I will be using solid pipe for both. They will be run inside the joist space above a finished kitchen/bathroom.

Someone told me that it's a good idea to insulate the first 6ft from the outside wall?

Cheers

Vent pipes running through any unheated space should be sealed at any joints and insulated along their entire length. This reduces the chances of condensation created when a warm duct contacts cold air. If that joist space is between two heated floors and does not have access to cold air, you might be able to forgo the insulation. They key is to keep the exhaust air as warm as possible until it exits the building.

You never see insulated flexible pipe and that is generally what's used for venting bathroom vents, while condensation is always a factor, it is not necessary to insulate the pipes. All they do is carry a smell.

I'll disagree with loridunla. In addition to smells, bathroom fans exhaust warm, moisture-laden air, especially during showers. Kitchen fans exhaust moisture-laden and grease-laden vapors from cooking. (Hopefully most grease is caught by filters.) This moisture can condense inside the vent piping as the piping runs through colder areas such as attics or other unheated spaces. That condensation can leak back into the building, causing damage and mold.

Cheap plastic flexible ducts can sag and create low spots, especially when run horizontally across joists. The low spots become great places to collect condensation and can actually restrict the air flow through the flexible duct.

I suppose plastic flexible ducts could be OK in very short runs or in warmer climates. I still recommend using insulated plastic flexible ducts, which are available by the way. There are also flexible metal ducts, but these should be insulated as well.

Insulating solid ducts or using insulated flexible ducts seems like an inexpensive bit of insurance against future damage.

The key is to get the moist (or smelly) air vented to the outside with as short a run as you can. Do not vent into your attic. The moisture will condense on your rafters and underside of your roof decking creating a haven for mold and rot.

Also, do not use cheap flexible plastic ducts for venting dryers. I know there are millions of homes with these. They create a place not only to capture condensation, but capture lint that can eventually become a fire hazard. Use smooth metal ducts that do not have the ridges and that can be more easily cleaned.

Do not use cheap cloth duck/duct tape to seal duct joints. Instead, use metal tape and make sure it is rated for the temperatures it will be exposed to in your application.

We haven't even discussed "make up air" for exhaust fans, dryers, water heaters, furnaces, and fireplaces, which is another subject all together.