I appreciate that there has already been some discussion on this and have read the various posts. I have had a (pricy) quote from an installer of one of the big propriety systems and felt that although they could do all materials and finishes they were essentially pushing the easiest system for them rather than the best for me (i.e. 50mm phenolic to ‘just’ meet building regs in the minimum, easiest-to-fit, thickness).

The Property
In Hertfordshire
Detached Victorian house which is solid brick wall.
There are lots of fitting on internal walls of the house including a fairly new kitchen so we are happy with the external over internal choice.
We are happy to end up with a white ‘cottage’ at the end of this so can cope with the change in character but would like to avoid it looking like a house which had insulation bolted on the outside (i.e. blends seamlessly into the eaves etc.).
The eaves overhang by 4-5” so while we appreciate that more insulation is better we don’t want to go to the point where we need to extend the eaves or add lots of prefab trim.
We are lucky to have no evidence of any, even minor, rising damp issues.
There is also already quite serviceable UPVC double glazing.
The loft is insulated with 300mm mineral wool.
The only suspended floor is insulated with 150mm mineral wool, the rest of the floor is solid.

Questions
From the previous posts I have read recommendations about creating ventilated gaps between the insulation the wall of the house and even mention of internal gutters etc. Given that the quote we have had so far for 50mm PU bolted onto the wall and masonry-paint/silicone-render is just over £15k creation of such a complex structure seems like it will be financially non-viable. Are these recommendations belt and braces for driving rain conditions or are such precautions really valid?

What are your thoughts on the best system? By best I’m not that fussed about the difference between stopping 95% and 99% of the heat flow but care greatly the about the lifetime and the general performance of the system in 20 years time and to a lesser degree the breathability. I am coming to the conclusion that 80-100mm or so of XPS bolted onto the walls covered in a ‘breathable’ silicone render might be the best compromise – Any alternative suggestions?

I would also like to reuse the current windows but reposition these to be at the level of the insulation to avoid cold-bridging/pricy-insulated-detailing around the windows. However I am likely to fit new, higher-performance, windows within the life of the EWI. I am struggling to find details of window repositioning and ease of replacing the windows in future once EWI is in place? The installers say that I’d need a glazier to reposition the windows but they couldn’t provide a spec for the position/fixings. I can’t imagine glaziers knowing a lot about positioning windows within EWI and at worst would immediately void the warranty of the EWI by drilling fixing holes into the EWI. What routes have other members taken with this issue?

Would also love to hear any success-stories/lessons-learnt from similar EWI projects, recommended installers or systems etc.

Thanks

John

Hi James,

Thanks for your thoughts.

Your comments make me thank that rockwool may be worth more of a look as it seems like it should last virtually indefinetly, and had the potential to dry out if it ever got wet. I will also have another look into EPS as my reading seem to show a moderate degree of breathability.

For your comment about window detailing and insulating the reveals won't this make it near impossible to remove the existing windows in future? It seems to me that if the EWI is just over the exterior faces of the walls (and not in the reveals) then the windows can be moved out giving a more simple construction and minimal thermal bridging. The only issue is what exactly do you fit the windows too! 8)

e.g L's of 30x5 Batstrap screwed to the outside wall face and projecting outward, there screwed to the perimeter face of the window frames - e.g. 2x down each jamb and 2x across the top - @ about 900c/cs The cill can often hang there if nec if that helps with detailing when there's nothing solid to rest it on or screw it to. That way, the window hangs in space outboard of the old wall face, ready to bring the EWI up to the sides of, and lap across the outside face of (jambs and head) as far as poss without fouling the opening light or hinges. It does wonders for effective insulation, to 'suspend' the windows within the insulation zone, well away from contact with anything internal (which is what the old wall becomes - part of the internal environment).

I met a great guy the other week who is a surveyor in the garden of England who regularly specifies 200mm eps for ewi.

I cant see any good reason to do it with a lower U value than that would give. central europe it is best practice and very common.

