Green Building Forum - Humidity buffering Fri, 01 May 2026 15:18:46 +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=249644#Comment_249644 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249644#Comment_249644 Tue, 08 Nov 2016 16:17:05 +0000 djh
The temperature inside this morning was 19.9°C; the temperature outside was just over 2°C; and the internal RH was 45%. And yes, the MVHR was running.

Clearly the indoor humidity isn't the same vapour as in the outside air. I reckon the walls are buffering it.

Or is there another explanation?]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249646#Comment_249646 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249646#Comment_249646 Tue, 08 Nov 2016 16:41:33 +0000 Ed Davies Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249649#Comment_249649 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249649#Comment_249649 Tue, 08 Nov 2016 17:28:58 +0000 djh
Hmm, how to quantify how much humidity a person adds? Breathing 400 ml /24 hours. Perspiration 400 ml /24 hours *. MVHR moving air from bedroom to ensuite extract. Bedroom doors mostly closed. Two people in bedroom and two cats somewhere. Humidity meter outside bedroom.

Another data point. When I got home today, a bit early, humidity was 44%; temperature inside 19.8°C and outside 4°C. One person in house plus two cats. Shouldn't RH be 32.5% by your theory?

Oh, and it wasn't much higher when there were four of us for the past few days.

edit: * link https://www.quora.com/How-much-water-does-a-person-lose-in-a-day-through-breathing]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249652#Comment_249652 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249652#Comment_249652 Tue, 08 Nov 2016 17:35:00 +0000 ringi
Buffering is more likely to be in all your soft furniture etc, unless you have unpainted walls, or have used spacial paint on new walls.

Also your RH meter may not be telling the truth....]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249654#Comment_249654 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249654#Comment_249654 Tue, 08 Nov 2016 18:00:51 +0000 Ed Davies Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249655#Comment_249655 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249655#Comment_249655 Tue, 08 Nov 2016 18:03:40 +0000 djh Posted By: ringiWhen the indoor RH is higher, the MVHR removes more humidity per second. When the indoor RH is lower, more humidity is given off by any exposed liquid water etc including cooking etc.
All true but we were asleep.

Buffering is more likely to be in all your soft furniture etc, unless you have unpainted walls, or have used spacial paint on new walls.

It's clay paint on lime rendered straw bales.

Also your RH meter may not be telling the truth....

I have two together that indicate pretty much the same. I also hired a calibrated meter a while ago that didn't show any significant discrepancies.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249656#Comment_249656 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249656#Comment_249656 Tue, 08 Nov 2016 18:05:10 +0000 djh Posted By: Ed DaviesWhat's your ventilation rate (m3/h), Dave?
Overnight it's 165 m³/hr to give the duct heater a chance. During the day it's 125 m³/hr. All probably a bit high.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249657#Comment_249657 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249657#Comment_249657 Tue, 08 Nov 2016 18:09:43 +0000 SteamyTea
If you were to condense all the water vapour in the air and thinly apply it to your wall surfaces, would you notice it?]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249658#Comment_249658 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249658#Comment_249658 Tue, 08 Nov 2016 18:11:05 +0000 ringi Posted By: djhIt's clay paint on lime rendered straw bales.

Is the lime render still drying out?

That wall makeup must be doing some buffering, however I expect your RH is control by the MVHR well enough that there is very little buffering going on over the time scales of a few hours.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249660#Comment_249660 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249660#Comment_249660 Tue, 08 Nov 2016 18:16:32 +0000 djh Posted By: ringiIs the lime render still drying out?
I don't think so. It's been there 18 months or so now. In the early days I used a moisture meter that showed moisture decreasing but it got to a point that seems to be stable a long time ago. The indoor RH is also pretty stable rather than decreasing.

That wall makeup must be doing some buffering, however I expect your RH is control by the MVHR well enough that there is very little buffering going on over the time scales of a few hours.

That's the point. If the MVHR were controlling it, the internal RH would be 20% or so as Ed said. So something else is going on.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249661#Comment_249661 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249661#Comment_249661 Tue, 08 Nov 2016 18:36:09 +0000 Ed Davies 2 °C, 100% RH, about 4 g/kg.

So each kg of air exchanged with the outside would get rid of 4 g of water.

180 kg/h (roughly 150 m³/h) would therefore be getting rid of 720 g/h or 17.28 kg/day. That does, indeed, sound like an awful lot.

Apart from anything, evaporating water at that rate would take nearly 500 W.

