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!


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.

  1. 100mm mineral wool has a U-value of 0.044
  2. 100mm EPS polystyrene has a U-value of 0.038
  3. 100mm sheet PIR/PUR (kingspan/celotex/xtratherm/quintherm) has a U-value of 0.022

Lower the U value the better, as we can see polystyrene is little better than mineral wool, and solid PIR/PUR insulation is twice as good as mineral wool. We would need 200mm of mineral wool to equal 100mm of PIR/PUR.

The only downsides to solid PIR/PUR sheets are noise insulation, fire rating and vermin resistance. Mineral wool is a very good sound insulator and has excellent resistance to fire and at least anecdotally vermin don't like it and certainly won't nest or chew through it. Vermin will quite happily chew and nest in PIR/PUR sheets.

The existing insulation is probably glass fibre (yellow colour usually) as this is cheaper than mineral wool. It does have similar sound insulating properties to mineral wool, but the vermin and fire resistance is not as good. Mice happily gather and nest in it.

If I were you I would put down 100mm PIR/PUR between the joists, undercut to fit and stuff the sides with the old wool or use squirty foam. I don't think it's worth the effort battening on top and adding more insulation, but with the U-values you can do the sums and see if you want to or not.

  • In addition to the above, the OP will need to ensure there is a decent vapour barrier under the insulation. With luck, the ceiling will be plaster-board with a foil back. If so, that's job done. If not, he should lay (thin) polythene sheeting on top of the ceiling before laying the PIR/PUR sheets. – Martin Bonner Jul 12 '18 at 19:48
  • Your U-values seem to be a factor of 10 out. The Kingspan calculator won't let me put in a value of less than 200mm of Kingspan, and that gives a U value of 0.1W/m2·K (which fits with a U-value of 0.2 for 100mm). – Martin Bonner Jul 12 '18 at 19:52
  • those figures are perhaps per square foot, but Americans use all sorts of oddball units, so that's just a wild guess. – Jasen Nov 20 '18 at 9:36

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