9 inch hollow block build - insulation

Timing belt

I’m trying to understand how best to insulate a 1970 3 bed semi detached house that is built using hollow 9 inch cavity blocks (big concrete blocks with 2 upright holes in the middle) and suspended wooden floors.

From reading online and learning looking at building regs it would seem that there are 2 methods (external insulation or internal insulation)

Similar houses I have seen have generally gone for external insulation and I can see the benefits as you don’t loose internal space and in theory should benefit from thermal mass so building stays warmer and benefit from a new outer water proof layer.

The bit that I am struggling to get my head around is that if the the external insulation is installed correctly the EWI will connect to the attic insulation and close off the cavity wall gap at the top of the wall. In theory this all makes perfect sense as the cavity in the block’s shouldn’t have vapour that needs to be vented. However in the event of this cavity holding moisture how does this get removed and not cause condensation within the wall.

The ways I can see moisture getting into the cavity block is from one of the following:

• -Damaged External insulation letting water in
• -moisture coming up from the foundation the wall is built on as there would be a thermal bridge here
• - humidity within the house pushing vapour into the cavity block

Is there something I am missing or would the moisture in the walls dry out via the internal wall as long as humidity is not to high within the house.

The other question I have is does the external insulation extend over the wall to Connect with the VCL layer and insulation in the attic and if so how does it work with the gable end where the insulation and vcl doesn’t go all the way up the wall. Does this part of the wall need PIR and VCL on the inside.

The other option would be internal insulation and more or less creating a timber frame house within the envelope of the building with a continuous VCL making the house air tight but then there is the risk of moisture from the outside getting into the cavity block and no heat coming from the house to dry it out. Could you use air vents in the outer layer of the block to get air in to dry it or would the draft etc cause to much heat loss

10-10-20

The links within this thread may be of interest as they contain an architect's eye-view on cavity blocks and instulation.

https://www.boards.ie/discussion/comment/119769773#Comment_119769773

Impressive bumph about the author here also:

https://www.tudublin.ie/explore/faculties-and-schools/engineering-built-environment/architecture-building-and-environment/people/staff/josephlittle.php