Roof Insulation, air gaps/ventilation??

[PJ_]

Hi,

I'm looking to insulate my roof, seeing various posts around this but just not understading what needs to be done so sorry if I'm repeating others questions.

Bit of background first, I have a 1950's build semi, double skin of engineering bricks so a nightmare for every tradesman who wants to drill a hole in to the outer walls for anything. The loft space is massive and when I moved in the only thing up there was about 100mm of insulation. In order to utilise this space I've used loft legs to raise the floor level, replaced the insulation with 270mm and boarded it all out.

As the DIY I've undertaken has grown so has my tool collection, and currently it's all living in the 3rd bedroom which is soon to be plastered and converted back in to a liveable room so need somewhere to store the growing collection. I don't have a garage so the idea is to use some of the loft space. The problem is it is very cold in winter, not the end of the world, but absolutely baking in summer so I want to insulate the roof to try and take the edge off.

So the the roof comprises from outside in, roof tiles, cross batterns, a membrane (the roof was re-done in the last 15 years, so this is a plastic sheeting rather than the oldschool felt) then 100mm deep rafters.

I figured 100mm rafters, add 100mm insulation boards between them, but am now seeing various mentions of leaving an air gap for ventilation but not sure which side of the membrane this is refering too, or both.

So with 100mm deep rafters, I should go 75-80mm insulation board, leaving an air gap of 20-25mm between the new board and roof membrane to allow for ventilation for any liquid that gets past the roof tiles and the membrane? Am i understanding that right??

Thanks,

PJ

 

[Notch7]

With a cold pitched roof you need to leave 50mm air gap generally.

Maybe look at 50mm rigid and perhaps a multi foil over the top.

 

[PJ_]

Thanks Notch7
So just looking at cold vs warm roof. So cold roof the insulation is between and inside the rafters. Warm roof insulation is between and on top of the rafters. So in the case that I don't plan to take all my roof tiles off cold roof is my only option. Think this is maybe where I was crossing my wires, and the 20-25mm gap I was reading about is between the tiles and the membrane or insulation in a warm roof install before you even get to joist level?

I'd priced up on 100mm boards and the total was a bit eye watering for what I will use the loft for, so dropping to 50mm makes it a bit easier on justifying the cost so works out well actually, not super insulated but doing it correctly so I don't end up having to have the entire roof re-done. Might just go insulation boards to start, see what difference it makes and maybe add the foil later.