I should say the building will be wrapped in UV Fascade and between the posts in the walls i will have 50mm internal battens, air guard, PIR (sandwiched between the posts), UV Fascade, Batten, Oak Cladding. There will be a 50mm air gap between the internal oak panelling (attached to the inner battens) and the air guard

Be-utiful.

I'd say the felt over the sarking should be chosen as sealed airtight but vapour breatheable (much better without a vapour barrier); don't use Kingspan, PIR or any foam plastic insulation (filthy stuff environmentally), instead 150 or so (why settle for less?) EPS (much nicer environmetally or wood fibre (not as good as it's supposed to be, environmentally) laid uninterrupted across the sarking boarding; then breather roofing felt flat over that; then downslope 35x50 counterbattens screwed thro the EPS into the rafters; then tiling battens as normal. Fix each downslope batten across the ridge to its pair on the opposite slope to prevent the tile slopes from sliding downhill; it's not the insulation that will slide.

Can talk more about what you do at the eave/verge to achieve your open-rafter look - excellent.

Do you need to meet building registration insulation levels ?

I don’t see that you need oak for counter battens but for no fascia or soffit I would fix stub oak rafters feet under the topmost batten , likely with the batten rebated into it say 200mm long with an oak tilting fillet that also closes off the underside of the sarking felt.

Tom always votes for EPS, and I always say woodfibre or hemp) as they are much better for the environment despite what the EPD for EPS might claim.

We can have that discussion elsewhere but we both agree that vapour permeability is best :-)

Unlike EPS or PIR, natural insulations also aid building moisture buffering, and add decrement delay as well as insulation value, which as we have seen recently is just as valuable.


My suggested build up would be:

- Oak sarking boards

- STEICOmulti cover 5 (or other black peal and stick rainproof VCM)

- Woodfibre / Hemp insulation sarking boards (certain brands are rainproof and don't require additional
membrane on top - although this can always be added for high weather exposure zones).

- Battens and counterbattens specced for the tile/snow/wind load and roof pitch. Speak to the woodfibre
manufacturer to confirm these requirements since you are screwing through the insulation and weight is
bearing on the boards. Not usually an issue except for flat roofs or heavy stone tiles.

Depending on the roof pitch and ventilation provided at eaves and ridge, a minimum of 40mm air gap is required above the insulation layer to allow wicking. Looks like your roof is 35-43 degree pitch, so this should be fine.


I understand if you have enough PIR insulation left over that is is tempting to use it, but remember, even if you do get a snug fit between the battens when installing (unlikely), it shrinks over time in high heat situations.

You will also need to ensure the VCL is impeccable, other wise you risk trapping excess moisture between the impermeable PIR and oak.

I wouldn't insulate the eaves far past the external wall insulation if I could help it, but depending on eaves length this might be cheaper and quicker than faffing around making up height with timber. This is an important consideration for us slow builders (13 years and counting for me!), allowing us to get a job fully finished a little qicker before moving on the the next task.

Simple is best, and a little extra on material is worthwhile if the labour is free.

Thanks for the feedback so far, I have revised my approach accordingly

Due to the fact I have sarking with 3mm gaps I cant go for an air tight construction. I need a VCL that is black on the underside as it will be visible through the sarking


So how about this

1) Screw 50mmx60mm oak battens on top of the sarking along the gable ends. The roof overhangs by a foot so this oak batten is well outside the building (i will be having fascia's at the gable)
2) Screw 50mmx50mm oak batten along the eaves (i wont be having fascia's at the eaves)

at this point i have an oak frame around the roof directly on top of the sarking

3) Lay a VCL taped (all i can find black is Reflectatherm Plus Reflective Vapour Control Layer) on top of the sarking between the new oak battens
4) PIR Kooltherm 107 on top of the VCL, the PIR will be taped. The VCL will be upturned at the gables and taped to the PIR finishing. At
the eaves i can upturn and tape but i am tempted to just leave it so no risk of it collecting dirt / moisture. I will screw the PIR to rafters using stainless screws in something like a Warm Deck Insulation Fixing

At this point I have a nicely wrapped up slab of PIR in between 2 gable battens and one eaves batten

5) Counter battens screwed through the PIR
6) drape roofshield
7) tile batten


Q1) I still need to work out the ridge, should my insulation be taped at the ridge to the opposite side?
Q2) should i be using those horrible over fascia vent strips somehow to allow air flow into the battened area.


Getting closer i feel.

Kooltherm is probably my preferred approach due to its ease of installation and i have plenty of it at 50mm. I also have the tapes. it is pretty good performance wise for its 50mm thickness.

so what do people think