Re: Insulation~Not all insulation is equal...
The current practice of...
I'm afraid you are simply wrong about some points, and haven't thought through some others.
Firstly, common practice in the UK (speaking as someone who has just built a house) is to install insulation to the equivalent of
- 150mm foil-faced expanded foam under the floor screed
- 75mm on the inner leaf of a cavity wall
- 150mm either on top of the ceilings (ventilated attic) or in the plane of the roof (attic within heated envelope of house)
It is not really possible to get away with any less and still meet building regulations, and the building regulations are due to be tightened again before too long.
Retrofitting that amount of insulation to an existing property is very difficult, except in the case of the attic. For the walls, an existing uninsulated cavity (if there is one, many older properties do not have one) may only be 50mm in width, so even full-filling one will not add a lot of insulation, though it's better than nothing. Care needs to be taken to fill it evenly (or risk cold spots / condensation) and with a product appropriate for the damp environment.
Without a cavity wall, or to bring a cavity wall up to modern insulation standards you would need to apply additional insulation either externally - foam boards or spray-foam rendered over - or some other form of insulation to the inside of each room, making the rooms smaller. Plasterboard with an insulation backing is one possibility, but high-tech solutions such as Aerogel are a possibility for the future. One friend of mine has installed cork insulation as a natural product not requiring a costly, Carbon-intensive manufacturing process.
Insulating under the floor is of course quite disruptive - you usually have to dig up the floor.
There are two common types of "wool" insulation. Rockwool (not RockWall - that is a board product) is quite literally made from rocks. More generic glass fibre insulation is what you are describing, it is similar in structure to the stuff used in fibreglass and concerningly similar to Asbestos at a microscopic level though the fracture patterns are different I believe, meaning it is an irritant but not considered a long-term health hazard once exposure stops.
Both are fairly good insulators (though not as good as foam), cheap to buy, can be formed into rolls (e.g. for attics), batts (e.g. for stud walls) and into fibres for blowing into cavities. Both are absolutely horrid to work with (speaking here as someone who used to crawl around attics for a living) and as a result, where we have needed a wool-type product in our new house, we have used Thermafleece which is almost as good an insulator, is only slightly more expensive, smells like damp sheep when you install it (but the smell soon goes) and not at all irritating, unless you happen to have an allergy to lanolin.
Other innovative products are available such as cellulose insulation - made from recycled paper, cotton-fibre insulation - made from recycled jeans, hemp insulation and "foamed glass", a product made from recycled glass which can be used as a combined hardcore and insulation layer under the floor slab. Try suppliers such as Ecomerchant or, in this neck of the woods, Tŷ Mawr.
And then you raise the whole question of materials used for the walls themselves. There are good arguments for building "light" structures from wood and board, packing the gaps with insulation - they are easy to manufacture off-site in a controlled factory environment and can be brought to temperature from "ambient" very quickly, but likewise there are good arguments for traditional "heavy" structures made from concrete, block, bricks and plaster. In particular they are easier to modify on-site, easier to make airtight (if you use wet plaster), easier to live with long-term (you can't just drill a hole in a timber-frame house for the power cable for that new outdoor light as you will break the vapour barrier and threaten the life of the timber) and, most importantly from our point of view, a heavy construction has a "damping" effect on the rate of rise and fall of temperature.
Somewhat frustratingly to the proponents of each style of manufacture, the total cost of each structure ends up being about the same and the total time from pressing "go" to having a completed envelope is about the same (timber spends more time in the factory and less on site, heavy spends pretty much all its time on site) and, of course, once complete to building regulations they are both just as efficient to keep warm. It's the dynamics of the heating that are the real differences.