Adamanski:

 

Can the vapour / moisture barrier be an aluminium foil?  Reflective so should be great for thermal - just querying it around electric cables?

 

I'm sure we have some surplus aluminium/kraft/aluminium at work that could be a deal if suitable, just need to check it's the non-perforated.  Let me know if you're interested and I'll take a look tomorrow.

 

The issue with aluminium foil is real, all you need is for one staple to go though to phase wire of the wiring or an accident with some powered device, and the whole sub-floor area will be live.  If you're down there when it happens, then you're also standing on bare and damp ground, so worse than the "zap" you may have typically experienced from 230v when standing on a dry insulated floor etc.  Now maybe with a new build, then all circuits except water heater and oven may be RCD protected, but OTOH most new builds are slab on grade, so there are few cases where sub-floor insulation could be added to a new house anyway. 

Another issue with aluminium is that thermal reflectivity reduces as it tarnishes or gets covered in dust.  The sub-floor insulation they were selling was perforated, dust does eventually cover the top side.  The perforations are presumably there so that if for example there's a leak or spillage onto the floor above, then there's still some air circulation so it can dry out, otherwise you'll have water trapped between the floor and insulation layer - the floor structure will rot. For that reason, you should never put an impervious film (ie plastic) attached to the floor joists etc.   IMO absolutely avoid unperforated alumium foil, but also avoid the "proper" sub-floor foil for electrical safety reasons and also for the reasons that it doesn't work well long-term, and if you're going to the hassle of installing sub-floor insulation then do it properly. Plastic film over the ground might be of some benefit if the soil is damp, but also make sure that sub-floor ventilation is adequate (someone might have blocked off many of the sub-floor vents by landscaping etc).

I have installed sub-floor insulation in our 1962 house.  But also increased ceiling insulation to R3.6 (+ existing old batts), plus wall insulation in all external walls, and as part of that work (full reclad) installed flashing systems to meet current building code around all window/door joinery (external).  The house was previously brick veneer, houses of this era the cavity is open to the sub-floor and may also be open to the roof space.  In this case, the only seal around joinery (stopping airflow from the open cavity) is from the architraves, in 50+ years even if those architraves had been flush, they've probably worked loose and/or warped.  On a windy day, then hold a candle near architraves to check for airflow.  Also check sealing of doors and windows, fit rubber seals as required.  Issues like this should be addressed before considering sub-floor insulation - IMO that (sub-floor) is a minor cause of heat loss in most older houses.