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