F-Factor for slab with insulation on top? - GreenBuildingAdvisor
Summary
The article discusses how to determine the F-factor for a slab-on-grade with insulation on top, addressing a specific query from a user regarding construction in Climate Zone 4C.
Why It Matters
Understanding the F-factor is crucial for energy efficiency in building design, particularly for new constructions. This inquiry highlights a gap in existing resources, emphasizing the need for tailored calculations in varying insulation scenarios, which can impact energy performance and compliance with building codes.
Key Takeaways
- The F-factor for slabs with top insulation is not well-documented.
- Existing resources typically assume insulation is below the slab.
- Calculating the F-factor is essential for energy-efficient building design.
- The inquiry is relevant for new constructions in specific climate zones.
- There may be equations similar to ASHRAE formulas for calculating this factor.
F-Factor for slab with insulation on top? don_lorenzo | Posted in Energy Efficiency and Durability on February 28, 2026 01:57am Does anybody know how to determine the F-factor of a slab-on-grade with insulation on TOP? I’m trying to find/calculate the F-factor of the assembly shown in the attached image (with and without perimeter insulation). The sources for F-factors that I’m aware of don’t include values for this condition — they all assume that the insulation is below the slab, which it typically is. Perhaps there’s an equation that can calculate this? Similar to the ASHRAE formulas used to calculate the U-factors for walls and ceilings. This is for a new construction project (not a retrofit) in Climate Zone 4C, zip code 98370 (near Seattle). Thanks! -Don Answer GBA Detail Library A collection of one thousand construction details organized by climate and house part Search and download construction details Join the leading community of building science experts Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field. Start free trial Related Questions F-Value for Uninsulated Slab How to Factor Vapor Retarders into Relative Humidity and thus Mold Risk calculations? Rigid foam insulation on top of a concrete slab K-factor and airflow measurement Replies Log in or create an account to post an answer. Sign up Log in
