Insulation and loft boarding

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Hi,
My loft has very little insulation (about 30mm), so want to top it up but also keep it as 70% boarded storage space.
Should I:
a) Add loft legs and wool insulation
b) use wool insulation to 100mm, then foam board insulation (50-100mm) on top where I want to board for storage (270mm wool where not boarded)
c) Increase height of joists where I want to board
d) only insulate to 100mm where I want to board, and 270mm for the rest - will it really make that much difference?
Any help appreciated!
Thanks
 
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I'd probably put in 30cm on all unboarded areas, then for boarded, which I assume must have some insulation below, lay 10cm insulation board down with chipboard on top. That would be the easiest way - although you will need to cut the boards to get them into the loft ...

maybe loft legs would be cheaper and not much harder, depending on how much boarding there is already.

How deep are the joists? 30mm insulation is practically nothing.
 
from (d) I am guessing that joist depth is 100mm. Certainly, use mineral wool to full joist depth. hoover it out before you start, and use only the brown variety treated with Ecose, which does not shed irritant dust and fibres (it is made by Knauf but also widely sold as an own brand. look for the Ecose name on the wrapper)

Where you don't, and never will want, to board, you can flop another layer of wool over the lot, but you will never be able to walk on it safely.

Instead of loft legs, you can lay rigid foam board slabs on top of the joists, and ply or chipboard on top (screwed down to the joists through the insulation so it cannot move). The board is stiff enough to take your weight in this way. Housebuilders actually pour concrete slabs on it. The foam slabs have the same insulation as twice the thickness of fibre, but are more expensive.

As Alan alludes to, you get diminishing returns with each additional layer of insulation.

For example if you lose 1kW, and apply a layer of insulation to halve the loss, you now only lose 500W.

Add another identical layer, to halve the loss, and you now lose only 250w

Add a third layer, and you halve it again, and lose only 125W

So you are saving hardly anything with increasing thickness.

I have 150mm of fibre, and calculated it is not worth increasing it.

Even the chipboard flooring reduces heat loss a bit.
 
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Great, thanks for your answers. I'll just make sure its up to the level of the ceiling joists, and I can always lop up later if I want to spent more money.

Will also make sure any gaps are sealed to stop draughts!
 
Will also make sure any gaps are sealed to stop draughts!

Don't do that. The loft, above the insulation, should be draughty to keep it ventilated.

Water vapour is lighter than air and will rise through the house until it is stopped, or finds something to condense on. Many lofts suffer condensation and damp, and water drips off the cold roofing felt.
 
Option e) Insulate the between joists (100mm) and ceiling of loft (PIR insulation)

Not the cheapest option but you will be able to keep the whole loft as storage and it should a bit warmer than outside.
 
No to Frenske idea.
Do as JohnD suggests.
Unless going for a proper loft conversion, keep the loft area draughty and ventilated.
SFK
 
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Don't do that. The loft, above the insulation, should be draughty to keep it ventilated.

Water vapour is lighter than air and will rise through the house until it is stopped, or finds something to condense on. Many lofts suffer condensation and damp, and water drips off the cold roofing felt.

I had this in a new roof (extension). Had to put in loads of vents in the felt. Should have had ventilated installed, but didn't!
 

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