Adding under-floor Insulation deeper than Joists?

[Beowulf]

Hi,

I'm looking at installing underfloor insulation:

1930s red-brick house
Suspended floor with crawl space
Good air brick ventilation
100mm depth floor joists

My intention is to use a combination of Rockwool and recycled plastic loft insulation.

The simplest/easiest option would be to use 100mm thickness between joists supported by netting. Would only 100mm be worthwhile? The latest regs suggest 170mm, I think. What would be the difference in benefit in reality?

Or, I could use 200mm of insulation that would be 'bulging' out below the joists.

Alternatively, I could use 100mm and add a second layer of 100mm across the joists. This would cover the bottom of the joists. Could covering the joists cause future problems with moisture?

Any advice would be gratefully received.

 

[JohnD]

why do you want to?

For a floor, 100mm is plenty.

 

[Beowulf]

why do you want to?

It was the building regs recommendation that I read.

For a floor, 100mm is plenty.

I'd have thought so too, but I was looking for other experiences/opinions.

Cheers

 

[tony1851]

Floor insulation is subject to the law of rapidly diminishing returns. As you have been advised, 100mm is plenty.
Any additional insulation is best put in the roof space.