I have investigated an increasing number of issues related to insulation, dampness, mould, and rot arising from improper workmanship and a failure to follow Building Standards or NHBC requirements in the new house roof void.
In the new constructions, I have inspected the perimeter insulation. It was not installed in accordance with the NHBC or Building Standards by the builder. This trend warrants further investigation to determine the extent of the installation issue. Is this due to a lack of oversight or insufficient penalties against the building company or its warranty insurance provider?
This required wall-to-roof insulation detail in a new home is often omitted, leading to significant heat loss, dampness, mould growth, and an unhealthy indoor environment.
From inside a modern roof space circa 2019
The insulation at the roof perimeter surrounding the house has been incorrectly installed along the wall plate, creating significant gaps that allow cold air to seep in. This misalignment not only causes cold bridging but also leads to excessive heat loss. Over time, the consequences can be severe, as these conditions foster the growth of mould and dampness.
Thermal image of the areas below due to incorrectly fitted insulation
The outside air was 11.2 °C with no wind. Then the outside temperature dropped to -2 °C. Two weeks later, heat loss and air leakage worsen due to improperly installed roof insulation in a newly built home.
It’s easy to blame the site worker. However, designers have not accounted for the limited workspace, which is why this defect occurs so frequently.
The client reported water running down the ceiling to the external wall junction at night. I recorded an external temperature of 9.7 °C, an internal dew point of 6 °C, and a surface temperature of 10 °C. The problem was a defect in the soffit construction, inadequate retrofit insulation, and inadequate mechanical ventilation in the home.
If you are buying a new home, try to get the builder to provide a written warranty. Ensure the insulation has been installed in accordance with Building Standards and/or NHBC requirements. The roof-space insulation of a new home must be checked, with the inspector’s signature and any relevant qualifications or experience.
Context (Cold Roof Space)
Traditional A-frame timber roofs were uninsulated in the UK.
These roof spaces are not heated and are not within the living area, and yet they do receive conductive heat from the ceiling and experience warm air leakage from the living space. This is due to construction defects, mainly relying on limited natural air movement for ventilation through ridge and perimeter openings. As a result, the space remains distinctly cooler than the living area.
The average temperature in roof spaces can vary significantly. It can reach over 30 °C in summer and drop to 5 °C in winter. This variation occurs because solar heat is gained during the day and lost at night. Wind can allow air to pass through, and during storms, damp air may be drawn in. Additionally, defects in slats, tiles, and flashings can cause small, intermittent leaks. Fortunately, these issues can often be mitigated by solar heat and ventilation.
Changes started with insulation.
Since the 1970s, we have been slowly insulating (retrofitting), but with lower performance (100mm insulation quilt) and the better we insulate, the less heat is lost into the roof space. However, this can affect its ability to vent moist air and dry the area. Therefore, we may need to improve roof-space ventilation to reduce condensation, especially in new homes with advanced roof insulation.
In England, the rafters/trusses have sarking felt laid over, followed by timber battens, then tiles/slats. The sarking felt helps reduce cost but is more subject to defects.
In Scotland, softwood sarking boards were traditionally fitted over the rafters, followed by roofing felt; this design is better suited to the Scottish climate but is more expensive and is being phased out by some developers.
Therefore, these roof-space designs buffer moisture differently, depending on the amount of timber.
