Green Building Forum - Moisture in internally-insulated walls: some real world results Mon, 01 Jun 2026 23:38:35 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256460#Comment_256460 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256460#Comment_256460 Tue, 28 Mar 2017 17:34:29 +0100 djh Posted By: lineweightUnfortunately I haven't got monitors in more joist ends just because of the timing of when they got covered up compared to when I managed to get the monitoring system sorted out. I am thinking of trying to uncover another joist end somewhere to install my "spare" monitor in it and see what's happening.
As I understand it, for timber moisture readings, all you need to do is bang a couple of nails into the wood, connect some bell wire to them and bring the wire to a place where you can later connect a moisture meter. I expect you have to get the nails some specific distance apart (the same as the ones on the moisture meter, presumably?) and possibly you have to think about materials to avoid galvanic couples?

So if anybody exposes joist ends, it seems to me to be worthwhile to stick the nails and wire in just in case you want to test the moisture at some later time.]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256461#Comment_256461 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256461#Comment_256461 Tue, 28 Mar 2017 17:37:12 +0100 djh Posted By: goodevansThe moisture content of the wood on each sensor is presented here (blue line) together with an predicted value in red based on the relative humidity of the air at the sensor.
Well done! The red lines look like an excellent match. How did you find the formula? Is it published somewhere or did you just experiment (SWAG)?]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256474#Comment_256474 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256474#Comment_256474 Tue, 28 Mar 2017 22:06:52 +0100 tony Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256556#Comment_256556 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256556#Comment_256556 Wed, 29 Mar 2017 22:00:14 +0100 SteamyTea Posted By: tonyI hate those prong things, I drill a hole, catch the dust, weigh it, dry it in a warm oven, re weigh, calculate the % moisture contentBut if you sample just twice a day, your house will soon fall down. :wink:]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256558#Comment_256558 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256558#Comment_256558 Wed, 29 Mar 2017 22:33:48 +0100 tony ]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256561#Comment_256561 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256561#Comment_256561 Wed, 29 Mar 2017 23:26:41 +0100 djh ]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256563#Comment_256563 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256563#Comment_256563 Thu, 30 Mar 2017 01:05:42 +0100 goodevans
In reality I suspect that the half life nearer the surface is going to be much shorter (maybe zero), at deeper levels much longer. At the surface as soon as the relative humidity reaches near 100% the wood moisture will be at around 20%. This will happen when it's foggy / raining. But it's not permanent - sooner or later the outside RH drops and the wood surface dries out.

All well ventilated rafters/battens in cold roofs will be affected in this way - but providing there is no leakage there should not normally a problem - However I have not been able to find out how much or how long or how often wood can sustain a RH events of more than 90% and not deteriorate for say 100 years.

given how closely the red line follows the blue line I think it is unlikely the offset is cause by the environment the wood is sitting in. We may never find out as the wood is inaccessible. My feeling is that the offset is more to do with the electrical characteristics of the wood - however there is a chance that the the offset is caused by each piece of wood suffering wetting/dampening due to rain on the brick side of the wood which could create a very slow response on the sensor side of the wood (but as a betting man I don't think the data indicates this).]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256594#Comment_256594 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256594#Comment_256594 Thu, 30 Mar 2017 23:41:53 +0100 lineweight
1. the RH measured in the cavity closely tracks RH from weather records
- therefore we assume that the cavity is essentially well ventilated
- therefore the humidity in the cavity is not driven by rain-driven dampness of the brickwork
2. the measured MC in the timber correlates with fluctuations in air RH
- therefore we assume that the timber MC is mainly driven by air RH
- therefore we assume that rain-driven dampness in the brickwork is not really relevant to the timber MC.]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256781#Comment_256781 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256781#Comment_256781 Tue, 04 Apr 2017 18:34:31 +0100 goodevans
Yes - I could not have put it better myself. I see no evidence for rain driven dampness in the wood.

It is possible that rain driven dampness on the far side of the wood is causing the different offsets in the moisture levels at each of the different sensors - however for that to be the case - rain driven dampness would have to have to effect the wood moisture content really slowly - i.e. a series of rain events/season would have to take a couple of months or so to affect the sensors - any faster than that and the red line would not follow the blue line.

I suspect that the brickwork moisture content is also RH driven - any wetness from rain is short term, shallow and when the rain stops the outer surface of the bricks dry to the moisture content dictated by the RH.]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256802#Comment_256802 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256802#Comment_256802 Tue, 04 Apr 2017 23:32:07 +0100 lineweight
Is it fair to say that you devised a formula for the amount/rate of moisture being transferred from air to wood, but that you fitted the formula to the results, rather than using a prediction based on observations elsewhere, and then seeing if it matched these results?

If your formula assumes that a certain amount of moisture is transferred, which is higher than the actual amount of moisture transferred, then is it not the case that when redrawn using that "actual" formula, your line would still go up and down in sync but would be flatter overall?

I'm wondering if it's plausible that rain riven dampness in the brick is making a contribution - but is broadly in sync with air-driven dampness because higher RH will tend to be associated with periods of higher rainfall.

