On Fri, 31 Jul 2020 12:15:20 +0100, Robin wrote:

Provided the boards are a tight fit with no big gaps they don't
need
to be foamed in, tapping will be fine. However as they are 25 mm
below the top of the joists taping the internal 90 degree joint
might
be a challenge.

It's recommended to have the insulation flush with the top of the joists
- AIUI so as to minimise air movement under the floor covering should
there be any leakage up the sides of the joists.

But with the joints taped there can be no "leakage up the sides of
the joists". I did think about the gap betwixt insulation and
flooring and sort of decided that a bit of warm side ventilation
wouldn't be a problem.

25x25 batten at the bottom and sides of the joists wouldn't be too
tricky to fit. I'd use a jig to set the postion though not rely on
the joists and/or insulation actually being the dimensions they
proport to be.

--
Cheers
Dave.