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WALLS
INSULATION
There is a fundamental difference between a single-skinned, uninsulated building and a layered, insulated one and a significant difference in cost, building time and effort. Making the right choice is essential. Rather like choosing a sleeping bag, you need to know what climatic conditions the article will have to function in. It is a serious mistake to try and extend the functionality of a two season bag into the colder months or higher exposures and it is the same with the hut envelope.
Choose the right type from the outset based on the planned occupancy pattern and the local micro-climate. Whilst it is not impossible to retro-insulate, it is a relatively difficult operation to carry out well unless you have very specifically designed the wall, floor and roof structure so to do.
In the hutting literature you will see a great many huts from much warmer parts of the world than Scotland that are completely uninsulated.
This approach will only be appropriate in Scotland for huts that will be used only in the warmer months. Uninsulated, exposed and mountain location huts for well-equipped hikers are often used in winter but this approach is unlikely to be a good one where occupancy is more extended.
PRINCIPLES & ENERGY
All over the world you can find uninsulated huts and bothies being heated (poorly) by solid fuel stoves. However, it is intended that hutting in Scotland should exemplify low energy, environmentally conscious design and it is therefore not good practice to install any form of heating without at least some insulation. Even locally sourced firewood has some embodied energy in the cutting (e.g by chainsaw) and it also emits some greenhouse gases during combustion.
You should therefore insulate if at all possible and, if the budget is tight, prioritise the roof over the walls and the walls over the floor. However, there is a law of diminishing returns with insulation so rather do a little of all of the envelope than a large amount in the roof and none in the floor.
Eliminate draughts whether insulated or uninsulated and ventilate in a controlled manner (refer Section 9).
This Guide is not prescriptive in setting specific energy targets for huts but rather steers the hutter towards understanding the principles and making rational decisions. It would be possible to build a hut so well insulated and air tight that it did not need any heating as passive solar gain and heat from occupants would be sufficient. If a stove was wanted for cooking and comfort reasons it would be sensible to apply at least a low level of insulation and care in construction to achieve air tightness. A hut used occasionally might not justify the investment in thick insulation and the slightly higher fuel use would not be a significant carbon impact provided it was from renewable sources.
MATERIALS
There are many types of insulation materials and the choice will be made on the basis of budget, environmental criteria, structural type and micro-climate. You may also have to consider rodent and insect threats.
Insulation should be well sealed within sheathed walls, floor and roof structures as draughts within these structures will significantly lower the performance of any insulation type. All voids should be filled. Some materials pack better than others and leave less voids. Some soft fills slump and others don’t.
There are many relatively new insulating boards on the market some of which incorporate significant racking resistance. These boards are often now used to “break the thermal bridges” in timber frame construction (see Thermal Bridging).
A wide variety of foil-faced polyurethane (PUR), polyisocyanurate (PIR) and polystyrene boards are readily available and used in house construction on account of their low cost, low conductivity and light weight. However the use of this type of insulation is strongly discouraged in this Guide due to the inflammable nature of some types and the high toxicity of fumes once ignited.
On no account should such materials ever be left exposed to the interior of a hut and any cavity where they have been used should be completely sealed. Due to the small volume of huts, toxic fumes generated in a fire may be of even greater consequence than in larger building types and good practice will therefore be to choose well known brands of certified non-combustible insulation.
An ecological approach to using materials is encouraged by this Guide in line with the Scottish Government low carbon and low impact definition of huts. It is allowable therefore in hut construction to use natural, recycled and waste materials for insulation such as straw, wood shavings, natural wools and scrap textiles. But remember that many of these are subject to insect attack and a fleece that at time of installation may seem perfectly adequate can become a pile of dust within a few years. Commercially available ‘natural’ insulations will generally be a safer option as they are treated against insect and fungal attacks.
U-VALUES &
R VALUES
Every insulant has a thermal resistance or r value. The higher the number the better the theoretical insulation performance per unit depth. However this value does not account for thermal storage nor the way the insulation performs in reality (in relation to water vapour, slumping, vibration etc) nor how well it was installed. So beware of simplistic judgements and calculations. Take advice from a builder or architect experienced in many insulation types.
