Seasonal Changes in Timber Moisture Content and condensation
Despite day-to-day fluctuations, the average outdoor relative humidity actually changes little from season to season. However, inside the home the relative humidity of outdoor air drawn inside is drastically altered by heating it and cooling it, without humidification or dehumidification. It is these extreme seasonal swings in relative humidity that may disrupt the wood moisture content and cause structural alterations of the timber through changes in dimensions. Our design incorporates and allows for this movement and may require adjustment depending on the amount of structural alteration that occurs
Condensation
The air which surrounds us in our homes, always contains water vapour, which is usually invisible. The warmer the air, the more water vapour it can hold. However, there is a limit to the amount it can hold for a given temperature. When that limit is reached, the air is said to be ‘saturated’. When saturated air comes into contact with a surface, which is at a lower temperature than itself, the air is chilled at the point of contact and sheds its surplus water vapour on that surface - initially in the form of a mist and, if excessive, eventually in the form of droplets of moisture. The increased incidence of condensation in today’s buildings is the direct result of changes in modern living conditions, which have led to warmer and more comfortable rooms. Modern buildings are designed to eliminate draughts and do not have the natural ventilation found in some older houses. Provided the rooms are heated normally, the solution will probably be found by providing controlled ventilation
Some examples of where the water vapour comes from:
- Breathing: Two sleeping adults produce 1½ pints of moisture in 8 hours, which is absorbed as water vapour into the atmosphere
- Cooking: Steam clouds can be seen near saucepans and kettles, and then seem to disappear. The clouds have been absorbed into the atmosphere. The cooker itself may be a source of water vapour e.g. an average gas cooker could produce approximately 1½ pints of moisture per hour
- Washing up: The vapour clouds given off by the hot water are rapidly absorbed into the atmosphere. • Bathing, laundry, and wet outer clothing: These are often the major sources of water vapour in the home
- Heaters: A flueless gas heater can produce up to 2/3 pint of moisture per hour. • Paraffin heaters produce 9 pints of moisture for every 8 pints of fuel burned. • Indoor Plants: A frequently unrecognised, but nevertheless significant source of water vapour
- New Property: The bricks, timber, concrete and other materials in an average three bed house absorb about 1500 gallons of water during construction. Much of this is dissipated into the indoor atmosphere during the drying out period
How to reduce the condensation forming on the inner (room-side) surface of the glass:
- Provide natural ventilation through an opening section of the window, or through a proprietary ventilating unit, or through an air brick
- Open at least one window in each room for some part of the day to permit a change of air
- Ensure ventilation of all rooms where gas or oil heaters are used
- Ventilate cooker hoods to the outside air
- Ensure extractor fans are fitted and used in bathrooms
- In cold weather, keep some form of heating on permanently in the house
- Condensation can be caused by isolating the inner glass from the warm room air with heavy curtains when drawn. To allow free passage of warm air to the glass, position curtains 15cm to 20cm away from the window, and ensure there are sufficient gaps at the top and bottom to permit continuous circulation
Importance of Humidity Control in the home
Moisture in house air can be a problem when there is either too little or too much
Air that is too dry can cause timber to shrink causing joints to open
Air that is too damp can cause timber to swell causing joints to crack, v-joint sealers to crack/blister, paint to blister/peel, stiff operation of windows and doors, condensation, timber to bow or twist, glass to crack and mould growth
Relative Humidity
Relative humidity is a percentage. It tells you how much moisture is in the air relative to the maximum amount the air can hold at that temperature. For instance, when air at a given temperature contains all the water vapour it can hold at that temperature, the relative humidity is 100 per cent. If the humidity is higher than 100 per cent, moisture will begin to condense from the air. If the air contains only half the water it can hold at that temperature, the relative humidity is 50 per cent. Warm air can hold more moisture than cool air therefore the colder the temperature the higher the relative humidity. The relative humidity of air will change as the temperature changes, even though the actual amount of moisture in air does not. For example, as air cools, the relative humidity rises
