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Retrofit: How to achieve the best thermal performance, with the lowest moisture risk

With Glasgow hosting the all-important COP 26 UN climate summit, the city has been keen to show the world what it can do to tackle emissions – including, its achievements decarbonising its buildings.

One of the projects to receive most attention – attention including an item on the BBC, a place in a COP themed international exhibition organised by the UKGBC, and even congratulations in the Scottish Parliament – is a retrofit currently under way in one of Glasgow’s  tenement blocks.

Many of the city’s streets date from the era of rapid industrial development and shipbuilding in the late 19th and early 20th centuries. The block that is being retrofitted, in Glasgow Southside’s Niddrie road, is a ‘walk-up’ block around 100 years old, and typical of thousands in Glasgow (and many more across Scotland). It is very much a key part of Glasgow’s distinctive look.

Much of Glasgow is built with locally quarried ‘blond’ sandstone. But with the coming of the railways, Glasgow’s builders were able to access a handsome red sandstone brought by rail from quarries in Scotland’s South West. This stone was preferred by builders because it is more consistent in texture and more workable than the local stone. It forms the façade of the Niddrie Road flats, and many other Glasgow buildings of the same age.

The retrofit is being funded as part of a research programme  by the Scottish government with the involvement of Glasgow University. The building is owned by Southside Housing Association and the design team is led by John Gilbert Architects (JGA). Their ambitions for the energy retrofit of the building are not half-hearted. They want to achieve Enerphit, the Passivhaus retrofit standard.

The choice of Enerphit it is not just about cutting carbon, but equally importantly, improving the lives of the inhabitants. Energy poverty is a serious problem in Glasgow, as it is across Scotland – and indeed, the whole UK. Buildings often poorly insulated or not insulated at all, incomes for many people are low, and it is cold, wet and windy a lot of the time. With its challenging targets for energy use and indoor environmental quality, Enerphit is as much about occupant comfort and health as it is about deep emissions cuts.

Thus the challenge was – could this building achieve emissions reductions fit to meet the COP 26 zero carbon challenge, protect the occupants from cold and damp – all without erasing Glasgow’s distinctive character and history?

Successful solutions at Niddrie Road have the potential to be shared widely, and to uplift the performance – and the lives of the occupants – of thousands of similar buildings across the country.

Reducing the emissions of a historic building frequently meets up with the conservation concerns, and this job was no exception. For deep retrofit like Enerphit, there is no alternative to  insulation of all the walls, the roof, and usually the floor too.

External wall insulation is technically more straightforward and lower-risk. But it was hard enough to persuade the planners to allow external cladding even on the flank and rear of the building. Externally insulating the street facade was clearly going to be a no-no. However, the façade still needed to be insulated – which meant internal insulation.

If masonry is insulated from the inside it becomes colder – which in turn, may lead to moisture build-up, which may prove harmful to the masonry, or the insulation and/or finishes inside – a serious concern for the design team.

Greengauge were therefore called in, to investigate options for insulating the fabric, and guide the team to achieving the best thermal performance, with the lowest moisture risk.

The choice of internal insulation is important – some insulation materials, for example wood fibre, are vapour open, and also hygroscopic (ie, moisture absorbent). Water vapour can still pass out into the room to be carried away by the ventilation, limiting moisture build-up, and the insulation can also move localised damp away through  capillary action.

Greengauge and the architects were already in agreement that wood fibre was likely to be an appropriate internal insulation choice. The question remained though – how much was it safe to install?

A deep retrofit requires a lot of insulation – but as  Chris Morgan points out, the advice when internally insulating historic fabric is – use a relatively thin layer  because of the possible moisture risk.  With this in mind, John Gilbert Architects asked Greengauge to test the construction with 60mm, 80mm and 120mm of woodfibre internal insulation.

To work out the moisture levels at  that riskiest point – the cold side of the insulation where it meets the masonry, Greengauge used the WUFI software. This carries out a simulation, modelling conditions in a wall over a period of years, feeding in the annual cycle of temperature, humidity and rainfall from outside, and the temperature and humidity inside, calculating the movement of moisture by capillary action, absorption and vapour movement, and giving an indication of what is likely to happen inside the wall as the years unfold.

In order for WUFI to run that simulation, it needs to know what the wall is made of and what influences are brought to bear. Weather averages, rain, indoor temperatures, and the performance of proprietary wood fibre insulation and other commercial products, are well documented. But a key element in the wall is the masonry itself.

With a concrete block, you can ask the manufacturer for information. With 100-year-old quarried Scottish stone, that data is not readily available (WUFI is a German software, so while some stone types are loaded into the programme, they are from German quarries.)

Fortunately however, it is possible to get some idea of the stone’s moisture behaviour by testing it in the lab. This is something Greengage recommended to the clients, as Greengauge building physicist Olivia De Sousa Costa explains:

“Building materials vary so much, so we undertook some basic material testing – it is really helpful to give us the more confidence in our simulations.”

Using small cylinders of stone drilled from the walls of the building, Greengauge tested the stone by sitting it in water, to see how quickly moisture soaked in, and also tested how much water it would hold at its wettest.

Tests showed that all the samples – but unfortunately, particularly the red sandstone of the facade –  took up moisture quite readily. Water soaked up through the full height of the samples in the space of 24 hours. There are German sandstones with similar properties already loaded into the WUFI software, so these were chosen to stand in for the Glasgow versions in the calculations.

