Retrofit Design for Moisture Open & Moisture Closed Constructions
Good retrofit design is about balancing the motivations, risks and solutions appropriate to each property. The designer has an important role in the process, to identify the right products to use and how they are best installed.
Understanding the sources of moisture, and whether a building has ‘moisture open’ or ‘moisture closed’ construction is key to a good retrofit. Unintended consequences will often result from a failure to consider the risks of moisture and where it is likely to form.
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BSi White Paper Moisture in buildings: an integrated approach to risk assessment and guidance
The best technical retrofit solutions seek to include insulation measures and ventilation systems in tandem. Both are critical to the way buildings manage moisture in order to keep the fabric dry and the internal environment healthy.
There are three main risks that can result from retrofit measures:
- Upgrading existing thermal elements will change the balance of heat gains and losses. With less heat escaping to the outside, the retained building fabric may now be colder as a result, placing it at greater risk of decay or deterioration, particularly with timber.
- A failure to anticipate where warm air will condense can lead to interstitial condensation becoming trapped with the construction layers.
- A warmer internal environment not adequately ventilated will suffer with high humidity, poor air quality and condensation risk. This can cause interior decay and deterioration of finishes.
This article will help explain the key differences between moisture open and moisture closed construction, how it affects the design process, drawings and specifications.
Definitions of Moisture Open and Moisture Closed
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p.17 Historic England – Energy Efficiency and Historic Buildings – Insulating Solid Walls
Energy Efficiency and Historic Buildings: Insulating solid walls | Historic England
A moisture open construction can be described as one where:
- the air tightness layer is on the inside face (e.g. the plaster) but does not act as a full vapour check,
- the building utilises natural materials (stone, clay bricks, timber, lime coatings or plasters),
- the fabric is designed to get saturated by the external weather and relies on a mixture of solar radiation and escaping internal warmth to dry itself,
- the insulation materials tend to be natural (mineral wool, wood fibre, hemp-based).
This describes older buildings or those of traditional construction that lack a cavity wall.
Moisture closed construction can be described as one where:
- the airtight layer is on the inside of the construction (either the wet plaster finish, full VCL membrane or taped board like OSB),
- one or more materials do not allow the outward passage of moisture,
- external rainwater is kept away from the internal skin via a separating cavity or rainscreen,
- it utilises highly processed modern materials, like polythene membranes, gypsum plaster, PIR or XPS insulation.
This description matches most modern buildings that have a cavity or typically anything past the mid-twentieth century.
Attempting to place all buildings into one of two boxes is fraught with difficulties and of course, exceptions exist. Modern buildings have layers that are vapour permeable, like breather membranes, and some older buildings may have been adapted to now include moisture closed layers which turn out to perform very well. Older buildings with modern extensions may have both solid walled and cavity wall elements.
As a designer, it is important you are tuned into the underlying principles of these approaches. Regardless of whether the building is old or new, a moisture open construction is designed to allow moisture to travel from inside to outside through all the constituent layers. A moisture closed construction will incorporate vapour barriers that seek to prevent moisture passing through. In a retrofit, your job is to match appropriate solutions to these scenarios.
Use these generalisations as your starting point, but if doubt remains, then further certainty can be sought using specialist hygrothermal simulations like WUFI or Delphin (using BS EN 15026). Work on a Listed Building would be an example where this approach would be encouraged to create a bespoke specification and avoiding harm as an intended consequence.
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Heritage Building Retrofit Toolkit p.35
City of London & Purcell
Managing Moisture Risk
When managing moisture risk in a property, the terms moisture open or closed have come to replace the term ‘breathability’ which relates to air movement more generally. Moisture open buildings can still be airtight, and rather than needing to breathe, ‘sweating’ is perhaps a closer human analogy.
However, note that alternative definitions exist too. BS 5250: 2021 Management of moisture in buildings – Code of practice refers to neither, and instead uses ‘vapour permeable’ and ‘vapour impermeable’ as its respective definitions for open and closed constructions. Both definitions seem commonly accepted across the industry.
There are many ways that moisture can affect buildings from both external and internal sources. Retrofit measures are predicated on the principle that repairs happen first, so the building should have already been made watertight from external weather. Moisture management of the internal sources is more difficult to manage as it involves the users’ behaviours around cooking, bathing and drying clothes. This must be managed somehow or it’ll form on the surfaces or become trapped within the construction layers, unseen. Good ventilation ensures regular air changes through the property.
Fig 2. A simplified version of water vapour movement in a mass masonary wall Fig. 3 Air movement in a traditional building
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Historic Scotland Fabric improvements for energy efficiency in traditional buildings p.5
Short Guide: Fabric Improvements for Energy Efficiency | HES | History
Hygroscopic and Hydrophobic
When it comes to material properties the definitions become less ambiguous as they have a scientific origin. A hygroscopic material is one that can absorb and release moisture over time, whereas hydrophobic materials repel water.
Woodfibre insulation is an example of a hygroscopic material which makes it suitable for internal wall insulation (IWI) within moisture open buildings. Its ability to absorb and release moisture means it can help the building regulate the internal humidity.
