Wood fibre insulation from STEICO and a diffusion-open EWI Pro render was chosen to support the low-carbon retrofit ambitions of the owner of a semi-detached house in Northwood in the London Borough of Hillingdon.
Furbnow, specialists in planning home energy improvements, carried out a survey of the property, including its existing building fabric, heating and ventilation systems. The Home Energy Plan produced detailed six measures to improve the energy efficiency of the building to raise the EPC score of 65 (a D rating) up to a score of 96 (an A rating). The plan included the need to top up the existing loft insulation to a depth of 300mm and install insulation on the external walls which would help minimise thermal bridging in the walls.
They recommended the use of the diffusion-open STEICOprotect L dry wood fibre external wall insulation with breathable EWI Pro primers and lime render.
Why was STEICO wood fibre insulation selected for the EWI system?
The diffusion-open EWI system supports the moisture profile of the solid wall construction, allowing moisture to pass through the wall system rather than forming surface or interstitial condensation. To ensure that moisture vapour within the home could diffuse through the solid walls, the internal plaster was removed and replaced with a breathable alternative.
The STEICO products helped to improve the calculated U-value of the solid walls from 1.7 W/m²K to 0.3 W/m²K.
As walls can account for up to 33% of a home’s total heat loss, improving their thermal performance significantly reduces the demand placed on the heating system. In turn, less energy is required to maintain a comfortable internal temperature, helping to lower operational carbon emissions as well as the cost of heating for the occupants.
Wood fibre insulation contains bound carbon dioxide
While insulating walls with an EWI system can significantly reduce operational carbon, each insulation type also brings its own embodied carbon to a project. For the client in this retrofit, embodied carbon was an important consideration, aligned with the wider ambition to deliver a low-carbon refurbishment.
EWI Pro lime render has a lower carbon impact than cementitious alternatives. During the manufacture of EWI Pro’s lime renders, less CO₂ is released into the atmosphere compared with modern cementitious products. EWI Pro’s lime range, used in conjunction with wood fibre insulation, is made from NHL lime, marble sand and clay. This means that there’s no harmful chemicals, no fly ash or slag and the products are completely at one with traditional construction. Once applied to the wall, lime-based systems also absorb CO₂ from the atmosphere as they cure, through a natural process known as carbonation.
Wood fibre insulation helps to mitigate overheating
Wood fibre insulation contributes thermal mass to the wall build-up, which can help moderate internal temperatures during warmer weather. Its combination of relatively high density, high heat storage capacity and low thermal conductivity helps to reduce the risk of summer overheating.
These properties enable the STEICO wood fibre insulation to absorb and store heat during the day, slowing the rate at which it is transferred into the internal room spaces of the house. This creates a time delay, meaning peak external temperatures do not immediately translate into peak indoor temperatures.
Importantly, these characteristics can be calculated and expressed as part of a building’s thermal performance. In much the same way that U-values measure heat transfer during winter, summer performance can be quantified using parameters such as thermal diffusivity and time delay.
Thermal diffusivity combines density, specific heat capacity and thermal conductivity to indicate how quickly a material responds to temperature changes. A lower diffusivity means heat moves more slowly, creating a longer time lag between external temperature peaks and internal temperature rise.
What are the benefits of adopting a whole house fabric first approach when specifying wood fibre EWI?
The detailed whole house approach taken to this retrofit helped protect the fabric of the building and increase the internal thermal comfort.
The Energy Plan indicated that the three measures installed so far – a top-up of loft insulation, decentralised Mechanical Ventilation and Heat Recovery (MVHR) system, and STEICO wood fibre EWI insulation would improve energy rating of the EPC by 10, raising the property to a ‘C’ rating. This increase in energy efficiency is predicted to deliver an annual saving of 1253 Kgs CO2 and would ensure the optimum air source heat pump size could be selected to maximise its operating efficiency, keeping costs to a minimum.