You may remember I installed external wall insulation (EWI) back in 2014 and unfortunately small gaps opened up between the insulation and the fenestration soon after completion. These gaps were sealed with silicone and it’s now been 3 winters and there’s been no further movement so it would seem everything has stabilised. So what happened and what can we learn. Firstly, the training provided by the system supplier was woefully inadequate (something I suspect the whole industry could do better on) and compounded by a refusal to make a pre commencement site visit which considering that neither myself nor the installer had ever installed this or a similar system was indeed unwise. Secondly (as seems to be common practice in the industry) the installer used the same contractors who undertake the rendering for the installation of the insulation and as this requires quite a high degree of craftsmanship, especially with the T&G woodfibre boards this became a painful process to get right. On a recent job for a client we had to throw the specialist EWI contractors off the site and decided to use carpenters for the insulation but this causes problems for the guarantee and raises another issue the industry needs to get straight. Thirdly, I pushed hard for a pull out test but neither the supplier or the contractor was prepared to do one which was unwise as the test performed after the works suggested that it was quite possible that some of the fixings may well have been inadequate and while in general this may not be outside the margin of safety for the system as a whole, it could be an issue locally and may well have been the cause of the excessive movement of the narrow strip between the door and window. Surely it’s worth spending a few extra pennies to have longer fixings to avoid this risk? Fourthly, the APU beads which form the first seal between the fenestration and insulation were specified by the system supplier but were unable to accommodate even small differential movement and so were not fit for purpose and indeed it may be wise to not fit them at all but I have not yet got to the bottom of this one, but the industry needs to – there’s no point fitting APU beads if they fail and the gaps need siliconing! The possible issue of highly damp walls causing the woodfibre to swell / move still hasn’t been resolved as the system supplier would not provide any data on the moisture performance of the insulation or assist in testing the installation – surely, if we are to retrofit our old building stock then EWI is a critical part of the solution and we need to ensure robust installations. However, the installation is a success and is working very well – it was just a bugger to make happen!
It’s been 3 winters since the works to the rear of the house were completed and the annual saving on the heating bill has been around £100 as predicted. An average (less eco minded) homeowner might see this figure double so let’s say £200 pa. Comfort has improved dramatically, especially the bathroom which is a mini ‘Passivhaus’ with high levels of insulation and airtightness with triple glazing and a single room extractor with heat recovery, and rarely drops below 18 degree even overnight in winter. How much is this extra comfort worth? Tricky question but let’s say £1000 annually then the total benefit is perhaps £1400 pa. If the incremental cost for the energy saving element over simply renovating is perhaps £10k then the payback becomes surprisingly quick. It’s definitely been worth it and becomes more so every year, and if one can separate personal gain from the benefit to the greater good, then even if we sell before we reach the payback period the interventions have been very worthwhile for mankind!
So the retrofit has now had a couple of winters and provisional figures suggest a saving in gas usage for space heating of around £100 a year, but let’s assume typical user might well use double the heat and so perhaps save £200 a year. The total works cost £26k so the payback is over 100 years (the sort of figure you get for double glazing) but the story is much more complicated. The rear elevation was failing with damp walls and crumbing windows so something had to be done. If I had simply re-rendered rather than insulating and replaced the windows with double glazed units then the cost might have been say £16k, suggesting an uplift for the energy improvements of the retrofit of perhaps £10k (still a 50 year payback). But there’s the less quantifiable benefits, such as comfort and air quality. Even though the retrofit was only to the rear elevation, the whole house retains its heat longer and is dramatically less draughty. The bathroom also has a single room heat recovery extractor and is basically a micro near – Passivhaus and the temperature is around 18 degrees when you wake up on winter’s morning. EWI and fenestration might last say 20 – 25 years, so only half the payback period if based simply on monetary terms. What about the increase in the value of the property? Very difficult to assess, but perhaps £10k. So in all it’s definitely been worth it, but more from a comfort, rather than financial, perspective. There are a lot of poor quality EWI jobs around and the industry desperately needs to address a multitude of issues (a subject for another time), but if installations fail after a few years or even 5 – 10 years then the sums really don’t add up and it can hardly be called ‘sustainable’ or indeed sensible. It does make a clear case for high quality kit, well designed and installed if we’re even to come close to being able to call such works ‘sustainable’. As an aside, the issues with the EWI mentioned in earlier blogs are still not resolved but I’ll address these another time.
As a temporary fix to stop rain entering the Diffutherm woodfibre insulation to the gaps at the jambs and head, silicone has been carefully inserted between the aluminum cladding to the fenestration and the back face of the plastic APU bead, making use of the weather seal. Obviously not a permanent fix, but should buy time to get to the bottom of the issues. Very disappointingly,the installer Downs Energy and the system manufacturer, NBT have now both walked away form the job and it my require the courts to get a resolution.
