Why Did My HERS Score Change Between the Preliminary and Final Rating?

One of the questions that can come up during a new construction project is: “The preliminary HERS Rating showed one score. Why did the final HERS Rating change?”

The short answer is simple:

A preliminary HERS Rating is based on the proposed design. A final HERS Rating reflects the home that was actually built, inspected, and tested.

That distinction is important.

During the design phase, a HERS model uses information from architectural plans, equipment specifications, insulation details, window schedules, and other proposed features to project how the completed home is expected to perform.

As construction progresses, some of those details may change. In other cases, the design remains the same, but the final measured performance—such as the blower door result—is different from what was originally anticipated.

Sometimes the final HERS score improves. Other times, it moves in the opposite direction.

The goal of preliminary modeling is not to guarantee a final number months before construction is complete. It is to give the project team a way to evaluate the design, understand potential tradeoffs, and identify whether the home is on track to meet its required HERS target.

Here are some of the most common reasons a final HERS Rating can differ from the preliminary result.

Product Specifications Change During Construction

Construction projects evolve.

A window package may become unavailable. HVAC equipment may change. A different water heater or ventilation system may ultimately be installed.

These substitutions may appear relatively minor, especially when the replacement product seems comparable to the original.

From a HERS perspective, however, the specifications matter.

A Real-World Window Example

On one recent project, the original window package was approximately:

• U-factor: 0.27

• SHGC: 0.40

The final window package changed to approximately:

• U-factor: 0.28

• SHGC: 0.28

Neither number changed dramatically when viewed individually. But together, the revised window specifications increased the projected HERS score by approximately two points.

The project then needed to make improvements elsewhere in the building envelope to recover those points.

That does not mean the replacement windows were poor-performing windows. It simply illustrates an important part of HERS modeling:

A product can be a reasonable substitute from a construction standpoint while still having a different impact on the overall HERS Rating.

This becomes particularly important when a project is already close to its required target.

*Learn more about window glazing, read our blog post titled ”How Glazing U-Factor and SHGC Affect HERS Ratings in MA”.

HVAC or Water Heating Equipment Changes

Mechanical equipment is another area where changes can affect the final model.

The preliminary rating may be based on a specific:

• Heat pump

• Furnace or boiler

• Air conditioner

• Water heater

• Ventilation system

If the final equipment differs from what was originally modeled, the final HERS Rating needs to reflect the equipment that was actually installed.

Even equipment that performs a similar function can have different efficiency ratings and energy impacts.

This is one reason it is helpful to communicate equipment substitutions before installation whenever possible.

If the project is close to its HERS target, running the revised equipment through the model can help identify whether the change has a meaningful impact while there is still flexibility to make decisions elsewhere.

*Learn why sizing heating and cooling equipment is critical in a high performance home, read our blog post titled “The Importance of Manual J Calculations for MA Homes”.

The Insulation Assembly May Not Match the Original Design Exactly

The preliminary model is based on the insulation levels and assemblies proposed for the home.

The final rating reflects what can ultimately be verified.

Changes can include:

• Different insulation products

• Changes in insulation depth

• Revised wall, roof, or floor assemblies

• Areas where the original assembly could not be installed as planned

• Installation quality that affects the final insulation grade

This is one reason the midpoint inspection is such an important part of the HERS process.

Once drywall and finishes are installed, many parts of the thermal enclosure become difficult or impossible to inspect or correct.

The midpoint inspection provides an opportunity to verify insulation and air-sealing details while they are still accessible.

The Final Blower Door Result Is Different From the Preliminary Assumption

This is one of the biggest reasons a final HERS Rating can change.

A preliminary model may use a projected or target air leakage rate—for example, a home may be modeled assuming it will achieve 2.0 ACH50.

But the final HERS Rating uses the actual blower door result.

If the completed home tests tighter than anticipated, the final score may improve.

If the home has more air leakage than expected, the final score can increase.

This is where construction details matter.

The home can have the correct insulation, windows, and mechanical equipment and still perform differently than expected if the air barrier is not continuous.

Common areas that can influence the final blower door result include:

• Transitions between different assemblies

• Penetrations through the building enclosure

• Connections between framing components

• Mechanical and plumbing penetrations

• Attic and roof transitions

• Rim and band joists

• Doors and windows

• Other difficult-to-access air-barrier details

A blower door test does not measure whether one individual detail was installed correctly. It measures the cumulative air leakage of the completed building enclosure.

That means multiple relatively small leakage pathways can add up.

Fireplaces Can Introduce Additional Air-Sealing Risk

A fireplace is not necessarily a major HERS penalty simply because a fireplace is installed.

The bigger consideration from an air-sealing standpoint is the additional complexity it can introduce to the building enclosure.

Fireplace installations can involve:

• Framing transitions

• Penetrations

• Venting interfaces

• Chases

• Connections between conditioned and unconditioned areas

Each of those interfaces needs to be considered as part of the home's continuous air barrier.

If these details are not properly sealed, they can contribute to additional infiltration and affect the final blower door result.

