The steady increase in insulation standards has many well-publicised benefits both to building users and the environment. However, increasing insulation is not without its attendant risks. The main risks are those associated with damp, cold conditions i.e. condensation which occurs both on the surface and within the structure itself (interstitial condensation). Adding insulation changes the way in which heat and moisture flows through the fabric, creating 'cold-spots' where moisture-laden air falls below its dew-point and deposits its moisture - this can then give rise to mould growth and decay within the structure which may take years to appear - or more dramatically results in deluges from the ceiling. Other problems arise with increased thermal stress placed on elements of the construction which under lesser standards of insulation would not be a problem. All these potential problems can be avoided if allowed for during design.
Moisture is held in air as a gas - water vapour. Air will only hold a specific amount of moisture at any given temperature. When air can no longer hold moisture it has become saturated. The temperature at which the air becomes saturated is known as the dewpoint. Below the dewpoint air deposits its moisture as condensation- like the dew on grass after a cold night. Air is not usually saturated but contains a proportion of its saturated state - which percentage is known as its relative humidity (RH). The RH increases through moisture being introduced from breathing, cooking, washing, water eveporation etc. It is reduced through ventilation provided that the incoming air does not bring in more moisture than it takes out. In cold damp climates (as in the United Kingdom during winter) increasing air changes above 1 per hour does not reduce the RH. There is a direct relationship between moisture generation, heating, ventilation and thermal resistance of the structure.
Air containing moisture vapour exerts a greater pressure than air without moisture - due to the increase in molecular concentration. The greater the humidity, the greater the pressure. These relationships are not linear- note that as the air temperature increases, the SVP (saturated vapour pressure) increases exponentially.
Walls, roofs and floors moderate the difference between external conditions (temperature and humidity) and the internal (design) conditions. The properties of materials that make up the wall, and their thicknesses and juxtapostions, affect the way in which heat and moisture flow between the inside and the outside. This varies from summer to winter, and is affected by deterioration of any of the materials within the envelope structure. Heat flows through the structure and is slowed down through the use of insulation creating thermal resistance. Similarly water vapour passes through the structure driven by differential pressure between the inside and the outside or vice versa. The passage of vapour is slowed down by the vapour resistance of the construction.
All building materials resist the flow of moisture vapour to a degree - even still air in a cavity. The vapour resistance of a structure is the sum total of the vapour resistances of its elements. Materials vary between those whose vapour resistance depends on thickness e.g. brick, plaster, concrete and which are the product of thickness and vapour resistivity and those which are sheet materials of negligable constructional thickness. Where these sheet materials have a high vapour resistance they are known as vapour checks or vapour control layers (the old term of vapour barriers which was applied to such materials was misleading as no materials is completely impervious to moisture). Sheet materials which are designed to have a low vapour resistance and help the flow of moisture vapour through the structure whilst preventing water penetrating the structure are known as breather membranes. The vapour performance of materials is often given as permeance and is similar to the lambda value used in thermal calculations.The permeance is the inverse of the vapour resistivity.
Manufacturers around the world give figures for vapour resistance, vapour resistivity and permeance in different units - some of which depend upon precise testing conditions. Test values which are made under laboratory conditions, can differ significantly from performance on site where gaps, tears and detailing play a significant role in performance.
When heat flows out of the envelope, the temperature of the internal surfaces is slightly lower than the internal air temperature due to the surface thermal resistance. The size of this temperature drop is related to the overall thermal resistance of the construction. Should the air carrying moisture vapour drop below its dew-point at the wall surface - excess moisture will be deposited. If this situation persists then surface mould will begin to form.
Moisture vapour and heat flow through constructions at different rates. Unless this is allowed for in the design it is possible, and quite likely, that moist air can arrive at a material in the construction the temperature of which is below the dew-point of the air itself. The result is interstitial condensation which in the long term can result in significant fabric decay. In the short term (particularly under metal roofs) damage can be done to ceilings, equipment and finishes. Interstitial condensation can be designed out by careful calculations using approved algorithms. Computer models have proved successful in analysing constructions to reduce risk to a minimum. The design of the fabric is achieved by balancing thermal and vapour resistance performance, with proper ventilation of cavities within the structure. The greater the thermal/moisture stress, caused by extremes of temperature and humidity, the greater the care that must be taken over the design. This is particularly the case with highly insulated, low-energy buildings.
This is a term applies to interstitial condensation which occurs when the usual design conditions are reversed. In the United Kingdom this happens during the summer when moisture can be forced back through the structure to condense on the back of internal linings. A particular case of this is condensation which can form on the underside of metal decking - in particular lead - during exceptional summer conditions of high humidity and high temperature which occur during thunderstorms. Condensate thus formed aggressively attacks the metal and can cause early failure. Correct ventilation is usually the way to avoid the problems associated with 'reverse flow' condensation.
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