When should you insulate your sub box?

[Bobbytwonames 5,000+ posts]

I don't mean poly-fil. I mean like the picture that I have attached. Where you attach the insulation to the inside of the walls of your box.

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[Jeffdachef 5,000+ posts]

Thats a different form of polyfill which is called batting that you can attach to the side of your box that looks like sheets of insulation rather than the loose cotton like stuffing used for pillows.
You can expect to get a 10% ish increase in airspace performance wise in a sealed box, that shit will just blow straight out of a ported box if your box is properly designed and loud. You'll have better results increasing bracing and increasing box layers along with a proper enclosure design based on your sub's electromechanical parameters and your car acoustics(cabin gain) in mind along with sub and port placement to make best use of that cabin gain keeping all the reflected and incident waves in phase.

 

[THATpurpleKUSH 5,000+ posts]

Unnecessary in ported boxes. Jeff explained it well, overall rigidity is more important.