Unfinished single-wythe concrete masonry walls are commonly used in warehouse and industrial settings to enclose the building. Single-wythe concrete masonry walls are relatively inexpensive to construct while simultaneously providing security and durability due to superior resistance to impact, fire, weather, insects, and corrosion. The main drawback of single-wythe walls is that they provide little thermal resistance. Under the recently superseded Model National Energy Code of Canada for Buildings (MNECB), single-wythe masonry walls meeting certain criteria were exempted from being insulated (NRCC 1997). The new National Energy Code of Canada for Buildings (NECB) has significantly stricter energy efficiency guidelines for building envelope design and provides no exemption for masonry walls (NRCC 2011). This, in addition to growing concerns regarding anthropogenic effects on the environment and increasing energy prices, has given rise to the need for an inexpensive and efficient option to insulate single-wythe masonry walls. The purpose of this investigation was to evaluate the effectiveness of POLY-MOR polyurethane foam as an insulating material and determine the best procedure for applying the foam. Three different application methods were investigated using two different foam densities and three different thicknesses. POLY-MOR foam was applied to unfinished single-wythe concrete masonry walls. The polyurethane foam was applied by spraying, formwork filling, or core grouting. Tests were conducted according to ASTM standards to determine the density and thermal conductivity of the foam in each application. A two-dimensional heat flow simulation was then used to determine the effective RSI-value of masonry walls with the different insulation configurations. Although the test specimens were concrete block walls, the results of the investigation could be relevant for insulating slabs, roofs, and other wall constructions.