External insulation for my own renovations

johnny_adidas

Time has finally come for a refurb of my own existing 70/80s mid terrace house and add a single storey extension to the rear. Planning is due to be lodged shortly (if i get the finger out this weekend) however i have some questions regarding external insulation for the construction phase which to be honest we do not use much of in work so have not had detailing or practical experience with it

For the existing house, uninsulated 300mm cavity wall with pebbledash, I have ditched the plan of drylining and will be getting quotes for external insulation and render. I will be taking the insulation down below the external ground level which is well below floor level. There is a cast soffit/fascia on the house and the attic is already sufficiently insulated.

Q1. Should the cavity be filled prior to ext insulation?
Q2. Should an insulated fascia and soffit cover be fitted? (however thin)

For the new extension im planning to use a 215 inner leaf with 200mm external insulation and render to finish. Given what ive been drawing for years (which i accept is a flawed method and totally reliant on top class blocklaying) im a bit worried about water ingress through a 'solid' structure albeit with water resistant insulation/render layer externally.

Q3 Am i right in saying there is no need to construct a cavity wall as the cavity needs to be ventilated and therefore negates the advantage of using external insulation?
Q4 Is the 215 structure sufficient for walls which will be 3.5m high max?

The insulation will be fixed up to the underside of a the roof deck which will be insulated under the single ply membrane so fingers crossed the completely wrapped extension wont have the cold isolation problem that some extensions suffer from.
I am on a tight site however and will be building almost to the property line. I have a neighbouring extension to one side (600mm left outside insulation to new walls) and a boundary wall on the opposite side (300mm left outside insulation to new walls). Leaving a working space to apply the insulation/render externally may require more space than i can afford.

Q5 Does the external insulation require the render in areas which will not be exposed? (below the eaves line of the adjoining ext)

I appreciate any/all opinions/suggestions. im sure i will have more questions however that is probably enough for now.

BryanF

q1: yes
q2: need to see detail, you must consider the zone between soffit to ceiling insulation (I've seen the lower section of roof raised a few inches, timber filleted and insulation installed to ensure this overlap)
Q3: again depends on details (boots and hat) + location + supervision etc
q4: engineer ( if you've been drawing for years ask your friendly engineer!)
Q5: sounds like you need to employ an arch tech and possibly consider another wall build-up, I can think of few but would need to see situation

johnny_adidas

im actually an arch tech myself, just been tied up for a long time with health and safety and contract admin work so havent really kept up to date with the external insulation side of things. In terms of detailing i will be quite happy to do those myself but just anxious as its a totally alien system to me.
I suspect i may be better off to go good old cavity wall on the new extension as there just wont be enough space for someone to drill fixings for the insulation externally. either way i will be closing off the space between both extensions at a certain height to prevent dirt buildup.