Now is the time to consider how you are going to select and install the insulation that is so important in any boat. Even fiberglass boats need insulation as they 'sweat' in the same manner as other boats, including metal ones.
If you are planning spray-in-place foam insulation then this should be installed after the deck and superstructure are in place, but before you start work in the interior joinery. An alternative is to install 'bats' or sheets of foam glued or held in place by the 'ceiling'. See below in the section covering lining materials for details on installing the ceiling planking.
My choice for foam insulation is the type of urethane foam that has fire resistant and non-toxic properties and is sprayed in place. In steel boats, the interior of the hull should be first grit blasted and primed, or built of pre-primed materials. The foam will now be sprayed to a depth between 1" / 25 mm and 2" / 50 mm; the thicker the better. It is a good idea to cover the stringers and they will most likely be about 1 1/4" to 2" / 30 to 50 mm deep. The foam will form a skin and should be of a type that is forms a skin on the surface that is impervious to water. Make sure you choose a variety that in the event of fire does not give off toxic fumes. You will also need to make sure that the foam is of the self-extinguishing type.
The supplier of the foam usually has all the equipment needed to install the material and charges by the cubic foot / cubic metre or by the pound / kilo. Make sure you obtain an estimate of the cost for the boat to be sprayed to the desired thickness. Be very careful that you get what you pay for. Measure the depth of the foam and check that it is reasonably constant thickness throughout the boat.
There are a variety of materials that can be used to line the interior of your metal boat. This is one area where the wrong choice can damn your boat and scream 'AMATEUR'. If you want to achieve a decent resale value, then you had best get it right.
If you are building a 'traditional' style metal boat such as a Spray replica, or if you like the warmth of an all timber interior, then ceiling planking can go a long way to achieving the right effect. Ceiling material should be 1" x 5/8" / 25 x 20 mm light colored fine grained timber that is rounded or arrased (beveled) on the outer edges. Install the planks longitudinally and space them at about 1/4" / 6 mm apart. You can plank only those areas that are visible after the joinery is complete, or you can plank the entire accommodation area and use the material as the lining for the various lockers. If you take this latter course then you had better reduce the spacing to say 1/8" / 3 mm so you will not lose small items though the gaps in the planking. If you only use ceiling where there are no lockers then you may have a problem getting a fair curve where there is no frame to attach one end of the planks.