This is a good example of the difference between something that directly changes a HERS model input and something that can affect the actual measured performance of the completed home.

The preliminary model may assume the home will achieve a particular blower door result. The final test determines whether it actually did.

Ventilation Systems Can Change

Mechanical ventilation is another area where the final installation may differ from the original design.

For example, a project may change:

• The ERV or HRV model

• The number of ventilation units

• The airflow strategy

• The final distribution approach

Because mechanical ventilation is part of the home's overall energy use, the final HERS Rating needs to reflect the system that was ultimately installed.

This does not mean every ventilation change will significantly move the score. But when a project is close to its target, seemingly small changes can become more important.

Duct Leakage May Differ From What Was Expected

For homes with ducted HVAC or ventilation systems subject to duct leakage testing, the actual test results can also affect the final rating.

The preliminary model may be based on an assumed or target level of duct leakage.

The final rating can then incorporate the verified performance of the completed duct system.

As with blower door testing, the result reflects the actual installation.

This is another example of why the final HERS Rating cannot always be determined solely from the plans.

A Preliminary HERS Rating Is a Design Tool—Not a Final Guarantee

A preliminary HERS Rating is extremely valuable because it allows the project team to understand the likely impact of design decisions before the home is built.

It can help answer questions such as:

• Will this window package work?

• How much insulation is needed?

• Does this HVAC system support the target?

• What happens if we change the water heater?

• How much room does the project have for changes?

• Is the home projected to meet the required HERS Rating?

But until the project is complete, there are still variables that need to be verified.

The final rating reflects:

• The products and equipment actually installed

• The insulation that can be verified

• The final building assemblies

• Measured blower door performance

• Duct leakage results, where applicable

• The completed ventilation and mechanical systems

That is why a preliminary score should be viewed as a projection based on the information available at that point in the project.

The Best Time to Evaluate a Change Is Before It Is Installed

One of the most useful parts of the HERS process is the ability to evaluate changes while there is still time to make decisions.

If a product or system changes during construction, it is often worth checking the HERS model before assuming the substitution will have no effect.

That is particularly true for projects that are close to a required HERS target.

A change to a window package may be manageable. A different water heater may be manageable. A revised HVAC system may be manageable.

But several changes together can have a larger cumulative effect.

The best time to find out whether a change affects the HERS Rating is while the project still has options—not after construction is complete.

How to Reduce Surprises Between the Preliminary and Final HERS Rating

The goal is not to prevent every change during construction. That is rarely realistic.

Instead, the goal is communication and coordination.

If any of the following change from the original design, let your HERS Rater know:

• Windows or exterior doors

• HVAC equipment

• Water heating equipment

• Ventilation equipment

• Insulation products or assemblies

• Conditioned floor area

• Fireplace or other building-envelope penetrations

• Major air-sealing details

• Duct design or equipment location

The earlier a change is evaluated, the more flexibility the project team has.

In some cases, the revised product may have little or no effect.

In others, the model may identify the need for an adjustment elsewhere in the home.

Final Thoughts

A preliminary HERS Rating and a final HERS Rating are part of the same process—but they serve different purposes.

The preliminary rating helps evaluate the home you are planning to build.

The final rating documents the performance of the home that was actually built, inspected, and tested.

A difference between the two does not necessarily mean something went wrong.

It may simply reflect:

• Product substitutions

• Equipment changes

• Changes to the building assembly

• Final insulation verification

• The actual blower door result

• Duct leakage results

• Other details that could only be confirmed after construction

The key is to keep the HERS model involved as the project evolves.

When changes are identified early, the project team has time to understand the impact and make informed decisions.

That leads to fewer surprises at the end of construction—and a final HERS Rating that reflects the home's actual performance.

FAQ: Preliminary vs. Final HERS Ratings

Is a preliminary HERS Rating guaranteed?

No. A preliminary HERS Rating is a projection based on the proposed plans, specifications, and assumptions available when the model is created.

The final HERS Rating is based on the completed and verified home.

Why did my final HERS score get worse?

A higher HERS Index means higher modeled energy use.

The score may increase because of changes to products or equipment, different insulation assemblies, a higher-than-anticipated blower door result, or other differences between the proposed and completed home.

Can my final HERS Rating be better than the preliminary score?

Yes.

If the completed home performs better than originally anticipated—for example, by achieving a tighter blower door result—the final HERS score may improve.

Should I tell my HERS Rater if a product changes?

Yes. Changes to windows, HVAC equipment, water heating, ventilation systems, insulation assemblies, or other significant components can affect the model.

How much can a window change affect a HERS Rating?

It depends on the size of the home, window area, orientation, and the difference between the original and replacement windows.

On one recent project, a change from approximately U-0.27 / SHGC 0.40 to U-0.28 / SHGC 0.28 increased the HERS score by approximately two points.

What happens if the final home misses its required HERS target?

The next steps depend on the project and where it is in the construction process. Identifying changes before the final inspection provides the greatest flexibility to evaluate potential improvements and determine the most practical path forward.