In general the insulation is done internally if it is retrofitted, or within the brick courses if it is a new build. The main products for retrofitting insulation on walls are made by the three biggest suppliers Kingspan, Celotex and British Gypsum. These boards come in two varieties, those backed with plasterboard and those not.
Normal Thermal Boards (Not plasterboard backed)
These are foil backed on both sides and are by far and away the most popular selling. The Celotex GA4000 range and Kingspan TP10 range are to all intents, indistinguishable, from a thermal performance perspective. The new regulations have meant that greater and greater thickness are now required to achieve the required U value. For walls this is likely to be in the region of 70mm of insulation but this is very much dependent on the wall construction (i.e. brick or block cavity or non cavity)
If you select a plasterboard backed product to go on your wall the main factors to think about are "How am I going to fit the product?" and "How thermally efficient is the product?":
You can either fix mechanically with screws or you can dot and dab (glue) the product with dry wall adhesive
With the Gyproc Thermaline range you can dot and dab or mechanically fix the whole range
Kingspan have divided their boards between mechanical fix Kingspan K18 and Dot and Dab Kingspan K17
Celotex provides a PL4000 board which can be glued or screwed
Where the choice is to mechanically fix, this is typically achieved by screwing through the insulation, either into battens or direct into the party wall. Screwing into battens is the preferred method as it easier to do and provides a more stable fixing. It also adds an air gap which is necessary where damp may be a issue. Typically a 25mm air gap is required. Dot and Dabbing is the term used to describe putting dollops of adhesive on the wall and pushing the boards into place.
Key to making the right board choice is understanding thermal efficiency of boards. Boards use different insulants which perform differently. These range from lower efficiency expanded polystyrene to high efficiency phenolic foam. The better the efficiency the more you pay; and the thinner the product is, therefore the less space you lose for the same amount of insulation.
Thermal Efficiency Of Boards Ranked Lowest To Highest
The least thermal efficient and therefore cheapest is Gyproc Thermaline Basic The insulator is expanded polystyrene
Gyproc Thermaline Plus - The insulation is extruded (a much more solid polystyrene)
Celotex PL4000 boards - The insulation here is PIR Foam
Kingspan K17 and K18 and Thermaline Super - The insulation here being phenolic foam, which is the most efficient of the insulating materials listed
Deciding On A Home Insulation Product
Your choice is often arrived at by weighing up the merits of space lost and the price of the boards. The better the performance of the insulation materials the higher the price.