What is the difference between low density, open cell SPF and medium density closed cell SPF?

1/2 lb Spray Polyurethane Foam (SPF)

Low density, open cell SPF refers to a generic spray polyurethane SPF that weighs between 0.4 to 0.6 lbs per cubic ft when fully cured. It is spray applied to a substrate as a liquid and expands about 100-150 times its original volume to form a semi-rigid/flexible, non-structural SPF insulation. The SPF has an R-value around 3.5 per inch and typically uses water as the blowing agent.

Medium Density, Closed Cell Spray Polyurethane Foam (SPF)

Medium density, closed cell SPF used in interior applications typically refers to generic spray polyurethane foam that weighs between 1.5 to 2 lbs per cubic ft when fully cured. It is also spray applied as a liquid to a substrate and expands about 35 to 50 times its original volume to form a rigid, structural SPF insulation with a compressive strength between 15 to 25 PSI. The SPF has an R-value of around 6.0 per inch (aged R-value) and uses high R-value blowing agents.

Similarities:

Both products are excellent air barriers in buildings, provide great insulation, can assist in the control of condensation within buildings and have great environmental benefits.