The foil is a vapour membrane. It will come with polyiso. Polyiso is a slab and generally comes in 8 x 4 sheets, however, Xtratherm will cut them to whatever size you wish before delivery, say 8 x 2 or 4 x 2 so you can get them through a loft hatch. Speak to their sales rep prior to ordering. I am sure most manufacturers will do the same.
Leave the insulation you have in situ, it will do no harm and will add a small bit of thermal property to the roof. If you are using polyiso with U value of 0.023, then you will only require a total thickness of 150mm. If you are putting in between joists, you will need to allow a further 15% in thickness to compensate for the loss in thermal conductivity from the joists.
Presuming your ceiling joists are 5 x 2's, I would put 125mm polyiso between joists and then a further 50mm perpendicularly across the top.
Don't forget to allow for air circulation around any recessed light fittings & electrical cables. Try to fit insulation below electrical cables, lying cables on top of insulation if you have enough slack.
You can also build a cw storage tank surround with the insulation, removing any insulation from below it and this will bring the tank into the heated space below. Insulate pipework also.
Fit a boarded walkway on top of the polyiso to tank and valves and anyother place you require access to.
No, as long as the vapour barrier is on the warm side of the insulation. Fibreglass is not a barrier for moisture, therefore it will not trap any.
...when you openned the trap door to attic, it used to be like a hurricane of cold wind was coming from the attic. This is no longer the case with the rockwool insulation
You still need to have the attic space above the insullation well ventilated or you will end up with condensation problems. Were the gaps at the eaves blocked by the insulation?
Leo
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