Building a New Home with Fossil Fuels in Massachusetts? Here’s What It Takes
Designing a new home with fossil fuels in Massachusetts is still possible in many areas — but not all.
Some communities have adopted local restrictions that require all-electric construction. If you're unsure whether your project falls into that category, start with our guide to Massachusetts fossil fuel ban towns before finalizing your design.
For projects where fossil fuels are still allowed, the reality is this:
You’re not just choosing a fuel type — you’re committing to a higher-performing building.
Between the 2021 IECC, Massachusetts Stretch Code, and HERS (Home Energy Rating System) requirements, homes with fossil fuel systems must meet stricter performance standards to comply.
Here’s what that looks like in practice.
The HERS Target: Why Fuel Choice Matters
Under Massachusetts Stretch Code, new construction homes must meet a HERS target.
That target depends on how the home is designed:
All-electric homes: HERS 45 or lower
Mixed-fuel homes (including fossil fuels): HERS 42 or lower
Lower is better — and that 3-point difference is significant.
What This Means in Practice
Choosing to include fossil fuels immediately tightens your performance target.
To hit a HERS 42, most projects need:
Better insulation
Lower air leakage
Higher efficiency systems
Fewer inefficiencies across the board
In other words, fossil fuel homes don’t just need to perform well — they need to perform exceptionally well to pass.
The Big Picture: Fossil Fuel Homes Require a Better Envelope
If your home includes:
A gas or propane furnace
A fossil fuel water heater
A gas range or fireplace
…the building must compensate with stronger thermal performance.
Target Benchmarks
ACH50: ≤ 2.0
Walls: R-30 or higher
Roof: R-49 to R-60
Floors: R-30 to R-40
These aren’t upgrades — they’re often what it takes to achieve a compliant HERS rating with fossil fuels in the design.
Windows: Performance and Placement Matter
Windows are one of the fastest ways to lose performance if not selected and placed carefully — but they can also work in your favor when specified correctly.
Key Metrics to Understand
U-value (U-factor): Measures how much heat passes through the window
Lower = better insulation
SHGC (Solar Heat Gain Coefficient): Measures how much solar heat the window lets in
Higher = more passive solar heat gain
Recommended Targets
U-value: 0.27 or lower
SHGC: 0.40 or higher
Why SHGC Matters in Massachusetts
In a heating-dominated climate like Massachusetts:
Higher SHGC allows more free solar heat gain in winter
This can reduce heating demand and improve overall performance
Design Tip
Limit window area on the north side of the home, where solar gain is minimal.
Too much glass in the wrong location can push a project out of compliance quickly.
HVAC: Efficiency Is Non-Negotiable
Even in fossil fuel homes, HVAC design must be precise.
If using heat pumps or mini splits, target ≥ 11.5 HSPF2 and ≥19 SEER2
Furnaces / boilers (AFUE): ≥ 96% efficiency
Always perform a Manual J calculation
HVAC systems are not plug-and-play. Oversizing leads to poor comfort, short cycling, and reduced efficiency.
Ventilation: Distribution Matters More Than You Think
As homes get tighter, ventilation becomes essential — but how it’s delivered matters just as much as how much.
A single ERV may technically meet airflow requirements, but often needs to run at higher speeds (for example, ~120 CFM), which can lead to:
Increased energy use
More noticeable airflow and noise
Less consistent distribution
Better Approach
Using multiple ERVs at lower airflow (e.g., two units at ~60 CFM each):
Reduces overall watt draw
Improves balanced ventilation
Enhances comfort room-to-room
The goal isn’t just meeting CFM — it’s delivering ventilation in a way that’s quiet, efficient, and consistent.
Basements: Completing the Thermal Boundary
Conditioned basements are one of the most common weak points in new construction.
Minimum Targets
Sub-slab insulation: R-10
Slab edge insulation: R-10
Why It Matters
Proper slab insulation:
Completes the thermal boundary
Reduces heat loss to the ground
Helps control moisture and condensation
Improves floor comfort year-round
Without it, the basement can quietly undermine the performance of the entire home.
Hot Water: Small Details That Add Up
Hot water systems are a common source of lost efficiency — especially in fossil fuel homes.
