In previous posts by our CEO Steve Hodgson (The principles and the pitfalls) and Technical Manager James Berry (What does this mean for Ventilation?), we have highlighted the PCA’s concerns about the Government’s Green Homes Grant scheme. We think it is a great boost to the construction sector as a whole and has extremely worthy objectives – jobs, reduced heating bills, carbon reductions etc.

Caution with traditional buildings…

But when a lot of money is thrown into a scheme like this and the sector has a narrow window of time to take advantage of it (Grants only available September 2020 to March 2021), it’s hard not to think of and anticipate a ‘Gold Rush’ mentality taking over. This may lead to failures, seen previously with other similar schemes, due to a lack of appropriate pre-installation checks and a failure to look at buildings holistically to ensure their functional design has is not been compromised.

In this blog, we attempt to explain in a bit more detail the importance of pre-installation surveys and how well-intentioned, energy-efficient upgrades can upset the fine balance (present by-design in all buildings when new) which ensures building timbers remain dry and free from mould/decay.

As we say, we think the Green Homes Grant scheme is great, but we hope all those involved can and will learn from past mistakes and ensure that buildings wrapped in a duvet still have adequate air-exchange rates and are protected from the elements!

The ‘green’ opportunity and the timber decay trap

If you are in the construction industry at the moment there are many reasons to be cheerful (cue ‘Ian Dury’). Firstly, despite hardships suffered due to the pandemic, the post-lockdown situation seems quite buoyant for the sector as a whole or, if there is a slump, it ought to be short-lived as house sales seem to be booming according to a recent RICS article and the news.

Secondly, the Government’s announcement that an extra £3 billion pounds will be invested in their Green Homes Grant scheme (up to £5,000 per household for energy-efficiency upgrades) is forecast to create new work/job opportunities for Trustmark-Approved builders etc., specialising in ‘green’ heating systems and various forms of retrofit insulation.

We definitely want this positive initiative to be successful for all involved but also feel justified in saying “let’s be careful how we do this”.

Although most of our PCA members do not specialise in areas like insulation, heating etc., they do know a thing or two about defect surveys and how to protect buildings from dampness and the consequential problems arising from a failure to do so, i.e. mould, timber decay and/or an increased risk of wood boring beetles.

So, let’s take them one at a time so we can see what the pitfalls of poor surveying/specification might be.

Don’t insulate over pre-existing building defects!

This one is easy and somewhat self-evident and fully embraced by all the retrofit insulation system manufacturers within their BBA certificate conditions (Design Considerations) etc. If there are pre-existing defects e.g. for walls: cracked or perished mortar joints, rising or penetrating damp, ineffective or poorly designed rainwater goods etc; then it is essential to have these rectified before proceeding. Ditto for floors where the system manufacturers highlight the importance of DPCs/DPMs and sub-floor ventilation. But we believe the pre-installation checks should be

• a) more extensive (include wall ties, timber defects, etc.) and
• b) be carried out by experienced and qualified surveyors (CSRT).

The reasons for this are fairly obvious – most solutions to damp migration and/or timber decay in buildings are easy to rectify when they are accessible, so having to uninstall an insulation layer to access a repair which should have been identified and resolved at the outset just adds cost and complications.

To summarise, we think the BBA Design Considerations are a good start but, for this scheme, should be expanded to require a broader assessment of Building defects to ensure there are no unintended consequences (see below) or missed opportunities to resolve long-standing defects before retrofit insulation systems are installed.

But let’s also look forward and assume the building is in good condition (or has been repaired) and the system is ready to be installed. Are there any other potential traps? Here we consider three potential impacts on the building or its occupants from alterations to the moisture balance unintentionally caused by poor design/installation practice.

