bigfoot and northern lite details on their construction

Not a freezing damage danger, but my furnaces had hot air from the exchanged directed first into outside cover before it turn around to go inside.
The sheet metal cover had no insulation and there was not much space for it.
What I did was spraying some of polyurethane foam on the cover, put plastic foil over the piping sticking inside and mount the cover - allowing the foam to shape around piping.
That probably will give it about R4 value, but at forced air compartment, where heat exchange goes into overdrive.