PANELVENT – High Performance external sheathing board, combining racking, weather resistance with excellent vapour permeability Panelvent is a 9.2mm thick sheathing board, developed specifically for the timber frame industry as an alternative to plywood or OSB. Combining the benefits of a category 1 racking strength, high water resistance and a diffusion open construction, Panelvent assists in the design of a healthy living environment. Manufactured entirely from wood waste, Panelvent does not use any glue in the manufacturing process, relying on natural lignin in the timber as the material binder. Because there are no glue lines, the product cannot suffer from delamination, which is a major advantage for a sheathing board. Natural waxes are added to the Panelvent formulation to achieve the required sheathing performance. Panelvent is easy to cut and install using the same fixing and cutting procedures as standard sheathing materials. With a thermal conductivity of just 0.08W/mK, Panelvent contributes significantly to achieving Building Regulations “U” values and minimises cold bridging in constructions where solid timber is used. Panelvent is a naturally healthy building material and has been awarded the Blue Swallow Standard by the Swedish Asthma and Allergy Association.

http://www.ecologicalbuildingsystems.com/products/pro-clima-windtight-systems/panelvent/

Panelvent has been replaced by a product called Timbervent now.

Thanks. Actually I don't know what air permeability should be - is this figure good or bad?

Incredibly, Fraunhofer Institute, home of WUFI, don't bother with air permeability, amongst all the numerous parameters they test manuf's products for, as necessary in WUFI simulation.

They've only recently discovered that slow all-over air permeation is at least as important a cause, if not more so, of interstitial condensation, by airborne bulk-water-vapour movement, as is the traditional focus on water vapour diffusion. So far Fraunhofer/WUFI only has a crude way of approximating that large risk, by dubious correlation with blower-door test results (by which time it's too late).

They seem satisfied with that, not busy collecting air permeability data of materials and products as foundation for a more rigorous methodology. This is a big shortcoming in WUFI and AFAIK all of its competitors.

I'm struggling to get my head round the physics (is that the right word?) of a timber frame construction, so you will have to bare with me on this one ---- so looking on a typica timber frame wall build up,

1 - plaster board layer
2 - a vapour impermeable layer (this stops the water vapour inside the house being pushed, pulled or otherwise drawn into the wall construction and hence stops condensation within the wall.
3- the timber frame and insulation
4 - wall sheathing
5 - a wind proof breathable layer (this is to allow any moisture within the wall to exit into the big wide world).
6 - a rain shield layer of some sort, cladding, bricks or block (rendered).

So do we need a sheathing layer and the wind proof breathable layer? What exactly are we trying to achieve?

1 - yes
2 - leave it out altogether - although still mainstream, recent research, WUFI modelling and bitter experience shows that vapour resistance at this (or any) point in the wall buildup is BAD. It's a myth that water vapour originating from inside is the problem in walls - walls can't help containing enormous amounts of moisture, hopefully safely in vapour form, not condensing to liquid at any point - and 95% of that originates from outside and ebbs and flows diurnally and seasonally. Don't try to block it - the drying out needs to happen both outward and inward. An inboard VCL halves the drying potential of the wall, which may never dry out, but increase its water content year by year. VCLs used to work, still just do for mere Bldgs Regs levels of insulation, but accidentally IMO, as an air barrier rather than as a vapour barrier as everyone imagined. Now, with v high levels of insulation, VCLs are bad news.
3 - yes - but only vapour permeable insulation i.e. not closed cell PIR, PUR etc.
4 - yes
4a - Now you need more vapour permeable insulation, right across the external face including the studs, which on the system you describe are a straight-through uninsulated thermal bridge. External insulation will almost entirely kill that bridge. You need 250-300mm of insulation; unless you have studs that deep, it can't all go between the studs; part must be in additional layer outside the sheathing.
5 - yes
6 - but masonry will require an outboard foundation, so vastly increasing your foundation dig, muck-away costs, lots more concrete, greater effective 'footprint'. Cladding will need to be hung from the eaves.

Instead of both 5 and 6, patent thin-coat render direct on the external insulation is by far the cheapest.

You need a layer that's impermeable to air (but very permeable to vapour) at some point in the sandwich - doesn't matter where.

In addition a windproof layer outboard, if your outboard insulation is fluffy or has an 'open' surface - EPS is a little bit 'open' so prob needs that; PIR/PUR are not 'open' so don't need that.

This windproof layer is usually also the 'breather felt' layer, laid close fitting at well lapped joints. Don't trust sticky tapes anywhere. The breather felt is well enough water proof to catch any water that may get thro the rain shield. It must be very impermeable to vapour; it won't be very air-impermeable but good enough to keep blasts of wind off the insulation face.

Don't take my word for all this unless paying me a consultancy fee - check it to your own satisfaction.