info here.
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6910&page=1#Item_17

very approx £100m2++ for retrofit EWI by supplier/installer ( x1000 wow!)

materials for most branded systems approx £20-30m2

Hi , any chance you fancy sharing/whispering wetherbys m2 price supply and fit , for your current project ?
I know some retrofit EWI companies looking for £150+m2 for just u value up to current regs
Cheaper the better i'd say to make EWI really happen big scale.
cheers Jim

Thanks guys.
I cant afford £100/m2 so going to have to look at DIY/local builders and get materials myself. Fostertom, I am going for air tightness as well, hopefully to the level where a MVHR unit would be required. Are you saying that the EWI is essentially airtight? I currently have sarking board on the old roof and was hoping to only replace what is damaged by leaks etc. My idea was to put all the insulation on the top just because people on the forum have talked about condensation problems if the airtight membrane is on the cold side of the insulation. My other thought was that if I get the air tight membrane down quickly then it would provide some protection against a freak shower (I live in N Ireland so these are common).

If the EWI is airtight do i just join it to the windows with a bit of tape and plaster over it?

My windows are all old sash and very leaky. My plan had been to DIY fix them structurally but seal them shut. I have never opened a window in my entire time here! Then I could put temporary film secondary glazing in until I recover from the EWI cost.

How should I conect the hanging eaves to the system? any ideas. I need to have this all drawn on the CAD and all the detailed parts so that if I use local builders it is simple to understand.

Thanks again

EWI isn't airtight - I suggested that OSB over rafters could be made airtight. Viking's been mastic sealing them - I favour gapfilling bubble glue-and-screw. So the EWI seal to windows needs to be weathertight but no attempt to make the seal airtight (nor vapourtight).

Posted By: pmagowanpeople on the forum have talked about condensation problems if the airtight membrane is on the cold side of the insulation

No - important to mentally separate airtight membrane/layer from vapour tight layer/membrane. They may be the same thing but frequently not. Airtight may or may not be vapour tight and vice versa. You've been told to put yr vapour tight membrane (if you have one) inboard of the insulation - but the airtight one can go anywhere in the sandwich.

I'm suggesting dispense with the vapourtight membrane, instead go for the alternative 'breathing' construction, in which all elements are of low to low-medium vapour resistivity, but the innermost ones need to have about 5x the resistivity of the outer ones - ideally a gradient of resistivities (note - we're talking resistivities not resistances). In fact I've found in condensation calcs that even that rule can be twisted a bit - as long as the highest (i.e. low-medium) resistivity is placed not further outboard than half way thro the insulation, then it's still OK (but satisfy yourself on this, don't take my word for it). This means that 6 or 9mm OSB3 can be laid over the rafters and approx equal amounts of insulation can be placed both inboard (between rafters) and outboard (over the rafters) of the OSB.

Ideally, in fact almost essentially, windows should be placed outboard of extg position (easy if renewing - your problem if not) i.e. mounted cantilever-bracketed outboard within the EWI insulation zone, not touching any of the extg masonry. If left where they are, they'll remain an efficient route for outdoor cold to almost directly contact the extg masonry (which has become part of the indoor environment, inside its 'tea cosy' of EWI/ERI insulation).

If planning to extend your eaves, cut off yr extg rafter ends just inboard of the wall EWI, before OSBing over them (incl the cut-off ends). Then lay new extension stub-rafters fixed on top of the OSB (within the ERI zone) cantilevering outward. Lay them out so as to hit-and-miss the extg rafters, but don't rely on the thin OSB to support them - use noggings. This way, the EWI/ERI 'tea cosy' is unbroken, not penetrated by rafter ends. You can hang a heavy cladding e.g. boarding (instead of patent render finish) from those rafter ends.

Thanks Fostertom, just trying to digest what you are saying! My current roof has tongue and groove pine boards on it. They are 15mm thick and about 150mm wide. They seem to produce quite a good smooth surface but I don't think they could be sealed like OSB.

From what you are saying could I put the insulation directly on to them, then put an air tight membrane on top of the insulation. Would this take away the necessity for two membranes. My impression is you need a membrane under the slates in order to prevent any leaks from getting in.

Your plan with the eaves, is it similar to the Viking house type. The cantilever rafter stubs cut in to the ERI.

I will continue re-reading your post as I need to digest this a bit more!

Thanks
Paddy

Your windows idea, Fostertom, would be much more effective but I don't think I could do it without costs spiraling. I have over 20 windows on the ground floor level alone! I think I have to accept a bit of compromise. I could chip back the reveals a bit to get more insulation in.

