Carpet Underlay as additional UNDERfloor insulation

[sherbo]

I have access to my floor void and am about to fit B&Q's cheap 170mm glass mineral wool insulation between the joists. I figure it will stay in place by itself, but am going to use the following (very cheap!) netting as a failsafe (stapled to the underside of the joists):
http://www.boddingtons-ltd.com/acatalog/Anti-Bird-Netting.html

HOWEVER, I've just had carpet fitted upstairs and there was some foam underlay (Cloud 9 Cumulus 11mm) left over. This got me thinking. Would underlay, stapled to the underside of the ground floor boards before the normal loft insulation is fitted, add an additional layer of low-cost, thermal insulation?

Will this be superfluous and have no great effect? Am I creating problems in terms of breathability/vapour/condensation?

Thanks in advance from a noobie.

 

[mointainwalker]

Anything - even a sheet of newspaper - has an insulating effect, so yes your underlay would improve the insulation.

I've no idea of the insulating property of this so unable to say if it is worthwhile but the effect is going to rather small I expect as insulation is subject to the law of diminishing returns as far as depth is concerned.

You will not be creating any problems.

 

[ladylola]

I've done this type of insulating before. One little tip, rather than try to staple the mesh to the underside of the joists cut it into strips a little wider than the span of the joists and staple it to the sides to form a "cradle". It's much easier than crawling around a floor void.
As to the underlay I'd go with mountainwalker on that one although I think trying to staple it to the underside of the flooring sounds akward at the very least.