An inwardly or outwardly opening sliding door or window assembly comprising: a. a frame having a bottom sill, said bottom sill having a sealing surface, b. a sliding panel moveable between an open position and a closed position, said sliding panel having a bottom sealing surface which is sealed against the sealing surface of the bottom sill of the frame in the closed position of the sliding panel to provide a sealing interface between the bottom sill and the sliding panel, c. a fixed panel arranged in line with the sliding panel in the closed position of the sliding panel such that the outer surface of the fixed panel is in the same plane as the outer surface of the sliding panel and/or the inner surface of the fixed panel is in the same plane as the inner surface of the sliding panel, d. a track, e. a wagon arranged to be displaceable along the track, and f. a displacement mechanism connecting the sliding panel and the wagon, said displacement mechanism arranged such that the sliding panel is displaceable with respect to the wagon in a direction having a vector component which is perpendicular to the plane of the sliding panel such that in an inwardly opening assembly, the sliding panel is displaced inwardly until the outer surface of the sliding panel is inside the inner surface of the fixed panel or in an outwardly opening assembly, the sliding panel is displaced outwardly until the inner surface of the sliding panel is outside the outer surface of the fixed panel, characterized in that said assembly further comprises g. a sealing plate which in the closed position of the sliding panel: i. is arranged between the wagon and the sealing surface of the bottom sill, ii. is provided with an upper sealing surface which establishes an upper sealing interface between the sealing plate and the sliding panel and which comprises a portion which is located above the uppermost edge of the wagon and iii. is provided with a lower sealing surface which establishes a lower sealing interface between the sealing plate and the sealing surface of the bottom sill and which comprises a portion which is located below the uppermost edge of the wagon, h. and in that the sealing plate is connected to the wagon and displaces together with the wagon when the wagon displaces along the track. a. a frame having a bottom sill, said bottom sill having a sealing surface, b. a sliding panel moveable between an open position and a closed position, said sliding panel having a bottom sealing surface which is sealed against the sealing surface of the bottom sill of the frame in the closed position of the sliding panel to provide a sealing interface between the bottom sill and the sliding panel, c. a fixed panel arranged in line with the sliding panel in the closed position of the sliding panel such that the outer surface of the fixed panel is in the same plane as the outer surface of the sliding panel and/or the inner surface of the fixed panel is in the same plane as the inner surface of the sliding panel, d. a track, e. a wagon arranged to be displaceable along the track, and f. a displacement mechanism connecting the sliding panel and the wagon, said displacement mechanism arranged such that the sliding panel is displaceable with respect to the wagon in a direction having a vector component which is perpendicular to the plane of the sliding panel such that in an inwardly opening assembly, the sliding panel is displaced inwardly until the outer surface of the sliding panel is inside the inner surface of the fixed panel or in an outwardly opening assembly, the sliding panel is displaced outwardly until the inner surface of the sliding panel is outside the outer surface of the fixed panel, a sealing plate which in the closed position of the sliding panel: i. is arranged between the wagon and the sealing surface of the bottom sill, ii. is provided with an upper sealing surface which establishes an upper sealing interface between the sealing plate and the sliding panel and which comprises a portion which is located above the uppermost edge of the wagon and iii. is provided with a lower sealing surface which establishes a lower sealing interface between the sealing plate and the sealing surface of the bottom sill and which comprises a portion which is located below the uppermost edge of the wagon, i. is arranged between the wagon and the sealing surface of the bottom sill, ii. is provided with an upper sealing surface which establishes an upper sealing interface between the sealing plate and the sliding panel and which comprises a portion which is located above the uppermost edge of the wagon and iii. is provided with a lower sealing surface which establishes a lower sealing interface between the sealing plate and the sealing surface of the bottom sill and which comprises a portion which is located below the uppermost edge of the wagon, h. and in that the sealing plate is connected to the wagon and displaces together with the wagon when the wagon displaces along the track.