Neil Turner, UK Technical Manager at Ecological Building Systems, brings extensive hands-on experience working with natural insulation materials when advising on loft upgrades. Here he advises on upgrading a loft with natural insulation.
“When it comes to improving the thermal performance of a home, upgrading the loft insulation is often seen as a relatively low cost but highly effective way to save energy. Loft insulation can be simple and straightforward to install and the benefits can be significant, in terms of improved comfort, as well as significantly reduced heating bills. It can also have a positive environmental benefit, by reducing the property’s energy and carbon footprint – especially if the loft insulation used is made from natural materials.
According to the Energy Saving Trust, up to 26% of heat can be lost through the roof.
Although this percentage varies between different buildings, it is a useful guide and demonstrates how a significant amount of heat energy can easily escape from a home.
When upgrading loft insulation, improving thermal performance and airtightness is a key consideration. As homes become more airtight, understanding how moisture behaves within the building becomes increasingly important. Everyday activities such as cooking, washing and bathing generate water vapour, which needs to be carefully managed to support long-term building performance.
How humid air gets into your loft
Warmer air carries more moisture. As the warm air rises through the house, it takes humidity with it, which means moisture vapour from the living space will eventually end up diffusing through the top floor ceiling into the loft, which could cause a problem.
This humid air might also enter the loft space via draughts around poorly fitting loft hatches, light fittings or cracks in the ceiling plaster, or anywhere there is a hole in the ceiling.
Ensuring continuity of the airtight layer at ceiling level is therefore essential, both to reduce heat loss and to manage the risk of condensation within the loft space. Based on our experience across a wide range of retrofit and new-build projects, we often specify Wellhöfer airtight attic hatches where reliable airtightness and thermal performance at ceiling level are required. In addition, we also specify an airtight vapor control layer at ceiling level.
Where the existing ceiling plasterboard cannot be removed then a moisture vapour diffusion variable membrane, such as Pro Clima Dasatop, can be installed from above the ceiling prior to insulating. Pro Clima Dasatop can be installed in a ‘sub and top’ configuration where it is run over the top and down the sides of the ceiling joists and across the cold side of the ceiling plasterboard creating a cradle for the insulation to be installed between the ceiling joists. Due to the moisture vapour diffusion variable properties of the Pro Clima Dasatop membrane then no moisture is trapped at the top of the ceiling joists. Hence the Pro Clima Dasatop provides air tightness at ceiling level and also manages moisture.
Cold ventilated lofts
The most common type of loft in the UK is the cold ventilated loft, where insulation is installed at ceiling level and the loft space itself remains outside the thermal envelope. In this configuration, warm, moisture-laden air from the living space below can move into the loft through gaps, cracks and service penetrations in the ceiling, as well as by vapour diffusion.
It is therefore important to remove water vapour in the loft to prevent it from condensing on cold surfaces like bitumen roofing felts or roof timbers, which could potentially create a problem.
A common feature of many older ventilated lofts is the presence of a traditional bituminous roof underlay beneath the tile battens. While this type of underlay is effective at protecting against wind-driven rain, it has a very high vapour resistance, meaning that moisture vapour cannot readily pass through it.
This is one of the reasons why good ventilation is needed, so that external air can freely enter, circulate and exit the loft space. This happens mostly via the ventilation gaps at the eaves, but also less commonly via ridge tile vents at the top or airbricks. A well-ventilated cold roof space ensures condensation risks are kept to a minimum.
Refer to Building regulations and guidance (BS 5250: 2021 Management of moisture in buildings – Code of practice) for more information.
To safely insulate a cold ventilated loft, the loft insulation is installed between (and above) the ceiling joists that make up the loft floor. The insulation stops at the underside of the rafters to maintain the ventilation gap between the insulation and the roof felt. The gap enables external air to easily enter and exit the loft space to remove any moist air before it can condense.
With a cold ventilated loft, the focus will be on increasing the insulation, maintaining good ventilation and eliminating unwanted draughts across the loft floor area.
How much insulation is required?
The rate at which heat is lost through a loft (or any other part of a building) is best described using U-values. U-Value states the overall rate of heat loss (in Watts) per square metre (of surface area) per 1 Degree Kelvin (temperature) difference between the inside and outside. The lower the U-Value the slower the rate of heat loss. When insulation is added to a loft, the aim is to reduce its U-value.
Loft insulation can make a dramatic difference insulation makes – the first 100mm of loft insulation can reduce heat loss by around 85%.
Why use natural insulation to insulate your loft?
Choosing natural insulation materials for loft insulation can offer important advantages in terms of moisture management and overall building performance. Wood fibre insulation, such as Gutex Thermoflex, and hemp/jute insulation such as Indibreathe Flex are hygroscopic, meaning they can absorb and release moisture vapour in response to changing conditions.
This buffering effect allows temporary increases in humidity to be managed within the insulation layer, rather than being absorbed by structural elements such as joists and rafters. Natural insulation materials can accommodate relatively high levels of moisture vapour without a significant reduction in thermal performance.
By helping to regulate humidity within the loft space, wood fibre insulation can reduce the risk of moisture accumulation and mould growth. Gutex Thermoflex is also inherently resistant to mould, making it well suited to use in roof and loft applications.
In addition, natural insulation materials are widely used in both self-build and refurbishment projects due to their ease of handling and compatibility with traditional construction.
How much can be saved by insulating your attic?
Adding additional loft insulation can significantly reduce heat loss and lower energy demand, leading to noticeable savings on heating costs over time. In many cases, the cost of upgrading loft insulation can be recovered relatively quickly through reduced energy bills.
Overall, natural insulation materials offer clear long-term benefits when used within well-designed building systems. They can improve energy performance, enhance indoor comfort and reduce the risk of moisture-related issues, while helping to protect the building fabric and support healthy indoor air quality.”