Key Requirements & Best Practices
All hot water piping must be insulated to ≥ R-3
This is a code requirement that is often missed
If installing a circulator pump:
A demand control switch or presence sensor is required
Constant recirculation will result in a performance penalty
Consider drain water heat recovery to improve efficiency
System Selection Matters
Heat Pump Water Heaters (HPWH):
Target ≥ 4.0 UEF
Require sufficient space (typically ≥ 700–1,000 cubic feet) to operate efficiently
Work best in basements or utility spaces with adequate air volume
Fossil Fuel Systems:
On-demand (tankless) systems generally perform better in HERS modeling than storage tanks
Electric Resistance Water Heaters:
Typically a poor fit for HERS compliance
It is unlikely a project will pass without significant upgrades elsewhere
These decisions can have an outsized impact on whether a project passes or requires redesign.
Appliances: What Actually Impacts Your HERS Score
Not all appliances are treated equally in energy modeling.
What to Watch
Multiple refrigerators (including wine fridges) are included in HERS modeling
These can significantly impact your score if not accounted for
Best Practice
Use ENERGY STAR® certified appliances wherever possible
Refrigeration is one of the few plug loads that can directly move your HERS number.
The Range Decision Can Change Everything
One of the most overlooked design choices is cooking fuel.
Switching from gas or propane to induction can:
Lower overall energy use
Improve your HERS score
Help bring a borderline project into compliance
In some cases, this single decision can avoid the need for additional upgrades elsewhere.
Where Projects Go Sideways
We consistently see the same issues:
Envelope performance is underestimated
HVAC systems are designed too late
Too many small inefficiencies stack up
HERS is treated as a checkbox instead of a design tool
How to Approach It the Right Way
If you’re planning to build with fossil fuels:
Start with the HERS target, not the equipment
Design the envelope and systems together
Model early and adjust before permitting
This approach avoids:
Costly redesigns
Permit delays
Failed inspections
Final Thoughts
Building a fossil fuel home in Massachusetts is still possible — but it requires a higher level of planning and performance.
At a certain point, the upgrades required to support fossil fuels start to look a lot like an all-electric home.
Understanding that tradeoff early is what separates smooth projects from delayed ones.
Not sure if your project is in a fossil fuel restricted community?
Start with our Massachusetts fossil fuel ban towns guide to understand your local requirements before finalizing your design.
Frequently Asked Questions
Can I still build a new home with fossil fuels in Massachusetts?
Yes — in many areas, new homes with fossil fuel systems are still allowed.
However, some communities have adopted local requirements that mandate all-electric construction. It’s important to confirm your project’s location early, as these rules are applied at the local level.
Do fossil fuel homes have to meet stricter energy requirements?
Yes.
Homes with fossil fuel systems typically need a stronger building envelope and higher-efficiency mechanical systems to achieve a compliant HERS rating.
This often includes:
Better insulation
Lower air leakage (≤ 2.0 ACH50)
Higher efficiency HVAC and hot water systems
What is most likely to cause a HERS rating to fail?
It’s usually not one big issue — it’s the combination of smaller ones.
Common problem areas include:
Too much window area (especially on the north side)
Poor air sealing
Lower-than-expected insulation levels
Inefficient HVAC or hot water systems
Multiple refrigerators or overlooked plug loads
Is a heat pump water heater required?
No — but it is often one of the easiest ways to improve performance.
Heat pump water heaters (≥ 4.0 UEF) can significantly help a project meet HERS targets, especially when fossil fuel systems are used elsewhere in the home.
They do require adequate space (typically 700–1,000 cubic feet), which should be considered during design.
Can I use a standard electric resistance water heater?
In most cases, no.
Electric resistance water heaters are typically too inefficient to support a compliant HERS rating without major upgrades elsewhere in the home.
Does switching to an induction range really make a difference?
Yes — more than most people expect.
Switching from gas or propane to induction:
Reduces overall energy use
Improves your HERS score
Can help bring a borderline project into compliance
In some cases, this single decision can eliminate the need for additional upgrades.
Are multiple HVAC or ventilation systems better than one?
Often, yes.
For example, using multiple ERVs at lower airflow can:
Improve comfort
Reduce energy use
Provide more balanced ventilation
The same concept can apply to HVAC design depending on the layout of the home.
When should I involve a HERS rater in the project?
As early as possible.
Projects that wait until permitting or construction often run into:
Redesigns
Unexpected upgrade costs
Delays
Early modeling allows you to make informed decisions before plans are finalized.
What’s the biggest mistake homeowners and builders make?
Treating HERS as a checkbox instead of a design tool.
The most successful projects:
Model early
Make trade-offs intentionally
Understand how each decision impacts the whole system
Still not sure what your project requires? We can model your home early and show you exactly what it will take to meet code — before construction begins.