BTW, my humidity meter in not drastically different conditions is showing about 40% now. I don't know how accurate it is but it at least seems cromulant with humidistat on my dehumidifier (not running now, of course).

[¹] https://upload.wikimedia.org/wikipedia/commons/9/9d/PsychrometricChart.SeaLevel.SI.svg]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249663#Comment_249663 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249663#Comment_249663 Tue, 08 Nov 2016 20:10:35 +0000 djh Posted By: Ed Davies20 °C, 45% RH: about 8 g/kg (water vapour vs dry air) [¹]
2 °C, 100% RH, about 4 g/kg.

So each kg of air exchanged with the outside would get rid of 4 g of water.
err, I wrote 'Agreed'. Then I actually checked the chart. Are you sure about the 8 g/kg? It looks more like 7 g/kg to me. Err but that's dry bulb; wet bulb is much higher. How do you read the chart?

180 kg/h (roughly 150 m³/h) would therefore be getting rid of 720 g/h or 17.28 kg/day. That does, indeed, sound like an awful lot.

Also agreed.

Apart from anything, evaporating water at that rate would take nearly 500 W.

Is that what cools my house? Turning the TV on is definitely enough to heat the living room. And I think that last year maybe my always on old PC was enough to heat the upstairs. Maybe I should be more aggresive in turning down the MVHR.

BTW, my humidity meter in not drastically different conditions is showing about 40% now. I don't know how accurate it is but it at least seems cromulant with humidistat on my dehumidifier (not running now, of course).

Does that suggest a different explanation?]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249666#Comment_249666 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249666#Comment_249666 Tue, 08 Nov 2016 21:56:47 +0000 Ed Davies Posted By: djhAre you sure about the 8 g/kg? It looks more like 7 g/kg to me.It looked like a tad over 7.5 so I rounded to 8. The other one might be a fraction over 4 g/kg, too. So overall, maybe 3.5 g/kg loss rather than 4.

Err but that's dry bulb; wet bulb is much higher. How do you read the chart?
Start at 20 °C on the bottom axis - it's the “dry bulb” temperature which is what you've quoted, presumably. Up the green axis to half way between the red 40% and 50% lines. Horizontally right to the right axis. That's awkward to do but you can zoom in a lot then hold the mouse (almost) still and scroll sideways to get reasonable accuracy. Looks to me to be just above half way from 0.005 to 0.010 groats of water vapour per groat of dry air.

Does that suggest a different explanation?
I don't know. This house has shiny wallpaper and multiple layers of paint everywhere (including on the carpets in places :sad:) so it's not going to behave like your strawbale and clay paint but on the other hand it might also be drying out a bit with me here with the heating on much of the time. One could speculate that humidity buffering is about shorter-term changes rather than anything seasonal which might be what this is about.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249669#Comment_249669 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249669#Comment_249669 Tue, 08 Nov 2016 23:09:17 +0000 djh Posted By: Ed DaviesStart at 20 °C on the bottom axis - it's the “dry bulb” temperature which is what you've quoted, presumably. Up the green axis to half way between the red 40% and 50% lines. Horizontally right to the right axis.
That's what I was doing but it appears to me to be well below the midpoint of the 0.005-0.010 groats/groat interval.

One could speculate that humidity buffering is about shorter-term changes rather than anything seasonal which might be what this is about.

Hmm, I'm not sure about timescales. It took months for the render to dry so moisture exchange is not necessarily a quick process. First google hit is http://www.conservationphysics.org/altisol/woolbuff.pdf which I will reread tomorrow.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249670#Comment_249670 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249670#Comment_249670 Tue, 08 Nov 2016 23:17:11 +0000 ringi Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249677#Comment_249677 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249677#Comment_249677 Wed, 09 Nov 2016 10:31:19 +0000 djh Posted By: ringiDo you cook with gas and/or have a open gas fire?
We don't have gas.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249678#Comment_249678 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249678#Comment_249678 Wed, 09 Nov 2016 10:36:17 +0000 Ed Davies Posted By: djhThat's what I was doing but it appears to me to be well below the midpoint of the 0.005-0.010 groats/groat interval.You're right - I was following the wrong red lines down for 40 and 50% RH so effectively measuring for 55% rather than 45%. Even when I traced the red lines with the mouse pointer I couldn't quite believe I'd done that: must be some sort of optical delusion.