Poor ventilation = Condensation and Mould

Forgive the simplistic overview, but condensation is a consequence of high atmospheric moisture levels and low surface temperatures (PCA Code of Practice for Investigation and Provision of Ventilation in Existing Buildings) and is generally designed-out of buildings through measures described in the Building Regulations Part F. But retrofit insulation systems can have unintended consequences and cut-off normal air movement in roof spaces/sub-floor voids (blocked air bricks, reduced ‘natural’ draughts).

Even a small increase in relative humidity (RH) over time can lead to timbers absorbing moisture from the atmosphere (wood is porous – hygroscopic – and will always be in equilibrium with the air around it). This is discussed below, but surface condensation usually arises due to rapid changes – when the ‘dew point’ is reached before such an equilibrium can be established.

Where there is condensation we find mould follows (on any ‘organic’ surface); at the very least this is an indicator of damp conditions that, left long-term, can be seriously detrimental to the wood. Also, we should focus on the mould spores produced which can circulate on air currents, cause irritation to airways and an undesirable ‘musty’ smell.

Finally, in chronically damp-air environments, condensation can accumulate at low points of rafters etc., and start to cause, with no other external moisture ingress, local pockets of decay. The wider implications of condensation/mould in occupied parts of the building have already been discussed before (What does this mean for Ventilation?).

Penetrating Damp = High decay risk

Penetrating Damp is the most significant likely cause of timber decay which might arise from poorly designed retrofit insulation. Traditional buildings tend to have floor joists built into walls both for support and to act as bracing (in tension), i.e. they are structurally essential for the integrity of the building and highly vulnerable to moisture penetration if the normal wetting/drying cycles of the masonry are interrupted.

Clearly, if energy-efficiency measures result in such a problem arising the designer of the system would be liable, but in practice, these impacts may take 5-10 or more years to manifest themselves in the form of a full-blown wet rot/dry rot infestation – by which time the damage could be very extensive and the contractor long gone.

Whilst we are on the subject of dry rot, lets not forget that it is a constant threat to buildings in our wet climate and many don’t realise that the only reason it is less common than wet rot (where sufficient water & wood is present) is that it requires poorly ventilated voids in which to grow over timber/masonry surfaces (see above!). So, might we see dry rot occurrence increase?

Atmospheric moisture = increased activity by wood borers

And not forgetting wood boring beetles. We know that all softwoods in buildings, even ‘dry’ buildings, are vulnerable to attack by e.g. common furniture beetle, and the risk and significance of this is increasing since softwoods used in construction today tend to have more sapwood as a proportion of the cross section. But, old or new timbers, it is also true that very dry wood (Moisture Content, MC < 15%) is significantly less palatable to the beetles (who, in the larval stage, have no other source of moisture other than the wood itself).

So, when the building is designed the assumption will be that the timber’s normal resting MC will be typically 14 – 16% (reflecting an atmospheric RH range of 50 – 60%), but if the average humidity in any voids increases to around 80% and is sustained, the resting MC in the wood will be closer to 20% – sub-optimal for wood decay but very attractive to wood boring beetles!

In roof spaces this can be seen fairly easily as the adults emerge in the summer but, with floors, such a problem can go unnoticed until carpets get changed for example, by which time a lot of damage has already happened and the floor may need to be replaced.

How can the PCA help?

PCA members include many experienced and qualified surveyors who can assess buildings in respect of weather-tightness, and who have an intimate knowledge of traditional building design and materials.

We strongly recommend that any company thinking of registering to provide services eligible for the Green Homes Grant scheme should always seek advice on these issues – as the functional performance of insulation systems may rely on the dryness of the external masonry and/or its impact on ventilation, both passive and active systems.

In general we believe it is useful to consider in each and every case whether there is a benefit or indeed a design requirement for these details to be incorporated in to any Grant application, and we believe TrustMark are considering this matter prior to the scheme going live for enquiries.

Controlling penetrating damp, rising damp, repairing timber defects/or upgrading timber durability in-situ together with assessments of the ventilation requirements of the building are all matters that need to be considered/assessed at the design/pre-installation stage, and PCA members are well placed to provide the competencies necessary.

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