18 mm loft boards over insulation

[murraymint]

Does anyone know the effect of 18mm boards laid over loft insulation, does it make it less efficient, is there any noticeable effect?

[Justme]

I think I am right in saying that as long as you are not crushing the insulation then it will actually improve its insulation value slightly (due to the extra layer of 18mm thick wood).

You might have to consider the effect on moisture that 18mm board will give. It could be that the insulation needs to breath if no Vapour barrier is fitted below the insulation.

[Stuart]

I'm sure I read 100mm overboarded is the equivalent of 200mm? as it stops the airflow

I had so much 100mm left i did another layer before boarding out anyway.

[w0067814]

I boarded a portion of my lost. I added 3*2 battens over the top of the existing joist to raise the level. I have 150mm of glass wool insulation under 18mm chipboard sheets.

If you are worried you can get silver backed insulation which would easily fit under the chipboard.

[Mudman]

ontop of the joists, i used waste chipboard (old kitchen units) to board most of the area for storage (small gaps just because it saved me doing too many cuts) then used some PIR yellow foam boards on top, finally the decent loft boards on top of that.
kingspan foam is approximately twice the value per depth of standard loft insulation which means i still have enough head room comapred to doing more joists and standard loft insulation like w0067814.
it's much much warmer in the house than when just had the standard loft insulation between joists.

[Mudman]

by the way 18mm chipboard will add very little insulation value.
K values
chipboard 0.14
mineral wool: 0.035
that's watts per degree per meter thickness (i think)

so it's four times less insulating than mineral wool.