The Right Moisture Levels
Generally, the ‘right’ moisture level - the relative humidity - in your house is less than 50-55 per cent. At less than 50 per cent relative humidity it is unlikely that mould will grow indoors. There are cases when 50 per cent relative humidity is too high. For instance, if there is condensation on your windows in cold weather, it’s a good idea to lower your relative humidity to as low as 30 per cent by increasing your inside temperature and ventilation. In regions where there are months of cool, damp weather or hot, muggy weather, ventilation just adds more moisture to indoor air. If excessive moisture is a problem then a dehumidifier is an effective way of dealing with this
Sometimes, reducing relative humidity won’t solve moisture problems. Defects in insulation or the air barrier in walls and ceilings can cause cold spots in your house. They show up as areas where there is always condensation, even if relative humidity is 50 per cent or less. A dehumidifier won’t solve this problem, you will need help from a qualified builder or insulation specialist
What Causes Moisture in Air
Moisture can come into your home from many places:Outside sources include the soil around your house, surface water drainage and damp outdoor air. Breathing and perspiration by you, your family and your pets are a major source of indoor moisture. Showering, bathing, drying clothes indoors, venting clothes dryers indoors, washing dishes and floors
Most houses have more than one source of moisture. Moisture can cause problems once in a while, or all the time. A little prevention can keep excess moisture out of your home’s air and prevent both occasional and continual problems
To check the moisture level - relative humidity - in your house, you can use a ‘hygrometer’. A hygrometer is an inexpensive, easy-to-use instrument, sometimes called a humidity sensor or relative humidity indicator. There are mechanical and electronic hygrometers. Electronic hygrometers cost from around £20 to £60
Responsibility for problems in timber products resulting from humidity extremes during occupancy rests with the homeowner
Redecoration Cycles
Care and Protection by user
With proper care and attention, factory finished door sets, windows and conservatories will give extended life between redecoration cycles. To achieve this performance, the following should be observed: • At least once per year all coatings should be washed with mild detergent and water to remove any surface pollution. • All hinge mechanisms and handles should be checked at least biannually for ease of operation and lubricated with light oil suitable for the purpose, as required. • Weather seals should be cleaned at least once per year to remove any dust or grime in accordance with the manufacturer’s instructions. • Ventilators should be cleaned at least once per year to remove any dirt or grime in accordance with the manufacturers’ instructions
First and subsequent re-decoration
All areas to be recoated should be washed down with a mild detergent solution & rinsed with clean water to remove dust, insects and other contaminants, which can form a base for algae and fungi growth. Where the coating system is intact but requires a cosmetic coat, the following procedure should be followed
- Using a good quality, long haired, synthetic brush, designed for use with acrylic paints, apply one or two coats of Aquatop 2600 in the appropriate shade, colour & gloss level. Allow to dry for four hours between coats
Where minor flaking affects small areas of the topcoat surface but the timber substrate is not exposed, the following procedure should be followed
- Abrade the damaged area with a fine grade abrasive paper to remove all unsound coating & feather out
- Clean down and wash the abraded area to remove dust, and allow to thoroughly dry
- Using a good quality, long haired, synthetic brush, designed for use with acrylic paints, apply a coat of Aquatop 2600 in the appropriate shade, colour & gloss level to the damaged area. Allow to dry for four hours & then apply a second coat
- If the damaged area is widespread, it is recommended that the whole frame is lightly abraded and repaired as described above with the second coat applied to the complete frame
Where moisture has penetrated joints, end grain, mitres or natural movement of the timber has opened shakes, treat as follows
- Abrade the damaged area with a medium and then fine grade abrasive paper
- Clean down and wash the abraded area to remove dust, and allow to thoroughly dry
- Treat bare wood, where appropriate, with a surface preservative, such as Teknol Aqua 1410, and allow to dry
- Prime with Aqua Primer 2900 base coat stain in the original colour for translucent systems or Anti Stain Aqua 2901 for opaque systems.
