The ultimate guide to insulating your loft

In this guide, we take a look at loft insulation. We'll begin by explaining what loft insulation is, where it is positioned and how this can create a cold or a warm loft space. We will explore the link between the thermal performance of loft insulation and the thickness needed to reach U-value requirements.

We will also look at where those U-value requirements come from. These vary between new build roofs and those loft spaces that already exist. We have included some handy tables to give a broad indication of thicknesses of different types of loft insulation including wood fibre batts and loose fill wood fibre insulation.

Next, we will cover the benefits that installing loft insulation may bring and what should be considered during and after installation. Finally we conclude by explaining where you can find out more information on wood fibre insulation solutions for loft spaces.

What is loft insulation?

Loft insulation is a layer of insulating material that helps to reduce heat loss through the roof of a building. Many types of insulating material are suitable to use as loft insulation, and they come in various forms including rolls, rigid boards, loose fill and flexible batts. As well as reducing heat loss from a roof in the winter, loft insulation can also help to keep a house cool in warmer weather by slowing down the transfer of heat from the loft into the rooms below.

Where is loft insulation positioned?

Loft insulation can be fitted in two positions.

• Between the joists in the loft - Often referred to as ‘at ceiling level’ because it is effectively being installed above the ceiling of the rooms below the loft.
• Between the rafters - Insulation is fitted along the line of the pitched roof at rafter level and is typically only suited to full loft conversions where the attic space is going to be used as a heated living space. It requires careful detailing to ensure that the pitched roof timbers are correctly ventilated.

How can loft insulation create a cold or a warm loft?

The position of the insulation determines whether a loft will be cold or warm.

• Cold Loft - Loft insulation positioned at ceiling level helps to retain heat in the dwelling. This leaves the loft space above as a cold ventilated space.
• Warm loft  - As described above,loft insulation positioned at rafter level creates a warm loft. This is usually only required when habitable loft space is required and is more commonly referred to as a room-in-roof conversion.

How is the thermal performance of loft insulation linked to the U-value of the roof?

The thermal conductivity of the type of loft insulation specified determines the thickness of insulation that will be needed to meet the U-value for the roof.

The thermal performance of the loft insulation

This is expressed as its lambda value and is a measure of the insulation’s thermal conductivity. The lower the lambda value the better the insulation performance because a material with a lower thermal conductivity conducts less heat.

The thermal performance of the insulated building element

The thermal performance of a building element is quantified by its U-value. This measures the rate of heat transfer through an element. It considers the effect on thermal performance of all the elements within that construction. In the case of an insulated loft at ceiling level, the joists, plasterboard, as well as the lambda value of the insulation itself, will all contribute to the U-value. As with lambda, the lower the U-value the better the calculated thermal performance of an element.

How much loft insulation do I need?

Once the lambda of the insulation and the loft’s U-value target are known, it is possible to work out the thickness of loft insulation required. The lambda value of the insulation should be available on packaging and supporting information from the manufacturer.

The U-value requirements for roofs can be found in Approved Document L – Conservation of fuel and power^[1]. This gives the U-value targets for various scenarios in new and existing buildings.

New build loft insulation requirements

For new build dwellings, housebuilders use the notional dwelling within the Standard Assessment Procedure^[2] (SAP) program as a U-value guide. The SAP program is part of the National Methodology for assessing whether a dwelling meets the energy performance and emissions requirements of Approved Document L. Aiming for U-values that are close to those of the notional dwelling gives the final specification a better chance of achieving compliance.

The notional dwelling roof U-value is 0.11 W/(m²_*K).

(The notional dwelling value can be found in table 1.1 from page 12 of Approved Document L Volume 1: Dwellings 2021 edition incorporating 2023 amendments).

As long as the overall dwelling specification still achieves compliance within SAP, there is some freedom to vary U-values. However, the limiting U-value of 0.16 W/(m²_*K) must always be met.     

(The limiting U-value can be found in Table 4.1 on page 24 of Approved Document L Volume 1: Dwellings 2021 edition incorporating 2023 amendments)

Existing buildings’ loft insulation requirements

As well as new builds, Approved Document L also covers work to existing buildings. These fall into two categories – adding a new or replacement thermal element to an existing building or renovating a retained thermal element.

A thermal element is a wall, floor or roof that separates a heated or cooled area from the outside, an unheated part of the same building, or a structure exempt from the building regulations, such as a porch or conservatory.

Adding a new or replacement element in an existing dwelling

When adding a new or replacement fabric element to an existing dwelling, such as a new extension, the U-value of 0.15 W/(m²_*K) should be met.

(The U-value can be found in Table 4.2 on page 25 of Approved Document L Volume 1: Dwellings 2021 edition incorporating 2023 amendments.)

Renovation of retained elements – limiting U-value

The U-value of an existing thermal element that is being renovated should be no worse than that of the element before it was renovated and should always meet the limiting U-value of 0.16 W/(m²_*K).

