RAAC is reinforced aerated concrete okay unless it gets wet, but as one comedian said "It fu*kin' rains!"  - aareated concrete was invented to make the builds more lightweight and reduce the dead loads, and make it easier to handle. Its a nice idea, but it is better to use something that can get wet; plastic for instance. 

We use PIR polyisocyanurate used in external insulation, and XPS used in wet room design. Order a sample, and leave it in your garden if you don't believe us. It can get wet, and is not affected by frost -  the fibreglass covering is just to go over the joins which are effected by frost, and make it more aesthetically appealing. 

Spray foam is a great way of insulating at a low cost, but it is not designed for timber roofing. When it is sprayed on a cold roof it makes it unmortgageable and sellable only to cash buyers. Some rouges are still doing it; and costs thousands to remove it. The foam traps the moisture not giving the timber opportunity to breathe. We use 'Sticky PU' which is not used for spray foam insulation but for adhereing plasterboards to wet walls; such as the house wall in a conservatory or an extension. The wetter it gets the better, and we apply water to the surfaces when we adhere the panels together (off site). The joins are affected by frost however, when water expands it will seperate the panels, so we cover it with fibreglass and resin, just like in wetroom design, or with boats and surfboards. 

It is forecasted that timber with quadruple in price in the next 10 years, and has tripled in price in the last 15 years.  We simply do not live on an island plentiful with trees, with populations expanding in places where there are a lot of trees; the UK, Brazil, Scandenavia, the import tariffs have risen as their own demand has grown. 

But the good news is; timber has many disadvantages: moisture control, airtightness, rot, and poor workmanship. In wetroom design we cannot use timber, so; we won't with your garden shed too.