The envelope

The dilemma in the design of the envelope is how to achieve a high degree of insulation with a minimal wall thickness and to avoid thermal bridges. Here, physics proves to be an unkind restraint.

Most insulating materials hinder heat flow by means of entrapped still air, which, compared to most solids, is a poor conductor. Examples include mineral wool, fibre glass, polystyrene, polyurethane and also natural insulation materials, such as cellulose, straw, sheep wool and cork. Unfortunately, achieving high insulation quality requires numerous still air cells - hence, thick walls and roofs. This thickness takes away from habitable space.

An alternative is to hinder heat transport via radiation and convection. The best imaginable example is the vacuum insulation panel (VIP). Several firms now market such panels. Whole houses have been insulated with VIP as demonstration projects; but the costs are still high for wider market penetration. Currently, special applications are attractive, such as roof terraces. Windows, the weakest elements of the envelope, also use this technology to achieve better U-values. Selective coatings of the cavity face of the glass inhibit heat radiation from the room side to the exterior. To further reduce heat losses, air in the cavity is replaced by Argon or Krypton.

Regardless of the insulation system, the construction must be detailed to minimize short circuits (for example, structural bearing elements penetrating the insulation) and be air tight. It is also, of course, desirable to protect the expensive investment in the envelope from moisture damage.

Finally, it should be emphatically stated that when housing is very well insulated, it is not inherently more prone to overheating in summer. Indeed, the insulation keeps out the heat, which builds up on the opaque envelope surface. The windows are critical. However, closed windows, whether they are single glazed (U = 6.0 W/m2K) or super glazed (U = 0.5 W/m2K), will trap too much heat in summer. Therefore, the obvious conclusion is that sun shading is essential - ideally, outside the window to keep the heat on the outside.

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