over rafter insulation

[1806mill]

Hello

I'm about to embark on an expensive journey into roof insulation, if anyone has experience relevant to the following scenario, I will be really appreciative.

I have an c1806 watermill which has two unconverted floors and an uninsulated roof so it acts like a heat chimney, hence the roof insulation issue. I have decided to go with over rafting insulation because the roof needs to be re-laid entirely anyway. My questions are based using multifoils for this purpose, such as Trioso Super 10, and to insulate between the two floors with Thermafleece or similar e-friendly stuff. This will get us closer to a decent overall U value. (If you are wondering why I'm not considering under rafter insulation it is because there are nice sarking boards, oak rafters and purlins which I want to keep exposed, it will keep the conservation dept. happy too).

So, here are the questions. What is the absolute minimum that I can raise the line of the roof to? Some say that I will need an air gap plus battens as well as the insulation which may raise the roof by 100mm, others say that I don't need a gap so the difference will be minimal. Does any one have a view on the product, the application and the roof height issue. (Obviously I want to change as little as possible).

Thanks in advance.

 

[Fluffster]

Are you converting this property if so then BC will be involved.
From personal experience my BC will not accept Trioso and insist on PIR silver backed sandwiched foam Celotex/Kingspan etc.

Ideally you need a 50mm ventalation gap betwen insulation and sark/underfelt.

Where is your mill obv not in london sounds like a great project.
Converted a barn in france a few years back howvwer no probs whatsoever with planning or bc just a few drinks and meals for the mayor.
Pete

 

[1806mill]

Thanks for that Pete

Since posting those questions we have discovered that over-rafting with insulation and a raised roof level will not get past the local conservation dept (understandably) and BC will not accept multifoils anyway as you say, so we are back to under-rafting. A u value of 2 or less is the usual target for a habitable space, but I am hoping to that BC will cut us some slack as its a listed building. Anyway, anything we add in terms of insulation will be better than none-which is where we are right now-(and its freezing).

We may investigate 75mm Thermafleece between 100mm rafters with a breather board/limeplaster or timber cladding over the top, but we are going to have to find a sensitive way to do it so as to preserve as much of the integrity of the inside of the roof as possible.

Any thoughts on the above much appreciated.

Andy