Like most things human, the New Museum aspires to the condition of eternal youth. Its new building on the Bowery, which opens to the public this Saturday, is the tectonic projection of an institution that, in its determination to remain always one step ahead of history, famously refuses to form a permanent collection.

But the youth that the New Museum wishes so desperately to cling to has already been lost, now that this institution has reached the ripe old age of 30. And yet, it refuses to face that fact. For bedeviling it at every step is the shadow of the Museum of Modern Art.

Once upon a time, when the words “modern” and “contemporary” were synonymous, MoMA, like the art it enshrined, was committed to stalking the chimera of Nowness. But in due course, as it aged and as its holdings soared in value, it became what it was destined to become: a repository of the past and of what would soon become the past. In architectural terms, its recent expansion, by Yoshio Taniguchi, is an admission of that fact. At this point, MoMA is about as vanguardist as the Pentagon. Everything fits and everything functions, and you can positively smell the money that went into it.

That feeling is exactly what the New Museum’s building, designed by SANAA, another Japanese firm, has striven with main force to avoid. And the formal contrasts between the two institutions are instructive. Both projects are fastidiously geometric and rectilinear. But Tanaguchi’s museum is a complex that binds together a number of pre-existing architectural events. SANAA’s New Museum, whose seven stories contain 60,000 square feet, has been built from scratch and reads as a series of six silvery boxes covered in anodized and expanded aluminum mesh. Located at 235 Bowery, between Stanton and Rivington streets at the intersection of Bowery and Prince Street, these boxes are stacked one atop the other and irregularly aligned along a north-south access, thus creating a highly distinctive sequence of recessions and cantilevers. Formally, the building to which it bears the most immediate similarity is the Whitney. Indeed, as the elevators open onto the exhibition spaces on the second, third, and fourth floors, it is hard not to feel a jolt of recognition, so closely does the experience recall the uptown institution.

But clearly the Modern is the father figure SANAA felt it needed to depose. And so, the most telling contrast is between the flawless polish of MoMA and the rough, almost seedy quality of the New Museum. When MoMA was being designed, Mr. Tanaguchi told the curator Terence Reilly that, if allowed, he could “make the architecture disappear.” In other words, everything would fit together so seamlessly that the visitor would be aware only of the abstract perfection of the spaces. And in large measure, he delivered on that promise.

When the plans for the New Museum were unveiled nearly two years ago, there was reason to expect from these austere, silvery boxes an effect of similarly otherworldly perfection. And let it be said that the vision of them rising up 174 feet into the brilliance of the wintry sun, over the lower-lying buildings that flank them, fully lives up to the promise of their renderings.

But it is in the details that the building ultimately disappoints. The architects themselves have said, “We don’t want to hide things behind gyp board. We want to show what the building is made of … This is why the building’s structure and guts are exposed.” But even though we are told that there is no disputing taste, I strenuously question the need to see every duct and sprinkler and all the puckered fireproofing foam agglutinated to an exposed steel truss.

On the exterior the gray, corrugated aluminum beneath the metal mesh fatally undermines the crystalline perfection that the renderings promised. Likewise, the decision to expose, from the exterior, the sixth-floor education center alone, simply feels like a mistake or an error of aesthetic tact, even given the “beautiful rough” look to which SANAA aspires. It has all the charm of an exposed elevator shaft.

Inside, the floors are polished gray concrete throughout, underscoring the dominant grayness of the building as a whole. The rough look extends to the exposed staircases as well as to the elevators, whose lime green corrugated metal walls are one of the few touches of color in the entire building. (The bathrooms in the basement, adorned with pinkish Bisazza mosaics, are another.) And the skylights formed by recessions in the irregularly stacked boxes provide no additional light worth having, even on the brightest day.

Ironically, despite the New Museum’s determination to be nothing like MoMA, there is a strong and probably unintentional similarity in the way both manage their lobbies. As at MoMA, one enters the New Museum through a 15-foot-tall curtain wall of clear plate glass, into an expansive, largely uninflected lobby space. At its eastern end, far removed from the street, is a video gallery. In between are a café and a bookstore.

It may be that the New Museum’s new home looks too downtown for its own good. With a rough-and-ready appearance that belies the vast wealth needed to build it in the first place, the New Museum reminds one of the sort of art world fixture who — though well into middle-age — insists on parading about with Kurt Cobain stubble and torn jeans, in a dreary stab at youth and street cred. How much better it would have been if this institution had owned up to its age and stature and had commissioned a building to match.