enter

I have a concrete floor in the loft. Looking to insulate with glass wool (in rolls) 100cm is there anything else I need except grafting.

Peter_in_Hungary

Do you really mean 100cm? and is that the thickness or the width?

enter

10 cm thick

Peter_in_Hungary

Given that you are going for 10cm thick then what you need is another 10cm !
I presume also that the loft will have a fair amount of wind blowing through it even if it has any roofing felt, in which case it would be useful to put a sheet of paper between the two layers of glass wool. This is to stop heat loss by 'wind wash' where the natural draft in the loft sucks heat through the glass wool by promoting air currents within the wool. Plastic must not be used as the layer needs to be vapour permeable. Also if you put in 2 layers of glass wool then off set them by 50% to avoid gaps at joins.

10cm of glass wool will give a U value of about 0.36, which means that for every m2 you will loose 0.36 W per degree difference between inside and outside. 20cm of glass wool gives you a U value of about 0.19

So if you have a 60M2 roof and a 25deg difference (20 deg in the house and -5 outside) then you will need 60 x 0.36 x 25 x 24 (hours) which gives a heat load of 12960W or about 13kWh per day
The same calculation with 20cm of insulation gives 60 x 0.19 x 25 x 24 which gives 6840W or about 7kWh. per day

(by the way assuming a ceiling of just 20cm of concrete gives a heat load of 121kWh per day)

Doubling the insulation doesn't halve the loss as you get diminishing returns when you increase the thickness.

Use onetime surgical gloves (from the chemist) and a zip up disposable boiler suit. Have a cold(ish) shower afterwards as a hot shower will open up the skin pores and the glass dust will make you itch.

Oh, and make sure you get the insulation well into the eaves otherwise the temperature difference created by the cold bridge at the corners can promote mould problems.

enter

cheers

wolfi

Interesting topic ...
I'm not an expert but want to mention the concept of a "vapor barrier" (Dampfsperre in German) which seems to be necessary in certain cases at least.
Hunmidity is abig problem in Hungarian buildings and I have the feeling (just from looking at some of them) that not all builders here manage this correctly, so beware!

enter

I assume the rolls of decorators thick paper would be ideal between insulation?

Peter_in_Hungary

Yes

fidobsa

One thing not mentioned so far in this thread is snow. I had problems when there was snow on the roof combined with strong wind. The snow would blow in under the roof tiles and sit on the loft floor until the next sunny period, when it would melt. I had plans for malting barley in my loft so I didn't want to insulate the whole area but I did do just the patch over my bedroom. I did this with 10cm thick slabs of expanded polystyrene. If this got covered in snow I could sweep it off before it melted. I have since had the roof overhauled and plastic sheeting put under the battens. Obviously melted snow on fibreglass would be bad news until it dried out.

Peter_in_Hungary

Good point about the blown snow if there is no roofing felt under the tiles. The same points I made above about the level of insulation apply and although expanded polystyrene is slightly better than glass fibre (insulation value) care must be taken to ensure NO gaps between the sheets.