- Seal any open joints with Teknoseal 4001 joint sealer applied by mastic gun. Wipe with a damp cloth or spatula to give a smooth joint and allow to dry to a clear finish
- Seal any exposed end grain with Teknoseal 4000 end grain sealer & allow to thoroughly dry
- Using a good quality. Long haired, synthetic brush, designed for use with acrylic paints, apply a coat of Aquatop 2600 in the appropriate shade, colour & gloss level. Allow to dry for four hours and then apply a second coat
Where damage has affected the full depth of the coating system, i.e. a deep cut or gouge, the full system requires repair
- Abrade the damaged area with a medium and then fine grade abrasive paper
- Clean down and wash the abraded area to remove dust, and allow to thoroughly dry
- Treat bare wood, where appropriate, with a surface preservative, such as Teknol Aqua 1410, and allow to dry
- Prime with Aqua Primer 2900 base coat stain in the original colour for translucent systems or Anti Stain Aqua 2901 for opaque systems
- Using a good quality, long haired, synthetic brush, designed for use with acrylic paints, apply a coat of Aquatop 2600 in the appropriate shade, colour & gloss level. Allow to dry for four hours and then apply a second coat
Where resin has exuded through the coating:
- The best remedial treatment is to allow it to weather until it dries and oxidises, forming a white crystalline powder. The dried resin can then be removed with a stiff nylon or natural bristle brush, and any remaining residues washed off with a cloth
- Water based coatings with their relatively high degree of moisture vapour permeability often allow the passage of resin to the surface without damage to the coating. If the finish is not damaged, by over-vigorous scrubbing during crystal removal, re-coating is often unnecessary
Although it may be unsightly, it is better not to remove fresh sticky resin. In practice, this can be very difficult, and the presence of sticky resin indicates that the exudation is still continuing. The remedial work for resin exudation is often best left until the first maintenance period, by which time the resin has normally fully crystallised. After removal as described above, the overall application of one maintenance coat of finish restores the general appearance of the timber and maintains its protection
When carrying out any coating work, do not attempt to paint when the temperature is below 5 degrees Celsius, or if the relative humidity exceeds 80% as the curing and performance of the coating may be impaired. If there is any doubt about the substrate or underlying paint film, apply the appropriate Teknos products to a small, inconspicuous area, allow to dry for 24 hours & then inspect for appearance and adhesion to substrate. Repair products can be obtained from Teknos (UK) Limited, and can be stored in frost free conditions for up to six months in sealed packaging
Discolouration of Coated Joinery
Discolouration of coated joinery is one of the most common site complaints on newly installed factory finished joinery
Milky discolouration in translucent systems is usually a sign of incomplete through drying of the coating and can happen in cold conditions, when the joinery has been incorrectly wrapped or is stored in damp conditions. This discolouration will disappear within a few days of the joinery being installed as the residual moisture dissipates to atmosphere and the coating achieves full cure
Staining of opaque coated joinery is a more complex phenomenon, and generally results from two main sources: soluble extractives or tannins; and resins, both of which are naturally present in timber. Tannins are the main cause of staining in hardwoods, whilst resin exudation tends to be more prevalent in softwood varieties, particularly around knots
As well as timber species, the severity of staining is also influenced by the following: - growing region; knots; preservation treatment; processing and exposure conditions. Moisture plays a major part in tannin migration through the coating system and site problems occur most often when dry joinery from the factory is exposed in warm moist conditions: the rapid uptake of moisture as the joinery conditions tends to mobilise the resin and tannins whilst heat and sunlight draw them to the coating surface
With good timber selection, care in the preservation process and appropriate coating selection, the problems can be significantly reduced and in some cases wholly eliminated
Soluble extractives and tannins
Many tropical and naturally durable timbers contain soluble extractives that are released when the timber is wetted by a coating. Such extractives can discolour the coating film, and the effect is most pronounced with traditional water borne coatings. Western Red Cedar is perhaps the most extreme example of a species prone to this type of staining, but grades of Iroko, Idigbo, Sapele, Brazilian Mahogany and Meranti can also show evidence of tannin staining as can individual batches of other timbers and modified timber products. Variation in growing region can produce significant changes in the mobility of extractives found in a specific timber species
Solutions
