In message , Robin
writes
On 31/07/2020 13:51, Dave Liquorice wrote:
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.

You may be able to achieve perfection but it is rarely within my grasp
Especially perfection that has to survive thermal cycling and
movement over the years.

But I can't swear that flush fitting is not just a myth that
manufacturers et al have bought into.

Can't comment on foaming but Cellotex has a surface release agent which
needs to be cleaned off for reliable sealant tape fixing:-(

--
Tim Lamb