Loft conversion insulation

[Ommm]

I have a 1960s bungalow with a 1970s loft conversion. The vertical walls are constructed roughly like a 1970s loft: plasterboard inside, then timber uprights with 80mm rockwool batts between, held in with string.  Performance of those isn't terrible, but there are inevitably gaps, and a chimney effect through the gaps.  Behind the wall is a cold loft, with venting at the soffits. The floor under the conversion (downstairs ceiling) is just the regular joists but with the gaps stuffed with rockwool.

One of the issues with the construction is its lack of thermal mass.  It gets very hot in summer and cold in winter as soon as night falls.  The walls are east and west facing so see a lot of solar gain in the morning and evening.

 

We're having a heat pump fitted soon so I've been thinking about insulation in general.  I've thought of a couple of ways to address the thermal mass issue and the gappy insulation.  One is to replace the rockwool with tight-fit woodfibre boards, which have more thermal mass and reasonable insulation.  Another is hempcrete blocks, although those are potentially harder to install.

 

What I'm trying to understand is the moisture implications.  Currently the plasterboard provides a moisture barrier from room air, and the timber is exposed so that it equilibrates with the loft.  I'm wondering whether the woodfibre or hempcrete would be more hygroscopic and so hold moisture close to the timber, which would seem to be bad?

 

I also wondered about using woodfibre for thermal mass and then adding PIR for further insulation (both conducted and blocking solar radiation via the foil).  But then the timber/woodfibre would be trapped between two impermeable layers - won't that be a damp reservoir?  (Given it wouldn't be feasible to entirely seal the edges)

 

Is there any other material I should look at for thermal mass and insulation that would be compatible with being butted up against timber in this kind of environment?