DIY BLOG - Principles of Traditional Construction
PRINCIPLES OF TRADITIONAL CONSTRUCTION
Although I am by no means an authority on traditional technologies (a lovely combination of words!), when faced with a large refurbishment project of a traditional, two+attic storey property on the Isle of Skye, I invested in a lovely new book and began some research. The following consists of notes prepared as regards interventions usually employed when refurbishing traditional buildings.
The goal is to understand how traditional buildings were successful in keep the inhabitant warm, dry and healthy and to avoid anything that may exacerbate damp or mould or result in potential sick building syndrome.
We may consider the technology of modern building techniques and materials as superior to traditional methods, that doesn’t necessarily mean they are better - in many cases, they are cheaper but they are not always compatible with traditional building methods. If it were more viable to employ stone masons and lime plasterers then surely we would seek to reinstate as best as we could but these options are usually too expensive and modern materials offer a cheap solution for the tight budget or avid DIY-er.
Some knowledge of Traditional and Modern construction methods and technologies provides an insight into their differences and understanding this is the key to making conscious choices to benefit you and your building. We may be tempted to pour insulation and vapour control layers into a cold and damp stone building to make it cosy and air-tight but this may cause other problems to arise. The careful balance of all interventions and their effects on each other serves as a reminder that refurbishing old traditional buildings is an under-taking, both in time, money and effort.
It is worth noting that some of the following points are based on stand-alone buildings and may not be so relevant to tenements however Historic Environment Scotland produce helpful short guides for tenements as linked to at the bottom of the page. Additionally, not all the points below will be relevant to your particular situation - some may not even be possible! In such cases there are property maintenance companies that can assist - look for those who specialise or have vast experience with traditional buildings.
One last consideration is whether or not your building is listed or in a conservation area. If so, you may require planning permission before making external changes and, in some cases, internal changes too. Check here to check if your property is listed or if you live in a conservation area.
Traditionally, stone walled buildings would absorb moisture from rain fall, the open structure of stone acting as a capillary, drawing rain water (especially wind-driven rainwater on those South-Westerly elevations) and ground water into it and at times in to the inner face of the building. The high thermal mass of the stone meant that the wall was able to dry out in the sunnier summer months, as well as from internal heat generated by open fires within the stonework itself - especially in the winter months. Internally, lath and lime plastered walls allowed the transfer of water vapour from inside to out when activities inside raised the relative humidity (washing, drying and cooking) across the internal wall from the rooms to the inside of the stone face. Moisture could also transfer from the wet stone face when the air was drier internally - such as the summer months - allowing the stone to dry out to the inside. This balance of vapour allowed for a healthier internal climate and one which could react to the seasonal weather conditions.
Traditional stone walled buildings in Scotland work on the principle of breathability (vapour permeability) and the ability and use of natural materials to balance vapour by absorbing and releasing it, without degrading the materials (Hygroscopicity). To this end, the principles of watertightness and vapour barriers are not considered appropriate for traditional refurbishments. For this reason, natural insulation, such as wood fibre, is considered more appropriate due to its ability to act as a moisture buffer. This is also true of clay and lime plaster - gypsum is less appropriate as it is not a material which happily takes on and releases moisture and can deteriorate over time. Vapour barriers or even vapour permeable layers can slow down and stop the transfer of moisture, leading to its build up in certain areas and potential problems. PIR-type insulation - rigid Kingspan, Recticel, Quinntherm, Celotex, etc., - is practically vapour impermeable and for this reason is not always recommended for traditional buildings. Cavities between the stone work and insulation are generally not recommended as they can create conditions for the potential for mould, especially where the cavities are poorly ventilated. Often, however, this may be the best solution due to ease of construction.
