I don't actually know the thickness of the foam insulation, its over 50 years old and being replaced in a couple weeks, so I might find out then (out of interest)
Our loft insulation is around a foot thick but the real reason for heat loss overnight is open windows. With no input from the CH, the house will generally maintain 16c at current outside temps, dropping to 14, maybe a little lower in the real cold.
It might be psychological but the house does feel colder since we reduced HW times...walk in the boiler or airing cupboard before, there was residual heat, now they're pretty cool in the mornings.
I suggested GD should get a desktop fan or similar to get air movement onto her gear, that might help.
Dehumidifiers are often mentioned...do they actually draw the moisture from clothing ? Or is air movement better ? Questions questions