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How Much Insulation Do I Need?

Last modified: 09.08.2023
This image shows a group of eco-friendly, low-energy homes built into the landscape with green roofs and large south-facing windows, designed to maximise energy efficiency and blend into their natural surroundings.

When it comes to energy efficiency in buildings, insulation is an essential component that often leaps to mind. The right insulation can significantly reduce energy consumption, improve thermal comfort, and contribute to an environment-friendly footprint. However, attaining superior thermal performance is not solely about insulation; various other factors come into play, especially when striving for the highest standards such as Passive House or fossil fuel-free homes. Despite this, it is crucial to consider how much insulation is required to achieve the best thermal performance and satisfy regulations. Thermal bridges, caused by more conductive materials that allow heat to bypass the insulation, like windows or structural elements, can undermine the overall energy efficiency of the building. Airtightness, ventilation, the use of thermal mass, and renewable energy systems also have crucial roles in creating a highly energy-efficient building envelope. In this article, we will delve deeper into the thickness required for different insulation types – namely, Expanded Polystyrene (EPS), mineral wool, and phenolic foam – to meet various standards and explore other vital components in achieving superior thermal performance.

Thermal performance is a critical aspect of building regulations in the UK, measured using U-values. The lower the U-value, the better the building component's ability to retain heat, contributing to energy efficiency. Under the current UK building regulations, the recommended maximum U-values are:

  • Walls (retrofits): 0.30 W/m²K
  • Walls (new builds): 0.18 W/m²K
  • Roofs: 0.20 W/m²K
  • Floors: 0.25 W/m²K

To meet these U-values, the required insulation thickness will vary. For external walls, the thickness of insulation needed to achieve a U-value of 0.30 W/m²K in retrofit situations and 0.18 W/m²K in new builds is approximate:

  • Mineral Wool: 110 mm for new builds
  • EPS: 90 mm for new builds
  • Phenolic Foam: 60 mm for new builds

These figures may vary slightly based on the specific product brand and other factors such as building design and location. It's important to remember that thermal bridges, created by elements like windows and door frames, can undermine insulation effectiveness. These should be addressed during retrofit projects and designed out in new build properties, to ensure the insulation performs at its best. The thickness of insulation required can seem substantial, but the energy savings and increased comfort it can provide make it a worthwhile investment. Whether you are retrofitting an older property or building a new one, understanding these values is a crucial step in designing an energy-efficient and comfortable home.

A Passive House, or Passivhaus in German where the standard originated, is a rigorous, voluntary standard for energy efficiency in building design and construction. The idea is to reduce the building's ecological footprint by creating structures that require very little energy for heating or cooling. The primary goal of a Passive House design is to achieve outstanding thermal comfort and energy efficiency only using a small fraction of the energy used in typical buildings, hence it could be termed 'passive'. The recommended U-values for the Passive House standard are as low as 0.15 W/m²K for all components. Achieving this calls for considerable insulation thickness:
  • EPS: 320-340 mm
  • Mineral Wool: 340-360 mm
  • Phenolic Foam: 180-200 mm
But insulation alone is not enough. Passive House standard places great importance on:
  • Airtightness: Buildings need to be virtually airtight, preventing heat loss through uncontrolled air leakage.
  • Thermal Bridge-Free Design: The design needs to minimise thermal bridges. This includes installing high-performance windows with insulated frames and using insulation consistently across all parts of the building.
  • Ventilation: Mechanical ventilation with heat recovery (MVHR) systems is a typical feature, allowing for adequate ventilation while maintaining indoor temperature by recovering heat from the exhaust air.
  • High-Performance Windows: Windows are an essential part of the design. In a Passive House, windows are usually triple-glazed with insulated frames to minimise heat loss.
[caption id="attachment_66611" align="aligncenter" width="1024"]This diagram shows how a passive house maintains energy efficiency through key features such as high-performance windows, airtight construction, and thick thermal insulation. It also includes a ventilation system with heat recovery that brings in fresh air and removes stale air while retaining warmth. The design reduces thermal bridging to prevent heat loss at junctions and maintains a consistent indoor temperature with minimal energy use. https://passiv.de/en/02_informations/02_passive-house-requirements/02_passive-house-requirements.htm[/caption] Fossil fuel-free homes aim to reduce the heating load to such a level that a conventional heating system becomes unnecessary, making insulation thickness crucial. To create a truly fossil-fuel-free home, the insulation thickness could be similar to, or even exceed, that required for the Passive House standard:
  • EPS: 350-370 mm
  • Mineral Wool: 370-390 mm
  • Phenolic Foam: 200-220 mm
Additional aspects to consider for fossil fuel-free homes are:
  • Thermal Mass: Building materials like concrete or brick with high thermal mass can absorb heat during the day and release it at night, providing natural temperature regulation.
  • Airtightness: High levels of airtightness, combined with controlled ventilation, can prevent drafts and heat loss.
  • Ventilation: Fresh air and controlled ventilation are important for indoor air quality and can be provided by systems like MVHR.
  • Renewable Energy Sources: Renewable energy systems, such as solar panels, wind turbines, or geothermal heat pumps, can provide all necessary energy for heating, cooling, and electricity.
In conclusion, while insulation is a critical factor in building energy efficiency, it's far from the only one. Achieving the highest standards of energy efficiency requires a holistic approach that also considers airtightness, thermal bridge-free design, use of thermal mass, controlled ventilation, and renewable energy systems. Each home is unique, so it's important to consider all these aspects in your design to reach your specific energy efficiency and sustainability goals. Through our close partnership with EWI Pro, we've had the privilege of working on the Howgate Close settlement. Currently, it is the most energy-efficient settlement in the UK and is classed as fossil-fuel-free. Dr Chris Parsons, a retired GP and second-generation farmer, brought to life an innovative project known as Howgate Close. The aim was to tackle multifaceted local challenges, including affordable rural housing shortages, biodiversity loss, and environmental impacts. The initiative spans across 10 acres of previously intensively farmed land, transformed into a habitat for nine rented homes, a 60KW solar panel installation, and over 10,000 newly planted trees and hedgerows. In doing so, Howgate Close stands as a testament to addressing climate change, restoring soil health, creating biodiversity, managing water resources, and enhancing community cohesiveness, all while providing affordable housing. The homes were designed by Dr Jerry Harrall, harnessing principles like southerly orientation, high thermal mass superstructure, super-insulated envelope, and renewable energy. Despite not including traditional elements like ground source heat pumps, cavity walls, roof trusses, or conventional foundations, these homes achieved the highest As-Built SAP Ratings in the UK, ranking them among the most energy-efficient dwellings in the country. Additionally, the project's Return on Investment (ROI) is 4.6%, with a payback period of 15 years. The site prevents 16 tonnes of carbon emissions annually through renewable energy, and the rewilding efforts sequester an additional 9 tonnes of carbon each year. The average energy bill for each home was less than £1 per day during the first 10 months. With a total project cost of £1.5m and build costs of £2,100 m², Howgate Close is a competitive model compared to conventional (£1,800 m²) or Passive House (£2,200-2,400 m²) dwellings. It is now a thriving community of mixed demographic residents, providing a safe, sustainable living environment.