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.

Well yes it *is*, actually. If there is any exposed joist, that is
*precisely* where condensation will happen. The foil doesn't just seal,
it is impermeable as well. You MUST ensure that the no-longer-ventilated
cold joists are taped over. It so happens that is simpler if the
insulation is flush with the joist tops.

Underfloor ventilation in an insulated scenario has no purpose: You wont
get condensation where its warm.

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 position though not and rely on
the joists and/or insulation actually being the dimensions they
purport to be.


Make up battens with slotted holes so you can get the batten the depth
of the insulation below the joist top, then screw in more screws to
stabilise.

Insulate, foam in any air gaps, and tape over.


--
"Women actually are capable of being far more than the feminists will
let them."