How should I insulate a 150 year old house to keep it cool?

Some good info
My family are moving some way south to an old house orientated north south but currently has no insulation. Would anyone have a source of info on how to "modernise" something maybe 150 years old, timber frame roof solid walls, to stay cool in the summer?
Lime with hemp on the inside of the walls is our plan to keep warm, but what to do with the uninsulated tiled roof is a harder question for me without sealing up the old timbers perhaps too much?

There are some good types of insulation that keep the warm in or the warm out depending on your climate, but still allow it to "breath" a little. I don't think it is healthy to seal up a house like a plastic bag.

Mineral wool
sheep wool
wood fiber
hemp wool
some kinds of cellulose

There is even a webinar on growing your own insulation using mushrooms somewhere on Permies, though I cannot find the post right now.

If you keep to the more natural materials and stay away from the fiberglass and foams you should be able to insulate just fine without over insulating.

There are lots of ways to insulate an old house.  If you have an attic, you might rent a blower and fill it with cellulose insulation or some of the other types of insulation.

While not the best permie appropriate way, the existing walls can be line with Styrofoam:  

https://permies.com/t/261477/Insulating-recycled-Styrofoam

https://permies.com/t/168557/permaculture-upcycling/bunch-styrofoam

I guess if they are moving south, within north America, the insulation issue is more about keeping the heat out?

If the house is situated north-south, does that mean the roof pitch is east and west? Likely. Over any doorways on the south side: a possible north and south facing pitched roof ...

To keep the roof cool, if aesthetics allow, one could add photovoltaic panels to any south surfaces, and anywhere not north: glycol panels (solar heat ) with a gap of a few inches between the panels and the roof.

Of course most insulating fails at the windows. In a hot climate, a reflective layer on the windows would help a lot, as would thermal blinds. In a cold climate, most likely window replacement, but if the walls are getting thickened up by the inside, one could double up some windows if some are the same size, and replace half -- the ones that don't open could be doubled, with a decent space in between.

I can picture the lovely window ledges once the hemp is added -- in the living areas, windows would be crying out for some really nice red cedar.

If you ever get a chance to post pictures, I am very curious!

The hemp lime is fireproof too so I love that idea!

jason holdstock wrote: without sealing up the old timbers perhaps too much?

This can be a real problem when changing the original design. I have seen a number of videos on YouTube regarding vapor barriers showing how problems can occur, as well as some real world mishaps leading to condensation, rot, and very short service life before needing repairs. A warm roof is assembled differently than a cold roof, and if there are points of the year where those temperature invert for more than a few days at a time, it can quickly lead to condensation. The main takeaway I got from it is that you really don't want to have a lot of wood of other material damaged by moisture or prone to rotting right up against the vapor barrier if possible.

There are different kinds of vapor barriers with various amounts of breathability, and using the wrong one or installing it incorrectly might be worse than not having one at all. Any insulation used could either draw up water and grow mold, or hold it up against something that will grow mold and/or rot. There is a good YouTube channel with a bunch of well explained info here: Steve Roofer - YouTube

To add to the advice already given, I would place insulation between the roof rafters to keep the heat out.  Even a thin reflective barrier will get results. Do keep in mind, this will not help to keep heat in if your attic is vented.  I would plant trees now for shade. I would plant a combination of fast growing trees along with slower growing ones.  This is assuming you are lacking shade trees now.  I have seen figures that one large shade tree is worth about 5000 btu.

Consider awnings on the south and west windows.

Hi Jason:
"...My family are moving some way south to an old house..."

You are on the Isle of Lewis, and are moving some way south on the Isle?  Or moving radically farther south?
A friend recently tried hemp/lime on the inside of a house in a climate similar to the Isle of Lewis (coastal Bay of Fundy, Nova Scotia), and it went mouldy.
I second comments to be careful about vapour barriers (or vapour resistant insulation like closed cell spray foam), and older houses.
It can be done, but it can also create a bag of rot.
Blown-in cellulose in wall cavities and attic can work with some constructions.

I thought I might get reprimanded for derailing the thread, but I was kind of following a couple of points already made on structural solutions as well as the more personal ones. Thank you for the replies any how :) Now I see it's in a thread all by itself!

We are moving a wee bit into France to be closer to family who moved there twenty plus years ago.

The roof pitch runs north south, all windows face east with wooden shutters, the roof starts about three feet above the first floor floor so most of the wall and ceiling upstairs is currently bare rafters. Most of the downstairs west wall has a secondary outbuilding attached but more like a lean-to or shed which would keep the sun off the habitation walls, and the south facing gable has next doors "hanger", or open sided tractor shelter, to keep the sun off that. The house is only one room wide apart from that lean-to area which has an earth floor, not habitation as such.

I could stuff insulation between the rafters, with one of those multi layered silvered sheets over or under but I think keeping the air around those timbers is a better idea than filling the gap. Maybe the best idea is to take the clay tiles off and put a weatherproof insulating layer between them and the rafters/battens but that's a lot of work, time and money. Or suspend insulation below the rafters, but that might get damp with minor tile damage without me knowing for some time?

I'm quite happy adding thickness with hemp on the walls. A previous house I spent time in had wattle and dawb making it very pleasant all year until someone took about half the thickness off so they could admire the timber frame underneath. Then too hot/cold and ruined it really. No damp though, thatched, no gutters, french drain along wall bottom.

Somewhere there is a thread around here about painting the roof with lime every spring.  The person had hot dry summers and a fall rainy season, so it washed the tiles back to dark in the fall.

Another option for cooling is to use sprinklers to hose down the tiles in the heat. I suppose there is a risk of cracking tiles if they are too warm when you do it, but I know it works to pull the heat out of the roof. Just enough water to wet the roof, not running off the eaves.

So relocating  ' a bit far South ' means moving continents!
Some photos will help also.
This knowledge will help others giving advise.
Hot tiles will not crack if sprayed with  water.
I used a water spray on the roof of a business, in Australia.
Because we were looking at 40 deg. C during the dy, I started the misters at 8am and let them run all day.
The roof never got a chance to heat up.
Today I would add either a second shade metal roof or a shadecloth cover 12 inches above the existing roof material.

Do you collect rainwater off the roof?
Painting it with lime wash may create a problem if you do.