Where a cavity is introduced between the existing stone wall and inner, insulated wall, the thermal mass of the wall is decreased as it can no longer benefit from the internal heat to help it dry out. This means there may be increased moisture on the inside face and conditions for potential mould growth are introduced when a cavity is formed. Mould occurs where relative humidity is >80% and where there is the presence of moisture and oxygen. Mould or condensation usually occurs on cold bridges (areas of thinner insulation or none at all) where hot moist air meets a cool surface. If the cavity can remain well ventilated, this will help moisture evaporate and reduce the risk of mould growth. This is why it may be important for a cavity to be formed on stand-alone houses, with additional air vents at solumn level. In addition, continuous eaves and ridge venting at roof level increase the potential draw of air to this cavity.
Cold bridging may occur where existing timber joists extend into and bear on stone walls. Wet rot can penetrate the stone wall to the end grain of the timber, with the capillary action of cellulose drawing the moisture deep into it. Where the timbers do not dry out, wet rot will destroy the integrity of the timber and lead to longer term structural issues. Wet rot is a fungus which propagates in wet areas where there is a lack of oxygen so where accessible, timbers should bear onto DPC's to reduce water uptake from the stone, or have the ability to breathe or be ventilated. Increased ventilation in the cavity will help this process.
To assist with excess vapour transferring to the cavity from internal processes, an IMEV (intelligent mechanical extract ventilation) can be adopted to work as a humidistat, helping to keep internal humidity low. By transferring it quickly to the outside, it thereby encourages further vapour transfer inwards and helps to dry the stone wall in wetter months and to maintain a low vapour content in the cavity.
Dehumidifiers may offer a short-term solution to assist in removing excess water from the air and helping to improve internal air quality.
Natural insulation such as wood fibre, can allow the transfer of water vapour, as well as storing it and acting as a moisture buffer. This is of particular importance with traditional buildings where vapour needs to move freely from inside to out. Where non-permeable materials are used, this can lead to a build up of moisture in the cavity: If moisture can penetrate the stone to the inside face but cannot move to the inside of the building due to vapour barriers or non permeable insulation, it becomes trapped within the cavity, leading to moisture build-up and the potential for mould. A balance must be struck between minimal use of insulation and over-insulating. Over-insulating will result in increased internal moisture levels and greater disconnect between internal conditions and the potential thermal mass of the stone wall. A U Value of 0.3-0.5 will be acceptable in most areas. This may not always be acceptable as far as Building Control are concerned but there can certainly be an argument made in support of this approach. This may also not be acceptable to the client if they are looking to reduce heating costs and comfort levels. Historic Environment Scotland have produced case studies which incorporate insulation levels much higher than those mentioned above however as far as I’m aware, there hasn’t been any further information on the long term effects to the fabric following such interventions, and whether they have been detrimental or not.
A typical ‘quick-fix’ for many properties with an attic space is to lay insulation between the attic ties and then another layer perpendicular and over the same. This creates a cold roof space within the attic and can lead to potential condensation issues if not adequately ventilated. Some DIY books suggest the installation of a vapour barrier below the insulation to prevent moisture transfer from rooms below.
Internal insulation applied 'on the hard' means that the thermal performance of the stonework can be retained. The risk of interstitial condensation is minimised with natural materials which are vapour permeable, allowing moisture to transfer in or out. In addition, the lack of an air filled cavity reduces the risk of mould appearing unseen. Cold bridging still remains an issue where detailing becomes tricky or materials cannot effectively form an insulation envelope however an insulated external render can be applied to the face of the walls as well as the returns of openings, providing a more effective, continuous insulated envelope. In addition, an external render can provide better rainwater protection to the building, especially from wind-driven rain water whilst still allowing the stone to dry to the outside. It's important that in all these approaches, natural, vapour permeable materials are employed. There are many suppliers of natural, vapour permeable products who can provide further information and in some cases, calculations to outline condensation risks.
