Blown-in or batt fibre insulation installed on the ceiling of an attic will typically have an installed R-value of between R-3 to R-4 per inch of thickness depending on the type, manufacturer, and density. See product manufacturers' published data for specific product information. To reach the desired R-value (typically in the range of R-40 to R-60), the insulation should be installed to the desired thickness per the table below. Note that the insulation thickness will be reduced at the eaves due to the slope of the roof sheathing and this will reduce the effective R-value of the ceiling plane. This deficiency can typically be made up by adding a few extra inches of insulation in the centre, unless the roof slope is particularly shallow. Guidance on the reduction calculation can be found in the Guide for Designing Energy Efficient Building Enclosures published by FPInnovations (see Resources).