Heat transfer through concrete block wall into a building is determined and comparison has been made with insulated hollow concrete block. The paper deals with use of different type of thermal insulations poured into the holes of the hollow concrete block wall and heat entry in building. The result of the study shows that nearly one tenth heat flow of concrete block wall takes place through PUF/ EPS insulated hollow concrete block. Similarly, 35% higher heat ingress is recorded by the layered thermal insulation used parallel to the concrete wall than the 12% insulated hollow concrete block wall. It is observed from this study that by increasing the volume of the voids (insulation) of hollow concrete block from 12% to 15 %, the heat ingress is further reduced by 22% approximately.
Dr. B. M. Suman, Principal Technical Officer, CSIR- Central Building Research Institute, Roorkee, Uttarakhand
Thermal properties of light weight concrete using agro-industrial and forest wastes can be enhanced by creating voids/holes and put good thermal insulation by pouring or spraying into it. Combination of textiles lining system with granulated mineral wool or glass wool may work as sustainable thermal insulating system suitable for false ceiling.
The concrete block wall allows higher heat flow into the buildings. Therefore, hollow concrete blocks are used for taking advantage of air gap thermal insulation. It is found from ASHRAE (1981)1 and a study2 made on achieving maximum thermal insulation value of the air gap. The result of the study shows that maximum thermal insulation was found when air gap is 38mm. For most of the cases, thermal resistances apply only to air spaces of uniform thickness bounded by plane, smooth, parallel surfaces with no leakage of air to or from the space. These conditions are not normally present in the standard building construction. For determination of accurate value of overall thermal transmittance of all types of construction with or without air space, the use of Guarded Hot Box apparatus3 working as per the code IS 9403 is essentially recommended. Keeping above in view, the qualitative thermal insulation replacing the existing air space in the concrete block gives better results. Heat entry into the building through roof and walls depend on their thermal resistances. As an example, when plastic foam was applied as superficial thermal insulation on mass concrete and effect of this is excellent. Construction with plastic foam is easy and the cost is also not very high so it can be used for long-term thermal insulation. Thermal resistance of multilayered concrete block (concrete block and thermal insulation) of a wall or roof will be the algebraic sum of thermal resistance of all the layers. Here, layer of insulation is used in between and parallel to the concrete block layer, therefore, the insulation is pressed enough to affect thermal resistance value. By pouring thermal insulation within the holes of the hollow concrete block may be termed as composite material, such case of pressure does not arise. Different types of hollow concrete blocks have been produced at Central Building Research Institute Roorkee. Hollow Gypsum Panels for using non-load bearing walls is one of the hollow blocks. Although, Gypsum has thermal insulation property but by adding good thermal insulation into the holes of Hollow Gypsum Panels, a better thermal-resistant material can be developed. Due to convective heat current within the holes of Hollow Gypsum Panels, its thermal resistance value does not improve, adding good thermal insulation into the holes is required to enhance its thermal resistance.
Novel insulations4 are both conductance-resistant and radiation-resistant. In the combination of conductance-resistant and radiation-resistant, first the insulation is characterized by heat transfer due to thermal conductance described by Fourier’s Law, and the second one is characterized by radiation heat transfer based on the Stefan- Boltzmann law. The combination depending on the use of bulk and particulate materials leads to an optimized and highly energy-efficient novel insulation design. Most examples of the combination are used for the application of high temperature difference. In case of building, thermal insulation is used for low temperature difference application and therefore, only conductance heat resistance is needed to enhance for building application.
The Hollow Concrete Block
Hollow concrete blocks are produced since long in our country for taking advantage of air gap in concrete block. But it is noticed that due to various other reasons and width of air gap is higher than prescribed value to start the convective current of heat in the air gap itself. Therefore, advantage of thermal resistance of air gap is not felt. For getting better thermal resistance, the good insulation materials like Mineral Wool, Glass Wool, Expanded Polystyrene, Expanded Polyethylene, Polyurethane foam etc., are poured into the holes of the hollow concrete blocks.
