It is known that the Advanced Construction Solution MK2, with D.I.T. 455R/20, is characterized by combining the structural capacity of reinforced concrete with the thermal insulation capacity of EPS, which allows solving the structural needs of insulation and enclosure of the building with a single construction system that scrupulously adapts to the requirements of the current CTE and EHE.
It is very remarkable, in this constructive system, the capacity to solve all the enclosure with a continuous insulation, even in the joints between horizontal and vertical elements, without the need to include structural elements such as columns and beams that may imply thermal bridges that may damage the insulating capacity of walls, floors and roofs.
This characteristic of the system allows that the thermal isolation that reaches the realized buildings is one of the most outstanding qualities of this system, allowing the best valuations in energetic certification in a much simpler way that with traditional systems. The continuity of the isolation and the facility to obtain the hermeticity of the different atmospheres with this constructive solution, put easily within reach even the attainment of a certification Passivhaus, according to studies of architects who work with this one.
But one of the most common questions we are asked is related to the galvanized steel connectors that maintain the integrity of the different layers that make up the system. Don’t they produce a considerable decrease in the thermal properties of the system? Don’t they represent an important thermal bridge?
The MK2 panels have 80 connectors per m2 of galvanized steel with a diameter of 3 mm, which are responsible for joining the steel grid on the exterior face with that on the interior face and for containing the EPS confined between the two meshes.
The distribution of the connectors on the surface is not concentrated in certain areas, they are distributed more or less evenly over the entire surface of the enclosure, which allows their effect to be dissipated evenly over the entire surface.
In addition, these are elements that are very “disproportionate” geometrically in that they have a very small diameter in proportion to their length, resulting in a very small contact surface with the outside in proportion to their length (7.07 mm2 versus 160 mm), while in the transverse direction they have a large contact surface with the adjacent EPS in proportion to their diameter (1,507.2 mm2 versus 3 mm).
It is logical to think that there will be a more relevant and rapid transfer and dissipation of heat transversally, with the surrounding EPS, than longitudinally along each connector.
But we shall now study the repercussion of this geometry numerically, from the point of view of heat capacity and thermal transmittance, in an MK2 wall made up of panels of the PN or PR 140 type (21 cm total thickness), the most commonly used for an enclosure:
The heat capacity or thermal capacity of a body is the quotient between the amount of heat energy transferred to a body or system in any process and the change in temperature it undergoes. In a more rigorous form, it is the energy necessary to increase the temperature of a certain substance in a unit of temperature.
It will be: C = c x m
Where:
“C” is the heat capacity of the material
“c” is the specific heat of the material
“m” is the mass of the material
A wall surface of the MK2 system of 1.00 m2 is then studied, without considering the effect of the connectors. For the materials used in the MK2 system, the following table is obtained
c (Kcal/kg °c) | m (Kg/m3) | V (m3) | C (Kcal/ °c) | |
EPS | 0,29 | 15 | 0,14 | 0,609 |
Concrete | 0,16 | 2.400 | 0,07 | 26,88 |
TOTAL | 27,489 |
The table below considers the variation in the heating capacity of the wall caused by the inclusion of the effect of the 80 connectors:
c (Kcal/kg °c) | m (Kg/m3) | V (m3) | C (Kcal/ °c) | |
EPS | 0,29 | 15 | 0,139921 | 0,6086 |
Concretew | 0,16 | 2.400 | 0,069988 | 26,8754 |
Steel Connectors | 0,12 | 7.950 | 0,0000904 | 0,08624 |
TOTAL | 27,57 |
The result is that the connectors have an effect of 0.29 % on the heat capacity of the wall, which is practically negligible.
The thermal transmittance U is the measure of the heat that flows per unit of time and surface transferred through a construction system formed by one or more layers of material, with parallel flat faces, when there is a thermal gradient of 1°C (or 1K) of temperature between the two environments that it separates.
It will be: U = 1 / Rt = 1/(Rsi+R1+…+Rn+Rse)
Where:
“Rt” is the Total Thermal Resistance (m2-K/W)
“Rsi” is the Indoor Surface Thermal Resistance (m2-K/W)
“Rj” is the Surface Thermal Resistance of each layer (m2-K/W)
Rj= λj/ej where “λj” is the Thermal Conductivity of the material (W/K*m) and “ex” is the thickness of each layer (m)
“Rse” is the Outer Surface Thermal Resistance (m2-K/W)
As in the previous section, the influence of the connectors will be assessed by comparing the transmittance value in an MK2 PN 140 wall without considering the effect of the connectors with one in which their effects are taken into account.
Thickness (m) | λ (W/K*m) | R (m2·K/W) | |
Outside | 0,04 | ||
Concrete Outer Layer | 0,035 | 1,8 | 0,019 |
EPS Layer | 0,14 | 0,039 | 3,59 |
Concrete Interior Layer | 0,035 | 1,8 | 0,019 |
Interior | 0,13 | ||
TOTAL | 3,799 | ||
Transmitancia “U” | 0,263 W/m2·K |
At the points of the connectors, it will be obtained:
Espesor (m) | λ (W/K*m) | R (m2·K/W) | |
Outside | 0,04 | ||
Concrete Outer Layer | 0,025 | 1,8 | 0,0139 |
Steel | 0,16 | 50,2 | 0,00319 |
Concrete Interior Layer | 0,025 | 1,8 | 0,0139 |
Interior | 0,13 | ||
TOTAL | 0,201 | ||
Transmitancia “U” | 4,98 W/m2·K |
It should be remembered that this high transmittance value is obtained in points of 3 mm diameter far apart from each other. To evaluate the effect of this transmittance per m2 of wall, an average thermal transmittance per m2 is assessed:
The surface of the connectors in each m2 is 80 x 0.07068 = 5.65 cm2
The enclosure area, per m2, that does not contain connectors will be:
100 cm x 100 cm – 5,65 cm2 = 9.994,35 cm2
To calculate an average transmittance, we’ll have
(9.994,35 x 0,263 + 5,65 x 4,98) /10.000= 0,266 W/m2-K
The difference between the transmittance value with and without the effect of the connectors is 0.003 W/m2-K, a very small value, also in this case.
The conclusion is that, due to their low impact on the properties of the facing from the point of view of heat capacity and from the point of view of thermal transmittance, the connectors do not represent a significant reduction in the insulation capacity of the system and cannot, in any case, be considered as a thermal bridge.