Other Internal conditions
Its important that any fire places which are blocked up are still allowed to ventilate inwards - this is usually achieved by installing a small air vent over any construction that has closed up the opening. This provides a flow of air, helping to evaporate any moisture ingress either from the chimney pot above or from the chimney wall through capillary action. The use of a multi-fuel stove in existing gable walls will help to transfer heat to the stone and provide a means to drying out any moisture. Where a flue liner is employed the chimney is then back-filled with small insulated' ‘chips’ or vermiculate. These do insulate the flue so less heat is transferred by the gases escaping than open fires. Also of importance and often the cheapest option to make immediate improvements are soft furnishings such as; heavy curtains; or the reinstatement of timber shutters across windows; carpets or rugs to exposed timber floors, etc,. Reducing draughts under doors, through keyholes and letter boxes and windows can make a huge difference This can often be achieved through draught excluders, foam strips or complete window refurbishments from companies such as Ventrolla.
It's important to ensure that rainwater is taken away from the building as quickly as possible. This means an effective roof, lead flashings and approved detailing, rainwater gutters that are maintained and unblocked, perimeter ground drainage that divert rain and ground water away from the building, solid walls with no cracks and pointed with appropriate mortar, effective windows and doors with sand mastic and cills capable of flashing water away, effective chimneys properly pointed with appropriate cowls to prevent rainwater ingress and retain ventilation.
Best fit Solution
Although there is no 'best fit' solution for the refurbishment of traditional buildings, there are methods which can reduce the risk of condensation or mould issues. Refurbishment of traditional buildings must take a balanced approach as often the introduction of one element will have both positive and negative effects which can impact the efficiency and health of the building
Long term monitoring
Monitoring of moisture levels in prone areas, such as South-Westerly elevations or at localised, problematic areas, can be accomplished with simple moisture sensors. Pairs of stainless steel screws are driven into a wood surface of the area in question, parallel with the grain and 20-25mm apart. Each pair of screws is wired to a terminal block in a convenient location where resistance readings can be checked from time to time with a moisture meter or recorded by a data-logger. Readings will likely make more sense over a calendar year where a full season of weather can be appraised. The benefits of such a simple meter means that concealed areas can be monitored without the need for exposing surfaces or construction. Where construction is not particularly accessible, simple moisture sensors can be made from small blocks of pine (typically 25 x 45 x 5mm thick) installed on the wall surface within the cavity, behind the insulation.
ENVELOPE SHORT LIST
The following is a check-list put together for a project on the Isle of Skye, though it is just as relevant to traditional buildings anywhere and may help identify areas which require care.
Assess condition of slates - repair/replace with new or reclaimed to match
Assess condition of sarking boards underneath - replace if required, retaining small air gap between boards
If removing all slates, consider fitting breathable membrane such as Tyvek or Protect VP400, to top of sarking to improve breathability of attic space
Check condition of Chimneys
Assess condition of lead flashings and replace if necessary with appropriate code lead to Lead Sheet Association details
Remove all vegetation
Assess condition of stonework and repair where required with appropriate lime mortar
Consider removal of any historic cement-based mortar repairs to allow vapour permeability of building
Any proposed external paint to be vapour permeable
Provide insulation to inside face of walls but retain ventilation void (frame out from wall)
Provide insulation ‘on the hard’ internally with installation support/advice from manufacturer
Consider natural, hygroscopic (absorbs and releases moisture) insulation is used such as wood fibre boards or batts
Remove all mould/fungus from stonework
If new windows or doors are proposed, ensure all/any cills, ingoes, heads and flashings are appropriately fitted to prevent wind driven rain and draughts from entering the building
Refurbish existing windows for ease of opening, draught-proofing, new ironmongery etc - take care not to paint over draught seals
Consider all windows and doors are fitted with trickle vents to allow passive air ventilation of building
Check sand mastic around junctions of window and door frames with stonework - if cracking or dry consider replacing with new
Check condition of external timber frames - if paintwork coming away or timbers showing signs of rot, contact local window companies for no obligation, free quotes to repair, repaint or replace
Ensure any cracked or leaked guttering and pipework is replaced with cast iron time-saver pipes
Ensure all existing gutters pipes are clear and unblocked