It is critical that the training, design, detailing and installation of an EWI system is well considered otherwise failure will be built in, as seems to be the case in many installations. Specifically the detailing around openings and the correct understanding of the weather detailing at jambs and sill is vital. There are many systems and solutions but a basic APU bead (if fitted) may well come away from the fenestration and then the expanding foam seal becomes the weather protection. It is thus critical that this foam seal is correctly specified and located tight to the edge of the insulation board otherwise driving rain can enter the structure. I welcome a discussion on this issue.
After only 6 months over winter there are issues with the EWI where gaps have opened up between the fenestration and the insulation, especially at the jambs. The exact cause is still being investigated but has anyone else had any issues with EWI, whatever the issue/system/insulation type, but specifically woodfibre/Diffutherm? Please get in touch.
Difficult to determine exact costs but the EWI was about £11k plus perhaps another £4k for associated works like reworking the gullies and soil pipe so a total of around £15k. It was a small job (32m2) but this makes the meterage rate around £450/m2! However, this was a Rolls-Royce option with woodfibre, thick insulation, below DPC, proper detailing, bespoke eave etc etc. You could easily add another 20% to these figures for project management and professional oversight. The windows, fitted would be about £9k on the open market. Luckily the Government GDHIF knocked £6k of this figure otherwise it would be prohibitively expensive.
Nanopor top coat looks great and you really can wash down it with a hose pipe. The insulation ‘step’ (about 160mm) between the property and the neighbour’s is barely noticeable. Maintaining the Victorian openings and aspect ratio of the first and second floors has worked well.
The Internorm triple glazed inward opening windows are performing as expected. Great flexibility with options to tilt (inwards at the top) for ventilation in the warmer months and the full turn for fully opening the window for access and cleaning. There’s also a trickle setting (with the handle set at 45 degrees) allowing background ventilation by just allowing the window to sit off the seals. The benefit here with retrofit is that rooms can be ventilated traditionally with the window providing trickle ventilation but if an MVHR is fitted at a later date then the window can be closed and there’s no trickle ventilator to solve either aesthetically or thermally. Great to wake up on a cold winters day and have condensation on the outside of the triple glazing – a sure sign that very little heat is being lost through the glass! You can see from the image that the condensation evaporates around the edge of the glass first where the spacers/frame are less thermally efficient than the glass itself. Great to see all the theory become reality.
An off the shelf option for rain water provision at eaves and verges with retrofit EWI is available from EWIPro
To reduce cold bridging at the eave the EWI needs to extend as high as possible and connect with the roof insulation. Luckily the ceiling rafters extend over the walls allowing insulation to meet the EWI. A bespoke gutter was specially designed and fabricated in one piece aluminium 4.7m long and fitted to the timber facia. Leadwork was carefully dressed over neighbour’s gutters. Note that loft conversion at a later date will address the roof ventilation.
The devil’s in the detail and it’s no good if the system fails because water gets in where the fixings penetrate the render. NBT specified Fisher Thermax fixings through the woodfibre insulation. The drill bit tends to wander all over the palce in the woodfibre because it’s so ‘soft’ but the Thermax worked well. Worth using stainless steel and note the plastic spacers on the RWP. Need lots of silicone carefully placed to seal back to the Nanopor render.
