Insulating flat roofs

When you insulate a flat roof that's part of your home's ceiling, you should fit the insulation both within the joist spaces and across the top of the joists. The exact quantity you'll require depends on whether you meet the specific U-value, a measurement of thermal efficiency, that the Approved Document specifies.

However, if installing this insulation would undesirably reduce your ceiling's height, you might receive permission to aim for a less stringent thermal performance goal instead.