One problem is that your saw will need to cut 4in (100mm) depth to handle a lot of insulation materials. I'm a carpenter and joiner and my biggest saw runs a 230mm (9in) blade and has a cutting depth of c.85mm, so unless you invest in a truly massive saw (at a truly massive price) you'll be limited in the thickness of material you can cut. Another major problem is dust. Cut any insulation with a power tool, especially one as aggressive as a circukar saw, and you'll generate substantial volumes of fine, cloying dust and fibres which are a known respiratory irritant. I've spent too many hot afternoons installing insulation over many years to want that ever again!
If you want to cut smaller volumes of insulation and don't want to resort to an old saw (and really rough cuts), it is probably better to consider something like an insulation kniofe, such as the ones by
Stubai or
Hultafors, or possibly the
Bahco insulation saw. I've had both the Stubai knife and the Sandvik saw for quite a few years and the Stubai is the most generally usable of the two, being capable of cutting both Cellotex and fibreous materials relatively cleanly and with fairly minimal dust/fibre release (you still need to wear a mask, though). It is faster and cuts fibrous insulations (e.g. mineral wool) far cleaner than the oft used cheap bread knife which makes it well worth the money IMHO
Incidentally I've cut the insulation for entire restaurant ceilings, library ceilings, etc with the Stubai knife - sufficiently quick, fairly neat and accurate cut, minimal dust (and noise), low (relative) cost and no power required (often an issue where you install) - in any case far superior to either a rusty old hand saw or a bread knife
If you need to mechanise cutting it is always possible to use specialist jig saw knives. Both Bosch and Festool sell similar blades. These are the Bosch ones:
I've had occasion to use the Bosch ones to cut the "patterned" foam rubber sound insulation used in recording studios. They work OK, but the cut needs to be done from the back and you often need to support the base of the jigsaw on plywood to prevent it digging into the insulation
Finally as you seem to be in the mood to spend money on machinery to cut deep insulation, there's always the
Festool ISC240 insulation saw (240mm depth of cut), a snip at c.£480
Personally, I don't see the need for circular saws to do this task, but then my needs might well be radically different to yours
