All ducting should be insulated EXCEPT --- the ducting that carries fresh outdoor air from outside to the HRV and the ducting that carries stale exhaust air from the HRV to outside.
What Size ducting ???
The difference in price between common smaller duct sizes, (as used in residential HRV installations), is negligible. The larger the ducting diameter, the better the airflow performance of the HRV, and the quieter the air flow noise. So make a basic principle to ALWAYS install the largest size ducting for as far as possible from the HRV. Only reduce ducting down to smaller sizes at the "take offs" to rooms where smaller ducting is appropriate, as close as possible to the final ceiling terminal outlet / inlet. Ducted models have outlet spigots designed to accept ducting that is 200 mm diameter. Plan on using as much 200 mm diameter ducting as possible. Then reduce to 150 mm ducting and finally the shortest possible lengths of 100 mm ducting.
Here are some ducting basics…….
One 100 mm ceiling terminal can service a room up to 40 m3 internal volume. If the room (say a large lounge on its own or a combination lounge, dining, kitchen, or large bedroom), is larger than 40 m3, then the ceiling terminals (Exhaust or Supply) must be increased to 150mm size, or two (or more) ceiling terminals of 100mm diameter.
The reason is simple ----- the ducting installation has to be capable of ventilating each individual room at the rate of .35 air changes per hour, and small ceiling terminals (say 100 mm diameter) have a limitation as to how much air can pass through.
All ducting must be installed fully stretched "out" so that the internal surface is as smooth as possible, to minimise air flow resistance.
Ducting must not be "bunched", or have very tight bends. Bends must be "sweeping curves" and keep the number of bends to a bare minimum. Bunching, and tight bends cause significant air flow resistance, and may cause the HRV to under perform.