flat roof airtightness question

esox28

hey all,

I am hoping to start looking into contacting some flat roof installers in the next week. from what I know about a flat roof there are two main types of insulation methods 'cold and warm' types. I am going to have a warm deck roof type installed:

weatherproof sheeting
150mm high density insulation 'pir'
22mm osb 3 board
firring pieces
225 x 50 rafters
38mm insulated board.

in this flat roof make where will the airtight membrane in this arrangement

thanks in advance

beyondpassive

Your roof membrane will have WPB ply over the PIR board as it is not trafficable. You will have a breather membrane under the ply. Glass wool between the rafters, back diffusion air tight vapour control membrane, then services cavity with 50 or 60mm battens, which can also be filled with glass wool or rock wool. I wouln't advocate using composite board around electrical fittings.

esox28

thats great thanks bp.

the main roof being a story and half, my structural eng has voiced concerns about some cellulose insulation not being vapour permerable and trapping moisture as it passes through the airtight vapour control membrane as I am hoping to put in 225mm of the stuff around the attic and celing space.

any taughts or opinions regarding cellulose fill?

beyondpassive

I generally specify a sarking board over the rafters when using cellulose fill, woodfibre board if budget allows, PIR if not, this will push the dew poin well outside the cellulose zone. Moisture shouldn't get through the vapour control layer anyway, the idea is that for the few weeks, that the outside is warmer than inside during the day, the membrane will allow the timbers to sweat out any vapour back into the room.

esox28

thanks bp, have you seen any extream cases where moisture has deterated the cellulose?


just after ok-ing the spec for rafters and main roof with structural eng.

2" x 1" counterbatten
2" x 2" batten
22mm gutex multiplex-top
225mm x 50mm rafters @ 600mm c/c
225mm cellulose insulation
15mm osb 3 board with airtightness taped joints
38mm insulated liner board.

my BER eng. has this spec as we speak and is inputting info into the deap.

our idea was to space out rafter to 600mm c/c in-order to fit the 2.4m x 1.2m osb board and using the board as the airtight membrane and structural support to the overall roof.
any taughts on this roof make-up or am I missing anything

beyondpassive

esox28 wrote: »

thanks bp, have you seen any extream cases where moisture has deterated the cellulose?


just after ok-ing the spec for rafters and main roof with structural eng.

2" x 1" counterbatten
2" x 2" batten
22mm gutex multiplex-top
225mm x 50mm rafters @ 600mm c/c
225mm cellulose insulation
15mm osb 3 board with airtightness taped joints
38mm insulated liner board.

my BER eng. has this spec as we speak and is inputting info into the deap.

our idea was to space out rafter to 600mm c/c in-order to fit the 2.4m x 1.2m osb board and using the board as the airtight membrane and structural support to the overall roof.
any taughts on this roof make-up or am I missing anything

Looks good but i would use a flexible wool to the services cavity and not the composite insulating board as it's not ideal with electrics.