Bob Minchin wrote:
I have a workshop floor that is 4" concrete on a DPM on hardcore/soil.
The plan is to put a wooden floor in to make it more comfortable to stand on
for long periods.
I don't want to lose too much ceiling height so i was planning to put tiling
batten (25mm) down as mini joists with mortar under to adjust the levels and
18mm ply as the floor surface.

Is it worth putting insulation underneath eg 25mm polystyrene or as the
space under is not ventilated, would the static air gap underneath be
virtually as good as the insulation?

The area is 30 sq metres so I don't want to waste money on polystyrene
unless it is going to make a useful difference.

It probably will.
Polystyrene conducts at a rate of .04W for a meter cube, with 1C
difference on the sides.
Divide by .025m, and that's 1.6W/m^2/K.
Multiply by 30, and that's 48W/K.
For 10K over ambient, that's 480W.
ISTR that a gap and a floor is about 4 times that - from some sums I did
a while ago.
The end result - when the shed is hot for a long time may be different,
as the ground underneath will heat up, and then insulate somewhat.
But this takes a long time, comparatively.

Do the sums.
How long do you use the shed?
How hot do you want it.

If you're going to be heating it 8 hours a day, 365 days a year, using
electricity, then you probably want to insulate the walls too.
If you're going to be using it for an hour every few weeks, it may well
be cheaper to simply put in a few heaters.

http://www.bre.co.uk/filelibrary/BR_443_(2006_Edition).pdf
contains all the information you need to work it out - there is probably
much easier ways.