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THERMAL INSULATION

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How come thermal insulation might be a problem to the acoustician? Well, there are quite a few points to beware of: To start with, quite a number of people will confuse an acoustic absorptive material and a thermal insulation layer, resulting in both materials being familiarly called insulation, with sometimes disastrous consequences (cf. Section 3.12.11).

Managing thermal comfort has been a constant requirement for man. While efforts were initially directed at heating systems, the scarcity of energy sources has prompted the legislator to issue regulations regarding the thermal insulation of buildings (e.g., [2-4]).

Thermal Insulation

Thermal insulation is the prevention of the passage of heat [5]. It may have to deal with the three modes of heat transfer:

  • • Conduction
  • • Convection
  • • Radiation

Usually a reflective or low-emittance sheet will not be considered a thermal insulation material, though it will reflect heat. Likewise, a radiant barrier or a vapor retarder will not be considered a thermal insulation material.

The thermal resistance R of a material is expressed in m2K/W. The higher the value of R, the better an insulation material it is. Its thermal conductivity K (X in some countries) is expressed in W/m.K and characterizes the quantity of heat that can be transferred into a material within a given time. The smaller the value of K, the more insulating the material is; insulating materials are considered to feature values under 0.06 W/mK.

The heat transfer coefficient U characterizes the quantity of heat through a wall under established flow per time unit, surface unit, and temperature unit differences between the ambiances on each side of that wall. It is expressed in W/m2K and is the inverse of the total thermal resistance of the wall. The lower its value, the better the insulation is.

Insulating the wall seems straightforward enough, doesn’t it? One simply has to apply a thermal insulation layer; however, applying that layer may result in downgrading the sound reduction index of the wall. For example, applying plasterboard and polystyrene doubling on a brick wall will usually result in a 4 to 7 dB loss on the sound reduction index. More to the point, applying a doubling on the inner surface of the fagade will complicate matters: The HVAC engineer will look forward to having a well-insulated envelope and will not accept that his interior thermal doubling will be interrupted. The acoustician will usually hate this particular doubling, as it will induce flanking transmissions; he will look forward to pushing the partition to the fagade (and incidentally, so will the safety engineer for fire control purposes), but this will create a thermal bridge (cf. Section 19.6.3). Another sore point will be the connection of a curtain fagade to the floors: The HVAC engineer will love the use of thermal bridge breakers, but both the acoustician and the safety engineer will loathe it, as it will create a secondary transmission path between floors (you have probably guessed by now that it will require mineral wool and a plate top and bottom, as displayed in Figure 19.1, to ensure the desired acoustic and fire result).

Incidentally, applying an acoustically absorptive material on a wall where there already is thermal insulation may seem a waste of space and money to the architect or the end user.

Example of floor and facade assembly

Figure 19.1 Example of floor and facade assembly.

Often a decision is made to implement a single material instead of the usual double layer of insulation material (two-thirds of the total thickness) and acoustic absorptive material (one-third of the total thickness, as displayed in Figure 19.2).

Unfortunately, this results in a material with a vapor barrier on the room side that will ruin the absorptive characteristics of the wall covering.

It is quite fashionable nowadays to make use of the thermal inertia of a building. This means that the walls and ceiling must be kept bare, but how does one achieve the required acoustic absorption then? Well, one could think of an acoustically absorptive material with a high thermal conductivity (e.g., a metal foam), but there are problems associated with its fixation on the walls and ceiling; more to the point, it may complicate matters when dealing with fire safety too. While such materials have actually been developed for specific applications (e.g., the absorptive interior lining of an electronic cabinet where one must try to

Example of the principle of thermal insulation combined with acoustic absorption

Figure 19.2 Example of the principle of thermal insulation combined with acoustic absorption.

reduce the noise while simultaneously taking care of thermal dissipation through its metal walls), their cost is yet prohibitive for regular building use [6, 7].

Another way is to use the convection effects to the fullest by having a suspended ceiling made of horizontal panels that covers only 60% of the upper floor surface [8].

Baffles can also be considered for such a purpose. However, there usually is a bit of a fight between the HVAC engineer, who will argue for a rather wide spacing, and the acoustician, who will prefer them much closer (a typical ratio will be 1 m2 of baffle per 1 m2 of floor). More to the point, the architect will understandably prefer to achieve a decent ceiling height, so the resulting expected performances must be carefully considered (cf. Section 19.6.13). On the other hand, baffles can also be used on the walls of large spaces (cf. Section 19.6.14).

Incidentally, there may be noise associated with objects dilating or contracting. While those objects will often be ducts or some fagade elements, some larger objects may also be concerned. Section 19.6.5 illustrates the point.

 
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