A concrete block is referred to as a concrete masonry unit (CMU) in the construction industry. Concrete blocks can be solid or hollow with two or three voids or holes. Concrete blocks are ideal for foundation and basement walls and partition walls in any home which can be put up quickly using hollow concrete blocks. The external wall can be made up by using concrete blocks with their cores or voids poring (filling) with good thermal insulation. Such hollow concrete blocks provide thermal resistance against cold and heat and reduces home’s energy consumption. The use of concrete block is economical due to dimensional accuracy and larger size of the hollow block leads to reduction in plastering and jointing costs. By pouring insulation into the holes of hollow concrete blocks, density becomes lower and lighter which reduces the dead load. Results of the study show that it has excellent thermal insulation properties. Since it is pre-cured product, it saves water during construction. There is no chance of efflorescence, therefore, reduction in maintenance cost.
Thermal Resistance of concrete block
Concrete blocks do not have good thermal resistance. By making these hollow, their thermal performance improves. But due to larger hollow and due to convective heat flow within the holes of the block, its thermal resistance does not improve much. Therefore, to improve its thermal properties good thermal insulation is poured into the holes of the block. Thus, its thermal resistance becomes higher. Thermal resistance of material is computed as electrical resistances which are combined in parallel or series and the resultant resistance depends upon whether resistance is in parallel or in series. Accordingly, resultant resistance is computed as,
Rseries = R1 + R2 + R3 +
Rparallel = 1 / R1 + 1 / R2 + 1 / R3 + --------- (1)
Computation of overall thermal transmittance (U) value
By taking the resultant thermal resistance (R) of the materials (hollow concrete block) and taking inside surface heat transfer coefficient, hi and outside surface heat transfer coefficient, ho respectively. The U-value5 of the hollow concrete block is given by the equation given below.
U = 1 / (1/hi+ΣR+1/ho) --------- (2)
Where, ΣR is either Rseries or, Rparallel hi = 9.36 & ho = 17.86 for building components.
The computed thermal conductivity of some building and insulation materials is given in Table 1. Similarly, the computed R and U are given in Table 2 and Table 3 respectively.
|Table 1- Thermal conductivity of building and insulation materials|
|S.No||Name of the material||Thermal Conductivity|
|3||Poly Urethane Foam||0.026||0.1092|
|7||Expanded Polystyrene (EPS)||0.036||0.1512|
|8||Exponent Polyethylene (EPE)||0.043||0.1806|
|Table 2 - Thermal Resistance of insulated hollow concrete block|
|S.No||Name of the insulation poured in the holes||Thermal Resistance(R) (m2K/W)|
|12% Hole size||15% Hole size|
|5||Expanded Polystyrene (EPS)||2.300||3.166|
|6||Exponent Polyethylene (EPE)||2.021||2.714|
|Table 3- Overall thermal transmittance of hollow concrete block|
|S.No||Name of the insulation poured in the holes||Overall Thermal (U)||Transmittance W/m2K|
|12% Hole size||15% hole size|
|5||Expanded Polystyrene (EPS)||0.407||0.301|
|6||Exponent Polyethylene (EPE)||0.459||0.348|
The diagram of hollow concrete block with two holes within the block is shown in Fig 1. Thermal conductivity of concrete, air and seven number of good quality insulations which can be (poured) into the holes of the block are given in table 1. Thermal resistance and overall thermal transmittance are computed as per equation 1 and equation 2, respectively. Thermal resistance values of concrete block with 12% and 15% inserted insulation by volume are given in table 2. Its curve is depicted in fig 2 as histogram. Similarly overall thermal transmitted values of hollow concrete block with 12% and 15% inserted insulation is given in table 3 and these values are depicted as histogram in the fig 3. The computed values of R and U are compared with the recommended values of thermal resistance and U values for roof, wall and window of the building As per Energy Conservation Building Code (ECBC)6. For multistoried building the exposed walls are more important for heat ingress into the building. The recommended U vales and thermal resistance for exposed wall are 0.440W/m2K mean U value of exposed wall should not exceed 0.440W/m2K for minimum heat ingress into the building. From table 3 it is observed that for 12% insulation of PUF, PIR, EPS satisfies the ECBC recommended value but remaining five thermal insulation of mineral wool, glass wool, EPE, gujwool, poly-isocyanurate do not satisfy with 12% inserted in the block. Therefore the percentage by volume of such insulation are increased from 12% to 15% for achieving the recommended value. It is now clear from table 3 that the ECBC recommended values are achieved by 15% use of such thermal insulation. By using 15% insulation of PUF, PIR, and EPS the recommended value of U for cold climate is also satisfied for exposed opaque wall assembly. The corresponding R-value and U-value of wall assembly is shown in fig 2 and also given in table 2. The quality of polyurethane foam, Expanded Polystyrene, Poly-isocyanurate insulation are superior than the remaining other traditional thermal insulations, and that is why performance of these insulation is better than traditional insulations. Even by using other four thermal insulation with 15% volume in the hollow concrete block is not a difficult job as these values satisfy the recommended value whenever it is said that 12% or 15% volume of the concrete block is inserted into the block means all the hollow concrete blocks used in the construction of wall should be insulation inserted concrete blocks. Then performance of wall assembly or whole building will improve.
