Definition of 'Inside Heated Envelope of Building'

[GB]

BS 9251:2014 states that sprinkler pipework needs to be insulated and provided with trace heating if its not within the heated envelope of the building.

So, if the pipework is under the loft insulation of the roof space - is that within the heated envelope of should insulation and trace heating provided even if under the insulation?

[Phoenix]

If it's under the insulation it should be ok as it is.  The point is, might the water in the pipes freeze?