The colder it gets outside the less effective Fiber Insulations such as Fiberglass, Cellulose and Rockwool are at keeping you warm inside. Polystyrene foam has a “real world” R-Value which is not affected by external forces like temperature.
Similar studies found the same effect with another external force: wind. Wind causes pressure differences across the exterior walls. Air Leakage compounds this effect as hot air leaks out the top of the building and cold air is sucked in at the bottom of the building. As a result of this pressure difference the polyethylene plastic vapor barrier is sucked into the wall cavity compressing Fiber Insulations such as Fiberglass, Cellulose and Rockwool. Depending on the strength of the wind R-Values can be cut by 40-80%.