How difficult to reduce thickness of 100mm Kingspan / Xtratherm

12 Sep 2019
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United Kingdom
I have a situation where 100mm insulation has been put on top of an unlevel concrete slab

Insulatiin boards are 400mm wide.

19mm T&G and 12mm laminate flooring to go on top.

Part of the floor will be too high, and I need to reduce the height.

Prefer not to have to grind concrete down.

How practical / easy is it to reduce the thickness of the insulation - with a hand saw, I presume?

The alternative is to apply self levelling compound to the concrete then use 70mm to 80mm insulation.

That has two negatives. Expense of buying more insulation. Lower insulation properties than 100mm (with some reduced thickness).

if I cut the insulation, it will not be a perfect contour for the uneven concrete slab so I will need to apply self levelling compound.

Should this go:

On top of the insulation (joints and edges sealed, surface primed)
On top the 19mm T&G (primed)

Or will it work on top of either. (My preference is on top of insulation)

thanks ColinK
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Handsaw kitchen carving knife all work along the board more difficult and less accurate but far more easy than nipping at concrete
If you're going floating floor it's needs to be dead flat so needs latexing then dropping inso thickness to 80 or 90mm
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Reducing the thickness of your boards will have the same effect as putting thinner ones in- ie worse thermal performance, maybe significantly worse (don't know how much of the R value comes from the foil surface). Building Control might have something to say about that (if they're involved).
If you do decide to go that road, a fine tooth handsaw will cut the stuff nicely (face mask and gloves, the dust is vile). You'd want to use a jig as well otherwise your cuts will wander & you need to be very accurate for a floating floor.
Grinding the excess concrete out will be a horrid messy job but do it then use SLC and you'll have a much better shot at a flat floor.
Dif between100 or 90 mm pir wil be minimal cutting the back out will result in a uneven floor plus it looses it's compression strength

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