Loft Insulation Difference between Top up and Base Layer

[Steted21]

Hi All

Im insulating the loft of our new extension I am able to get hold of 200mm top up and also 100mm Top up insulation would this be ok to put down or do I need to install a base layer is this a must or would these two layers be sufficient

 

[cdbe]

It's all the same, just use whatever fits your joists etc better at the lowest price. I believe the "standard" is being increased from 300 to 450mm - the savings are marginal but every little helps.