Hi All,

Some advise if I may. 10yr old house with, std brick & aerated block construction. Single, integral garage with bedroom over. Bedroom is cold and this i what I am trying to fix.

Last night I removed a plasterboard panel in the garage to investigate why. In the pics below I have not moved the mineral insulation in any way, this was 'as found'..

N.B Engineered joists are 230mm deep with chipboard to the bedroom above, so my plan is to..

Fill joists to 200mm deep with mineral wool
attach 35mm (approx.) celotex board or similar to underside of joists (i.e. garage-side).
Foil tape to seal.
Plasterboard over top.

Any problems with the approach above? Something I am not sure of is if there needs to be an air gap at or on one/both sides of PIR?

Thanks in advance

I reckon that you will still have the problems with cold if you do that! It is a given that outdoor air will be blowing about under your floor. see http://readinguk.org/draughtbusters/?page_id=268 for more info

If it were mine I woild seal both ends of all the joist gaps including along the length long sides of the room between then joist and the wall. blockwork is not air tight.

Then I would fill the void with mineral wool or batts, cheapest and replasterboard.

Ideally there should be a vapour barrier on the warm side of the insulation and no draughts in the void inside the insulation barrier, no need for ventilation to the underside of sheet insulation if you use that.

no, such a void will be within the thermal envelope of the building so will be as warm as cupboards etc would be. I dont like it that much as it is sure to be draughty eventually.