loft insulation under roof slates

[hathaer1]

Hello, I have bought a house recently and I want to inprove the loft insulation. So far I've found all the information I need rgarding the loft floor insulation and boarding in the folum but I cant find anything regarding insulation attached to the roof itself. My property has had insulation, about 25mm deep, fitted under the roof slates in between the joists that run under the slates. Is this ok? and if so can I include that as part of my total loft insulation when trying to make up the 270mm?

There is still room between the surface of this insulation and the bottom of the joists (about 25mm). If I fitted plaster board to the joists would this be enough for a ventilation gap if I kept it all round?

thanks

 

[freddiemercurystwin]

Unless you have a breathable membrane or no membrane/felt at all or you have continuous ventilation along the ridge then this insulation should be removed. Otherwise the void is not vented properly and the rafters will not be able to breathe. Keep all your insulation at floor level ion the loft.

 

[hathaer1]

I've had another look and found that there's a 50mm gap between the insulation and the tiles of the roof. Can I leave the insulation in place?

Also I'd like to renew the insulation between the floor beams but found them to be 70mm. I cant seem to find insulation that thick any more. Is it still possible to get mineral wool this thick?
Is it pointless to compress 100mm down to this size?

 

[freddiemercurystwin]

Unless you have a breathable membrane or no membrane/felt at all or you have continuous ventilation along the ridge then this insulation should be removed.

Your old insulation will not have deteriorated much in terms of its thermal resistance, its pretty pointless replacing it.

 

[hathaer1]

Its falling apart and patchy.

 

[freddiemercurystwin]

Well if its that old and tired fair enough. 100mm will squash into 70mm space no problem without a significant loss of efficiency.

 

[Chri5]

Well if its that old and tired fair enough. 100mm will squash into 70mm space no problem without a significant loss of efficiency.

Be mindfull that old plaster ceiling below could buldge or if weak collapse.

Forcing anything on to a weak surface could be an issue.

Celotex / Kingspan might be a better option, you can get it in 50mm upward depths and it has approc twice the thermal value, so 50mm equals 100mm of fibre glass