Roof insulation & venting for warm roof?

[darceybutton]

Dear All - Please can you assist. My existing unventilated pitched roof structure is of warm roof (slate, waterproof sarking felt, rafters, 75mm mineral wool & 9.5mm ordinary plasterboard).

I wish to introduce foil insulation in a few locations - typically above the exposed timber purlins (where i cannot paint EMF shielding paint - nearby mobile phone mast) and I would like to know the installation precautions I need to take.

2 products appear suitable - Ufoil by Euroform products or Astrofoil (www.astrofoil.co.uk) - the multi foil quilts (typically 400mm width) do not appear suitable as i have to install between rafters and in my very old house the spacings are irregular - some 320mm, some 370mm, etc.

I was thinking of stapling this foil membrane to the rafters and then refitting the existing mineral wool BUT I am not sure whether the foil should go above (directly beneath sarking felt) or below the quilt - no existing problems with condensation staining but the quilt is damp in places - picking up damp from underside of sarking felt.

THANK YOU FOR YOUR HELP

 

[chappers]

Think you may be creating more problems for yourself. the reason your quilting is damp is because you don't really have a warm roof. Warm moist air is passing up through the quilt and condensing on the sarking felt.
You could try fitting a vapour barrier to prevent warm air getting between your rafters but then you will probably just create problems in your loft space.

ventilation ventilation ventilation.