Hi folks,

I'm going round in circles here trying to find the best way to insulate my loft which I'm converting to the kids bedrooms.
The house is an old mediterranean house, the roof has no vapour barrier or anything like that, just 'mechanical' tiles. I've fitted 12 air tiles (3 top and bottom of both faces of the roof to allow air to circulate.

The way the roof is constructed is a bit different from UK roofs:

24x12cm purlines running horizontal, spaced 1m apart
7x5cm rafters sat on the purlins running vertically (perpendicular to the purlins), spcaed 40cm apart
2.5x2.5cm battens on the rafters running horizontally, spaced ~35 cm (length of the tile)

I want to insulate between the purlins, leaving as much of the beam showing as possible (aesthetics but also head room).

I was going to put triso super10, but now I'm having big doubts about its efficiency.

The roof is in full Mediterranean sun all day and without the insulation, the loft easily reads 40-50oC in the height of summer.

What's the best way to do this? Cost not a pb, but the space (headroom) is so really don't want to use the whole 24cm depth of the purlins.

I was thinking 100mm PUR, 3cm space, some reflective thin thing, 2 cm space, foil-backed plasterboard.

Any thoughts?

Many thanks

Cost not a pb? Then I'd go for an aerogel insulating board such as Spacetherm. The addition of a single layer of foil above this and below the tiles would probably make a worthwile difference to stopping incoming heat.

Of course you could stop most of the incoming heat by putting PV panels above the tiles. This will shade your roof, keeping it cool. Oh, and you'll get some electricity as a free by-product.

Forgot to add. Then to get your character back fit false purlins beneath the ceiling boards thro to the purlins of any thickness that you desire.

If there's doubt about multifoils in temperate climates, there's much less doubt when rejection of summer solar heat is a main priority. A layer of foil at least, or better, multiple layers, does an incredibly superb job of eliminating the solar heat beating on a mid-day summer roof - and it doesn't matter where the foil layer is, the inner or the outer layer, relative to other e.g. between-rafter insulation.
Assuming your 7x5cm rafters are 7cm vertically, my suggestion, loosely based on UK practice, would be max 35mm Cellotex between the rafters (35mm bottomside airgap), multifoil to the rafters' underside, at least 35mm high counterbattens beneath, plasterboard (don't bother about foil-backed, nor any other kind of vapour-check, to the plasterboard). This way, you lose only 45-50mm of the purlins' visual height.
If you've got under-tiling felt that has to drape, then reduce the Cellotex thickness to provide topside drape space, or preferably use breather felt flat across unreduced Cellotex/rafter tops, downslope battens on top of that, tiling battens across that.
Note that multifoil, being 30mm thick, really requires 2x35mm space, not 25mm as generally recommended.
If the multifoil is the imperforate type, not the stitched type, then it will provide a bit of a vapour check. Or it can be a serious vapour and/or airtight barrier, given lots of detail work at all edges, all mechanically clamped e.g. by screwed battens right into every difficult corner-meeting; no reliance at all on sticky tape, which lasts a year before begining to fail.
The 70mm high multifoil space provides a services zone inboard of the vapour/airtight membrane, extending to 105mm high here and there, for example to fit a heat-downward (or LCD) recessed spotlight (cut away the Cellotex 200diam locally and use a 100mm length of 110 pipe with 20mm ventilation holes drilled in its side, to prop the multifoil over the fitting - plan where to leave slack in the multifoil).
With this spec you won't need a/c on account of solar gain through the roof, and in winter should be equivalent to 190mm of Cellotex.