a wide institutional framework that includes more factors for evaluating the energy efficiency of a building.
However, building techniques that were established for over three decades have failed to adjust to new regulation demands. Nowadays constructors try new techniques that may present better results concerning energy efficiency and building construction.
Major concern is usually the thickness and location of the thermal insulation layer. Although the regulation indicates a standard numerical result, application technique remains a challenge. The technique of positioning the insulation layer in the core of the confined masonry and on the exterior side of the bearing structure – as it was applied in Greece for years – seems to encounter problems and it is not always the appropriate technique. The new regulation sets higher requirements for the thickness of the insulation layer and takes under consideration the created thermal bridges (the old regulation did not refer to thermal bridges).
A thermal bridge is the point in a building shell where the heat flow appears increased compared to heat flow of the adjacent area or where the thermal resistance appears reduced in that point compared to thermal resistance of the adjacent area.
This might be due to shell geometry or by a discontinuity of the insulation layer. The cause of discontinuity may be a construction failure, building defects, material faults, omissions or negligence, deterioration or other extraneous factors.
To calculate heat loss or heat gain in a building, we ignore the 3D form of heat flow. We usually adopt that heat flow is a one-dimension value that has a vertical direction to the building material surface. This simplifies the calculations and the numeric result does not differ from reality. It is true that in each point of a flat surface the heat flow that is not vertical is neutralized by the heat flow of adjacent areas. On the contrary, in thermal bridge points where homogeneity is interrupted or building geometry is different (e.g. on a two- or three-side angle) the fore mention adoption cannot be realistic as it is revealed that the heat flow is multi-dimensioned in the mentioned point.
In Greece, the previous Building Insulation Regulation (BIR), as the new Regulation for Building Regulation Energy Efficiency (RBEE), also adopts the assumption that heat flow is a one-dimension value to facilitate calculations. But the BIR did not take under consideration the building geometry and the thermal bridges, facts that are considered and calculated by the RBEE.
The RBEE introduces a value in order to calculate heat flow on a thermal bridge point. This value is the factor of linear heat permeability (Ψ) that is calculated by the difference between 2D and 3D thermal flow and is equal to the unit of length in the thermal bridge point. The factor of linear heat permeability (Ψ) is measured in W/ (m•Κ) and multiplied by the thermal bridge length (l), gives the extra heat loss with the heat transfer factor (Ηθ/γ) described by the relation: Ηθ/γ = Ψ × l [ W/K ].
The RBEE describes values for the factor of linear heat impermeability either through cross sections and their respective values of Ψ, either through simple value tables. In both cases, values are described just for only two materials: in concrete and in brickwork, indicating the same value use for all other material, considering them approximately identical to one of the two aforementioned based on their conductivity or density. For example, metal or stone are considered as concrete, while wood, gypsum and other light material are considered as brickwork.
Performed Control Methodology
The results of this method present a certain divergence from real values, either small or big. Nevertheless, they are considerate as efficient for the calculation of building energy efficiency and they do not present a problem to the constructors or the researchers.
On the other hand, a widely presented problem to the researchers and the constructors is the correct position of the insulation layer in order to minimize thermal bridge effects. There is the concern whether positioning the thermal layer in the internal, external or in the core of the construction effects and to which extent the heat loss, and specifically, what is the thermal flow percentage resulting from thermal bridges present in the construction shell.
An answer to this question may be given by a calculation control. The control is about calculation of heat loss (according to RBEE) through heat transfer factors and presentation of a percentage on the total of the heat loss of a building shell in every different positioning of the insulation layer. The results of such a calculation control are presented in this paper.
More specifically, the elementary construction unit (hereinafter the “module”) was taken as a basis for the control study. The module is a ground-floor building of a virtual square floor plan, free from all sides, made of concrete and confined masonry of brickwork. The characteristics of modules are presented in the following context.
Control has been conducted on three building types:
– Building on ground surface (without a basement).
– Building on pillars with an open space (pilotis).
