Insulation’s ability to resist heat flow is measured by its R-value. “R” stands for “resistance.” Like many numerical values, there are two R-values and we refer to both as “R-value.” The metric R-value is also known as RSI or R for resistance in the “Systeme International” — French for the metric system. The RSI is the American R-value divided by 5.678.

Performance Factors

The combination of insulation and other building materials creates the R-value (RSI) in any building assembly and depends upon these factors.

  • Air leakage and convection from voids, gaps, or low insulation density.
  • Thermal bridging or thermal shorts in the assembly.
  • Type and density of the insulation.
  • The outdoor temperature.
  • Water’s presence inside the assembly.
  • Mass of the insulated assembly.

Air leakage is the most potent moisture-carrying mechanism that causes condensation in building cavities. Vapor diffusion, a less potent mechanism, moves water vapor through permeable materials like insulation, drywall, and masonry. Low-R building materials, combined with water-absorbent building materials, create many moisture problems in buildings.

Types of R-Values

There are two types of wall R-values: clear-wall R and whole-wall R. Clear-wall R is the average R of the cavity between framing members in hollow walls. Whole-wall R or RSI is the area-weighted average R-value or RSI of the wall including structural materials. We can apply these  performance factors to other building assemblies like roofs, ceilings, and floors.

Density’s Effect on Insulation Performance

Some common insulating materials have an ideal density, where the R-value per inch or RSI per centimeter is at its maximum. Selecting and installing fiberglass, mineral wool, or cellulose to an ideal density maximizes their R-values per inch or RSI per centimeter. Below or above that optimal density level the thermal resistance decreases.
Foam insulation exhibits the same density effect. Foam insulation with a higher density usually has greater thermal resistance per inch or RSI per centimeter than foam with lower density. Density is also a practical consideration for foam insulation. Greater density provides greater strength — for installation on flat roofs and underneath foundations, for example.

Water’s Effects on Insulation Performance

Absorbed water decreases the R-value (RSI) of insulation. Water fills the insulation’s air spaces and conducts heat far faster than the air it displaced. Water and ice also damage insulation. Wet insulation corrodes metals and supplies water to insects and microorganisms that feed on organic building materials.

Mass Effects on Insulation Performance

The mass of building components, particularly walls, affects the heat transmission through them. Especially in sunny climates with large daily temperature variations, massive building assemblies absorb surplus heat from both the indoors and outdoors, slowing heat transmission. In this way, the mass tends to average the indoor and outdoor temperatures, moderating the effect of outdoor temperature.

For more information

Insulation Performance Factors Mini Course

Insulation Materials and Equipment Short Course

Energy Auditor Field Guide

Building Shell Field Guide

Residential Energy