Green Building Forum - Humidity buffering Fri, 01 May 2026 16:34:21 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249932#Comment_249932 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249932#Comment_249932 Sat, 12 Nov 2016 21:27:46 +0000 WillInAberdeen
Ed, 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?]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249936#Comment_249936 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249936#Comment_249936 Sat, 12 Nov 2016 21:56:08 +0000 djh
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?]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249938#Comment_249938 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249938#Comment_249938 Sat, 12 Nov 2016 22:11:21 +0000 Ed Davies 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?Might give that a go tomorrow depending on the weather. If you want to try playing with it I'd suggest reducing the vapour resistivity of the outer render so it takes more water vapour away to “simulate” condensation. Tweak the resistivity until the calculated vapour pressure and the equilibrium pressure are about the same. Remember to press “Plot” to update the results.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249943#Comment_249943 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249943#Comment_249943 Sat, 12 Nov 2016 22:27:01 +0000 djh Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249945#Comment_249945 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249945#Comment_249945 Sat, 12 Nov 2016 22:40:39 +0000 djh
dunno. I haven't addressed buffering yet.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249946#Comment_249946 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249946#Comment_249946 Sat, 12 Nov 2016 22:46:30 +0000 djh
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.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249947#Comment_249947 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249947#Comment_249947 Sat, 12 Nov 2016 22:51:04 +0000 djh
I have no idea, really. Plants, pets, cooking, washing up, showering, clothes drying, what else?

WillInAberdeen also said "To be fair, I don't have a better figure! "

Me neither. That's what I found but I'm very open to alternative numbers.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249948#Comment_249948 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249948#Comment_249948 Sat, 12 Nov 2016 22:52:34 +0000 WillInAberdeen = 2.592 * 264 g/day = 0.7 kg/day]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249949#Comment_249949 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249949#Comment_249949 Sat, 12 Nov 2016 22:53:23 +0000 djh
Well that's why I want to see what happens if I change the initial conditions, but first I need to evaluate your postulate about capillary pumping (although supposedly straw is not very capillary active).]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249950#Comment_249950 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249950#Comment_249950 Sat, 12 Nov 2016 22:54:47 +0000 djh
IIRC the EU standard figure is 0.048 W/m·K after correction for moisture content etc.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249951#Comment_249951 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249951#Comment_249951 Sat, 12 Nov 2016 23:05:04 +0000 djh Posted By: WillInAberdeenPerhaps wall area 264m2 not 464?
= 2.592 * 264 g/day = 0.7 kg/day
Ah, thanks :shamed:

Dunno how that got in there, but that's why I asked for everybody to check :cool:]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249958#Comment_249958 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249958#Comment_249958 Sun, 13 Nov 2016 10:08:34 +0000 Ed Davies 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.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249960#Comment_249960 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249960#Comment_249960 Sun, 13 Nov 2016 11:36:57 +0000 djh Posted By: Ed DaviesLucky we don't see 2 °C, 100% RH for more than a few days very often, I suppose.
I think that's right. Before we put the silicate paint on, it was a limewash surface which very clearly showed when it was 'wet' as opposed to 'dry' by a strong colour change. It was only 'wet' for fairly short periods during and after rain. The silicate paint shows much less colour change in rain, and I haven't got my moisture meter out recently to check surface moisture levels. So I doubt the external humidity is really 100% for long.

It's currently 6°C and 80% outside for example, although it rained all of Saturday.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249963#Comment_249963 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249963#Comment_249963 Sun, 13 Nov 2016 13:10:56 +0000 ringi
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….]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249964#Comment_249964 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249964#Comment_249964 Sun, 13 Nov 2016 14:18:11 +0000 Ed Davies
How reliable that humidity reading is is, of course, open to question. Still, it would be interesting to collect measurements around buildings in different conditions (rather than in Stevenson screens).]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249968#Comment_249968 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249968#Comment_249968 Sun, 13 Nov 2016 21:15:50 +0000 djh 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.]]>