Retrofits are a bit like belly buttons. Are you an Innie or an Outie? This is the key decision you need to make at the beginning of your retrofit.
Though there are examples where it is a bit of both, to upgrade the performance of the building we need more insulation while at the same time improving the airtightness. You can do this from the inside or from the inside. Most likely you will be replacing the windows. Upgrading windows has generally the biggest impact.
Which you choose might be because of the type of construction, because it is a heritage building, the other work you are doing, or some aspect of the current design. If you are an outie you are affecting the cladding of the building. If you are an innie wall linings and having to redecorate.
Either way, you need to think about where your control layers go. There will be moisture management considerations and potentially difficult details at the junctions. The airtightness/vapour control layer needs to be virtually continuous. Specific modelling of the junctions may be required.
With a masonry building, most retrofits are on the outside of the building. This means all of the building fabric will be warm. If for heritage or other reasons, you install the insulation internally there might be moisture issues between the cold masonry and insulation where some sort of ventilated or vapour permeable solution is needed.
Large apartment buildings are often retrofitted from the outside, sometimes with prefabricated units. The trick is getting the airtightness to work. It can have a detail that is reliant on sealant which isn't accessible. You can watch more about a Casa Pasiva retrofit, a New York example here https://www.youtube.com/watch?v=g2hv7rN-nZQ.
There is currently one Enerphit certified house in New Zealand. It is a masonry Outie - https://sustainableengineering.co.nz/casestudy/piha-enerphit/. There is also a webinar about it here https://www.youtube.com/watch?v=NdGY7HVQjAw&t=3430s. It had to get an exemption for its existing uninsulated concrete slab, where only an insulation upgrade was possible at the edge.
There is a timber framed house retrofit that reached the full Passive House Standard - https://sustainableengineering.co.nz/casestudy/till-cottage/. It is also an Outie. This meant an external airtightness/vapour control layer.
With our project, the external cladding is in reasonable condition, but 40% of the wall linings had to be replaced due to fire damage so it was decided to do an Innie. There were also some layout changes we wanted to make.
This means the wall construction will look a lot like a typical new build high-performance timber frame construction with airtightness on the inside of the timber frame and the introduction of a services cavity. We lose 55mm of the area of the house on all the exterior walls.
Keeping the existing cladding doesn't provide ideal double external water control layers with a cavity so there is a risk of moisture. Retrofitting a membrane with a ventilation mesh would be one option but this is incredibly time-consuming, and would always be imperfect as you are working between the studs. The water needs to go somewhere. We decided on blown insulation designed for retrofits and exposure to moisture. This is putting a lot of faith in the product.
Below are some of the initial sketches, with comments we did to test key junctions and allow us to discuss them with Sustainable Engineering, Proclima, and Knauf. We also needed to work out how the ventilation routes might work. To do this we introduced an area of dropped ceiling.