It is customary to express the insulation value of a whole construction element such as a wall, roof, window or door as a U-value. Thermal transmittance (U) equals the reciprocal of the sum of all the resistances.
As well as each material in the construction, the resistance of external surfaces, internal surfaces and cavities can be added from a table. Some insulation boards and membranes, added to control moisture movement in the construction, have foil facings and these can dramatically improve U-values by reducing radiation in cavities.
Just for comparison the weighted average U values in W/m2K for a building of less than 50m2, to comply with Scottish Building Standards, are 0.22 for a wall, 0.18 for a floor and 0.15 for a roof. For more detail on calculation methods see the BRE document BR 443 - ‘Conventions for U-value calculations’.
THERMAL BRIDGING
Insulation is most effective in a continuous layer. This means however fixing through the insulation to the building. This only works with some board insulations and special fixings. Other insulants will be placed between the structural elements meaning the supports act as thermal bridges, which more easily conduct the heat out of the building than the insulant. This effect can be calculated and reduced by designing the building to narrow the bridge or make a break in it.
In the context of simple huts, heat loss is not required to be calculated. It will be most effective to focus energy conservation on air tightness (Air tightness ), a reasonable level of all round insulation and space heating that is as near carbon neutral as possible (eg from local firewood).
AIR TIGHTNESS
Air tightness in buildings has been afforded a high priority in recent years in order to reduce energy use and carbon emissions. Whilst it is important to eliminate unwanted draughts therefore even in a hut, it is equally important to ensure good ventilation in small spaces for human health, for moisture control and the longevity of the structure and also in order to be absolutely sure that a solid fuel stove is able to draw all the air for combustion that it requires. (Refer Stoves, flues and chimneys).
Air tightness can be achieved with various mastics and silicones but proprietary air tightness tapes are now preferred and should be applied to any junctions that may leak both on the outside of a frame and on the inside. Areas to give special attention to are around door and window frames, the floor to wall junction and the wall head to roof junction.
CONDENSATION RISKS
The inhabitants of buildings give out large amounts of water vapour in their breath and also from cooking and washing. The higher the temperature in the building the more moisture it can hold in the air and a positive vapour gradient almost always exists from inside to outside in an occupied building even in the most humid external conditions.
In a confined space such as a hut, the water vapour content can become very high although this can be alleviated by good ventilation, easily achieved in small spaces by opening a window or door! However, there is still a strong tendency for water vapour to try and pass into the fabric of a building and in insulated timber-framed construction there is a risk of condensation forming within the insulated structure. This can lead to decay in the structure due most usually to fungal attack of the timber and ultimately to structural failure.
This condensation risk in the frame can be eliminated or at least considerably reduced by well-informed design and good workmanship. (Condensation risk analysis as well as U-value calculations are generally conducted at no charge by insulation manufacturers if provided with drawings of intended envelope build-ups).
There are two main design approaches to eliminate the risk of this ‘interstitial’ condensation. The most common is simply to try and prevent moisture passing into the wall and roof in the first place by use of a suitable vapour barrier e.g. polythene just behind the interior lining of the wall or roof. Any penetration by plumbing or electrical installations severely compromises the effectiveness of this barrier and frequent puncturing of it will cause total failure of the strategy. Therefore, such penetrations should be sealed. This approach (of a sealed wall) includes the use of preservative treated timber in the frame.
An alternative approach involves the design of a vapour permeable wall or roof, which promotes the escape of water vapour from inside the frame out to the external environment. Also known as a “breathing” wall or roof this is achieved primarily by ensuring that the inner lining (facing into the inhabited space) is relatively much more resistant to the passage of vapour than the external sheathing. The inner lining could be OSB, hardboard or plywood (with all joints well taped) while the external sheathing should be a proprietary vapour diffusive board designed for this purpose. A properly engineered vapour permeable wall does not require the frame to be treated. This may be particularly important therefore to hut builders using home-grown and or home-processed timber. Figure 11A shows the design of a vapour permeable wall and exactly the same principles and build up can be applied to an insulated roof. In contrast figure 11B shows a conventional construction.
