Hi glad to see this site is still going! Planning to retrofit a 20 yr old house with external insulation, triple glazing and a heat pump. I had an airtightness test done which came back as around 9 ach which seems to suggest I need to do some serious airtightness work to make this all worthwhile. Its brick and block with a 70mm cavity filled with glassfibre and dot and dab plasterboard. Upstairs the two bedrooms have dormer windows with sloping ceilings from 170 cm above the floor. I was thinking of an airtightness membrane on the outside of the brick joined to the foundation which crosses the wallplate and continues up the inside of the rafters to make a warm roof. I was thinking of externally insulating with a larsen truss with 40mm woodfibre boards externally and blown in loose woodfibre (gutex) with XPS EWI below the dpc down to the foundations. I realise this isnt perfect as theres a leakage path under the slab between the foundation and blocks below ground level but was hoping I could still get a pretty good airtighness result.Would welcome any thoughts on this!

What are your target u-values, airtightness - I love 3g good decision.

Perhaps an easier way to get external air tightness would be to do a conventional EWI with EPS and foam between the slabs of EPS either as a post EPS application or perhaps easier foam between the slabs as they are applied to the wall (squirt foam on the 2 sides to be mated then press the next slab in position). After the EPS is on the wall then the normal layer of adhesive and glass mesh then thin film render over.

With your proposal above you don't say how you would prevent water getting into the wood fibre board and blown in wood fibre. Any moisture that did get in would degrade the insulation and probably have difficulty in drying out.

For the down to the foundations insulation I would use EPS rather than XPS. XPS is closed cell but will eventually get water absorption and will not dry out. EPS on the other hand will release any water that did get absorbed. There is at least one study that showed (after a 15 year period if I remember) that EPS out performed XPS in below ground use.

If injecting the foam after I would use thin spacers between the EPS slabs to make injecting easier and give the foam somewhere to expand into.

Adjacent EPS panels even if touching won't be air tight.If a slight gap is created (say the width of the diameter of the foam gun) then it will be easy to inject the foam which can then expand as needed.

Posted By: Peter_in_HungaryAdjacent EPS panels even if touching won't be air tight

True

Posted By: Peter_in_HungaryIf a slight gap is created (say the width of the diameter of the foam gun) then it will be easy to inject the foam which can then expand as needed

Prob gd idea

Yes, Tom, Not worth EWI when you have CWI.

PinH - Good point - 9 ACH! eek
- and especially as he is looking at a warm roof and suffered a internal retrofit.

According to greenbuildingadvisor.com Eps is an effective air barrier if it is at least 5.5" thick but the adhesive + mesh coat followed by the thin film render all add to the mix to obtain air tightness.
Putting a membrane under the EPS would obviously solve any issues if the EPS is thinner but then fixing the EPS needs thought

A hellava lot easier to glue EPS blocks direct to the masonry and render it. Or woodfibre boards instead above ground-splashback level.

Second-guessing whether or how EPS may be harmlessly salvaged and in some way recycled or sequestered in 60yrs time is fruitless.

Posted By: Doubting_ThomasSince you are EWI-ing you may need to consider extending your rafters. This is sometimes called a 'bobtail', and you need to consider the impact on high-level windows that open out (see the section on 'sprockets')

I think you're confused about what a bobtail is. A bobtail truss is where the truss is cut back because of an indent in the shape of the building, or the whole truss is raised up (made thicker vertically) to make more room for insulation. Sprockets is a better term for the topic under discussion.

All solved if there's reason to re-fix or replace the slates/tiles while the scaffolding's up - board over the rafter tops, carry the EWI up and over the boarding and over the ridge (as well as insulating between the rafters), leaves the attic cosy, draught-free, useable, completely seamless insulation at the eave. Cut the existing rafters off flush at the eave, lay probably longer 'rafter extensions' embedded in the over-roof EWI (handsome exposed sawn untreated doug fir in the pic). The 'extensions' are thus higher up than any previous rafter projections, so will clear any windows - in fact on elevation the gutter line is still approx same as before, despite larger overhang, just the ridge is higher on elevation.

Stub rafters? Eave rafters? And then there's the verge!