Another question would be what evidence could prove that rain-driven dampness was making a contribution. Would a few months' record of actual rainfall on site tell us anything useful?

I suppose that if I were to do something to the brickwork externally - paint it with a moisture-repellent finish for example - then observe a change in results over the following year(s), then that might suggest that rainfall is relevant.


But assuming you're right - I think that would suggest that (as long as we are confident that we are safe from interstitial condensation) the ventilation cavity is not really necessary, and it would probably make sense to insulate right up to the brickwork... would others agree?]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256898#Comment_256898 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256898#Comment_256898 Fri, 07 Apr 2017 20:47:40 +0100 goodevans
First, rain driven dampness would cause the wood next to the brick to be wetter than the moisture than a high air RH would deliver - but any wetting at the far side of the wood would take it's time to reach the sensor.

And I would have to agree that the formula I used has been created to fit the data - not a good way to go (however dividing the RH by 5 is a common estimation of equilibrium wood moisture content).

Filling the cavity may still have potential issues however - at present the cavity air is dominated by the outside air. If this air is prevented from circulating in this region there is a chance that the moisture content could be dominated by your internal environment (which in winter is approximately 2 gpkg wetter). In addition this cavity always allows the internal surface of the bricks to be able to dry to air copiously sourced from the outside (and it is these bricks your joists are supported by - you need these bricks to be as dry as possible).

However we are getting into real areas of uncertainty here. Some of the wood does already indicate quite high levels of moisture content, I am no expert - I just like unravelling real data sets to get to nuggets of information.

Ideally some of the other regulars here should give an opinion - (insulate on the outside of the wall - no-brainer) on the inside - not so sure.

sorry I can't be more positive.]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256906#Comment_256906 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256906#Comment_256906 Sat, 08 Apr 2017 10:47:14 +0100 lineweight
It's something I could record relatively easily now as I have a bit of a weather station set up for other purposes.]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256909#Comment_256909 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256909#Comment_256909 Sat, 08 Apr 2017 12:10:15 +0100 tony
I would insulate outside of wooden members, (this makes me worry about sips)]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256912#Comment_256912 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256912#Comment_256912 Sat, 08 Apr 2017 12:52:13 +0100 djh Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256913#Comment_256913 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256913#Comment_256913 Sat, 08 Apr 2017 12:52:44 +0100 djh Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256915#Comment_256915 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256915#Comment_256915 Sat, 08 Apr 2017 14:07:34 +0100 SteamyTea Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256918#Comment_256918 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=256918#Comment_256918 Sat, 08 Apr 2017 17:24:24 +0100 goodevans
Perhaps a sensor attached to the inside of the wall and a 1 meter square polythene barrier over the sensor sealed air/moisture tight at the edges against the wall. Compare that sensor with one of the others (except sensor 3) - any lumps and bumps in this new sensor not correlated to the standard sensor(s) would indicate a non-airborne water source (i.e. rain) permeating through some part of that 1m2 brickwork. If you get lumps and bumps attempt to correlate with local weather reports - (rain, direction, wind, sun etc from the Weather underground site to confirm - at least 2 stations report within a mile or so of you.)

The 1m2 barrier would emulate a worst case scenario of filling the cavity - any lumps and bumps would be bad news - it would mean your joist ends would be damper than they are at the moment. My gut feeling - its a risk - ventilation limits the detrimental effects of small leaks.]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257126#Comment_257126 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257126#Comment_257126 Fri, 14 Apr 2017 19:47:50 +0100 fostertom Posted By: SteamyTearecord wind velocityand direction - or rather, the direction and velocity of driving rain. Wind doesn't always contain rain, and contains more or less rain, from different directions. The 'wind rose' is different in shape and magnitude from the 'driving rain rose'.

Lack of driving rain info is the only drawback to using Meteonorm software to generate otherwise excellent weather data for any location - like PVGIS does for insolation. For a few £100 you get any number of the reqd weather data files for anywhere, for use in WUFI for example - instead of £100s for every location's file if bought from Met Office or other source, in privatised UK/Eire. Almost anywhere else in the world this info is free, public property. WURI comes with many dozen weather files free from all over the world - with a big blank only for UK/Eire.

Meteonorm is primarily for agricultural purpose, which apparently doesn't need driving rain info. I'd love to know how others manage to reliably approximate around that lack, in WUFI.]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257127#Comment_257127 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257127#Comment_257127 Fri, 14 Apr 2017 21:20:45 +0100 djh Posted By: fostertomMeteonorm is primarily for agricultural purpose, which apparently doesn't need driving rain info. I'd love to know how others manage to reliably approximate around that lack, in WUFI.
I'd have thought that any design needs to cope with a 'moderate' amount of driving rain from any direction. Where 'moderate' means that the rainscreen can resist the onslaught at the time and dry itself out to resist the next onslaught before it happens.