Timber selection may help to minimise the problem, though in many cases it can be difficult to predict if tannin staining will occur. Fortunately, with the appropriate selection of coatings, preventing tannin staining is relatively straightforward in most situations. Water based isolating primers, such as Teknos’ “Anti Stain Aqua 2901”, which form a barrier against tannins and help inhibit their mobility, will in most cases eliminate the problem, and in extreme cases a solvent based Teknal isolating primer will give almost total protection. When applying stain inhibiting primers it is important that subsequent sanding is kept to a minimum to avoid removing the protective layer and this is particularly important on profile edges and rounds
Resin staining and exudation
In addition to soluble extractives, the cell structure of wood contains groups of chemicals, often referred to as exudates. In softwoods, the principle exudate is referred to as resin and can show itself in coating discolouration, viscous liquid or crystalline solids on the surface of the timber or coating, or a combination of both. Resin staining and exudation is commonly seen at knot margins, but is also found in resin ducts and sometimes on latewood bands. Timber species, age, growth conditions, season of harvest, the ratio of sapwood to heartwood and the number of live knots all have an influence
The staining associated with resin is chemically different from the tannin stains of hardwoods and many hardwood isolating primers offer relatively ineffective protection against resin staining
Resin exudation is usually initiated by a combination of heat and moisture, which changes the resin from a solid to a liquid and in severe cases to a gas. In joinery processing, drying equipment, heated storage, stretch wrap packaging, and heating systems in buildings post fitting are common initiators
In service, specific weather conditions can trigger the problem and discolouration will occur most rapidly on warmer southern elevations when humidity levels are high. Darker coating colours, which absorb more energy, may initially disguise staining, but can accelerate resin migration leading to blistering and adhesion failure
Preservative solvents can also be a factor. If insufficient drying time is allowed, residual solvent that has absorbed resin, colour and natural extractives from the timber can discolour subsequent coatings. The problem is often seen around knots, where the preservation fluid will partly dissolve resin concentrates, bringing them to the timber surface, producing severe staining and blistering even when knotting solution is used
To reduce the problem, allow sufficient drying time between preservation treatment and subsequent coating or switch to a surface applied, water based preservative such as Teknol Aqua 1410. Preserved timber will dry faster if air is allowed to circulate the surface of the substrate, and stacked timber should always be separated with spacing bars
Solutions
There is no foolproof solution to the problem of resin exudation, though timber selection to exclude knots from decorative surfaces is increasingly common and effective. Finger jointing or laminating can offer an economic route to knot free surfaces and with careful selection of basecoats an aesthetically pleasing result can be achieved even with translucent finishes
When a coating solution is required, Teknos’ preferred method involves filling and sealing the knots with Teknofill 5001 fine surface filler, sealing the knot area with Teknos knotting solution, and over-coating with a specially formulated isolating primer, Anti Stain Aqua 5200. This system will not prevent the exudation of liquid resin in extreme cases, but has proved to be highly effective in minimising resin stains and restricting the affected areas
Other common coating solutions include: -
- Primers with barrier pigments such as leafing aluminium offer fairly effective short-term protection against staining, but only delay the exudation process and because they embrittle the film can lead to other failures
- Two pack epoxy and polyurethane coatings form a dense coating layer that encapsulates the timber restricting movement of extractives and resin through the paint film. Short term, these coatings offer good protection, however, pressure from resin exudation can, over time, push these coatings off the timber surface, leading to total adhesion failure
RESIN EXUDATION REMEDIAL MEASURES
When resin has exuded through a permeable coating, the best remedial treatment is to allow the resin to weather until it dries and oxidises, forming a white crystalline powder. The dried resin can then be removed with a stiff nylon or natural bristle brush, and any remaining residues washed off with a cloth.
Water based coatings with their relatively high degree of moisture vapour permeability are more likely to allow the passage of resin to the surface without damage to the coating, and if the finish is not damaged by over-vigorous scrubbing during crystal removal, re-coating is often unnecessary.
Although it may be unsightly, it is better not to remove fresh sticky resin. In practice, this can be very difficult, and the presence of sticky resin indicates that the exudation is still continuing
The remedial work for resin exudation is often best left until the first maintenance period, by which time the resin has normally fully crystallised. After removal as described above, the overall application of one maintenance coat of finish restores the general appearance of the timber and maintains its protection
Care and Maintenance Guide kindly reproduced from Bereco consumer literature, for use with their products
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