Figure 1: Diagram of concrete block with inserted thermal insulation
Figure 1: Diagram of concrete block with inserted thermal insulation
There are number of ways to use thermal insulation in buildings. In some buildings, thermal insulation is used as a layer on external exposed surface of the buildings, somewhere it is used as on internal surface of the building and somewhere it is used as a sandwich panel. All these systems of thermal insulation can be used as layered form. But in the present study, thermal insulation can be used by inserting into the holes of the block to get better thermal performance of wall. Result of the study shows that inserted insulation in wall show better result than multilayered insulated wall system. By taking an example of glass wool using 12% volume in the concrete gives overall thermal transmittance value as 0.448W/m²K where as by using the same glass wool in multilayered system with 5cm thick glass wool and 20cm cement concrete block gives overall thermal transmittance 0.707 W/m²K. Thus multilayered insulation system gives almost 50% higher heat flow showing its lower performance than the composite inserted insulation system.
The study made on combining of thermal insulation poured into the holes of hollow concrete blocks, following facts have been observed.
Thermal resistance of concrete block used in the wall is very low. Its thermal insulation value is enhanced by using thermal insulation as parallel layer to the block in the wall. Such arrangement gives higher thermal resistance and low thermal transmittance.
If the same thermal insulation is used by pouring into the holes of the hollow concrete block, far better thermal resistance and lower heat transmittance is observed. It is found from the study that by using thermal insulation in layered form parallel to the concrete block gives 35% higher thermal transmittance than using poured thermal insulation into the holes of the concrete block.
Further study states that by increasing the volume of the hole for insulation from 12% to 15%, heat transmittance is further reduced by 22% approximately. Thermal insulation using parallel to the block is called multilayered section and by using insulation into the holes of the concrete block may be called as composite section.
The paper is submitted for publication with the permission of Director, CSIR-CBRI, Roorkee. Acknowledgment is due to Mrs. Lakshmi Sindhuja Naidu for preparing the manuscript of the paper.
- ASHRAE, handbook of Fundamen- tals, American society of Heating Refrigeration and Air conditioning Engineers, Inc, ATLANTA USA (1982).
- Suman B.M and Srivastava R.K,’ Effect of air gap on thermal performance of composite wall section’, Indian journal of science and Technology V.1, No5(Oct, 2008), pp.1-4.
- IS: 9403, method of test for thermal conductance and transmittance of built up sections by means of guarded hot box (1980).
- Valentini B, Plankinsteiner A. and Grohs C, ‘New Design Solutions for Thermal Insulation System for high temperature furnaces,’ 18th Plansee seminar (2013) held at PLANSEE SE,6600 Reutter, Austria.
- IS: 3792, Insulation Guide for non- Industrial Buildings(1978), p.30.
- ECBC 2007, ‘Energy Conservation Building Code ‘ (2007), p.7.