Saving the chart and measuring pixel positions in Inkscape I make it 6.47 g/kg for 20 °C, 45% RH. Similarly, for 2 °C, 100% RH I get 4.35 g/kg so a net ventilation extraction rate of 2.12 g/kg which, at 180 kg/h comes to 9.1584 kg/day. Still a lot: 11 people's worth going on those 400 ml/person·day figures for perspiration and breathing.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249679#Comment_249679 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249679#Comment_249679 Wed, 09 Nov 2016 10:42:22 +0000 Ed Davies Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249685#Comment_249685 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249685#Comment_249685 Wed, 09 Nov 2016 11:44:52 +0000 djh Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249692#Comment_249692 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249692#Comment_249692 Wed, 09 Nov 2016 13:39:08 +0000 djh http://www.conservationphysics.org/altisol/woolbuff.pdf which I will reread tomorrow."

I've just finished reading it. It wasn't one I'd read before as it turns out but it is very interesting. Timescales vary from minutes, through hours to days and even months apparently, depending on the various materials and their thicknesses. Tom (FT), I think you would find it interesting among others. The ones I had read previously were from 2009 and 2011 by Padfield and Jensen.

It seems like my construction of a few cms of lime and then a few 100 mm of straw might be pretty good for buffering. But apparently the paper surfaces of my internal plasterboarding are quite important and probably my wool carpets too, as well as house contents as ringi suggested. So I'm pretty much convinced there is very significant humidity buffering going on.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249695#Comment_249695 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249695#Comment_249695 Wed, 09 Nov 2016 14:17:59 +0000 ringi
Our walls are normal brick+plaster with many coats of normal paint on them, we have nothing to buffer RH apart from the carpets and furniture (and lots of books).

Logic says our RH should be reducing down to a level that is too dry, thankfully it is not......

My conclusion is that a normal home has enough RH buffering without having any contribution from the internals of walls etc.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249698#Comment_249698 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249698#Comment_249698 Wed, 09 Nov 2016 14:32:22 +0000 djh Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249699#Comment_249699 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249699#Comment_249699 Wed, 09 Nov 2016 14:46:10 +0000 ringi
Also for example, our showers may be longer and hotter when it is cold outside, and I do make more cuts of tea at this time of year along with more hot meals.

I think buildings are more forgiving them we give them credit for until they reach a critical point with the way they are being used, and their design.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249703#Comment_249703 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249703#Comment_249703 Wed, 09 Nov 2016 15:16:38 +0000 Ed Davies
20 °C, 45% RH: 6.63 g/kg specific humidity.
2 °C, 100% RH: 4.43 g/kg specific humidity.

So a reasonable match to reading off the graph.

At 06:30 this morning, just before the heating came on, the temperature in my living room was 13.4 °C and the RH 44% so 4.25 g/kg SH whereas by late morning it was 19.7 °C and 42% RH so 6.1 g/kg. By that time I'd been breathing in the room for about 3 hours which might account for the rise in SH but I suspect it's more to do buffering.

It's something to look into a bit more but it's pointing in the direction ringi says with buildings being reasonably forgiving. I wish I'd had that COâ‚‚/pressure/temperature/humidity sensor earlier in the year: it would have made an interesting contrast to do the same measurements in the static caravan I was in.

Also be interesting to see what happens later in the winter.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249716#Comment_249716 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249716#Comment_249716 Wed, 09 Nov 2016 21:07:00 +0000 WillInAberdeen
In contrast AIUI any porous materials that bridge across the construction will absorb water from high RH areas (outside) and desorb it in low RH areas (inside) so act to pump water into the house, against the prevailing vapour pressure gradient, a capillary action similar to many biological systems. I don't know how to estimate how many kg/d this brings into the house but sure someone will.

To buffer a large quantity (many kg) of water, the porous materials in the house must be absorbing and releasing kgs of water, in relation to relative humidity changes.Without changes in RH, there can be no change in water adsorption in the porous materials so no buffering.
Have you measured much change in RH, eg between summer/winter, it sounds quite stable day/night? Any sign of timber warping shape, internal doors sticking, consistent with water being adsorbed/desorbed?

In pre-mhrv houses (sadly including mine) the ventilation rate is boosted in summer by opening windows, to compensate for the lower abs humidity difference between inside/outside hence the lower drying capacity of ventilation air in summer. Do many folks with mhrv typically adjust the ventilation rate in summer/winter? If not, it will dry the house more in winter than summer.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249724#Comment_249724 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249724#Comment_249724 Wed, 09 Nov 2016 22:25:41 +0000 Ed Davies Posted By: WillInAberdeenIn contrast AIUI any porous materials that bridge across the construction will absorb water from high RH areas (outside) and desorb it in low RH areas (inside) so act to pump water into the house, against the prevailing vapour pressure gradient,…On first reading that sounds awfully like one side of a perpetual motion machine but it's also acting as a pretty effective heat pipe in the opposite direction: releasing heat on the outside as the vapour is adsorbed then absorbing heat on the inside as it's evaporated again.

