Slab Insulation and HERS Ratings in Massachusetts: What You Need to Know

May 1

Overview

If you're building or renovating a home in Massachusetts, you're likely required to meet strict energy codes—especially in towns under the Stretch Code. One area that often goes overlooked but carries significant weight in a HERS rating is slab insulation. Whether you're an architect, contractor, or homeowner, understanding slab insulation's role in energy performance and code compliance is essential.

This article explores how slab insulation affects your HERS score, when it's required under the 2021 IECC, and what best practices can help you avoid costly mistakes.

What Is Slab Insulation?

Slab insulation refers to the thermal barrier installed around or beneath a concrete slab foundation. Its primary purpose is to reduce heat loss through the slab, especially in colder climates like Massachusetts. There are two main types:

Under-slab insulation (beneath the entire slab)
Slab edge insulation (along the slab perimeter)

These techniques help maintain consistent indoor temperatures and improve overall energy efficiency.

How Slab Insulation Affects a HERS Score

The Home Energy Rating System (HERS) measures the energy efficiency of a home. A lower score means better performance. Slab insulation contributes to a better score by:

Reducing conductive heat loss through the foundation
Improving the thermal envelope
Lowering heating and cooling demands

Energy loss through an uninsulated slab can be significant—addressing this area leads to measurable improvements in your HERS score.

When Slab Insulation Is Required in Massachusetts

Massachusetts follows the 2021 IECC for energy code enforcement, with most towns adopting the Stretch Code. Slab insulation requirements depend on several factors:

Required:

Slab-on-grade foundations in conditioned spaces
New construction homes and additions with conditioned slab floors

Not Typically Required:

Unconditioned basements
Garages or outbuildings that aren’t heated

Under the 2021 IECC, slab-on-grade floors must include R-10 insulation. The insulation must be installed vertically, horizontally, or a combination of both.

Benefits of Slab Insulation

Besides improving your HERS score, slab insulation provides long-term comfort and cost-saving advantages:

Reduced heat loss in winter months
Lower energy bills year-round
More consistent indoor temperatures
Better indoor air quality by minimizing moisture intrusion

Best Practices and Common Mistakes

Best Practices:

Use rigid foam board insulation rated for below-grade use (e.g., extruded polystyrene or EPS)
Seal joints and edges to prevent thermal bridging
Protect exposed insulation with a termite barrier or durable cover

Common Mistakes:

Skipping slab edge insulation
Using improper insulation materials

How Spectrum Energy Helps

At Spectrum Energy, we factor slab insulation directly into your HERS rating model. Whether you're in the planning stages of a new home or preparing for an addition, we:

Guide you on when slab insulation is required
Help ensure it’s installed to IECC and Stretch Code standards
Integrate it into your HERS rating to help you pass local code

We make sure your project checks every box—cost-effectively and code-compliantly.

Frequently Asked Questions

Does slab insulation really make a difference?

Yes. In Massachusetts' cold climate, uninsulated slabs can lose significant heat. Insulating the slab perimeter and underside dramatically improves efficiency and comfort.