How Glazing U-Factor and SHGC Affect HERS Ratings in Massachusetts

May 21

Overview

If you're building, renovating, or adding to a home in Massachusetts, choosing the right windows isn't just about aesthetics—it's a key part of meeting energy code requirements and optimizing your HERS (Home Energy Rating System) score. The glazing U-factor and SHGC (Solar Heat Gain Coefficient) are two critical metrics that directly impact your home's energy efficiency, comfort, and long-term utility costs.

This guide breaks down how these window performance factors affect HERS ratings, what Massachusetts code requires under the 2021 IECC, and how smart window selection can support both compliance and performance.

What Are U-Factor and SHGC?

Glazing U-Factor: How Well a Window Insulates

The U-factor measures how much heat transfers through a window. In colder climates like Massachusetts, a lower U-factor means better insulation, which translates to less heat loss during winter. U-factor is a critical value in the HERS modeling process because it affects heating load calculations.

Typical U-factors for energy-efficient windows range from 0.20 to 0.30
Lower U-factors = higher performance in cold climates
Required for Stretch Code compliance

SHGC (Solar Heat Gain Coefficient): How Much Solar Heat Gets In

The SHGC measures how much solar radiation passes through a window. A higher SHGC means more heat enters the home from sunlight. While this can benefit passive solar heating in winter, it can be a liability during warmer months.

Ranges from 0 to 1
Higher SHGC helps with passive solar heating
Lower SHGC reduces cooling loads in summer

Note: Under the 2025 Massachusetts Stretch Code, there is currently no maximum SHGC requirement, giving more design flexibility for cold-climate energy strategies.

How Window Performance Affects HERS Ratings

HERS ratings are calculated by modeling the thermal performance of the building envelope—including walls, roofs, and windows. Glazing plays a significant role in:

Heating and cooling load calculations
Overall energy demand
Compliance with the 2021 IECC and Stretch Code

Impacts on HERS Score:

Lower U-factors reduce heating loads and improve the HERS rating
Optimized SHGC can contribute to solar gain in winter (beneficial in MA)
Improper window selection may force trade-offs elsewhere in the design (e.g., more insulation or better HVAC efficiency)

Massachusetts Window Code Requirements

Massachusetts follows the 2021 International Energy Conservation Code (IECC) with state-specific amendments for Stretch Code communities.

Key Requirements:

Maximum U-Factor: 0.30 or better for windows in conditioned spaces
No SHGC limit under the 2025 Stretch Code
Applies to new construction, additions, and major renovations

Why Glazing Matters More in Cold Climates

In cold climates like Massachusetts, the building envelope’s ability to retain heat in winter is crucial. Windows represent one of the weakest points in that envelope.

Poorly performing windows can lead to:

High heating bills
Drafts and discomfort
HERS score penalties

High-performance glazing can provide:

Measurable gains in HERS modeling
Lower heating demands
More consistent indoor temperatures
Better long-term energy savings

Common Misconceptions About Glazing and Energy Ratings

"SHGC doesn’t matter in Massachusetts."
Not true—it matters strategically. High SHGC can be an asset for passive solar heating, even if it’s not a code requirement.

"All ENERGY STAR windows are equal."
ENERGY STAR is a baseline. For Stretch Code compliance and HERS optimization, you may need performance levels beyond minimums.

"Better windows cost too much."
The upfront cost of high-performance glazing can often be offset by smaller HVAC systems, better HERS scores, and lower utility bills.

Best Practices for Selecting Energy-Efficient Glazing

Choose windows with a U-factor of 0.30 or lower
Use higher SHGC values on south-facing walls (when designed for solar gain)
Confirm your window specs match your HERS energy model inputs
Ask your rater or builder for window performance documentation (NFRC label)
Avoid assumptions—window brand and model matter

How Spectrum Energy Helps

At Spectrum Energy, we integrate window performance directly into your HERS rating model. We:

Review your window specifications for code compliance
Advise on optimal U-factor and SHGC combinations based on orientation
Run energy modeling scenarios to maximize your score
Help ensure you pass local energy codes with cost-effective strategies

Frequently Asked Questions

Do all windows need to meet the same U-factor in a home?

Not necessarily. You can mix performance levels based on orientation and budget, but the total impact must meet code and HERS requirements.

What is the minimum U-factor required in Massachusetts?

Per the 2021 IECC, windows must have a U-factor of 0.30 or better for Stretch Code compliance.

Is there a SHGC limit in the 2025 Massachusetts Stretch Code?

No. The 2025 Stretch Code removed SHGC limits, allowing more design flexibility, especially for passive solar strategies.

How do I know if my windows are compliant?

Check the NFRC label on your window specs. Spectrum Energy can verify compliance as part of your HERS rating.

If you're planning a new home, addition, or renovation in Massachusetts, contact Spectrum Energy today. We'll help you navigate window performance requirements, optimize your HERS score, and ensure your project complies with the latest energy codes.