Wall insulation, cavity+external?

[unclebob1]

Good Afternoon All,

I'm looking to extend my house to the rear, the main house is solid wall, and at some point in the future i will be adding external wall insulation.

As part of the rear extension build, i will have a cavity, the question is, should i build the exterior wall in line with the existing wall, with the cavity coming into the extension volume, Then add external insulation as i do the rest of the house to keep it a straight flat wall?

I understand adding more insulation will reduce energy/heat loss, are there any down sides? the once outside wall of the cavity will now be insulated once, the inside wall will be insulated twice. Any ideas on moisture issues?

Also, if i am looking to do this, any issues with me building both the inner and outer walls in thermal or otherwise blocks?

Alternatively, should i just insulate the cavity, and leave it stepped once the external wall insulation is applied to the rest of the house?

 

[xdave]

Unless the extension is obviously subordinate then it may look better finished inline with the rest of the property. When I had my bungalow externally insulated (120mm PIR, ply, barrier and shingles) it went over three different wall types thanks to various extensions over the past 50 years. One was 50mm cavity with no insulation, one was 50mm cavity with blown insulation and one was 100mm cavity with full-fill fibreglass insulation. Visually it tied everything together. I had a thermal imaging survey done in January. In all three areas the walls were all within 1 degree inside and out, so from a thermal point of view the original cavity insulation made very little difference to any heat loss.

 

[unclebob1]

Thanks, in which case, i should probably build in line with the existing brickwork, and render now. and when external insulation is done, cover hack off the render on both the existing and extension and finish off again the same.