General Principles of Thermal Insulation

Fabric Heatloss

All thermal insulation materials work on a single basic principle: heat moves from warmer to colder areas. Therefore, on cold days, heat from inside a building seeks to get outside and on warmer days, the heat from outside the building seeks to get inside.

Insulation is the material which slows this process. Rigid phenolic and urethane insulation materials have tiny pockets of trapped gas, these pockets resist the transfer of heat.

Heat transfer occurs when a hot surface is surrounded by an area that is colder, heat will be transferred and the process will continue until both are at the same temperature. Heat transfer takes place by one or more of three methods; conduction, convection and radiation.

In order to perform effectively as an insulant a material must restrict heat flow by any, and preferably, all three methods of heat transfer. Most insulants adequately reduce conduction and convection elements by the cellular structure of the material. The radiation component is reduced by absorption into the body of the insulant and is further reduced by the application of a bright foil outer facing to the product.

U-Values & Thermal Looping

Thermal Looping

The thermal performance of a plane building element, within a particular construction, is described by its U-value. This is a measure of the heat transmission through the element per degree of temperature difference (W/m2K) (degrees Celsius denoted as degrees Kelvin to signal temperature difference) between the internal and external environments.

The lower the U-Value, the slower the rate of heat transfer and the better the thermal performance of the construction element under consideration.

A U-Value is a theoretical measure of the thermal properties of a construction and may not necessarily provide a true reflection of the actual thermal performance, which will be larger dependant on how the Insulation is fitted. No matter how thick the insulation is or how good the material is, if it isn’t fitted correctly and an air gap is formed then the performance of the structure is drastically affected.

An example of this is might occur is where an air gap occurs between the insulation and the cold masonry/block surface, air can circulate from the warm side of the insulation to the cold side. The heat is pouring out at this point. This air movement is known as ‘Thermal Looping’. This is critical as thermal looping dramatically reduces the effectiveness of the insulation and can result in the actual U-Value being double that of the calculated value (therefore only 50% of the intended performance).

Potential benefits of thermal mass are negated as the ventiated air in the cavity is contantly cooling the inner leaf block. The wall never builds up a store of heat as it continually fighting to maintain a constant temperature. The rapid cooling effect of the ventilated air in the cavity carries any heat stored in the block rapidly away using the air path created by thermal looping

Advantage of Timber Frame construction

In Timber Frame construction the problem is completely eliminated, as there are no air gaps to allow thermal looping to occur within the insulated part of the wall, as the insulation is positioned entirely within the internal leaf (warm face).

Indeed in timber frame U-Values are often found to be on average 10% better than their calculated values. Kingspan Century Timber Frame construction also incorporates factory fitted insulation, ensuring insulation is fitted precisely under stringent ISO quality control conditions to provide the ultimate guarantee of thermal performance

Effects of Build Quality on Thermal Performance

In cavity wall construction poor build quality and workmanship defects such as mortar droppings and air gaps in insulation can significantly reduce the calculated thermal performance of walls.

Research has indicated that U-values increase markedly when gap sizes between sections of insulation exceed 10mm. Air gaps are a serious issue with current forms of cavity construction, and can affect all types of insulation products.

Research carried out between 1998 and 2000 showed that true (measured) U-values were higher than calculated U-values, even when thermal bridging and wall ties were taken into account. The difference depended upon the type of construction, and the differences were found to be as follows:

  1. For internally insulated cavity walls, 0.05 W/m2K (approx.)
  2. For fully filled cavity walls, 0.05 W/m2K (approx.)
  3. For partially filled cavity walls, 0.10 W/m2K (approx.)
  4. For timber frame walls, agreement between measured U-values and calculated U-values tends to be close,
  5. For sloping ceilings with insulation in the slope of the ceiling, actual realised U-values can be extremely high.

These are major discrepancies and are highly significant in thermal terms. In addition to this, as insulation standards for new housing are nominally increasing, the difference between theoretical and actual U-values is likely to become greater and could have serious implications.

Kingspan Century

Kingspan Century Limited

Clones Road


H18 TX09

+353 (0)47 38400

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