– Building with a non-heated basement.
All three building types were studied on three different construction types, with an insulation layer being:
– on the external side of the building element,
– on the internal side of the building element,
– in the core of the building element on the confined masonry and externally on the elements of the main structure.
For the internal insulation study, it is assumed that in all interval pillars, a vertical thermal bridge occurs from the transverse masonry towards the outer shell that interrupts the insulation layer throughout the construction element height.
This basic unit was multiplied for the three dimensions of the building, defining in this way all small or big buildings (one-floor or multi-floor buildings, detached houses or apartment blocks).
Control study results
Basic building unit
On the basic building unit (ground floor building) the control study showed that, in terms of heat loss through thermal bridges, the best conduction is presented by a building in which the insulation layer is placed internally. This of course is expected, since the insulation layer, being disposed inside the set of building components (perimeter shell, floors and ceilings) does not present any discontinuity but only at the intermediate column positions, which support an internal wall transverse to the outer shell.
Building without overhangs
During the control the fundamental building unit (ground floor building) only with windows and without overhangs the price of heat loss through thermal bridges in a building with internal thermal insulation is almost always negative. This means that we overrated the amount of heat loss from the flat surfaces and must be lessen in the calculated amount (Ηθ/γ) in sections of thermal bridges. Actually, the positive values of heat transfer factors (W × l) of the vertical thermal bridges at positions where the internal walls encounter transversely the outer shell (unique places where the heat insulating layer is interrupted) are lower than the negative of other thermal bridges, i.e. the horizontal and vertical angles.
In a same building (no overhangs, with windows), when the insulation is placed externally, control results are very satisfying as the total heat loss does not exceed 15% of total heat loss of the building.
In the case of a one floor building onto a pilotis, the rates of heat loss are higher and present 20% of total heat loss.
By contrast to the two insulation positions (in the internal or the external of the shell), placing the insulation into the core of the masonry, on a building with windows but no overhangs, presents high heat losses due to thermal bridges that reach 40% of the total heat loss.
Building with overhangs
On the contrary, on a ground floor building with balcony doors and overhangs the percentage value of thermal bridge sum is positive when the insulation is placed internally, but remains well below that of the other two types of thermal insulation (external and core). This positive but small value of the rate of heat loss due to thermal bridges observed in all three types of test buildings with insulation internal (on ground, on pilotis and on underground non-heated space). This positive but small value of the rate of heat loss due to thermal bridges is observed in all three types of test buildings with internal insulation (on ground floor, on pilotis and for underground non-heated space). Increased heat loss on a building with overhangs is generally due to the fact that the overhangs are extensions of the horizontal floors, which necessarily interrupt the continuity of the insulation layer of the vertical building components.
When the insulation is placed outside the same building with overhangs and doorsteps that rests on the ground, the rate of heat loss due to thermal bridges reaches 25% and it is higher in 10 percentage points than in a building without overhangs. This percentage goes up a little more and reaches 28% when the building is based on pilotis.
Finally, when the insulation is placed in the core of masonry, the rate of heat loss increases considerably and reaches 34%. The proportionate share of heat loss due to thermal bridges is the same in a building that has a basement unheated space. Nevertheless, the percentage of thermal loss due to thermal bridges is less than that of a building without overhangs. However, in absolute terms, the heat loss in all types of buildings with overhangs is greater than the corresponding types without overhangs.
Heat capacity of structural elements.
It is clarified that, although the building with internal thermal insulation exhibits a smaller heat loss, it lacks in heat capacity (key element in buildings) as the desirable is to maintain the heat when the heat system is shutdown. For example, in a residential building it is desired to maintain heat for as long as possible after we turn off the heating, as opposed to an office building, in which after office hours, the reservation of high temperature is not desired.
Moreover, we should remember that the risk of condensate formation due to diffusion of water vapor inside a structural element is much more likely when the insulation is placed on the internal side than on the external.
