Source: David Mann / Andrew Footwear
These boots have leather and fabric uppers. This means that their performance primarily comes from the construction of the sole and welt, which is the manner in which the sole is attached to the uppers. Injection welted boots have rubber soles that are typically stiffened with a nylon or plastic shank. They are joined to the leather uppers by way of a gluing or injection type of process, hence the name. Most lightweight hiking boots seen these days also use an injected welt construction. So, one way to identify an injected welt is that they look something like modern hiking boots. Injection welted boots do not have visible stitches along the welt.
Some have questioned the durability of this type of boot type and it is unclear if they can be resoled in a cost effective manner. Still, the injection welt and modern nylon reinforced shank make these boots lighter, more watertight and better turning boots than their Norwegian welt counterparts. In general, I would recommend a injection welted boot over a comparable Norwegian welted boot for these reasons.
These leather boots look like old fashion hiking boots with big stitches around the welt. This used to be the primary construction type and today this type of boot is still being produced by Garmont and Alico. Typically, the boot derives its torsional strength from its midsole. While these boots can be resoled in a cost effective manner, they have a well earned reputation of being torsionally soft. An old timer once advised to not use leather boots to ski hard pack with skis any wider than 65mm at the waist for just this reason.
Norwegian welted boots are also prone to getting wet, even when used with glued on rubber-randed super-gaiters. The reason for this is that it is practically impossible to entirely waterproof the welt and mid-sole (between the outer sole and the stitches). If you do decide to use a Norwegian welted boot and keeping your boots dry is important, there are some things you can do to minimize (but not eliminate) water intrusion. First, seal the stitches with good sealant. My favorite goop for boots is Seam Grip. It stays flexible for years and resists being pulled away from the boot. Before you apply the seam grip, clean the stitches and leather thoroughly with a solvent like acetone to remove any grease or wax from the leather, which prevent the Seam Grip from bonding with the leather. Note, the heavy solvent may permanently strip or change the color of your boots. Pour the Seam Grip into a syringe style infant medicine dispenser which are available and most pharmacies. This will allow you to lay the seam grip into the seams accurately with a minimum of mess.
The second thing you can do is to use rubber-randed super gaiters, which are available in both insulated and non-insulated versions. I glue mine on at the beginning of every season with Shoe-Goo to prevent them from pulling off of the toes of the boot and to create a more water-tight seal. The gaiters are tight, they may curl the boot toes upward toward the sky. Some folks don't glue their gaiters on for this reason. Personally, I get so annoyed by gaiters popping off in the field, that the risk of curling my boot is worth it. I glue my gaiters on at the beginning of the season and leave them on till the last of the spring skiing. Note, removing glued on gaiters will almost certainly mar the finish of your leather boots.
A good Norwegian welted boot will last for many, many, many years, especially if they are repaired and resoled by a good cobbler. But, their weight, the general lack of torsional stiffness and lack of waterproofness compared to injected welted boots makes it hard for me to recommend them.
Back in the day, the ultimate in warmth was provided by double leather boots. These had a inner bootie that was placed inside of a outer leather shell. While heavy, these boots had good reputation for being warm. Today, these boots have practicly disappeared and have been replaced by plastic double boots. The plastic shells of these boots are generally lighter and more durable than their older leather counterparts. They are also waterproof.
For more detailed information about boot construction, check out Part II of this article right here.