Hello,

I have just noticed a big cold bridge issue in our extension (see attached pic). We are building onto neighbour's party wall - however they have a flat roof, and we have a pitched roof (already being built as we speak so cannot change the design). The issue is that because we aren't going to the same height as their roof, we are going to have a thermal bridge through the brick wall.

Whether it's right or wrong, I am less worried about heat loss at this stage, and more about the wall being cold and causing condensation and mould.

I think it would be difficult to do EWI on the wall properly because we will have a gutter close to the wall. Would a small amount of IWI help keep the surface of the wall warmer - and if so, would breathable (wood fibre) be best, as opposed to a foil backed insulation?

@mike, gutter isn't there yet, but the rest of the roof structure is underway, and there's only a small space left alongside that wall to fit the gutter. So EWI wouldn't leave enough space.

At this point I feel like I am getting very stressed and I just want an effective 'enough' solution rather than the best solution. I know EWI is optimal, but do you think a small amount IWI would generally also solve the problem? I just want to ensure the surface is warmed slightly so it doesn't cause condensation. We will also have UFH in this area and nothing covering the wall, so I hope that will help.

I am totally amazed they haven't objected to having such a design, as in effect if there is a gutter or valley failure you are flooding their property.

As far as I can see the gutter between the wall and roof will have to be a valley gutter. It should be simple enough to put EWI on the wall as thick as you want chamfered at the top to shed to water into the valley gutter and chamfered at the bottom to fit the roof and then use the EWI as you would have used the wall for the valley gutter.
Valley gutters work best when they have a flat bottom - less prone to blockages which cause side overflows. EPS can also be used to provide the base for the bottom of the valley gutter profile.