Model: RDH1507TCW

Studying the heater's innards, I noted there is the heat unit box that sits inside the outside wall mount box. I noted no back clearance and the heat unit lays flat against the back of the outer box. Normally a clearance would provide a separate air convection for circulation such as found in the older fireplace chimneys. The wall hollow is 4inches to the drywall of the next bedroom and there's no insulation.

So I read the instructions for insulation of the box surround that's required. None.

It says only this concerning insulation.

"Insulation surrounding the back and sides of the recess
mount box will not affect performance. Ensure insulation is rated for temperature up to 85 deg/C". (180F I think).

So is that saying temperatures can reach 180 deg within the wall? I don't want to buy a 50$ bag of Roxul to get 4 sq ft of insulation for the unit.

Any advice would be appreciated.

Thanks.