As well as subjecting the virtual wall to the known wind and weather of Glasgow, Greengauge always  add in some ‘just in case’ factors to stress test the design. “It it important to assume your building is not perfect – as buildings never are,” Olivia de Souza Costsa explains. Some ‘likely imperfections’ are therefore built into the simulation.

The actual 1920s mortar composition was not known for certain, so preliminary simulations were run with some different mortars, and the worst-performing (most moisture-absorbent) option was selected to be used for  subsequent calculations. The stone was also assumed to have cracks that allow some of the rain, to penetrate directly into the core of the masonry. And the construction, though aiming for full airtightness, is assumed to have some gaps that allow warm relatively humid air from inside to escape into the wall.

The WUFI simulation was run to predict conditions in the wall over a period of several years as it settled down after the retrofit. This showed that as expected the stone would generally retain more moisture after being insulated. However, the moisture content of the very outer millimetres of wood fibre, where it sits against the wall, did not reach a risky level.

The “cut off” above which wood and wood-based materials such as wood fibre insulation may be vulnerable fungal decay is considered to be 18-20% by mass. The modelling suggested that even with the thickest proposed insulation layer (120mm), moisture levels in the insulation would remain below 18% – and, if the wall is in good enough repair to prevent rain blowing into the structure, below 17%. (This confirms the benefit from repairing and repointing the walls – as is being done at Niddrie Road.)

So it looked like the wall and the insulation were going to be fine. However, there are also timber joists, set into  the stone to support the floors. This is a classic point of concern in internal insulation. The main part of the joist sits in the nice dry, and newly warm, interior of the building. But the ends  poke out through the insulation, into the cool and potentially damp stone wall.

Greengauge had to work out what  conditions the joists would experience where they sat – a few inches into the thickness of the stone – and determine whether this was safe for timber.

Again the actual timber – and particularly, its current condition in the wall – was not known, so for safety, a standard spruce (one of the less durable timbers) was assumed. For spruce that critical 20% moisture by weight is reached when the surroundings are at a relative humidity of 85%. Greengauge revisited the WUFI models, but this time, looked at humidity 100mm in to the stone – approximately how far a joist would penetrate.

With the 120mm internal insulation, even closest to the inside surface, humidity in the stone was above 80%. Further into the stone,  where the ends of the joists would sit, it was higher – in fact it sat at 100% for a part of each year. This was not OK.

Were the team going to have to use only a thin layer of insulation after all? Greengauge checked the humidity at the joist end zone with 80 and 60mm wood fibre. But even with the thinnest modelled layer of insulation – 60mm –  the stone’s humidity sat consistently above 85%.

There was a further option proposed by Greengauge which was the addition of a hydrophobic coating, on the outside of the stone. These coatings act a bit like a high-tech raincoat for the masonry: rainwater beads up and roll off the surface, rather than soaking in. But while the coating resists wetting by liquid water, it is vapour open, permitting moisture in the stone to dry out when the weather is better.

The WUFI model showed that with the hydrophobic coating on the outside, humidity on the inside of the masonry, at the joist ends, would be lower. With 60mm insulation, humidity levels pushed below 85% for most (not quite  all) – of the year.

In these circumstances, as the stone, and therefore the timber, would dry out across the summer each year, so this option was arguably acceptable. Pushing the insulation thickness up to 80mm the model showed that the joist ends timber would still be below 20% moisture for more of the year than not. This might be a feasible construction if  a mineral anti fungal treatment such as boron paste could be injected into the joist to increase the protection.

However, Greengauge acknowledge, hydrophobic stone treatments in particular are contentious in historic buildings, because the treatment is not reversible, and the impacts are still being researched, including by Greengauge Director Toby Cambray who is undertaking a PhD on precisely this topic.

So, rather than deploy chemicals and still have only 60 or 80 mm  of insulation on the facade (meaning the rest of the building had to be even more insulated, to compensate) JGA decided to remove the problem altogether, by taking the joists out of the wall and supporting them within the warm dry interior. This enabled them to revert to the preferred 120mm wood fibre. It also made it easier to eliminate potential thermal bridges and air leaks behind the floor edges, and simplified the whole insulation deisgn.

It also meant there was no need for any chemical treatment for the timber – an important additional benefit, the architects believe.

The calculations were very helpful in clarifying the decision making, Chris Morgan says. “The results from the modelling made me more confident that it was the right thing to take the joists out, and secure the client’s agreement to do this.”

Studying the WUFI results also  helps bring a deeper understanding of the challenges faced by a building standing up to the weather year after year – particularly when the results are a bit counter-intuitive, as  Chris Morgan observes. “WUFI reminds us that despite our conceptual map, where we think of  warm moist interior air being “the” moisture threat, in fact an awful lot of the moisture in a wall comes from outside.”

“It focuses your mind on issues like the guttering and overhangs” – essential ingredients if a building is to have a long healthy life.

Olivia de Souza Costa agrees. “Even in the south of England, a lot of water can enter a wall from outside – not necessarily what people expect,” Olivia points out.

Here at Niddrie Road, WUFI calculations confirmed the architect’s proposal of wood fibre as an appropriate system for internal insulation of the historic facade. It also highlighted the need to address the issue of joist ends. The client and design team were therefore able to take an informed decision: the expense involved cutting back the joists was necessary and justified, if the exposed stone facade was to be preserved as they wished. This way, the work is honouring Glasgow’s past while fitting its buildings for the future.

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