Green Building Forum - Best Spec for EWI retrofit Thu, 11 Jun 2026 19:22:53 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102261#Comment_102261 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102261#Comment_102261 Sun, 13 Feb 2011 01:18:05 +0000 pmagowan
Most of my walls are 50cm wide random stone and mortar, the newer parts are 28cm wide block. I cant get the cost of the different EWI systems online. I have calculated I will need approx. 1000m2 so it will not be cheap no matter what. Which system is best and what do they cost for materials? (i know, subjective!). Which render etc?

I think the best system is the continuous insulation and air tight membrane, with cantilevered eaves (see dia). Do I just mechanically fix the EWI or do I also use adhesive? Is the final render airtight and can it be joined with the airtight membrane on the roof or is it the adhesive coat between the wall and the EWI that performs this function?

Can I use the same insulation board on the roof, above rafters (as in diagram) or is their a different type? Looking on the web it looks like the whole system is fiddley but not technically difficult. I was thinking that getting scaffold up and a few handy people you might be able to keep the costs down. Any one with any experience in this?

I have the surveyor coming on wednesday so I would like to be prepared and have an idea on spec. I estimate my internal area to be about 800m2 so which MVHR and at what cost? I have really good access to all rooms for ducting so I should thing this would be an easy DIY. Also I will be doing ground work so underground inlet would be easily installed.

I am using a shareware CAD program for the mac to draw up plans and detailing. If i use local builders then I will need to be very clear on what needs done as I dont think they will be used to this system.

Thanks again everyone
Paddy]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102285#Comment_102285 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102285#Comment_102285 Sun, 13 Feb 2011 10:57:28 +0000 jamesingram 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]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102293#Comment_102293 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102293#Comment_102293 Sun, 13 Feb 2011 11:18:52 +0000 fostertom http://thinjoint.com/ is v helpful as supplier/supervisor (not Wetherby), will travel far from base.

Same material can be used for both - but 200-plus thick on masonry walls, only say 100 thick over rafters (or studs) because there it's presumably only topping up between-rafter/stud insulation, preferrably blown-in cellulose fibre (old newspapers) http://www.excelfibre.com .

Gd idea to OSB over your rafters/studs (can be yr airtight roof membrane) and fix the ERI (external roof insulation) to that nice flat surface.

Subject to condensation check, no need for vapour membrane inboard of the insulation, provided you're using low(ish) vapour resistance stuff like EPS (ordinary white EPS (Expanded polystyrene) or grey 'Platinum' EPS - not to be confused with coloured XPS (extruded polystyrene)) or wood fibre http://www.natural-building.co.uk . Given low(ish) vapour resistances throughout, you can go for a 'breathing' construction in preference to one that has a strong inboard vapour resistance.]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102294#Comment_102294 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102294#Comment_102294 Sun, 13 Feb 2011 11:23:36 +0000 jamesingram 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]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102298#Comment_102298 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102298#Comment_102298 Sun, 13 Feb 2011 11:44:03 +0000 fostertom Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102306#Comment_102306 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102306#Comment_102306 Sun, 13 Feb 2011 12:25:02 +0000 pmagowan 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]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102308#Comment_102308 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102308#Comment_102308 Sun, 13 Feb 2011 12:38:17 +0000 jamesingram also posted eleswhere on forum
http://www.viking-house.ie/timber-frame-warm-stud.html
some more costing info here
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3025&page=2
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3025&page=3]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102311#Comment_102311 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102311#Comment_102311 Sun, 13 Feb 2011 13:08:29 +0000 fostertom
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.]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102312#Comment_102312 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102312#Comment_102312 Sun, 13 Feb 2011 13:09:36 +0000 pmagowan I dont know how I missed viking houses design in all my searching. I think it must be because it is timber frame, i just skimmed by. Do those rafter extensions not produce a thermal bridge or is this negated by the between rafter insulation? I was looking more at standard batons over the insulation, helping to hold up the eaves which would also be attached to the EWI somehow. I was still thinking of having all the insulation on top of the rafters.]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102314#Comment_102314 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102314#Comment_102314 Sun, 13 Feb 2011 13:25:45 +0000 pmagowan
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]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102315#Comment_102315 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102315#Comment_102315 Sun, 13 Feb 2011 13:30:36 +0000 tony Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102317#Comment_102317 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102317#Comment_102317 Sun, 13 Feb 2011 13:35:38 +0000 pmagowan Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102318#Comment_102318 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102318#Comment_102318 Sun, 13 Feb 2011 13:38:17 +0000 pmagowan Thanks]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102319#Comment_102319 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102319#Comment_102319 Sun, 13 Feb 2011 13:39:54 +0000 fostertom Posted By: pmagowanI put the insulation directly on to themSounds 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]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102321#Comment_102321 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102321#Comment_102321 Sun, 13 Feb 2011 14:02:22 +0000 pmagowan
I don't know about the air-tightness of the walls. I was hoping I could incorporate it into the EWI somehow. Do you have ideas?