If you are designing a vapour permeable wall or roof, then the type of insulation used will be particularly important as it too has to allow vapour transfer readily. Sheep’s wool, wood fibre and recycled newsprint insulations are all suitable in this application. Rigid synthetics, plastics and mineral wool (glass wool) should not be used in this type of wall or roof.
For further information see: BS 5252:2011 ‘Code of practice for control of condensation in buildings’ and NHBC Foundation NF16 ‘A practical guide to building airtight dwellings
INTERIOR LININGS
Interior linings of a hut can contribute to thermal performance, vapour control and rodent control. Many huts will only be occupied for short periods and some may be left unoccupied for whole seasons. In these cases plasterboard will be a poor choice as an internal lining as it can support mould growth in a damp environment. However well detailed the building, because of their intermittent use, huts will frequently be cold and therefore require more robust materials than plasterboard. Timber linings are generally superior in all of the above respects and can take a variety of forms. OSB, fibre cement, plywood and solid timber (eg as a tongue and grooved board) are all suitable as an internal lining.
Tongue and grooved boards will not keep water vapour out of a frame on their own so should be backed either by a vapour control membrane or layer of OSB/plywood. (This internal sheathing layer may be installed in any case to achieve racking resistance (See Superstructure).
Surface spread of flame should be considered in general for internal lining material for walls and ceilings including any treatment thereof to restrict flame spread and its rate of growth. Where such linings are exposed to the room they should be medium risk as specified in Table 2.8 of Annex 2B of the Domestic Technical Handbook.
11A
SOLID STRUCTURES
Although not covered in this guide, it is anticipated that the walls of some huts will be built with natural or machined logs or types of solid stacked interlocking timber blocks. As the material becomes more affordable, some huts of the future will even be made of laminated timber panels of one sort or another. (eg ‘crosslam’ or ‘nail-lam’) which may form complete structures of floors, walls and roofs. In all solid timber structures, the structure can double as the interior lining and, when depths reach 100 mm or more, the amount of insulation required on the outside can be quite minimal. Often a layer of rigid wood fibre is all that is needed on the external face of a solid timber construction.
The excellent hygroscopic (moisture-absorbing and releasing) and thermal storage capacities of all sorts of solid timber construction make them an ideal choice for cold and wet climates (this is not accounted for in standard Scottish heat loss calculations). It is no surprise that northern countries, where log buildings have always played a major part in domestic architecture are now focusing on laminated timber panels (eg ‘crosslam’), which bring solid timber into the 21st century, combining air tightness, huge carbon fixing potential, lower embodied energy and off-site construction into one extraordinarily efficient and beautiful material.
11B
CLADDING
For the purposes of this Guide, wall and roof cladding (detailed in Roof cladding) is considered as a non-structural skin to protect a frame, insulated wall panel and/or inhabitants from the elements including wind and wind-driven precipitation. It is also taken to include a support system of battens and possibly counter-battens.
Cladding (and roof covering) cannot be considered entirely in isolation from structure. Choice of wall cladding type will be influenced for instance by whether there is an insulated wall behind it or not. Its design also needs to take account of the prevailing micro-climate - is it subject to frequent wind driven rain, is it deeply shaded and sheltered within a woodland?
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This Guide does not cover wall constructions of either masonry or straw bale (either structural or non-structural) nor does it cover any exterior covering, rendering or cladding systems appropriate to these construction types. The Guide does cover light-weight timber and sheet material coverings. Other types of coverings will require engineering expertise. This Guide may only be followed for wall claddings, which do not exceed 6 metres in height at their highest point from the entry floor level (generally at the gable peak).
SUPPORT STRUCTURES
(APPLIES TO ALL LIGHT
WEIGHT CLADDINGS WITH
VENTILATION GAP)
The support structure of battens and sometimes counter-battens provides a critical ventilation and drainage void in an insulated structure (see Section 11). Cladding may be fixed directly to a post and beam or stud frame in uninsulated frames but, in these cases, special care will need to be taken that water ingress is minimised which could wet the structural frame itself. Drying of this frame to prevent rotting will then be dependent on adequate ventilation of the internal space itself.
