Why Volume vs. Floor Area Matters in Your HERS Rating
Understanding the Tradeoffs of Conditioned Attics in Massachusetts Homes
Overview
When planning new construction in Massachusetts—especially in Stretch Code towns—understanding the relationship between Conditioned Floor Area (CFA) and Conditioned Volume (CVA) is essential to achieving a competitive HERS score.
Many homeowners and builders assume that insulating and air sealing the roof deck of an attic will automatically improve performance. But depending on how the attic is handled in the design and energy model, it could actually hurt the final HERS rating.
Let’s explore why.
What Is CFA vs. Volume in a HERS Model?
Conditioned Floor Area (CFA): The square footage of space in your home that is intentionally heated or cooled.
Conditioned Volume (CVA): The total air volume inside the thermal and pressure boundaries of the building.
Why it matters: The HERS index score is influenced by both values. An increase in volume without a proportional increase in floor area can reduce the overall efficiency of your home on paper—especially when it comes to heating and cooling loads.
When Conditioning Attics Becomes a Problem
Insulating the roof deck (rather than the ceiling plane) brings the attic inside the thermal envelope—but unless that space is also actively conditioned with HVAC supply and return, you may face a modeling penalty.
Here’s why:
The model sees added volume that must be maintained at indoor temperatures.
But if there’s no HVAC equipment serving that space, it assumes it will behave like an energy sink.
Worse yet, if the space isn’t directly conditioned, modeling will not permit counting it as Conditioned Floor Area—so you get stuck with more volume and no credit.
That’s a red flag in the HERS model, and it could drag your score down several points.
How HVAC Affects the Outcome
Many new homes include HVAC ductwork in attic spaces, and both building science and energy modeling software agree: ducts should be inside the thermal boundary. If you’re already insulating the roof deck, you should go one step further and size the HVAC system to serve the attic directly—even if it’s not a living space.
✅ In this case, the space becomes conditioned
✅ You’re credited with both CFA and CVA
✅ Ductwork is inside the envelope, reducing losses
✅ Your HERS score improves or remains neutral
But...
❌ If the attic is unvented and insulated but not directly conditioned with heating and cooling, it becomes a modeling liability—one with more volume, no CFA, and higher energy assumptions.
Best Practices for Managing CFA and Volume
To make the most of your HERS rating in new construction:
Design with intention
Don’t condition attics by default—do it purposefully and only when HVAC will be installed.
Match insulation to HVAC
If you insulate the roof deck, plan for supply and return ducts to serve that space.
Confirm if CFA increases
If the attic is insulated and brought inside the envelope but not directly conditioned, it will likely increase volume without being credited as CFA—resulting in a penalty.
Model early with your HERS rater
Collaborate during design to model different attic and HVAC scenarios. A simple change could improve your score.
Keep ductwork inside the envelope
Whenever possible, route ducts through conditioned space to avoid energy losses and scoring penalties.
FAQs
Can I just insulate my attic and not directly condition it with heating and cooling?
Yes, but if it’s unvented and part of the envelope, your HERS model will increase the volume without crediting it as floor area—potentially hurting your score.
What happens if I insulate the roof deck but don’t add HVAC to the attic?
The attic becomes part of the modeled envelope, increasing volume but not floor area. Unless it's directly conditioned, the result is a potential HERS penalty.
How can I avoid a penalty for an insulated attic?
Make sure the attic is fully served by HVAC (both supply and return). This turns the space into conditioned volume and eligible conditioned floor area, helping your score.
Why is this more important under the Stretch Code?
The Stretch Code mandates a maximum HERS rating for new homes. Design decisions that increase modeled energy loads—like adding unconditioned volume—can make or break your compliance.
Final Thoughts
Conditioned attics aren’t automatically better—they’re only an upgrade when designed holistically with HVAC strategy and floor area in mind. By collaborating early with your HERS rater and keeping energy modeling in the design loop, you can avoid volume-related penalties and build a home that performs as well as it looks.
