What Is the F-Factor? 🔍

When modeling slab-on-grade floors in energy simulation software like eQUEST, EnergyPlus, or TRACE 700, you've probably seen references to something called the F-Factor. But what exactly is it?

The F-Factor is a simplified way of describing the heat loss rate through a slab-on-grade floor based on its perimeter length, not its area. It’s defined in units of Btu/hr·ft·℉ and allows modelers to account for ground heat transfer without needing to get into complex soil thermodynamics. That’s why it's so useful — and why it's required by ASHRAE 90.1 for certain modeling paths.

Where Do You Find F-Factor Values? 📚

ASHRAE 90.1 provides a handy table with predefined F-Factor values based on insulation levels, slab configuration, and climate. For example, in the ASHRAE 90.1-2019 , the relevant table is:

• Table A6.3.1-1 – Assembly F-Factors for Slab-on-Grade Floors

This table lists F-Factor values for common floor assemblies by climate zone and insulation condition. To use it:

1. Determine your project's climate zone
2. Identify whether the slab is heated or unheated
3. Choose the correct insulation configuration (e.g., vertical, horizontal, depth)

Once you’ve got the F-Factor, you can enter it directly into many simulation programs when modeling slab heat loss. This standardized method ensures consistency across models and simplifies ground heat loss calculations.

Why Is It Important? ⚙️

If you skip over modeling the slab correctly or ignore the F-Factor, you could end up with inaccurate heating loads — especially in cold climates where ground losses are a big deal. The F-Factor method helps you get consistent, standards-based results that pass review.

Want to Learn More About Modeling F-Factors? 🎓

Our in-depth energy modeling training covers how to model slabs, choose the correct F-Factor, and avoid common pitfalls that can cost you time or even get your model rejected.

👉 Check out our training courses at Energy-Models.com