Vapour Barrier

[FRAN1870]

For my loft conversion do I need to vapour barrier the whole room or just the roof and gable ...

Like the purlin and party walls for example ?

What is your Preference over foil backed PB or polythene ?

 

[jeds]

The purpose of the VB is to prevent vapour entering unventilated voids within the structure - particularly voids on the cold side of the insulation. If the room is a bedroom, it will be subject to relatively high levels of vapour and positive pressure during the night when people are sleeping. Pressure will drive vapour into the voids where, if the temperature is lower than the internal, it will condense to water. So, you need a vapour barrier on the warm side of the insulation - particularly on voidy type structures such as roof and timber frames.