How to prevent condensation & mould

This article was originally published in issue 18 of Passive House Plus magazine. Want immediate access to all back issues and exclusive extra content? Click here to subscribe for as little as €10, or click here to receive the next issue free of charge

“What can I do about the mould on my walls and ceiling?” I was asked by a member of the audience at a recent public meeting. My first response was to ask when it was built. Had it been a new house, or a recently refurbished one, my answer would have included a suggestion that they seek legal advice.

This is the regulatory minefield we, as designers of buildings or retrofits, now walk.

So the topic I wish to explore today is how to ensure that we avoid liability, should mould appear in any of our buildings. And yes, I am constructing a legal defence, because I believe that a legal defence is going to be required, particularly in Ireland.

We live in a culture where mould in buildings is normalised. That culture tells us that it’s the occupant’s fault: they’re drying clothes on the radiators, or they are taking baths and not opening the windows, or they’re not turning on the central heating. Any number of excuses are offered, but the attitude persists that it is definitely not the designer’s fault. That culture is changing.

There are two main causes of mould in otherwise healthy buildings: bad fabric design, and bad operation/use. The designer of a building — be that a qualified building professional, or an unqualified person acting in that role — is responsible for the first of these. The requirements of building regulations on both sides of the Irish Sea are clear, and derive from the same international standards. BS 5250:2011 states: a surface temperature factor of not less than 0.75 is considered to be sufficient to avoid mould growth, given the range of conditions in UK buildings and the UK climate (p86).

The means of implementing the standard is very different in each jurisdiction, with assigned designers carrying significantly greater risk in the Irish Republic as a result of the insured self-certification regulatory model.

The Irish Technical Guidance Documents are also less proscriptive and more focused on ISO Standards to define surface temperature factor compliance.

In the UK, Approved Document C seeks to establish minimum acceptable U-values for planar elements as a means of satisfying the requirements and use of Accredited Construction Details as a means of limiting surface condensation at junctions. In Ireland, there is a national register of thermal modellers who are there to certify bespoke constructions beyond the scope of ACDs, or manufacturers’ certified details.

The surface temperature factor defines the limit of the building designers’ responsibility.

Designers should embrace and apply it, as it gives them a clear way to limit their liability for mould, should it appear in one of their buildings. Unfortunately, this has yet to happen.

Few building designers have heard of surface temperature factor, fRsi, let alone how useful it may prove to be in protecting their professional indemnity cover. This must change, preferably before a change is forced by a wave of damages awards secured through the courts. Earlier this year we saw a claim of €52,000 (£45,000) settled in advance of the court for health impacts and personal damages caused by mould in one Dublin City Council maisonette. This sum was no doubt dwarfed by the need to rehouse the family, refurbish the offending dwelling and pay legal costs. It is not hard to calculate that the total liability accruing in such a case could be as much as €150,000 (£130,000).

Affinity Solution Ltd’s 2013 study of 102 retrofits, representative of 76% of English housing, indicates that incidences of mould, damp and condensation following retrofitting far exceed the baseline English House Survey’s 6% incidence. The implication is that mould risk may actually be increasing as a result of energy retrofitting. Reducing air leakage, insulating the easy flat bits of walls, floors and ceilings but failing to address thermal bridging (the grant-aided solutions favoured by both governments) increases the risk of mould forming at the difficult junctions.

This approach actually drives condensation towards the weakest points in the construction.

And those points are predictable: they are where the surface temperature factor is less than 0.75.

It is time to embrace fRsi, time to learn what it means in practice, and how to manage it.

Designers may even begin to sleep soundly at night, content in the knowledge that they have done their bit to prevent mould forming in their buildings. Others have their role to play too, but so long as you can point to a temperature factor calculation showing compliance, the design will not be at fault.

So what is a temperature factor? Temperature factor is a deceptively simple concept perhaps best explained by a simple example. It’s a frosty winter’s night and the air temperature outside is 0C. It’s toasty warm inside with the ambient air measuring 20C. The thermometer hanging on the inside of the external wall reads 16C and the wall feels slightly cold to the touch.

The surface temperature factor is established by dividing the measured surface temperature (16C) by the difference in temperature between the inside and outside ambient air (20C). This gives us a surface temperature factor, fRsi = 0.8. This exceeds the minimum acceptable, or critical, surface temperature factor, fRsi = 0.75, which is required in most buildings to ensure that condensation will not occur on a surface (if the building is being operated correctly). In some buildings, like swimming pools, a higher temperature factor is required to meet the standard.

So, you might well ask if all it takes is three on-site temperature measurements, what’s all the fuss about? If you can arrange for exactly 0C outside for a few days and ensure exactly 20C is maintained by the heating system over the same time period, then your on-site surface temperature measurement will be reasonably accurate.

But real life is not like that and buildings live in a dynamic environment, internal and external temperatures fluctuate, sunshine hits the façade. Similarly, thermal conductivities and thermal mass influence heat flows through the construction, in three dimensions. Consequentially, we have to resort to modelling heat flow using digital simulations. You have now entered the specialist world of the thermal modeller, and may need to buy in their services to prove compliance.

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