Where pipework penetrates the ground ensure all bends are clear
Consider French drains if water collecting at foot of wall or consider sloped landscaping to draw water away from the building
Consider a soak-away if hard-standing areas are waterlogged
Ensure ground is not built up above internal floor level around the building
Ensure under-floor ventilation is clear and operating efficiently (no blockages from vegetation or proposed insulation
Fit additional cast-iron vents in walls
Short Guides for home improvements - Historic Environment Scotland - Free downloads
Text below taken from Historic Environment Scotland Short Guide 9, ‘Maintaining Your Home’, and Historic Environment Scotland Short Guide 1, ‘Fabric Imporvements For Energy Efficiency in Traditional Buildings’, respectively
“...for the benefit of owners of traditionally constructed dwellings, which generally, but not exclusively, means those of pitched roof and solid wall construction built before 1919. Owners need to be aware that the fabric of such houses behaves differently from the fabric of those constructed using modern methods and materials, and generally comprises natural materials sourced locally or regionally. Many of these components, such as masonry, timber and lime, are vapour open (sometimes referred to as ‘breathable’) and this important property should be kept in mind when inspecting the building and planning repairs. Table 1 below outlines some differences between traditional and modern construction. Figure 1 gives a graphical interpretation of water and vapour movement in traditional construction. Of course maintenance is essential for all buildings, and many of the general principles will be the same whatever the age of your house. Generally work to traditional buildings requires the use of a range of materials that are compatible with the existing traditional fabric. This allows the vapour open nature of the construction to be maintained so that moisture can be freely dispersed. These materials, which for the external structure are normally lime, stone, timber and slate, by virtue of their material properties or the way they are put together allow the movement of water vapour through the material or via moving air around them (Fig. 3). The use of modern impermeable materials in repairs may compromise this dynamic and lead to concentrations of water or areas of high humidity, both of which will provide favourable conditions for mould or timber decay. Areas where inappropriate materials are often used are repointing walls with cement-based mortars and painting stone walls with masonry paint. Sometimes on their own such interventions may not cause problems, but when coupled with other shortcomings in maintenance, such as blocked downpipes or defective stonework on a chimney, the fabric can rapidly start to suffer. Persistently damp or wet masonry can have structural implications.”
“Research by Historic Scotland supports the view that there are two key principles for improving thermal performance in traditional buildings, and these underpin the advice given in this guide: firstly that the materials used should be appropriate for the building, and in most cases water vapour permeable, and secondly that adequate ventilation should be maintained to ensure the health of the building and its occupants. Breathable Construction. Traditional buildings are often referred to as being of ‘breathable construction’ that acknowledges the fact that the materials used for their construction have the ability to absorb and release moisture. Such materials are often referred to as ‘hygroscopic’. This property is of benefit when seeking to buffer the peaks of humidity created through the daily tasks of occupation (Fig. 2). Exactly how much water vapour is moved through component materials and at what rate will depend on the type of stone (igneous or sedimentary rock for example) which the wall is made from, the voids in the wall and the external condition of the masonry. In retrofit work, using materials and construction methods that are appropriate for traditional buildings will ensure that energy efficiency improvements are technically compatible with the building fabric and will therefore reduce the risk of damage from inappropriate interventions. Furthermore such compatibility will ensure that the upgrades are durable and long lasting. Ventilation. Traditional buildings were constructed in a way that allows modest air movement through vents, windows, doors and chimneys, circulation through rooms, stairwells, and through gaps under floors and behind wall surfaces (Fig. 3). This natural ventilation is important for managing moisture accumulation and clearing humid or stale air along with other vapours produced in buildings. However, when air movement becomes excessive it reduces internal temperatures and has a negative effect on thermal comfort. The difficulty is finding a balance, because if ventilation is restricted, air carrying water vapour cannot properly escape, leading to increased humidity, condensation build-up and undesirable consequences such as mould growth.”
Images above taken from Historic Environment Scotland Short Guide 9, ‘Maintaining Your Home’, and Historic Environment Scotland Short Guide 1, ‘Fabric Imporvements For Energy Efficiency in Traditional Buildings’, respectively