The same cannot be said of a hydrophobic PIR board, which when fully taped, acts like a vapour barrier. This allows the use of hydrophobic products below DPC level where they will not be damaged by water penetration.
Retrofit Risk Management
The government’s PAS2035 retrofit standard is not so much a ‘how-to’ guide for retrofitting a home, but a risk management process to follow for a greater chance of success. It is mandatory for government funded schemes like ECO4, SHDF or GBIS. Moisture management is embedded into this document, making frequent reference to BS 5250: 2021.
As the technical solutions vary so much, whether a building is moisture open or closed is determined early in the PAS2035 process, often as part of the Retrofit Assessment data gathering stage. From this, a Risk Assessment is followed to establish the overall risk path of the building. The fifth criteria of this risk assessment considers the building’s construction, with most properties falling into one of two categories: conventional or traditional. The exceptions to this are system build or high-rise units.
Definitions Based on Age
Although not exclusively so, ‘conventional’ describes modern cavity wall properties (moisture closed) and ‘traditional’ covers non-cavity buildings (moisture open), so there’s an immediate divergence in approach based on property age. Any properties that are listed or in a Conservation Area will be classed as ‘protected’ and are also typically moisture open.
Property age is therefore a key determinant for classification of a building’s construction. 1930 is the typical cutoff date, with anything before assumed to be solid walled and anything after having a cavity, owing to the change in trends around this time. Note however this is not a fixed rule, and properties between 1930-50 may have no cavities or very thin cavities that cannot be usefully filled with cavity wall insulation.
How to Identify the Walls?
If you’re initially unsure, then there are a few non-intrusive investigations you can try. Firstly, you can measure the wall thickness and then try to estimate the likely construction based on what you see. The EPC for the property might also give a guide, although sometimes the Domestic Energy Assessor (DEA) will have also made an assumption. Spotting the tell-tale penetrations used to retrofit cavity wall insulation is a good way to prove the presence of a cavity that is at least 50mm wide.
Thermal imaging can be used to help identify filled vs unfilled cavities, or even if previous cavity wall insulation has slumped. A small inexpensive endoscopic camera attachment on your phone can be inserted into any holes or gaps around waste pipes as another way to examine the walls without resorting to intrusive drilling.
If the walls are masonry, then familiarity with certain brick bonds may help too. Bonds that use brick headers suggest they are tying back into the inner masonry as traditional construction. A wall entirely of stretcher bond is highly suggestive of a standard cavity wall with metal wall ties.
If walls have cavities that are wide enough to be filled with cavity wall insulation (CWI) then chances are there were moisture closed and will remain so afterwards. As an example, an unfilled cavity wall from the 1960s is still ‘conventional’ and ‘moisture closed’ after CWI is added. Even though it may be leaking air from the inside, the inner finishes were designed to be airtight and use gypsum and cement-based building elements. These layers were never designed to get wet and dry in the same way as a thick moisture open wall with natural stone and lime plaster.
Examples
Returning to the three main risks at the start, all apply to both scenarios although the first two are more likely in moisture open buildings (change of the heat balance & interstitial condensation) and the third more prevalent in moisture closed (warmer internal environment and lack of ventilation). Let’s look at some examples of these below.
Risks With Floors – Change of Heat Balance
Upgrades to a suspended timber floor is a good example of a risk associated with a change in heat balance. Solid walled properties are more likely to have suspended timber floors. Cavity walled properties are usually complemented by solid floors (maybe with insulation depending on their age). Floor insulation and flat roof insulation both score highly on the PAS2035 risk matrix due to the associated moisture risks.
As timber joists sit into the solid masonry, they are subject to external moisture from the weather passing through the walls as well as potentially moisture rising from the ground. To keep floor timbers dry, they are heated by the warmth within the house and ventilated via the subfloor void. This creates a happy balance where timbers can get wet, but also have the ability to dry naturally.
When a suspended floor is insulated, there are two main aims: to stop heat loss through the floor and reduce draughts through the floorboards from the cold sub-floor void. In energy efficiency terms these are admirable aims. However, the effect is that as heat no longer passes through the floor, the timbers will become colder for longer and may not be able to withstand the moisture levels, eventually leading to decay.
Moisture which gets into the building fabric from either inside or outside can cause damage to organic materials like timber embedded in the walls, as well as rust in some metal components and spalling in external masonry from the freeze / thaw cycle
These joists are in reasonable condition but the masonry around is damp, so a sub-floor new vent is required and each joist end has been wrapped.
The joists in this hallway were rotten, so new joists have been inserted, with protected ends and sitting on a new DPC.
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Pebble Trust SUSTAINABLE RENOVATION – improving homes for energy, health and environment
Left p.33 right p.106
230814_SRG-ed2_Digital-V2_compressed.pdf
This can be mitigated in two ways. Firstly, before the floor is insulated, the timbers can be inspected for any signs of deterioration. Repairs can be made before the next stage. Secondly, timbers set into the walls can be treated to reduce the risk of future decay.
Fixing Insulation From Above Floor
Here the insulation is supported by boarding carried off the joists which gives more scope for the type of insulation material that can be used.
