The Science of Heat Resistance
Whether you are trying to keep the blistering summer heat out of your living room, or trying to trap the expensive winter heat from your furnace inside your house, the effectiveness of your home's thermal envelope relies entirely on Insulation.
However, you don't buy insulation based on its physical thickness; you buy it based on its thermal performance, known as the R-Value.
If you are finishing a basement or building an addition, local building codes mandate strict minimum R-Values for walls, floors, and ceilings. Understanding how to calculate the total R-Value based on the material type and thickness is critical for passing inspection and lowering your utility bills.
What is R-Value?
R-Value stands for Resistance Value. It is a scientific measurement of a material's ability to resist the conductive flow of heat.
- Higher R-Value = Better insulation.
- Lower R-Value = Poor insulation.
Every building material has an inherent R-Value per inch of thickness. For example, standard window glass has an R-value of roughly 0.14 per inch. Solid wood has an R-value of roughly 1.0 per inch. Fiberglass insulation boasts an impressive R-value of roughly 3.14 per inch.
Understanding Material R-Values
To calculate the total thermal resistance of an insulated wall or attic, you must know the specific R-Value per inch of the material you are using.
- Fiberglass Batts: ~3.14 to 3.5 per inch. (The industry standard for 2x4 and 2x6 framed walls).
- Blown-in Cellulose: ~3.5 to 3.8 per inch. (Treated recycled paper, excellent for blowing thick layers onto attic floors).
- Expanded Polystyrene (EPS) Foam Board: ~4.0 per inch. (The cheap, white "beadboard" foam).
- Extruded Polystyrene (XPS) Foam Board: ~5.0 per inch. (The dense pink or blue rigid foam, highly water-resistant, ideal for basements).
- Closed-Cell Spray Foam: ~6.5 to 7.0 per inch. (The absolute highest performing, most expensive insulation available. It expands to seal all air leaks and provides massive thermal resistance in thin wall cavities).
How to Calculate Total R-Value
Calculating the total R-Value of a single insulation layer is a simple multiplication equation.
The Formula
- Identify the specific insulation material and find its R-Value per Inch.
- Measure the total Thickness (in inches) of the insulation you plan to install.
- Multiply the Thickness by the R-Value per Inch.
Quick Example: Insulating a Basement Wall
You are insulating the concrete walls of an unfinished basement. You plan to glue sheets of XPS Rigid Foam Board (Pink Board) to the concrete. The foam board is 2 inches thick. You know that XPS provides an R-Value of 5.0 per inch.
- Thickness:
2 inches - R-Value per Inch:
5.0 - Multiply:
2 × 5.0 = 10
Your basement wall insulation will have a total thermal resistance of R-10.
Compressing Insulation Destroys R-Value
The R-value of fluffy insulation (like fiberglass and cellulose) does not come from the glass or paper itself; it comes from the millions of microscopic air pockets trapped between the fibers. Still air is a fantastic insulator.
If you take a 6-inch thick piece of fiberglass (R-19) and physically compress it down into a 3.5-inch thick wall cavity, you squeeze out all the trapped air. The fiberglass will no longer perform at R-19; it will drop down to roughly R-13. Never compress fluffy insulation. It must remain fully expanded to achieve its rated R-Value.