Due to their functions, bathrooms are exposed to a lot of water and humidity. Their design and construction should take this into consideration to prevent mold growth and heat/energy loss.
Insulating wall cavities prevents the transfer of hot or cold air to or from your bathroom along with any moisture that accompanies this air. Spray foam insulation is a good choice for wall cavities as it doesn’t allow any air to flow through it (which is a characteristic of fiberglass insulation). Sealing around windows and fixtures such as pocket lights will help prevent air leakage to the attic or to the outside that could offset your efforts of insulating the walls of your bathroom.
Wrapping pipe insulation around water pipes that flow through unheated or un-insulated spaces can help to prevent energy loss, especially if these pipes are carrying hot water. Putting an insulating jacket on your tank water heater can also help to increase its efficiency, preventing heat loss and the necessity for re-heating as often when in standby mode. Of course, the eventual replacement of your tank water heater with a tankless or on-demand water heater is the best way to improve both water use and energy efficiency when heating water.
Any wall insulation should include a vapor barrier. This should be installed on the warm side of the insulation to prevent condensation from occurring when hot air meets cold air. Water proof housing for any light fixtures above the shower protects electrical wiring from steam. The backing of showers or bath areas (what goes behind tile) should also be waterproof to prevent molding within the wall.
All material choices in your bathroom ideally need be water resistant. This includes flooring, countertops, cabinets and even wall covering. Ceramic tile and natural stone flooring are good water resistant options, and can be sealed with products low in toxins and VOCs. These same materials can also be used for countertops. Cabinets should be exterior grade plywood (with higher rot resistance than low grade) or solid wood. Cabinets can be also be sealed with low VOC wood finishes or painted with a durable low VOC paint. Wall paper should be avoided if possible in your bathroom, as moisture can cause mold build-up behind it or warping of the paper. Instead, paint your walls or consider tiling them.
Ventilation is also key for managing humidity! A ventilating fan that exhausts air to the outside should be included in each of your bathrooms.