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My distribution box for the MVHR  ( sometimes called spigot box or manifold) will sit beneath a flat roof , but above a ceiling.

The roof build up from top down is;

  • Alvitra Avalon over
  • 270mm of insulation on 
  • some super expensive vapour stuff - forgotten the name - the price was so traumatic
  • 22mm OSB3
  • POSIs above the ceiling build up which is 
  • Vapour control layer 
  • 25mm insulation
  • 12 mm plasterboard
  • 2mm plaster skim.


There's enough space for  spigot boxes to  sit comfortably under the POSIs. That means they are outside the heated envelope. (just)


The supplier says not to worry : but they would wouldn't they? Everyone has that problem, said the rep,


What do you think?


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I think, do you need the 25mm insulation? Is it going to be make a significant different to target Uvalue.


The supplier is probably right, the temperature in the void between the posi's will be very close to room temp. Chance of condensation very minimal I would have thought.

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