Vapour Diffusive Walls using Gypsum Fibre Boards

[Bruce Newlands]

I'm looking for any advice / experience of using gypsum based boards externally & internally on a vapour diffusive construction.

We're keen to explore alternative options to using OSB internally because of potential VOC's (price of low VOC OSB) but also just to reduce the number or board layers in the construction.

We also want to maximise the recycled content in the wall construction, hence the gypsum board preference.

We're aware of woodfibre boards for external insulated sheathing but again, these can be quite expensive and we've had issues before about the robustness of the fixing these boards to timber frames and subsequent weathering issues when they are left exposed in the West of Scotland.

We're keen to pursue an offsite closed panel technique, so building the entire walls in the factory for delivery & fast erection, mainly due to remote rural location(s).

The proposal is therefore to use a timber frame which is structurally braced using diagonal timbers, so that no racking is required from the sheathing / lining boards.

The frame includes an internal batten arrangement to try and minimise repetitive thermal bridging.

I've came across a number of relatively new gypsum based boards for external sheathing use from Lafarge, Fermacell & British Gypsum (albeit they seem to be cement based) they have µ value of 10-19 which as I understand it (using the 1<5 rule of thumb for layers) could potentially be used in conjunction with something like a SISALKraft Building Paper internally with a µ value of 40-60 and therefore work as a vapour diffusive wall.

The proposed insulation will be a natural fibre, although undecided yet between wool or cellulose.

I'd be keen to find out if anyone has seen / used anything similar to this.

Just to reiterate the layering of the proposed construction is a follows:


- Windtight Breather Membrane
- 12.5mm Robust Gypum Based Board µ 10-19
- 190mm Timber Frame (Self Bracing / No Sheet Racking required) / Natural Fibre insulation µ 5
- SISALKraft Building Paper or equal µ 40-60
- 45mm Service Zone / Natural Fibre insulation µ 5
- 12.5mm Gypsum or Clay Based Lining Board µ 10-40



Thoughts?

Is the external board to high µ for the insulation we'd like to use?

Does anyone offer WuFi analysis as consultancy?

[Graham Clyde]

Hi Bruce,

I've been looking into something along the same lines as you.

Whilst at Scotframe - a timber frame manufacturer, we've came up with a breathable wall solution. But it was kinda hard to stick to only having 5 layers. The outer layers consisted of pavatex and diffutherm with a render system.

Woodfibre based sheet materials do tend to get damaged very easily, we've experienced this at Scotframe. Hence the need to produce a closed panel timber frame system in the factory.

With regards to wufi ORNL - see here: http://www.wufi.de/index_e.html

I've been looking at purchasing this software, but not had the demand for its use. Could you be the first?


Regards

Graham

[fostertom]

Have you though of say 120x50 studs crossbattened with 70x50s - that way thermal bridging from your 190 through-timbers is v effectively killed and the cross-battening gives something for the Warmcel to lock onto and not slump.

Racking resistance - rather than adding more (diagonal) timbers that reduce insulation, how about galv steel flatstrap nailed diagonally across the studwork face, maybe at the stud/crossbatten interface. Not sure if Engineers and Regs will acknowledge anything lighter than 5x30 Batstrap but to me 100x0.7 dryliner's flatstrap wd be fine.

Having used OSB for that struct purpose, plus with all edges supported, glued and screwed also serving as a robust airtight membrane, I'm v interested in substituting gypsum board, which requires something else to do the racking resistance, but I' still wondering how gypsum board could be installed as robustly airtight as OSB can.

[Mark Siddall]

Hi Bruce
I think that you'll find the ORNL version of wufi is locked (so you can not add weather stations.)
If that's the case then ease be aware that Joeseph Little does WUFI analysis
http://www.josephlittlearchitects.com/

cheers
Mark

[Bruce Newlands]

Thanks Graham, your probably thinking along the same lines as me that the sheer performance of hybrid/sips is fine but I've got a niggling doubt that many clients, especially public sector ones are looking for solutions that are both affordable, high performance and carry some degree of ecological performance in terms of materials / locality to the project...... Tom, thanks, I know you've commented before, it really does seem to be a bit of the holy grail to find a board that doesn't cost the earth and can handle the practicalities of exposure whilst performing ina  vapour open way, I'll come back to this once I've completed my search.....

Mark, cheers for that, I spoke to Jospeh a few weeks back about possibly doing some thermal bridging / wufi analysis and we may yet get him onboard, thanks for the tip.