(The U-value can be found in table 4.3 on page 26 of Approved Document L Volume 1: Dwellings 2021 edition incorporating 2023 amendments.)

Renovation of retained elements – threshold U-value

The threshold value is the absolute maximum U-value and is there to allow for renovations where achieving the limiting value may not be technically or functionally feasible or would not achieve a payback time of 15 years or less. If this is the case, then the element should be upgraded to the lowest U-value that meets both these requirements – the threshold value of 0.35 W/(m²_*K) should only be used as a last resort.

*Please note – All values above are indicative values intended as a guide only. Actual results will depend upon the lambda value of the insulation and the exact build-up of the loft element to be insulated. Always contact the manufacturer to obtain a project specific U-value calculation that will determine the thickness of insulation required.

What are the benefits of installing loft insulation?

There are several benefits of installing loft insulation in a home.

Helping to improve energy efficiency

Installing loft insulation can help improve the energy efficiency of a property, thus saving heating energy, which can also have an impact on heating bills. The possible savings can be found on the Energy Savings Trust (EST) website^[3].

Helping to increase a dwellings EPC and the value of your home

Making improvements to the energy efficiency of your home, such as installing loft insulation, can help to make the property more valuable. A Government report showed that making energy saving improvements could increase a home’s value by as much as 16% on average^[4]. It stated that improving a home's Energy Performance Certificate (EPC) rating can significantly increase its value. On average, upgrading from band G to E or from D to B can add over £16,000 to a property’s sale price.

The impact of installing loft insulation on a home’s EPC will vary depending on the current EPC rating and the energy efficiency measures that are already in place. Whilst loft insulation may not result in a jump from one band to another, it can help as part of a package of measures that can be found in your home’s current EPC report.

A government report^[5] on the energy efficiency of UK homes looks at the highest rated EPC (A and B) and lowest rated EPC (G) dwellings. It states that only 11% of the worst G rated properties have more than 250mm of loft insulation. Showing that loft insulation installed in these properties would be a sensible option to help increase their energy efficiency.

Helping to mitigate overheating

As well as helping to retain heat within a house, loft insulation also helps to slow down heat from entering during hot summer days. Loft insulation such as wood fibre boards have a beneficial  combination of density, thermal conductivity and specific heat capacity that could act like a ‘buffer’ in summer. The heat from the day can be stored in the roof insulation material for hours. A large proportion of the heat does not reach the interior at all and can be dissipated outside again during the colder nights. This may help to mitigate against the effects of overheating and could support a reduction of internal temperature fluctuations for a more comfortable internal climate.

What are the main considerations when installing loft insulation?

Loft insulation should be installed without gaps to maintain the continuity of the insulation layer and help reduce paths for heat to escape. It is important that the loft insulation does not interfere with the cross-ventilation provision in a loft space, so should not be fitted right up to the edge of the loft where the roof meets the wall, known as the eaves. If eaves ventilation is compromised this can lead to build-up of moist air in the loft space. This warm moist air can condense on cold surfaces and has the potential to cause long term damage to joists and rafters through a build-up of mould and moisture that could result in rotting timbers.

If the loft is being insulated at ceiling level, then it should not be used for storage as this can compress some insulation types and reduce their thermal performance. If storage is required, then extra flooring should be added above the insulation layer. This can then provide an area where storage can be supported without placing a load on the insulation below. If additional flooring is placed above the insulation there should be a minimum air space between the flooring and the insulation of 50mm in order to allow for ventilation.

Where loft insulation is installed in a previously uninsulated loft, the temperature in the roof space will be colder than before. Therefore, any pipes or water tanks should be insulated to prevent issues with freezing during the winter.

It is worth considering whether your loft hatch needs some attention. Where the loft space is cold, an uninsulated and draughty loft hatch can lead to some of that cold air making its way into the home. Ensuring that the loft hatch is insulated and tightly fitted can potentially save energy and help to increase thermal comfort.

Where can I find out more information about wood fibre insulation solutions for loft insulation?

Wood fibre insulation from STEICO is made from more than 80% natural wood sourced from PEFC certified forestry. STEICOflex 036 is a flexible thermal insulation batt that can be used as loft insulation in both new and existing buildings.

STEICOflex 036 in combination with STEICOtop can be used where loft storage is required.

To enquire about using our wood fibre loft insulation solutions on your next project, please contact our expert team who will be happy to help.

[1]assets.publishing.service.gov.uk/media/662a2e3e55e1582b6ca7e592/Approved_Document_L__Conservation_of_fuel_and_power__Volume_1_Dwellings__2021_edition_incorporating_2023_amendments.pdf

[2]bregroup.com/expertise/energy/sap

[3]energysavingtrust.org.uk/advice/roof-and-loft-insulation/

[4]www.gov.uk/government/news/energy-saving-measures-boost-house-prices

[5]researchbriefings.files.parliament.uk/documents/CBP-9889/CBP-9889.pdf