An assembly comprising: - a structural element (33,63,77,89,91,93) interposed between an external environment and an internal volume to be thermally and/or acoustically protected, - and an inner lining member (39,390,391,393,395,397) which is lining the structural element, radially from the inside, the inner lining member comprising at least one thermal insulating element which is three-dimensional and under vacuum, the thermal insulating element comprising: - a compressed three-dimensional porous structure, - an envelope closed in an airtight manner: -- comprising a thermoformable and airtight barrier wall, -- in which envelope is enclosed the porous structure whose shape is followed by the barrier wall, and -- in which envelope, at outside ambient temperature and pressure, a pressure of between less than 10 5 Pa and more than 10 -2 Pa prevails, the barrier wall being thermoformed and the porous structure, which is three-dimensional, being bent-shaped and/or having reliefs and/or depressions, with, in the envelope, zone(s) (10a1) of a reduced thickness located away from zone(s) where the envelope is airtightly closed, characterized in that the porous structure has thick zones (10b1) having a determined thickness and, as said reduced thickness zone(s), thin zones (10a1) the thickness of which is less than the thickness of the thick zones (10b1), the thin zones (10a1) having a porous material density greater than a porous material density of the thick zones (10b1), the minor surface of a wall being to be understood as the area where sheets, or portions of a sheet, forming the barrier wall are inter-bonded and where they are sealed together to close the envelope, and complementary, the major surface of the same wall being to be understood as the area of the envelope delimited by said bonding area. - a structural element (33,63,77,89,91,93) interposed between an external environment and an internal volume to be thermally and/or acoustically protected, - and an inner lining member (39,390,391,393,395,397) which is lining the structural element, radially from the inside, the inner lining member comprising at least one thermal insulating element which is three-dimensional and under vacuum, the thermal insulating element comprising: - a compressed three-dimensional porous structure, - an envelope closed in an airtight manner: -- comprising a thermoformable and airtight barrier wall, -- in which envelope is enclosed the porous structure whose shape is followed by the barrier wall, and -- in which envelope, at outside ambient temperature and pressure, a pressure of between less than 10 5 Pa and more than 10 -2 Pa prevails, the barrier wall being thermoformed and the porous structure, which is three-dimensional, being bent-shaped and/or having reliefs and/or depressions, with, in the envelope, zone(s) (10a1) of a reduced thickness located away from zone(s) where the envelope is airtightly closed, -- comprising a thermoformable and airtight barrier wall, -- in which envelope is enclosed the porous structure whose shape is followed by the barrier wall, and -- in which envelope, at outside ambient temperature and pressure, a pressure of between less than 10 5 Pa and more than 10 -2 Pa prevails, the barrier wall being thermoformed and the porous structure, which is three-dimensional, being bent-shaped and/or having reliefs and/or depressions, with, in the envelope, zone(s) (10a1) of a reduced thickness located away from zone(s) where the envelope is airtightly closed,