An essentially invisible lace weft hair extension comprising a first layer of lace, extension hair knotted into the first layer to define a weft base, a knot sealant/bonding agent applied to the weft base, and a second layer of lace bonded to the first layer over the weft base. The weft base is double-bleached (both lace and knots), preferably before the knot sealant/bonding agent is applied. Methods for constructing the weft, and for applying the weft to a wearer's scalp using adhesive, are also disclosed.