Why Continuous Exterior Insulation Improves Your HERS Score in Massachusetts
Overview
When building a new home or major addition in Massachusetts—especially in a Stretch Code town—energy efficiency isn’t optional; it’s code. One of the most impactful ways to meet these requirements and lower your HERS score is through continuous exterior insulation (CI).
Used correctly, CI significantly reduces thermal bridging, improves envelope performance, and makes passing your final HERS rating much easier. In a cold climate like Massachusetts, the benefits are even more pronounced.
What Is Continuous Exterior Insulation?
Continuous insulation refers to rigid insulation (like polyiso, EPS, or mineral wool) installed on the exterior side of the structural framing, uninterrupted by studs, rim joists, or other framing members. It can be used on walls and roofs—especially effective when insulating an attic at the roofline.
Unlike cavity insulation, CI wraps the building in a thermal blanket, limiting thermal bridging, which occurs where heat bypasses insulation via framing.
Why Thermal Bridging Hurts Your HERS Score
Wood framing isn’t a good insulator. In a typical wall, framing makes up 20–25% of the surface area, allowing heat to flow more easily out of the home in winter and into the home in summer.
The result? Higher energy loads, greater HVAC demands, and a higher HERS score.
By adding CI:
You reduce the wall/roof UA values
Lower heating and cooling loads
Improve the effectiveness of your HVAC system
Reduce air leakage when paired with taped sheathing or fluid-applied membranes
All of this helps bring your HERS rating down—and keeps your project in compliance.
Where to Use CI for Maximum Impact
Wall Assemblies
Even 1" of CI dramatically boosts R-value
Helps meet prescriptive and performance paths under 2021 IECC
Especially beneficial on complex facades or homes with large window areas
Rooflines (for Conditioned Attics)
Insulating at the roof deck—especially in homes with HVAC equipment in the attic—can be a smart strategy for bringing that space into the thermal envelope. Adding continuous exterior insulation above the roof sheathing further improves performance by increasing R-value and limiting thermal bridging.
But there's another important benefit: condensation control.
In unvented attic assemblies, the underside of the roof sheathing can become a cold surface in winter. If warm indoor air migrates into this cavity (even in small amounts), it can condense—leading to moisture buildup, mold risk, and long-term durability concerns.
Adding properly sized continuous insulation above the roof deck helps keep the sheathing warmer, reducing the risk that it will drop below the dew point of interior air during cold weather.
This isn’t just theory—it’s embedded in the 2021 International Residential Code (R806.5), which permits unvented roof assemblies only when the exterior rigid insulation meets a minimum R-value, depending on climate zone and the type of interior insulation used.
In Massachusetts (Climate Zone 5), code typically requires at least R-20 of rigid insulation above the deck if air-permeable insulation is installed below (like fiberglass or cellulose).
When done correctly, this roof assembly not only boosts energy performance—it also reduces moisture risks and improves long-term durability.
Best Practices for Continuous Insulation
✔ Use high-performance materials like foil-faced polyiso or mineral wool
✔ Tape seams or use fluid-applied membranes for added airtightness
✔ Detail around windows, doors, and penetrations carefully
✔ Combine with cavity insulation for balanced wall assemblies
✔ Coordinate with your HERS rater early in design for best modeling outcomes
FAQs
Does CI replace cavity insulation?
No—CI is typically used in addition to cavity insulation. Together, they provide higher total R-values and reduce thermal bridging.
Is CI required in Massachusetts?
CI isn’t always mandatory, but it’s often the easiest path to meeting Stretch Code performance targets under the 2021 IECC.
How much CI should I use?
In Climate Zones 5 and 6 (all of MA), 1" to 2" of polyiso or mineral wool is a common and cost-effective strategy for improved performance.
Will CI help with my blower door test?
Yes. Properly installed CI helps seal the envelope—especially when combined with exterior air barrier systems—and can reduce air leakage for a better test result.
Final Thoughts
Continuous exterior insulation is one of the smartest envelope upgrades you can make. It keeps your home warmer in winter, cooler in summer, and helps ensure you meet HERS targets under the Massachusetts Stretch Code.
If you're designing a new home or major addition, CI should be on the table early—and modeled into your HERS rating from the start.
Need Help Optimizing Your Envelope?
Spectrum Energy provides HERS Ratings, energy modeling, and code compliance support for residential projects across Massachusetts. Contact us today to learn how exterior insulation can lower your HERS score—and improve your home’s long-term performance.
