KA230
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 needs to be 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.
When insulation is added to a loft, the aim is to reduce its U-value. Loft insulation can make a dramatic difference – the first 100mm of loft insulation can reduce heat loss by around 85%.
Why use natural insulation for the 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, which means 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 new build and renovation projects due to their ease of handling and compatibility with traditional construction.
Energy savings with loft insulation Upgrading loft insulation can significantly reduce heat loss and lower energy demand in a home, 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.
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