To rate insulation, its resistance to heat flow is measured, and products are labelled with both an RSI value (Résistance Système International) and an R-value. The R-value is the imperial measurement, and the RSI-value is the metric measurement of thermal resistance.

Use the following equation to convert an R-value (imperial) to RSI (metric).

Conversion of an R-value to RSI

R-value/5.678 = RSI

Example: 20/5.678 = RSI 3.52

The higher the resistance value, the slower the rate of heat transfer through the insulating material. One brand of insulation may be thicker or thinner than another, but if they both have the same RSI value, they will control heat flow equally well. Chapter 3, "Materials," describes insulation materials and their RSI values.

Follow these common guidelines where you install insulation:

• Ensure that the insulation fills the space completely and evenly. Blank spots or corners allow convection currents to occur, sometimes letting heat bypass the insulation completely.
• Minimize thermal bridging. A thermal bridge is any solid material that connects the warm side of the envelope to the cold side (e.g. a stud in a wall). When insulation such as foam board is installed on one side of the thermal bridge, it acts like a roadblock, reducing heat flow.
• Loose-fill insulation must be installed by your contractor at the proper thickness and blown in at the proper density. This is especially important for wall and cathedral or flat roof applications.

The nominal insulation value is the insulating value for the insulation itself (e.g. RSI 3.52 batt [R-20] as purchased). The effective insulation value accounts for all the other building components and what portion of the structure they represent.2.2.2 Nominal RSI values and effective RSI values

For example, wall studs and top and bottom plates will reduce the effective insulation value of an insulated wall; while sheathing, drywall and the exterior finish can increase a wall's thermal resistance. In practical terms, for example, RSI 3.52 (R-20) batt insulation in a 2 x 6 wall might yield an effective insulation value of only 2.99 (R-17).

How much insulation?

You must install at least the minimum levels according to local building codes, but you can exceed these values where it is practical and economical. How the house is built will determine how much you can practically add, and how much insulation is already there will help you decide how much you can economically add. If you are doing other renovations, this may make it worthwhile to add higher insulation levels.