So the airtight membrane could go in pretty much any of the obvious locations and then I also need a breathing membrane under the slates. Would this need to be held away from everything between batons and counter batons? (too many membranes, my head is fried).

I am still getting my head around the hit and miss, idea. I had thought to directly overlie so that you could screw into the sub rafters giving good support. The only way you could do this without bridging would be my original idea of the rafter stubs being thin (essentially batons) and overlying the insulation. Maybe this would not be strong enough! What holds it up in your example? Is it the counter batons over the top, as well as attaching to the sarking and with a noggin at the eave?

I have not even though about the verges yet!

Thanks
Paddy

PS Thanks for the eaves idea, that has just saved a few quid.]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102323#Comment_102323 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102323#Comment_102323 Sun, 13 Feb 2011 14:13:10 +0000 fostertom
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).]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102324#Comment_102324 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102324#Comment_102324 Sun, 13 Feb 2011 14:16:32 +0000 pmagowan
Do you have anything up your sleeve for the walls and their air tightness? I was hoping the two could be joined together like the whole house wrapped in cling film and with a big wooly jumper on.

Thanks]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102325#Comment_102325 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102325#Comment_102325 Sun, 13 Feb 2011 14:40:19 +0000 pmagowan
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]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102328#Comment_102328 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102328#Comment_102328 Sun, 13 Feb 2011 15:50:19 +0000 jamesingram I think by cutting back rafters flush with wall plate as Tom suggested bringing OSB airtight barrier on top of rafters down to meet external wall behind EWI and seal this join with plate mesh and parge render ?

What the cantilever bit re. stub rafters ? got any drawing/pixs]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102330#Comment_102330 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102330#Comment_102330 Sun, 13 Feb 2011 16:17:06 +0000 MarkBennett 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.]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102331#Comment_102331 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102331#Comment_102331 Sun, 13 Feb 2011 16:51:28 +0000 jamesingram Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102332#Comment_102332 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102332#Comment_102332 Sun, 13 Feb 2011 17:21:17 +0000 pmagowan
I reckon this could be strong enough?]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102334#Comment_102334 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102334#Comment_102334 Sun, 13 Feb 2011 17:31:42 +0000 fostertom Posted By: pmagowanmake up a floating eave attached to a sheet of OSB/ply. But this up to EWIDo you mean a sheet of OSB/ply laid flat on top of the ERI? It wd have to be pretty strong - like 25 ply or more - because there's considerable cantilever carrying wind/snow/ladder loads as well as deadweight, and it wd put all that load onto the top outer corner of the EWI/ERI. I think it wd have to be solidly supported at that top EPS corner, or wd crush it in time, leading to eave sag. Still, it's a possible alternative to stub rafters - the downslope battens wd indeed help.

Either way, the length running inboard of that point shd be about double the cantilever length - that wd be a large girth of 25 ply! - presumably laid over EPS 25 thinner than the upper roof.]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102343#Comment_102343 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102343#Comment_102343 Sun, 13 Feb 2011 18:49:44 +0000 pmagowan
What do you think?]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102346#Comment_102346 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102346#Comment_102346 Sun, 13 Feb 2011 19:07:10 +0000 fostertom Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102364#Comment_102364 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102364#Comment_102364 Sun, 13 Feb 2011 22:35:56 +0000 Viking House Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102366#Comment_102366 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102366#Comment_102366 Sun, 13 Feb 2011 22:39:44 +0000 jamesingram how about something like a wall plate strap under or on top of down pitch batton to strengthen ?]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102368#Comment_102368 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102368#Comment_102368 Sun, 13 Feb 2011 22:48:48 +0000 fostertom Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102372#Comment_102372 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102372#Comment_102372 Sun, 13 Feb 2011 23:39:00 +0000 pmagowan Paddy]]> Best Spec for EWI retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102374#Comment_102374 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6967&Focus=102374#Comment_102374 Mon, 14 Feb 2011 00:11:33 +0000 fostertom