I did not insulate the cabinet. No need to. I only had one return in the rear berth right into the unit. The boat is open everywhere except the v-berth. So you just need to keep that door open for return. I had three supplies. One in the rear berth (I made a duct out of wood and had the grill towards the entry into the berth), one under the nav station and one under the port settee. I did not run a duct in the v-berth but could have.Thanks Sailgunner, I'm taking your advice. Curious though, did you insulate the cabinet you constructed? Did you route the return air through the head? I'm a little leary of having the return air come through the rear cabin, your thoughts would be apprciated.