Why Windows Saved $3.3M and Walls Saved $677K at HERS 45

If you are value engineering a large multifamily project in Massachusetts, the first thing most teams reach for is the wall assembly. Strip a layer of continuous insulation, drop the cavity spec, reframe the detail. Walls are the easy win. We modeled three compliance paths to HERS 45 on a 530-unit project, and the numbers said the easy win was the wrong one. All three paths passed code. The cost spread between them was eye-opening, and almost all of it lived in the glazing.

The Real Numbers

The project was a 530-unit multifamily building in Massachusetts, and the target was HERS 45. We modeled three compliance paths to that same number so the only thing that changed between them was the envelope spec. Every path cleared code. The question was never compliance. It was cost.

Path A used a premium wall assembly and triple pane tilt-turn windows. This is the high-performance envelope, the one that looks right on paper and reassures everyone that the building will perform. It was also the most expensive of the three by a wide margin.

Path B kept the same triple pane tilt-turn windows but downgraded the wall assembly. That single move saved $677,000. A real number, and the kind of savings that makes a value engineering meeting feel productive.

Path C kept Path B's downgraded walls and swapped the triple pane tilt-turn windows for double pane low-E vinyl windows. That move saved an additional $3,317,000. Same HERS 45. Same compliance. Roughly $4 million in construction cost separated the most expensive path from the least, and the window swap was responsible for the lion's share of it.

The Glazing Cost Lever

Here is the part most teams miss. When value engineering starts, architects and general contractors almost always go after the wall assembly first. It is the most visible line in the envelope budget and the easiest to picture cutting. Strip the continuous insulation, drop the cavity spec, and the savings feel tangible.

The model says otherwise. On a building with this much glazing area, the windows are the dominant cost lever by a factor of five. The wall conversation is a rounding error next to the glazing conversation. Path B's wall downgrade saved $677,000. Path C's window swap saved nearly five times that, $3,317,000, on the same building hitting the same rating.

The reason is area and unit price together. A 530-unit building carries an enormous quantity of glazing, and the jump from a double pane low-E vinyl window to a triple pane tilt-turn unit is a large per-square-foot premium. Multiply a big premium by a big area and the number dwarfs anything the walls can give back. The wall assembly is a smaller area at a smaller unit-cost spread, so even a meaningful percentage cut moves a smaller absolute dollar figure.

The energy model rated all three paths at HERS 45 because the envelope can hit the target through more than one combination. A triple pane window buys performance, but on this building that performance was not required to clear the rating. The same HERS 45 was available with the cheaper glazing, which means the triple pane premium was buying margin the project did not need.

When the Premium Glazing Still Wins

Triple pane tilt-turn windows are not a mistake on every project. There are buildings where the premium glazing earns its place, and cutting it would be the wrong call.

Buildings that cannot hit the target without it. On a project with a tighter HERS goal or a less forgiving envelope, the triple pane window may be the lever that closes the gap. If the cheaper glazing fails the rating, the premium is not optional, it is compliance.

Projects with acoustic or comfort requirements. Triple pane glazing and tilt-turn hardware buy sound attenuation and a tighter air seal that a double pane vinyl unit does not. On a building near a highway, a rail line, or an airport, that performance is a program requirement the energy model does not score.

Owners holding the building long term. A premium window changes the operating cost and the occupant comfort over decades. An owner-operator who keeps the asset may value that differently than a developer who sells at stabilization.

High-end product positioning. Some projects sell on finish and feel, and the window is part of the product the buyer is paying for. On those buildings the glazing is a marketing decision, not just an energy one.

The Decision Framework

If you are chasing HERS compliance on a large multifamily project, model the window package before you touch the walls. That is where the budget actually lives. The instinct to cut the wall assembly first is understandable, the walls are visible and the savings feel concrete, but on a glazing-heavy building the walls are the small lever and the windows are the large one.

The mistake is sequencing the value engineering by what feels easy to cut rather than by what the model says costs the most. On this 530-unit project, a team that cut walls and kept the premium windows would have left $3.3 million on the table while congratulating itself on a $677,000 save. The order of operations was backwards.

No two buildings put the lever in the same place. A project with modest glazing area might find its biggest cost in the walls or the mechanicals, and the window swap that saved $3.3 million here might save very little there. The factor of five is specific to this building, this glazing ratio, and this target. The only way to know where your dollars are concentrated is to model the paths against the same rating before the value engineering meeting starts, which is exactly why we built three of them on this one.