines life cycle assessment and thermal dynamic simulation to assess alternative retrofit strategies for the roof and exterior walls of two dwellings (from the beginning of the twentieth century), in the historic city center of Coimbra, Portugal. A comprehensive analysis of alternative insulation thicknesses (no insulation, 40, 80, and 120 mm of expanded polystyrene) was made to identify optimal thickness levels minimizing life cycle (LC) environmental impacts for a single-family house and an apartment.

Methods

Embodied and operational impact trade-offs were calculated for six impact categories: climate change, ozone depletion, terrestrial acidification, freshwater eutrophication, marine eutrophication, and non-renewable primary energy. The operational energy was calculated using a dynamic thermal modeling software (EnergyPlus). The functional unit selected for this study was 1 m² of living area over a period of 50 years.

Results and discussion

The single-family house embodied impacts account for 26–57 % of total LC impacts. For insulation thicknesses larger than 80 mm, the embodied impacts are greater than operational impacts. For the apartment, embodied impacts account for 25–49 % of total LC impacts. The environmental benefits of additional insulation are very low (<3 %) for thicknesses of more than 80 mm for both roof and exterior walls. For thicknesses above the tipping point (where total LC impacts are minimized), the marginal impacts of additional insulation are higher than the benefits. The results for the apartment show that optimal insulation thicknesses (LC tipping point) range from 30 to 40 mm for the roof and from 60 to 80 mm for the exterior walls. The LC tipping point for the single-family house is achieved by combining 80–100 mm of roof insulation with 60–80 mm of exterior wall insulation.

Conclusions

Extra insulation levels in temperate climates can lead to higher embodied impacts, without significant reduction in operational impacts, which can result in higher total LC impacts. The results show that a tipping point can be identified, and recommendations are provided for the roof and exterior wall retrofits of buildings from the beginning of the twentieth century.