fibreglass on new flat roof

gruby

I wonder if anyone came across flat fiberglass roof in terms of built-up. Speaking to few installers their approach is very simple: 18mm OSB over timbers and its ready to apply fiberglass. Insulation goes in between joists and rafters (latter to create the slope).

how about the vapor barrier? any air gaps needed for ventilating the insulation?

its a new built. the 16m long, 1.5m wide corridor is to separate old bungalow+outbuildings and new extension.

BryanF

gruby wrote: »

I wonder if anyone came across flat fiberglass roof in terms of built-up. Speaking to few installers their approach is very simple: 18mm OSB over timbers and its ready to apply fiberglass. Insulation goes in between joists and rafters (latter to create the slope).

how about the vapor barrier? any air gaps needed for ventilating the insulation?

its a new built. the 16m long, 1.5m wide corridor is to separate old bungalow+outbuildings and new extension.

Fibre glass
Separating layer
On 2 x 100mm thick layers of rigid PIR
On ply wood, all joints siliconed & edges sealed to wall, laid to 4x the min fall on fibre glass iab cert,
On joists firred to suit fall
With taped & sealed vapour barrier beneath
Plasterboard finish with backing boxes on any fittings
Gutter & Eaves/parapet detail requires special attention, details agreed by architect prior to commencement -you really need to carefully consider the look of the eaves/gutter detail + wall insulation overlap

What your installers suggest May cause thermal bridging manifesting in mould at the ceiling level, also with that method there needs to be ventilation.

gruby

Thanks BryanF,

I can't have "fat" or "warm" roof as it will intervene too much with the look of traditional irish cottage to which it adjoins. Have to keep the thickness to a minimum.
I probably will be reducing the cold bridge by using good quality breathable membrane like Solitex Plus, it actually adds extra insulation to the roof, but I would put it under the plywood (on top of rafters). Would provide 2" air gap between the membrane and with the ventilation opening at both eaves.

BryanF

gruby wrote: »

Thanks BryanF,

I can't have "fat" or "warm" roof as it will intervene too much with the look of traditional irish cottage to which it adjoins. Have to keep the thickness to a minimum.
I probably will be reducing the cold bridge by using good quality breathable membrane like Solitex Plus, it actually adds extra insulation to the roof, but I would put it under the plywood (on top of rafters). Would provide 2" air gap between the membrane and with the ventilation opening at both eaves.

Solitex reducing thermal bridging? Rubbish.

2" ventilation is essential.

16m corridor? How are you keeping the heat in this?

Consider letting solitex sag the 2" between rafters, venting above and fit insulation below this with vapour barrier underneath

Due point clac should be done, with maybe further insulation (obeying the 2/3's rule) beneath the rafters

BryanF

Has your Arch done a drawing of this corridor and detailed the roof /wall junction?

Drift

Out if interest Bryan - what's the reason for letting the solitex sag and venting above it?

BryanF

Drift wrote: »

Out if interest Bryan - what's the reason for letting the solitex sag and venting above it?

The felt is breathable so why not vent outside of the insulation zone.

gruby

thanks BryanF

"Has your Arch done a drawing of this corridor and detailed the roof /wall junction? "
I'm still waiting for the drawings from architect but I like my "homework" to be done as well

"16m corridor? How are you keeping the heat in this?"
both long walls are internal, ceiling is only 2250mm, plus I may actually do the morning run there to heat it up )

a vapour barrier goes on the very bottom of the insulation (warm side) , am I right?

can you explain "2/3's rule"? googling does not help