The support structure for timber cladding should be at least as durable as the cladding itself (see 6.4 below) although, at the scale of hut design, adequate longevity may be achieved by a number of different specifications.
Cladding battens are usually vacuum treated softwood or untreated larch heartwood. Surface treated softwoods are acceptable in hut building but never use non-durable softwood without some form of treatment as the void between cladding and an insulated envelope is damp and invisible.
METAL & FIBRE CEMENT WALL CLADDING
Sheet steel (or other metal) and fibre cement wall coverings are admissible and it is essential to follow the manufacturer’s instructions with regard to fixing spacings and types, which vary from profile to profile. Of metal claddings, sheet steel is the most economic solution and typically each sheet covers a one metre width. Generally, sheet steel will be fixed to a timber support structure (either vertically or horizontally) with 45-65 mm self-drilling proprietary fixings with integral neoprene washers. Some profiles are best fixed in the valleys and others in the ridges. Plain galvanised is the most environmentally sound steel sheet in reducing coatings to a minimum. Coatings containing PVC are best avoided on environmental grounds and cannot be cut readily without edge damage. There is a wide range of polyester paint coatings with varying guarantees reflected in their relative costs.
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Where sheet metal or fibre cement cladding is used to protect an insulated wall, it should be supported on battens to provide an adequate ventilation and drainage gap. The minimum depth of this gap will be 20 mm. In some weather conditions condensation will occur on the back of metal in particular and must be able to run down and out of a gap at the bottom of the wall.
When carefully designed, sheet metal or fibre cement wall cladding can make a very cost effective, aesthetically pleasing and technically proficient form of external envelope. Sheet steel (formerly tin) is also a strong part of the vernacular in many parts of rural Scotland. It may be particularly appropriate with single-skinned buildings in high exposure sites to prevent leakage and subsequent frame wetting.
TIMBER CLADDING SPECIES CHOICE
Larch, oak and cedar (usually of Canadian origin, sometimes home-grown) are the commonest durable cladding timbers in the UK and may be used untreated. Pressure treated Scots pine (redwood) and spruce are also acceptable for hut building. Accoya and other specialist treated timbers will mostly be beyond the budget but are, of course, ideal.
In hut building it will be acceptable to use less durable timbers, monitor their performance and replace them when rotten. This will allow re-use of materials and the avoidance of toxic preservatives, which meets other hutting objectives such as low impact construction.
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It is good practice to apply a hierarchy of preference when choosing timber for external applications in all timber building in Scotland. In declining order of preference :
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​Modified e.g. Accoya
- Naturally durable material e.g. oak or larch (but note parts of tree have different durability)
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Pressure treated (note pine sapwood takes treatment better than spruce)
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Less durable but preservative treated and surface coated in workshop
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Less durable but preservative treated and surface coated on site
​​There are two other critical provisos in selecting timber.
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The sapwood of all softwoods is non durable and will ideally be treated or coated in some way. Even naturally durable timbers contain sapwood.
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In the case of larch, juvenile heartwood is also non durable so that careful grading/selection of heartwood is required to achieve the full durability potential of the material.
TIMBER CLADDING PATTERNS
A large number of timber wall cladding types have been used over the centuries in different countries and may be found in the hut literature. This Guide however focuses on those that are deemed the most appropriate to the Scottish climate and the materials most readily available to the contemporary hut builder in Scotland.
The guidance that follows utilises the findings of extensive research by Napier University involving long term cladding trials and which is reported and analysed in detail in ‘External Timber cladding: design, installation and performance’ - see key reference at end of this Section.
Six types of timber cladding are recommended and illustrated below (see figures 6A and 6B).
Horizontal types: shiplap, weatherboarding, rainscreen. Vertical types: board on board, batten on board, gap boarding.
The use of durable timber shingles is also allowable but not illustrated. For the open rain screen
cladding types you must use a proprietary waterproof and UV stabilised type membrane.
