Current UK guidance recommends a depth of around 270mm (about 10.5 inches) of mineral wool in an open loft. That figure is what's needed to hit the thermal performance modern building standards aim for — older homes were often built with far less, or none at all.
Insulation works by trapping still air, and more depth means more trapped air and less heat passing through the roof. Returns taper off as you go deeper, and 270mm is the widely accepted sweet spot between cost and performance. Going much beyond it gives diminishing benefit; falling well short leaves easy savings on the table.
If your existing insulation is dry and in reasonable condition but simply too shallow (many homes have 100mm or less laid decades ago), it can usually be topped up to 270mm by laying a second layer across the joists. If it's damp, compressed or contaminated, it's better removed and replaced. Either route gets you to the recommended depth.
Boarding straight onto the joists squashes the insulation and undoes much of its benefit. If you want usable storage, a raised boarding system keeps the full 270mm working underneath — see our loft boarding service.
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