Posted By: WillInAberdeenDo many folks with mhrv typically adjust the ventilation rate in summer/winter?
Good reminder - why does DJH have such a high ventilation rate? He mentioned boosting it at night to allow the duct heater to work better but would cutting it significantly during the day (to allow the RH to rise a bit) make a saving?]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249728#Comment_249728 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249728#Comment_249728 Wed, 09 Nov 2016 23:09:52 +0000 WillInAberdeen
Likewise, water diffusing from inside to outside increases in entropy.

Back to the OT : can we explain where are the many kg/d of water coming from, is Djh's straw drying out for the winter?]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249736#Comment_249736 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249736#Comment_249736 Thu, 10 Nov 2016 07:55:04 +0000 SteamyTea http://gharper.co.za/Interests/One-Wire/GHowSAW/GHowSAW_Humidity_Calibration.asp

The reason I mention this is that some chemicals are very tolerant of temperature, lithium chloride does not vary at all between 5° and 25°C.

Now I have no idea of the make up of these straw bale shacks with lime render, but is there something in there that controls the RH i.e. weed killer, anti-fungal or just plain potassium carbonate.
PC seems to be pretty stable at around the reported RH levels for this house.

For more about the difficulties of using an RH sensor:
http://www.vaisala.com/Vaisala%20Documents/White%20Papers/lsh-Trouble-with-Humidity.pdf

I still fail to see what this is all about, your RH is your RH, and short of a small water leak, there is probably little that can be done about it.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249745#Comment_249745 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249745#Comment_249745 Thu, 10 Nov 2016 10:07:03 +0000 Ed Davies Posted By: SteamyTeaNow I have no idea of the make up of these straw bale shacks with lime render, but is there something in there that controls the RH i.e. weed killer, anti-fungal or just plain potassium carbonate.But Ringi and I are also seeing fairly stable RHs in very different houses.

For more about the difficulties of using an RH sensor: http://www.vaisala.com/Vaisala%20Documents/White%20Papers/lsh-Trouble-with-Humidity.pdf
Bit of FUD and misdirection in there:

Relative humidity is simply the measure of the amount of water vapor in the air compared to how much it can possibly hold at that temperature.
No it isn't. I wish people would stop using this misleading short hand.

For example, a sensor rated to measure 10 to 90% RH from -40 to 70C must perform in humidity
conditions ranging from 10,000 parts per billion to 200,000,000 parts per billion. The dynamic range this represents is 20,000:1,…
But RH sensors measure RH, not ppb.

Also, they're looking to measure RH to relatively high levels of absolute (haha) accuracy. We're more interested in ballpark figures and smaller changes which makes a big difference to the calibration requirements. If my meter says it's 49.9% it doesn't matter if it's really 45% or 55% (or, perhaps, even 40 or 60%) so long as when it says it's dropped to 44.9% it's really gone down by something between, say, 4 and 6%.

I still fail to see what this is all about, your RH is your RH, and short of a small water leak, there is probably little that can be done about it.
Since, apart from cost, water in the building is the biggest constraint on keeping energy in the house I'd say that having a good understanding of this is right at the middle of what GBF is about.]]> Humidity buffering https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249763#Comment_249763 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682&Focus=249763#Comment_249763 Thu, 10 Nov 2016 12:13:52 +0000 WillInAberdeen DJH is extracting 9kg/d of water from his house, plus an unknown amount that is diffusing out the very vapour-open walls.
He can explain 2kg/d, as coming from humans/felines. Maybe a little more from cooking washing etc.


Given the apparent large excess (>7kg/d) of water-extracted compared to water-generated, where is the excess coming from?

Maybe it is drying out the fabric by >7kg/d?. This is not consistent with the RH staying stable not decreasing, but as DJH mentioned in the OP the straw construction does have large buffer capacity. Maybe the fabric will absorb this water again in the summer in a stable cycle.

Maybe the >7kg/d is capillary wicking in through the porous construction?
Maybe the >7kg/d is entering due to a water leak somewhere? (Hope not!)
Maybe the >7kg/d is coming from something else, what have we missed?

In summer the water extraction rate will be much less - what happens then?

If the mhrv rate were decreased now, what would happen?

What does it mean for constructions with less buffer capacity?

All valid GBF questions... I'm interested!]]>