See link below for a table of Vapour Permeability (r) of a range of materials (page 2) in this article making the case for Breathable constructions and the breathability of insulation products
EPS 150 MNs/gm

http://www.natural-building.co.uk/PDF/Breathability-Matters-response-to-Kingspan.pdf

If you go the non-breathable route, using the lower lambda value insulations, you might as well increase insulation thickness ( say 80mm phenolic) . As discussed on other threads, insulation cost is only a small percentage of the overall cost when keeping to a thickness that doesn't require additional works

Tom, the notion that EPS is vapour permeable, "breathable", showing little resistance to the passage of water vapour even if using Neopor is contrary to what Springvale says in its literature and even in its BBA where it quotes "significant resistance to water vapour transmission" http://www.springvale.com/ni/uploads/bbacertificates/platinumwallshield_bbacertificate.pdf see 11.1

In the coldstore industry where EPS wall and ceiling panels were once the norm the very suggestion that EPS was more vapour permeable than XPS and therefore more reliant on the performance of the sealant between joints in a high vapour drive environment would send the EPS manufacturers into a state of apoplexy

just found the link below when looking for the breathability of silicone render (p4 - option B).

http://issuu.com/the_building_centre/docs/8864_pdf7

This gives the vapour resistance of 'FRA grade' EPS as 6.9MNs/gm. That's about 20 fold less than the figure used by the www.natural-building.co.uk/PDF/Breathability-Matters-response-to-Kingspan.pdf article.

Uggh, but not remotely surprised. Not all high prices though; one of the guys on my Eco Refurb course last year was London-based, and doing 60mm phenolic for £90/m2.

Not wishing to highjack the thread but there is a general relevance to window detailing within external insulation.
Any suggestions as to how you would detail support for a window unit positioned outboard of masonry that weighs somewhere close to 800 kg. Window approx 2 m x 3.7 m, triple glazed 8 mm toughened glass to all panes. Or am I stuck with locating within the masonry envelope possibly located within a load bearing insulation collar.

Nick, your last comment was along the lines I was considering. Extend the depth of frame back into the masonry opening and locate it within a insulation collar, Foamglas or similar. Also wondered if a continuous length of angle section resin bolted beneath window with load bearing insulation above and window sitting atop.

Tom, no particular requirement for continuous support to cill, this will in itself be sufficiently robust as a section. Just acutely aware of the weight of these damn things !

I am somewhat wary of entrusting our house to these outfits, insofar as it's very difficult to monitor and control the quality of their work in real time, while after the fact it's all concealed and sealed beyond inspection by the render!

Isn't that true of v many building processes? Esp as Architect, making weekly site visits! A lot hangs on selection and trust of the 'right' contractor/subcontractors - and ultimately on enforceable contractual descriptions of work if faults discovered later (if you have a taste for litigation!). It's a miracle how it generally turns out OK-enough.

It's a conundrum without a watertight answer. Right now I'm considering tenders - two firm-price tenders for the work as voluminously specified, and one 'fair estimate' of what a cost-plus contract might come to (unsurprisingly, the lowest of the 3 figures). The fair point made by the cost-plus man was that it removes all temptation to cut corners, which a fixed-price in theory promotes. First thought is 'speak for yourself' - an insight into that man's conduct when on a fixed-price. Despite all that, I'm not dismissing the cost-plus proposal, because I was impressed by that man's apparent eco-knowledge, enthusiasm and track record, which I would check out further before going that route. So we 'feel our way into' a solution - but it all takes time and can't be formularised.

And I added more while you were typing!

As to choosing between tenders , I think trusting ones intuition usually seems to work.
I do a fair bit of 'fair estimate' work , it takes alot of the pricing/loss making pressure off and allows me to concerntrate the work side rather than the money making side. Truth is I usually make more on price work
as I have to stick a % lump on for arse covering, dont like it though , can be a stressful distraction.