40"W X 6"H [Duct Opening] Steel Air Vent Return Grille - HVAC VENT DUCT COVER DIFFUSER - [Outer Dimensions: 41.75"W X 7.75"H] w8h8 and the second number represents

and the second number represents the height

1/2" White insulation will give you a good R value of 3

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Aluminum Return Grille Spec Sheet

+ Product Details Size – 4"w X 12"h is the size for the duct opening measurement only

40"W X 6"H [Duct Opening] Steel Air Vent Return Grille - HVAC VENT DUCT COVER DIFFUSER - [Outer Dimensions: 41.75"W X 7.75"H] w8h8 and the second number represents+ Product Details Size 40"w X 6"h is the size for the duct opening measurement only, actual front end outer dimensions are approximately 41. 75" width X 7. 75" height. Design Easy unrestricted airflow for your HVAC system 50 angled to provide maximum airflow. High Quality Solid stamped steel with counter sunk screw holes (screws included) and no sharp edges. Powder Coat Finish Smooth, shiny, scratch resistant, beautiful. Looks like it was just waxed.