The study also showed that in the four zones of the RBEE the differences are too small. In some cases also there are no differences. For this reason, we selectively present the results of Zone C only, considering that about the same applies to all zones.
In height extension of basic building unit
Internal heat insulation
However a significant change of the above image is presented in a multi-floor building, as the basic structural unit multiplies in height with the addition of more floors.
Thus, when the thermal insulation is placed internally, ceases to have a negative value in the heat loss through thermal bridges. Heat loss in a two-floor building is limited from 10% to 12%, when it comes to building without overhangs and is approximately the same as the percentages of het loss in a building with external insulation, while reaching a percentage of 15% in a two-floor building with overhangs and are lower than the rates of heat loss of a building with external insulation (about 25% to 27%). This increase in heat loss is due to the separation ground of two floors, which now interrupts the continuity of the insulating layer over its entire area, creating large thermal bridges.
Obviously, as the number of floors increases, the percentage of heat loss gets higher in a multi-floor building with internal insulation, because there is more heat loss from each floor added. As percentage - based, this increase is not linear and continuously decreases as the levels are increased, because the big increase of heat loss is made when adding the second floor and the appearance of the first partition plate that has interrupted the insulating layer and disproportionately increased thermal loss. Gradually the added new rates are "softened" as they are incorporated in the total heat loss from the remaining building material, which display stable heat loss on every floor. This increase tends to be minimized and the rates of heat loss from the positions of the thermal bridges in this building with internal heat insulation tend to stabilize at a rate of around 20% to 25% depending on the building type (on ground, on pilotis, on a non -heated underground space, with or without overhangs).
External heat insulation
In contrast, the rates of heat loss in a building with external insulation remain constant in all types of buildings, indifferently of the number of storeys, with a minimum reduction of 1% when floors are rising in buildings with no overhangs. A differentiation exists only in buildings with pilotis, where there is a bigger reduction of thermal loss of around 4% to 6%, added to the first added floors, on buildings with or without overhangs. However, on higher buildings on pilotis (over 4 levels) we observed again a stabilization of thermal loss with a small downward trend of 1%. The stability in the rates of heat loss values due to thermal bridges is also observed in buildings with thermal insulation in the core of masonry regardless of the number of floors. The rates range around 40% to 47% when it comes to buildings without overhangs, and around 32% to 36% when it comes to buildings with overhangs.
Note, however, once again that these values are proportionate share (of heat loss due to thermal bridges) of the total heat loss from building shell. In absolute values, the heat loss from buildings with overhangs is higher than these form buildings with no overhangs.
It is not possible to provide a unique answer concerning the correct application of insulation layers as to prevent thermal loss due to thermal bridges. Criteria are related directly to the building type and geometry and should be considered differently in each case.
In general, it is proved that heat loss due to thermal bridges is more restricted when the insulation layer is placed on the internal face of the construction material. Nevertheless, that is not absolute because it depends on the amount of the transverse construction material towards the outer shell as in such points the insulation layer is interrupted and a strong thermal bridge is created. As floors are added, choosing internal insulation is not the best choice as major thermal bridges are added, situated to the points that the floors meet the outer shell. Because of this fact, in multi-floor buildings external insulation is better than internal insulation. Moreover, it should be noted that the thermal bridges can be limited in low-rise buildings, but it also limits the heat capacity of components as they remain thermally unprotected. The insulation layer protects only the indoor air, so when the heating system is shutdown, the indoor air also cools quickly. On the contrary, external heat insulation is more efficient in all building types examined and heat loss rates remain relatively low. The heat loss from thermal bridges increases in a form of a percentage of total heat loss in buildings with overhangs, as a break in continuity of the partition plate between floors contributes to increasing the value. Finally, buildings where the insulation layer is in the core of the masonry, present the biggest heat loss due to thermal bridges. In this insulation type, more thermal bridges are created due to continuing interruption of the insulation layer in every point of interlock between masonry and bearing structure (bars, pillars, walls and floors) or even in the cement ring points.