Nanopor top coat is now on and looks fantastic – photos to come when the scaffolding is down. Here’s an image of the sill, hopefully all the effort and attention to detail will make it robust for many years to come…
Woodfibre insulation boards need to be carefully laid out, cut accurately and detailed around windows and are also tongue and groove and so require a surprisingly high level of skill to fit properly, such as provided by a skilled trade such as a carpenter. Woodfibre is much more difficult to fit than EPS which is square edged and gaps can be simply foamed. I would make sure any woodfibre EWI installation uses appropriately skilled trades and don’t assume the regualar EPS system installers can handle a woodfibre job. We also had a bit of a run around with the number of fixings and locations required for the NBT Diffutherm system so worth getting this resolved upfront. As Brighton is Zone 3 for rain and wind, NBT required 6 fixings per board with 3 pe board along any exposed edges. Never did get to the bottom of weather a pull out test was required and who should be making this call. Also consider which fixing is suitable once the substrate is known. Nice to use wood though…
The works became a bit unstuck during the installation of the NBT Diffutherm insulation. The SVP, windows/sills and bespoke gutter were all located, sized and ordered based on a 200mm thick layer of insulation. All these items rely on the final finished surface of the render being where it should be within a tolerance of perhaps 10mm. The first issue was that the levelling coat, which was supposed to only take out any low points, became a full 20mm thick parge coat, which sured up the brickwork but moved the whole insulation/render out by 20mm. The second issue was that the brickwork wall was not plumb in any direction and the levelling coat had levelled locally but not across the whole facade so there were still high points, again causing the insulation/render to be a further 20mm further away form the wall than as designed in some locations. This meant that the carefully located SVP/Gutter and sills were now around 40mm closer to the render than designed and the sills especially must have a drip of at least 30 – 35mm for weathering. The solution was either to relocate/replace the SVP/gutter/sills or reduce the insulation by 40mm. In the end we manually removed 20mm form the 100mm Pavatherm base layer (the boards are glued in 20mm layers and this was tricky but not too bad) and swapped out the 100mm Diffutherm for 80mm which cuased a delay and was a real hassle. The answer is to bear this in mind when detailing, survey the facade and keep a close eye on the knock on effects of each stage! Example of a completed sill detail with the correct overhang:
Windows are massively important for Passivhaus and sills seem to be disproportionately important for retrofits, especially for woodfibre insulation which needs very careful detailing to avoid water ingress. If water is going to find it’s way in then the sills are the most likely entry point. Sills are Bug aluminium fitted directly to the Purenit packing which Internorm use for transporting the windows but offers good thermal performance and can be fixed to, a special material like Foamglas. Strange ‘Pacman’ butyl corners ensure no driven rain gets past the end caps. The plywood liners are proud of the levelling coat and require blocks to support the the brackets. Silicone between the windows and the tops of the end caps complete the weather tight construction. Quite a lot of work and careful detailing to ensure durability.
The garden walls both abut the main building allowing moisture to migrate into the building and are also a terrible cold bridge. One garden wall was removed and replaced with a fence but the other side had to be retained on request from the neighbour. To minimise the impact, the wall was rendered in lime to allow it to dry out and a lead saddle was installed again to keep the local part of the wall near the building dry. The lead was dressed into the wall at an angle to assist in water running off away from the insulation (on the right in this photo). You can also see the injected vertical DPC, the holes of which will be covered by the insulation
The brickwork was is such a poor condition and out of level that Baumit MP69 render system with reinforcement mesh was applied across the whole elevation. The MP69 is lime based but does have some cement but has a similar breathability as pure lime with a mu of 10 – 15 so won’t impede the breathability of the wall as a whole.
The windows and door have been installed using plywood liners to allow the windows to be located outside the brickwork and in line with the insulation. This also allows easy installation of the airtightness tape. The fenestration by Internorm is triple glazed, triple sealed inward opening tilt and turn allowing the insulation to be wrapped around the frames and was supplied and fitted by the excellent CS-L
Just measured the moisture in some of the brickwork at 100% and the timber joists around 15% with some localised readings around 25% where the timber has rotten away… so this all ties in with the visual information.
The original Victorian external lime render and internal plaster was a breathable construction but the lime has been replaced with cement externally and gypsum internally with vinyl paints applied to both sides to really make sure that water vapour can’t move through the structure! Perhaps acceptable on a modern building but risky on a solid walled Victorian property with limited/No DPCs, dodgy eave details and garden walls etc abuting. Cracked render then allows moisture in, especially driving rain, which is then trapped behind impervious layers resulting in soaked brickwork. The real issue however, is the timber joists which in my case bear into the brickwork on the rear elevation. The brickwork to the top floor bedroom was soaked due to faulty rainwater goods over 7 years ago and has never dried out. Surprisingly the joist seem to have survived considering their prolonged exposure to damp brickwork but perhaps even though the house was very poorly built, the timber may have been much better quality than used today and/or the timber was able to wick away the moisture. Not the case for the timber noggins placed in the walls for fixing the skirting, which has disintegrated. The ground floor kitchen window had timber lintels installed probably in the 80’s/90’s and these have totally rotted through where they bear onto the brickwork. It’s all about moisture and being vary careful not to trap it within the building structure. Images below show the rotten timber lintel and damp brickwork (inside face)
Over the next few months I am renovating the rear of my house, adding external wall insulation (EWI) and replacing the fenestration. The house is a Victorian mid terrace, 3 storeys, 9″ solid brick walls and is unusual in that there’s no return/extension at the rear so it’s flat across the back making EWI much simpler (relatively!). The insulation will be 200mm NBT Diffutherm woodfibre with a lime render providing a breathable construction and will extend below the DPC. There will be 5 new windows and a door all Internorm triple glazed inward opening tilt and turn. All works will be airtight and minimise cold bridging where possible with a bespoke gutter to stradle the EWI. The scheme will benefit from a £6000 government grant under the Green Deal Home Improvement Fund (GDHIF), unfortunately now closed to new applicants. The next installment looks at the damage the cement render has done to the building fabric!