Hi,

I've just bought a 1920s 3 bedroom 2 storey semi, and am planning a significant retrofit (incorporating rear extension and loft conversion - probably new roof), preferably before actually moving in. Full scaffold, and bathroom move is going to be needed (so this mitigates a chunk of external wall insulation costs).

I'm pondering what to do about the walls - particularly the side wall, which is the largest in terms of area (I plan to do a hip-to-gable change on as part of the loft conversion).

Current construction is (in-to-out).

Plaster + 100mm brick + 75mm cavity with blown mineral CWI + 100mm brick + 20mm pebbledash (+white paint).

There's not much in the way of existing architectural details, and the eaves overhang is 170mm. The location is Brighton - so wind-driven rain can be significant in winter, but it's away from the sea-front (and up hill a bit - so probably minimal blown salt-spray), and probably slightly more than-averagely-exposed for the south coast. The pebble-dash is not in great condition, and permitted development case-law seems to suggest that I can replace it with render if I like. Planning were a bit fuzzy about the "permitted-developmentness" with respect to the thickness of cladding, suggested I make a full permitted development certificate request.

It is as air-tight as the average tent, or any other unimproved house of the era (where the average numpty kitchen fitters etc. has been in).

Internal insulation looks impractical, because it would narrow the existing staircase, and several of the rooms against the side wall too much.

The side wall only has a narrow access-way to the back garden - 900mm wide, so I don't want to thicken the wall too much on the side, at least not below shoulder-height.


Some general thoughts:

I don't know how well the blown fibres have been installed (they seem to be white but I don't know much about them except that hey were installed about 5 years ago) - some can be seen having spilled out from a hole in the inner leaf under the ground floor. I'm a bit concerned about the cavity as-is from a thermal-bypass point of view. Stopping air flow through the cavity might be difficult. Is DPC bridging likely to be a problem with the blown fibres in the future, if the vapour barrier ends up on the outside of the wall?

Given that Brighton gets a lot of sideways rain the winter (Approved Document C puts it in the "Severe exposure to driving rain" zone - and I'd tend to agree), is some sort of breathable outer skin the best way to go? e.g. Tyvek->battens->cementatious boards->render or similar?


So I suppose these are the basic options:

1. Leave largely as-is - improve air-tightness a bit, and make-good the pebble-dash if poss.

2. EWI without disturbing the existing wall much - just remove the pebbledash first, add a vapour barrier on top of the existing outer skin. I suspect the thermal bypass caused by air movement through the cavity will seriously impact the EWI effectiveness tho'.


Some more left-field ideas - don't know how practical any of these are likely to be. Many of them probably aren't practical at all.


3. EWI, but remove the blown fibres first, and replace with in-situ closed-cell PU foam (prevent DPC bridging is probably tricky) or "platinum" EPS beads - to improve air tightness and U value.

4. Take down the outer brick skin entirely, in-situ pour (high GGBS?) concrete by using new EWI sheets as concrete formers, and pouring against the existing inner skin (sticking in vapour barrier somewhere).

5. Take down the outer skin, and build periodic blockwork buttresses to brace the inner skin. Fill over the lot with insulation. Dunno if that'll work, would need a structural engineer, or for me to dust off my maths.

6. Like 5., but use steel or something else for the bracing?

7. Take down the outer skin entirely, widen foundations, and build new block-work outer skin (filling the new cavity with PU sheets etc. with a 20mm air gap, plus extruded PS below DPC, and low thermal conductivity wall ties).


I've just spent most of the last 12 hours reading up on this (including about 35 EWI and related threads here), but a lot of the material was a few years old, and the use of direct-rendered EPS etc. in driven rain areas didn't seem to have been addressed fully in the threads I saw... Maybe I missed the "key" thread tho'?

Tim.