IWI vs CWI

Barmy_Army_92

I am seeking some advice, as I am going insane over how best to insulate/more efficiently heat my property before next winter. 

House:

1890's mid-terrace, in the NW [risk of driving rain], in a built up residential area - at the rear there is a semi-detached extension with pebble dash render. This extension is original to the property and is SW facing. 

The house has cavities, presumably because of the weather. 

Questions:

Cavity wall insulation is the no-brainer option, for speed, cost and ease of installation. I would choose a CIGA approved installer, have a survey done prior to installation and would opt for PVA bonded beads. But, I have read so much about potential issues that I am increasingly considering going down the far slower, and costlier route of internal insulation. 

I have recently purchased this property and so I plan to move through the house and renovate one room at a time, so this is something I am prepared to do - but with a small child and work commitments, it will probably be many years and winters before I will have finished insulating the external walls. 

What are the forums views on this topic?

FreeBear

If there is a cavity, it is going to be very narrow. At most, 50mm. Filled with polystyrene beads, the level of insulation is not going to be particularly high, and may cause problems particularly around any air vents for underfloor ventilation.

My preferred option is insulating the walls internally after stripping the plaster back to bare brick. 65-75mm of Celotex/Kingspan will get you the current recommended u-value without loosing too much floor space. With a little care, 25mm can be put round the window reveals to reduce cold spots in those areas. The downside to IWI is things like radiators & sockets need to be moved - Assuming you have decent double/triple glazed windows, the radiators could be hung on an internal wall..

Insulating the walls internally in a kitchen has its own set of complications, especially if you want wall cupboards - Rather than finishing the walls with plasterboard, fixing 12-18mm plywood up gives you something to screw wall units to.

stuart45

Some of the Victorian cavities were bigger than 50mm. I've worked on some in coastal regions with cavities around 3 inches. There were some solid walls built in stretcher bond around back then which are mistaken for cavity walls.
Companies won't normally fill a cavity less than 50mm.

Bendy_House

IWI is quite simple the most effective. 
If 65-75mm takes up a tad too much space, then even 25mm will be transformative. 50mm if you can.
Next thing - seal off any drafts from the floor! Add insulation under it too, if easy to do. If not, then cover the top with a layer of whatever is the best thin insulation, possibly just fibreboard? But SEAL it between sheets, AND around the wall perimeter.