A key to being a good insulating agent is having a high thermal resistance since this is that property which helps your insulating agent form an unbreachable barrier against the invasion of hot, humid and cold air.
Thus Thermal Resistance Is An Issue Of Vital Importance
Thermal resistance of any substance is defined as its ability to obstruct the transfer of heat. It is expressed in mathematical terms with the letter R with units (m^2*K)/W. In the US, the units are expressed as 1 h·ft2·°F/Btu. The greater the R-value of a substance, the better is its ability to resist heat flow.
Conventionally used insulating materials like fiberglass, cotton and cellulose have significantly low thermal resistances (or R-values) when contrasted against the high R-value of spray foam. The testing methods which determine the R-values of various insulating agents available in the consumer market and fix them are established by the American Society of Testing and Materials.
Now the R-value of fiberglass(loose-fill) ranges from 2.5 to 3.7 per inch, that of cellulose(loose-fill) is between 3 to 3.8 per inch, whereas cotton batts have an R-value of 3.7 per all of which are much lower than spray foam’s.
Open cell spray foam insulation
This variety of spray foam consists of interlocking polyurethane cells filled up with a low-conductivity gas. Due to the nature of its porous walls, open cell foam is unable to keep out water vapor successfully and can offer a thermal resistance or R-value of up to 4.4 per inch depending on the quality of constituent materials of the foam. But despite its weaknesses, open cell foam is a much superior insulating agent than fiberglass, cellulose and cotton.
Closed cell spray foam insulation
The cells of this variety of spray foam, as the name suggests, are completely sealed thereby preventing any leakage of gas and completely pre-empting passage of air and water vapor. Closed cell spray foam is known to assume varying shapes and densities, filling up intractable corners of your house. Depending on the reputation of the manufacturers and proper materials used, closed spray foam can last a whole lifetime when installed without glitches. Since this form of spray foam blocks out entry of moisture and other pollutants, it checks the growth of mold, algae and mildew.
Thus the thermal resistance seems to be the deciding factor in taking both open and closed cell spray foam up by a few notches on the utility scale of all Attic insulating agents.