The garden room is a modified summer house. Summer houses are built to be airy and lightly timber framed and as the name suggests they are not built for winter. Air tightness is the main issue: it doesn't matter how thick the coat Is: if there are gaps a much thinner coat keeps you more warm.
And this air tightness problem isn't just from poor assembly. One hard truth is that the customer demands more fenestration than is suitable. A Garden Room is exposed on all sides: thus leaving one whole side glazed, and exposed to the elements increase the thermal conduction by a lot. Further in most use cases a curtain wall would increase the solar gain, and the thermal mass, in buildings we like to lay concrete in areas where the sun will hit it, so that the concrete absorb the heat during the day and releases it over night keeping the temperature quite stable. Garden Room are usually always built on suspended timber floors so there is no heat gain from fenestration, only heat loss.
Insulting a Garden Room usually means cutting sheets of PIR/PUR to fit in between the timbers. Firstly these timbers are so thin in a lightweight garden room, that unsuitable thin insulation is often used: leading to complaints. And secondly the workmanship here is always poor: as Garden Rooms are not a regulated construction practice the builder can cut with the right type of saw, (timber hand saws take less time than specially designed insulation hand saws) and they will always leave a gap, perhaps they will tape over the gap with IR reflecting tape but it is rare to see insulation properly adhered in with expanding foam, and lately this practice has been shown to trap moisture so is generally avoided now.
And lastly no consideration is made for shape of the Garden Room. Think of when you sleep in a cold bed, you will likely sleep in the foetal position, and when you are too hot you will star fish, igloos and museum huts and many other primitive dwellings understand that the shape plays a huge role. Garden Rooms are always boz shaped. This means their SA/V ratio is as high as 1.4 when the ideal shape for low thermal conduction is a sphere with a SA/V ratio of 1.
The larger a garden room is the worse the IR (heat radiation) performance will be, and in many cases user buy the Garden Room to use on their own: but find the room too lofty and large to be heated with body and technology heat: so the users go inside to use the most efficient radiators rather than an expensive space heater.
All Garden Igloos are built with uninterrupted insulation, with even the small gaps in the joints forming part of the insulation. They are arched, domed, barrelled and come in a variety of sizes to suit the use.
