An aerogel is a solid with nanometer-scale pores which is less than 1/3000th the width of a human hair. The porosity of an aerogel is in excess of 90% and even can be as high as 99.9%, so the density of the aerogel can be as low as 3 kg/m³. Essentially, an aerogel is “puffed-up sand” and often being termed “frozen smoke”. Therefore, an aerogel has the lowest thermal conductivity (0.014 W/m.K at room temperature) of any solids and it also has good transparency. In addition, an aerogel has the lowest density, the lowest speed of sound, the lowest refractive index, and the lowest dielectric constant of all solid materials. These unusual properties of the aerogel lead to their potential wide applications in the built environment. This paper reviews current and potential applications of the aerogel in the built environment and illustrates the applications of the unique properties of the aerogel, such as the low thermal conductivity for window panes and building thermal insulation, optical transparency for window panes and covers of solar collectors, acoustic property for noise abatement of building, adsorption and catalysis for indoor air purification, non-combustibility (inorganic aerogels) for fire retardation board in kitchens. In a word, aerogel products could help buildings become safer, more energy efficient, and healthier.