Roof insulation or not ?

[PipM]

Hi. Complete amateur/DIYer here. 
My 1970s brick built detached house is bloody freezing and costing a fortune in heating. 
We have attic floor insulation (although I’m sure it could be improved) but the interior of the roof has nothing and it’s bloody drafts up there (see pic). 
What would happen if I fitted some foam insulation/board and fibre glass between the rafters, leaving an air gap between the insulation and felting ?

Is this a quick and cheap way to keep heat in or will I be doing more harm than good ?

TIA

[ProDave]

It is supposed to be cold and well ventilated up there.  The loft insulation, the pink stuff at floor level, could do with some tidying up to make sure there are no gaps through to the actual ceiling (likely where it is lifted in that picture)

[Radian]

You seem to be aware of the potential for causing problems which is a good thing. As you suspect, there are pitfalls. First you need to be clear about your objectives here. Roof spaces such as yours have been designed to be well ventilated - and by that I mean very noticeably draughty. The function of the draught is to expel any excess moisture in the air and allow the timbers to dry out if they do develop condensation on them. Most plans such as your proposal work against this.

 

I notice that you have a large boarded area. This is a problem in two ways: it means there is limited fiberglass insulation underneath and any moisture coming through from below is trapped and unable to dissipate through the ventilation. You need at least 300mm of uncompressed fluffy insulation in all areas and boarding over will be problematic. If you have any alternative storage space I'd advise moving your stuff there, lifting the boarding and going over the entire surface with additional mineral wool insulation to bring it all up to 300mm.