Insulating barn walls with lean-to extensions

[Rupert]

A common feature of old barns is an external lean-to structure added to one or more walls, often retrofitted, but not necessarily insubstantial. Here's a lovely example at the gable end of "The Long Barn" in Ceredigion:

 

 

Here's a cross-section of a similar structure, this time at the side of the barn with the upper roof overhanging slightly:

 

 

My question is: would you recommend insulating the entire inner wall floor-to-ceiling, or could you get away with just the dotted areas? I'm trying to preserve as much exposed stonework inside as possible, but without giving myself an insulation / condensation problem.

 

Edit: assume that the lean-to is completely insulated roof and exterior walls.

Edited May 27, 2022 by Rupert

[George]

Yes you only need to insulate the external thermal line.

 

 

If the lean to isn't insulated or a habitable space and is effectively ambient air temperature then you should treat the house wall as the 'outside' and ideally insulate all the way to the ground. However, any lean to does create some level of protection so would need to be judged on a case by case basis.

 

 

As you have shown, you need to go beyond the actual join point of, say, 600mm, (or the distance of the wall which gives an equivalent u-value).