AN AIR AND WATER-TIGHT BUILDING ENVELOPE IS ESSENTIAL TO THE ENERGY EFFICIENCY OF THE BUILDING.

Indeed, without it, air leaks make the insulation wholly ineffective. In addition, the waterproofing protects the structure from potential seepage. A tight building is much more comfortable and durable.

Air tightness is a key element in both the comfort of a dwelling and the effectiveness of the insulation.

Indeed, air infiltrations / leaks make any home uncomfortable and raise the heating and cooling costs.  Moreover, when warm air leaks out of a building envelope it is replaced by cold air. Condensation within the wall cavity may take place which, in turn, would damage the insulation and likely create mildew. Without air tightness, the insulation of a dwelling is wholly ineffective. There are simply no benefits to insulation when windows are kept open!