Image Credits
Energy Efficiency and Historic Buildings: Insulation of suspended timber floors | Historic England p.14
Risks With Flat Roofs – Interstitial Condensation
Ceilings and roofs tend to have fewer differences between moisture closed and moisture open buildings. Timber structure and plaster finishes are the standard, although the materials may differ from lime-based in traditional to gypsum-based linings in conventional situations. The main decision is whether you are dealing with a warm roof or a cold roof.
In a warm roof, the structure is within the thermal envelope, topped with insulation and a finish. This is better for energy efficiency as no structure passes through the thermal envelope and causes a thermal bridge. A warm roof is ideally matched to an external wall insulation (EWI) solution as they can be joined to give a complete warm jacket around the building.
In a cold roof, the structural members (e.g. flat roof joists) do create repeating thermal bridges and rely on a ventilation layer below the roof finish. This allows any moisture that condenses on the outer face of the insulation layer to be ventilated away and not cause damage to the roof. As this happens within the construction layers of the roof, this is a form of interstitial condensation. Upgrading flat roof are considered a high-risk measure under PAS2035.
Warmer Internal Environment – Surface Condensation
As we make our buildings warmer and more airtight with insulation and draughtproofing measures, we seek to raise the internal temperatures for greater comfort. Unless ventilation is also supplemented alongside this (‘no insulation without ventilation!’) then the air quality will reduce and become more humid. This is an unintended consequence that can lead to property damage and health issues.
Condensation occurs when warm air carrying water vapour reaches a dew point and turns back into liquid water. This warmer, moist air will find cold surfaces like windows and external walls to settle on causing condensation. The conditions that lead to condensation may then in turn produce mould and other bacteria.
Common areas for surface condensation are:
- against external walls,
- in the corners of rooms with two external walls,
- in cupboards and alcoves that are poorly ventilated or heated,
- on external walls behind furniture,
- around windows reveals (where thermal by-pass of the cavity may be happening)
- and on glazing panes.
In extreme cases, whole surfaces will become damp with surface condensation causing dangerous and unsightly mould. Unfortunately, in many early retrofits this was an unintended consequence of adding insulation without considering the importance of good levels of ventilation.
Image Credits
Photos by Alex Bell
Specification Tips for Designers
When specifying a new build or retrofit construction that involves complex layers, it is always good practice to engage with the insulation manufacturer. They can help produce dew point calculations to test the risk of interstitial condensation forming. Err on the side of caution and do not exceed recommended thickness, particularly with IWI. It is better to have a thin layer of insulation working well than a thick layer creating untold problems down the line.
Using hygroscopic materials in combination can have added benefit. An IWI wall system that uses a clay plaster base with woodfibre increases the overall moisture storage capacity, leading to a buffering effect, regulating the passage of moisture and heat transfer. Have the manufacturer help guide you towards appropriate solutions and robust specifications. This is the best way to avoid interstitial condensation and match the solution to each unique situation.
It is also good practice to reflect your design principles clearly on your drawings, always using coloured annotations to show the airtightness layer and how the continuity between walls, floors, windows and roofs is formed with tapes or other sealing material.
A clear air tight line around the whole of the habituated areas of a home is desirable.
It can be on the outside of a home, but is more commonly on the inside where it is more protected from the elements.
Image Credits
Devon CC Retrofit Guide p.15
The Devon Retrofit Guide – Energy Saving Devon
Remember – The Four Cs
To help designers manage the risks with moisture, the BSI introduced a simple reminder using The Four Cs, outlined in Moisture in buildings: an integrated approach to risk assessment and guidance (Neil May and Chris Sanders) 2017, BSI/UK/899/ST/0816/EN/HL. To quote from this document:
A whole-building approach must integrate fabric, services and people and place them in context while also dealing with uncertainty. In regard to moisture risk this approach can be broken down into four principles.
The key principles are:
- Understand the context of the building and the building project and ensure compatibility of the design with this context.
- Ensure coherence in approach and detailing.
- Build in capacity in the design and construction phase for mistakes, uncertainties and future challenges.
- Ensure that caution is taken in the use, maintenance and after care phase where there are ongoing requirements of care and uncertainty of outcomes.
Understanding the differences between moisture open and moisture closed and following the principles of Context, Coherence, Capacity and Caution will help you create better retrofit solutions for your clients. Remember to work using a Whole House Approach, and ensure good ventilation is present.
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Featured Resources
Energy Efficiency and Historic Buildings: Insulating solid walls | Historic England
Heritage Building Retrofit Toolkit
Short Guide: Fabric Improvements for Energy Efficiency | HES | History
230814_SRG-ed2_Digital-V2_compressed.pdf
Energy Efficiency and Historic Buildings: Insulation of suspended timber floors | Historic England
Energy Efficiency and Historic Buildings: Insulating flat roofs | Historic England
Author
Written by Alex Bell, Retrofit Architect, Retrofit Coordinator and Passivhaus Designer with 25 years’ experience of design and construction. Alex practises low energy, sustainable design using the Passivhaus principles, EnerPHit or the PAS 2035 Retrofit approach.