Roof insulation not as effective ?

[omph]

I had some draught coming in from a light circuit hole in an uninsulated roof. I then proceeded to insulate the roof with about 400-500mm of roll on mineral wool, ensuring it was pushing into the joists and each layer being laid perpendicularly. I thought that was the end of the draught, but to my surprise I could still feel what was quite a significant breeze through the lighting hole.
Am I doing something wrong here ? I get the whole moisture thing, but if cold wind is blowing through the insulation then how effective can it really be ?

 

[Harry Bloomfield]

I get the whole moisture thing, but if cold wind is blowing through the insulation then how effective can it really be ?

Insulation of that type, is only intended to reduce convention and conduction of heat, it is not designed to stop air flow completely. You need to look at other solutions, to stop air flow via lighting holes

 

[big-all]

400-500mm 16-20" thick insulation ??

 

[Swwils]

400-500mm 16-20" thick insulation ??

450mm is a standard detail now to get around 0.10 W/m2K

 

[big-all]

450mm is a standard detail now to get around 0.10 W/m2K

wow thought it was still 227-270mm i am so out off touch

 

[Swwils]

Well at least you don't have to do it twice !