What is the construction of the wall? That is, does the exterior have siding, sheathing, and a vapor barrier?

I'm not sure about that. I know that the outside is some sort of sheeting like plywood that has grooves routered in it to look like beadboard, but I don't know about vapour barrier. Unfortunately, we are not there right now to check.

We are in an extreme climate (we go south to Fargo if we want to warm up) and my father in law says it was insulated to the standards of the day when it was built (1965).

It has only ever been used for the 3 summer months, but we would like to start using it for occasional winter weekends. right now it takes up to 8 hours for it to warm up enough to be (barely) liveable.

That sounds like T1-11 siding, which is usually applied over regular 2x4 framing, or hopefully 2x6 up where you are. If so, yes, you should be able to remove the interior paneling and add insulation. Just use the paper backed(one side) insulation. That type should be fine, since the paper acts as a vapor barrier in both hot/cold conditions.

You may find drywall behind the paneling.

Thanks handymac. We will look for the paper backed type.

DavidR, in some places where it meets the ceiling, we can see behind the panelling and it is the only thing there - well, except for several generations of mouse droppings.

> mouse droppings

Ewwww. Been there. Wear face masks and have lots of ventilation.

Don't forget to insulate the roof as well!