Lots to think about and answer there; chaps. Thanks for the input.

Posted By: WillInAberdeenDidn't quite follow 30ug/s/m2 being 1.2kg/d

30 µg/s/m²

= 30*60*60*24 µg/day/m² = 2592000 µg/day/m² = 2.592 g/day/m²

= 2.592 * 464 g/day = 1202 g/day = 1.2 kg/day

yes?

I expect there's condensation at the outboard side. I don't see that as a problem. If you think about what happens when it rains on a lime-rendered straw bale, it's not entirely unexpected to find liquid water there.

willinaberdeen said: ""The outside render is almost half of the total vapour resistance and is outboard of the insulation. A conventional simplistic analysis would say that would mean condensation on the inside face of the render. As the temp on this inside face is virtually external, and the external RH =100%, the RH on the inside of the render is also 100%. As the vapour pressure gradient across the render is virtually nil, no vapour can diffuse through the outside render. In contrast the plaster layer inside the wall, experiences almost the entire vapour pressure delta (300Pa inside to outside), so vapour flow through it will be about twice the figure calculated here (so 300/5.6) most of which will condense in the straw. To be honest I think this shows up the flaws in the conventional CRA methods, not in your wall! "

Yes, there's almost certainly condensation inside the external render and probably for some way through the straw (but we're into guesswork there I think). And yes, I don't think conventional analysis has much to offer; we're into the marginal cases and a full FEA would be the way to go.

Perhaps wall area 264m2 not 464?
= 2.592 * 264 g/day = 0.7 kg/day

Ed Davies said "Assuming the thermal conductivity of straw is 0.040 W/m·K"

IIRC the EU standard figure is 0.048 W/m·K after correction for moisture content etc.

Posted By: WillInAberdeenEd, if you set the vapour pressure at the render/straw interface to be 100%RH due to condensation, what vapour flow does that give through the internal layers?

Tweaking the straw bale thermal conductivity to 0.048 W/m·K as DJH suggested results in:

Thermal resistance 10.770833333333334 m²·K/W
Thermal conductance 0.09284332688588007 W/(m²·K)
Temperature difference 17 K
Heat flow 1.5783365570599612 W/m²
Vapour resistance 11.379826446280992 GN·s/kg
Vapour permeance 0.08787480237247441 µg/(N·s)
Vapour pressure 326.826894426334 Pa
Vapour flow 28.719848757723657 µg/(m²·s)

…

Interface Straw bale / Lime render (outer)
Temperature 2.9864603481624776 °C
Vapour pressure 870.3599103637765 Pa
Equilibrium VP 757.2798006791074 Pa

So RH at that interface of 114.9324%.

Deleting the outer two layers (silicate paint and outer lime render) and setting the “outside” conditions to temperature 2.98646 °C and 114.932% RH does, as one would expect, give the same heat and vapour fluxes to an appropriate number of significant figures. Reducing the RH to 100% then gives:

Thermal resistance 10.145833333333334 m²·K/W
Thermal conductance 0.09856262833675564 W/(m²·K)
Temperature difference 16.01354 K
Heat flow 1.5783365913757699 W/m²
Vapour resistance 5.654826446280992 GN·s/kg
Vapour permeance 0.17684008687086583 µg/(N·s)
Vapour pressure 275.4858886614811 Pa
Vapour flow 48.71694848259399 µg/(m²·s)

So the vapour flow has increased from 28.7 to 48.7 µg/(m²·s) implying a net condensation rate of about 20 µg/(m²·s) or 1.728 g/m²·day. Lucky we don't see 2 °C, 100% RH for more than a few days very often, I suppose.

We all know that Straw Bale homes don’t rot quickly (within 30 years) if they have a good set of boots, and a good hat. Long term we know little, as older buildings had so many drafts and where heated with open fires. (But we do know that rot is very slow when it is cold, as anyone trying to make compost in winter will tell you.)

The boundary air next to the silicate paint is lightly to be a little warmer then the “outside” air whenever the sun is out, and hence have a RH a lot lower than 100% when the sun is out. Also most of the time there will be some air movement from wind against the silicate paint.

What do the calcs show if the boundary air is assumed to be 2c above the rest of the outside air?

I think setting up detailed monitoring of walls in a Straw Bale house could make a nice PHd project for someone….

Posted By: ringiI think setting up detailed monitoring of walls in a Straw Bale house could make a nice PHd project for someone….

Yes, there has been some but it's surprising how little, and how little has been openly published.

I have some logs from embedded RH+T sensors in my walls and roof, but the system is currently offline. Hopefully will be back in December.