6A
6B
TIMBER CLADDING DETAILING
There is more to designing and installing timber cladding that will last well than the inexperienced designer or builder may know. Along with the considerations addressed in Cladding to Timber cladding patterns (above), there are a number of fundamental design principles that need to be incorporated in order to achieve longevity of timber cladding as follows:
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Cavity depth should be sufficient to allow good ventilation. If wind driven precipitation is frequent, then the depth should be increased.
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Batten spacing will generally be 600 mm to coincide with timber studwork but also to provide sufficient nailing of cladding boards
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Vertical batten sections should be 19 x 45 mm minimum.
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Where horizontal battens are required (e.g for vertical timber cladding) then these should be 45 x 45 mm minimum on 600 mm centres and fixed to 38 x 19 vertical counter-battens to maintain the drainage gap of 19 mm.​
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No timber cladding should be installed nearer than 150 mm from finished ground level​
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Use vermin and insect meshes where appropriate especially at the base of cladding
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It is best practice never to place the end grain of timber up against solid material in a wet or damp environment. This will prevent it drying and lead to rot. Always try to leave a 2-3 mm gap between the end grain of cladding and drips, sills and flashings whether of metal, other timber or masonry.
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Timber cladding should always be able to accommodate some movement, shrinkage and swelling. Profiled boards that slot over each other must have appropriate tolerance for swelling.
OPEN CLADDING
‘Open’ cladding systems, sometimes known as rainscreen (see drawing 6A type C and 6B type C), are used extensively in Europe but only occasionally in Scotland, though gaining favour rapidly. In such systems, cladding boards are separated from each other and there is therefore very free airflow around them and any support system. They should only be used in combination with modern waterproof, vapour permeable, UV stabilised membranes (which may be visible behind the boards) and are clearly not usually appropriate for single skinned accommodation.
Such open cladding patterns performed relatively well in the Napier University cladding trials and can be recommended but, as with all cladding, subject to the timber either being inherently durable or the performance of less durable material enhanced by treatment and/or coating.
The support system will be frequently wetted with open cladding and should be made of timber at least as durable as the cladding itself. It is usually painted in the same colour as the cladding if a colour is being used on the latter as it will be visible as an albeit subtle pattern on the facade.
The best open cladding boards - if laid in the horizontal orientation - are trapezoidal or rhomboidal in section to promote rapid run off from the top surface of each board. The section is often relatively deep compared with closed cladding systems to confer the necessary stability. The Napier University trials also showed that vertical boards can be laid in an open pattern but this has not been at all common in practice. It is however often used on farm buildings where ventilation without rain penetration is important. It would be appropriate for use on woodsheds which have a similar requirement.
CLOSED CLADDING
One of the simplest and therefore commonest closed cladding patterns is called board on board (batten on boards is also quite common in Scotland) and scored reasonably well in the Napier University trials. It receives recommendation in particular for windy sites but is generally a safe bet in most micro-climates and is particularly appropriate where waney edged boards are being used either to reduce processing or for their rustic appearance.
Random width boards can be used; an overlap of at least 50 mm is recommended in typically exposed sites. A relatively flush exterior face is achieved with a gap of 5-10 mm, or a more strongly patterned facade with a wider gap, see figures 6B.
Such variables are partly a matter of taste and partly to do with available materials and processing capability.
Many profiled and overlapping horizontal patterns are available ranging from a simple weatherboarding to a sophisticated modern profile with expansion gaps. Only two overlapping profile types are shown in figure 6A. It is interesting to note that neither of these scored as highly as the open systems in the Napier University trials, but all are nonetheless acceptable with the above provisos on material treatments and detailing.
FIXINGS
Corrosion of metal fixings in wet and green timber can be significant and it will be important to use stainless steel nails or screws. Close to the sea, use marine grade stainless steel. Use proprietary ring shank cladding nails to get a good grip. These can be hand nailed or collated for nail guns. Black staining from ordinary or even galvanised nails can be both disheartening and lead to early failure.
To avoid splitting, a minimum distance of 5 times the nail diameter from the edge of the wood and 10 times the diameter from the end of the wood should be adopted.
Key references
External Timber Cladding: Design, installation and performance. Ivor Davies and John Wood. Arcamedia 2010. ISBN 978-1-904320-08-1