Posted By: pmagowanI put the insulation directly on to them

Sounds good, no need for OSB.

Posted By: pmagowanthen put an air tight membrane on top of the insulation

Wherever in the sandwich is best to join up with the wall airtight layer. What are you intending to be yr wall airtight layer?

You still need the normal underslating breather 'felt' in addition to all we've been discussing.

Posted By: pmagowanYour plan with the eaves, is it similar to the Viking house type

Yes - but no need for 2 layers of insul over the sarking boards (depends on yr intended thickness) - the cantilevering stub rafters can fill the whole over-sarking insul thickness e.g. 100x50 stubs in 100 EPS. As long as the stubs hit-and-miss the extg rafters, bridging is avoided - but noggings reqd to support the stub rafters at the eave and to prevent uplift of their inboard ends.

If extending eaves, how are you going to extend the verges likewise?

You don't need all that timber forming a boxed eave (and verge) - your stub rafters can be fully exposed - v attractive - don't even need a downstanding fascia board, instead lay a 75x50 along the top outer ends of the stubs and fix rafter-top galv gutter brackets over it. Nice boarded soffit also laid across the tops of the stubs.

Like http://i30.photobucket.com/albums/c301/fostertom/2005-11-10017.jpg

Many people use internal wet plaster as the airtight layer on masonry - but a challenge to link that up with roof membrane - and it has to be unbroken across the entire wall surface incl abutting partitions and floor thicknesses and fully accessible so inevitable hairline cracks can be filled as they occur. The latter means you can't rely on external render to do it, if it's hidden under EWI. Don't expect EWI to form airtight layer. It's a problem.

Lay breather felt flat over the ERI, lay downslope battens over that (long-screwed thro the ERI) and your slating battens across that. Alternatively, downslope battens first, breather felt draped over that (i.e. sagging between the battens) and your slating battens across that - all normal stuff.

Strongly fix noggings between the stub rafters, which will run across and bear on the extg rafters (thro the OSB or in your case T&G sarking boards).

We are slightly out of sync there, with editing and what-not.

I really was hoping that there would be a way to make the EWI system do the air tightness. I have heard people talk about an initial 'parge' coat of some special stuff but I can't find details. I cant make the house airtight internally, it would just require too much destruction.

There must be a way. Externally air tightening a house avoids all the fiddly bits which are bound to fail with time.

Thanks

Posted By: fostertomMany people use internal wet plaster as the airtight layer on masonry - but a challenge to link that up with roof membrane - and it has to be unbroken across the entire wall surface incl abutting partitions and floor thicknesses and fully accessible so inevitable hairline cracks can be filled as they occur. The latter means you can't rely on external render to do it, if it's hidden under EWI. Don't expect EWI to form airtight layer. It's a problem.

Hmm. I am primarily counting on the EWI to form the airtight barrier. I believe that an external basecoat with the insulation boards glued to the basecoat and taped/glued to each other would for a pretty effective airtight layer, even with hairline cracks in the basecoat. using some form of mastic to jon the basecoat to the window/door frames should cover that area.

In this scenario, do you really think that hairline cracks in the external render would make much difference?

Note I am also planning internal wet plaster, again sealed to all openings as a backup. I wasn't planning to try to seal behind the electrical boxes though.

Some kind of variation on that, yes, James. The trouble is working out which would be best. I still think there is a possibility my original idea would work. 200mm of EWI coming up and joining to 200mm of roof insulation. the air tight membrane could be on the underside of the roof insulation and the 'parge' coat of the EWI. Then just make up a floating eave attached to a sheet of OSB/ply. But this up to EWI and attach through the whole system to the OSB/rafter ends under the insulation. The batons running down the rafters (on top of the insulation) would t hen continue down the rafter stubs and help secure them.

I reckon this could be strong enough?

No I mean the OSB attached to the rafter ends where they were cut off at the wall edge. I would envisage putting similar fixings through as you would need to screw the batons (running with the rafters) through the insulation onto the sarking. In essence the top part of the EWI would be sandwiched between two pieces of OSB/ply. I would expect that the way the batons would cantilever the whole unit the forces on the EWI would essentially be compression forces an I would expect it could hold up to this.

What do you think?

Hi Patrick, Did you considered putting new short rafters on top of the ply?

Still lots of bendy sag potential, looks like 400mm horizontal cantilever, so the EPS will sooner or later end up taking lots of vertical crush, if it doesn't de-bond from the wall face first.

Not so much the deadweight - that's trivial beside the snow load, ladder load, wind load/buffeting that shouldn't be underestimated during a long life! I'd really beef it up, make everything solidly supported rather than relying on squishy materials longterm.