Have a 1970's 3-bed detached house with quite a low pitch roof – thus making loft space and accessibility less than easy.
We are UK based.
We've got bare minimum insulation at present (seems to be 100mm rockwool/fibre wool) and we're starting to get a little condensation/mould on our master bedroom ceiling.
We've only lived here 2yrs, so slowly getting round to some jobs and this one is really getting my main mission.
We use the loft for some storage and would like to keep using it if at all possible.
I'd like to know if I can replace all the fibre wool with polystyrene or some other foam but also get the best insulation value at the same time.
I'm aware that thickness of material is going to be a factor but the fact it's a fairly low pitch roof (about 1m high) and we would like some storage I'd like to know if there is some fairly super-duper solution that's not very deep???
Can I put polystyrene sheets between the joists (directly in contact with the ceiling boards below)?
If I can then I guess I'd also need to put similar sheets at 90-degrees to cover the joists and bring the thickness up?
Would it be easier to use mineral wool between the joists and put polystyrene above?
The recommended thickness (UK 270mm) is essentially eating into loft storage space. If I have to lose storage space for a better insulated home then so be it.
Thank you all in advance!
Best Answer
You can put polystyrene between the joists or across above them.
The super-duper products for this do get you equivalent insulation to fibre wool at a lesser thickness but at a higher cost in £££s. Search for Celotex or Kingspan insulation boards. They are available at a wide range of thicknesses (25mm, 50mm, 75mm, 100mm). They should be cut to the width of the gap and friction-fitted. For maximum insulation at minimum thickness, you would use 100mm between the joists and another layer (your choice of thickness) above the joists.
To maintain storage space you should apply batons at right angles to the joists (installing insulation boards) and then board over the top with tongue and groove chipboard floorboards (use wood glue on the tongue/groove interface and wood screws into the batons). The required spacing of the batons will depend on the thickness of your chipboard - approx 400mm between centres for 18mm chipboard or 600mm centres for 22mm chipboard.
A good head torch and electric screwdriver will be invaluable.