I would expect that in locations where there is more than moderate driving rain, there is local knowledge and custom that is best observed. But a balanced rainscreen in front of a double drainage plane will cope with almost all conditions, I believe.]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257130#Comment_257130 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257130#Comment_257130 Fri, 14 Apr 2017 21:50:03 +0100 lineweight Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257133#Comment_257133 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257133#Comment_257133 Fri, 14 Apr 2017 22:05:30 +0100 djh Posted By: lineweightI have been wondering what the wettest part of the UK that has a tradition of brick built architecture is.
Brick is modern. Traditional architecture is stone, in those parts of the country that have stone and especially where there is exposure.]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257137#Comment_257137 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257137#Comment_257137 Fri, 14 Apr 2017 23:51:38 +0100 fostertom Posted By: djhany design needs to cope with a 'moderate' amount of driving rain from any direction. Where 'moderate' means that the rainscreen can resist the onslaught at the time and dry itself out to resist the next onslaught before it happens.That's the moot question - many common constructions (incl modern) don't, in some locations, on some faces - what WUFI's for.

Posted By: djha balanced rainscreen in front of a double drainage plane will cope with almost all conditions, I believe
Probably - but again, many common modern constructions don't do that - and are fine, in most locations, hopefully on all faces.]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257139#Comment_257139 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257139#Comment_257139 Sat, 15 Apr 2017 06:53:08 +0100 SteamyTea Posted By: fostertomand directionThere is speed and Velocity.
One is scalar and the other is a vector, learn the difference.:wink:]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257146#Comment_257146 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257146#Comment_257146 Sat, 15 Apr 2017 10:08:56 +0100 fostertom
What it is with Meteonorm - it gives wind speed but not velocity i.e. ignores wind direction, which apparently doesn't matter to farmers.

Also, in saying the 'rain rose' is different from the 'wind rose', that's taken care of if the weather file records both wind velocity (i.e. incl its direction) and rain intensity by time of day. That then defines when 'driving wind' also becomes 'driving rain'.

But oddly, for our building purposes, the intensity of the rain while 'driving' is not so important - it's that it's happening at all, at any intensity. Duration (and direction) not intensity. Because a light sluicing makes the wall just as wet as a heavy sluicing.

But we still need to know which of the 4 walls gets the highest duration of sluicing, hence need for direction data. Then WUFI will tell whether say the SW wall nees to be rain screened as djh suggests, but the other 3 walls can reliably get away without. Or all 4 walls OK without.]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257148#Comment_257148 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257148#Comment_257148 Sat, 15 Apr 2017 11:19:07 +0100 djh Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257149#Comment_257149 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257149#Comment_257149 Sat, 15 Apr 2017 11:30:02 +0100 fostertom Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257150#Comment_257150 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257150#Comment_257150 Sat, 15 Apr 2017 11:55:23 +0100 lineweight Posted By: djhBrick is modern. Traditional architecture is stone, in those parts of the country that have stone and especially where there is exposure.

Posted By: djhThe traditional solution is to build the building, then observe which if any walls get wet enough to cause a problem and then add a rainscreen to such walls. It's only the modern vanity of the planners' aesthetic that requires prediction.


I don't think I understand what point you're making. Brick is not just a modern thing; there are many areas of the UK which have used brick for many centuries. Indeed the more exposed parts of the UK tend to use stone and yes I expect this is to do with it being a more weather resistant material but it's also the case that much of the UK's west coast is to some extent mountainous and using stone is a result of the local geology as well as climate.

The reason I wondered which of the UK's traditionally-brick areas is the wettest is that there might be building methods particular to those areas which might give some clues as to whether brickwork becoming saturated through driven rain is a real and likely thing. If, for example they use a noticeably less porous brick then that suggests that it's something that's been recognised as a problem historically, and therefore something that we should be worrying about when we make changes such as adding internal insulation. If on the other hand there are areas of the UK which have a lot of rain but also have historically used brick that is similar to that common in drier areas - then maybe it's not such a worry, and it's the nature of brickwork that only the outermost part gets saturated, and other things being equal, timber embedded into its inner portion is fairly safe.

I can't think of many/any traditional UK building methods that use a rainscreen - let alone one added post-hoc - what are you thinking of?]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257151#Comment_257151 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257151#Comment_257151 Sat, 15 Apr 2017 12:00:40 +0100 lineweight Posted By: fostertom

But we still need to know which of the 4 walls gets the highest duration of sluicing, hence need for direction data. Then WUFI will tell whether say the SW wall nees to be rain screened as djh suggests, but the other 3 walls can reliably get away without. Or all 4 walls OK without.

Is WUFI accurate/reliable, though, when you're dealing with historic brickwork? As I understand it, different types of brick (and mortar) can show different behaviours. And the type of brick may not even be the same throughout the wall (I can attest to this from fiddling around with the walls in my place; some bricks are much harder/softer/porous than others). Perhaps there can be an established "worst case scenario" but if there is, have WUFI simulations been tested against real world results?]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257153#Comment_257153 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257153#Comment_257153 Sat, 15 Apr 2017 12:34:53 +0100 tony
Lime wash has been used as a protective layer and can act as a rain screen, often applied every autumn.]]> Moisture in internally-insulated walls: some real world results https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257154#Comment_257154 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&Focus=257154#Comment_257154 Sat, 15 Apr 2017 12:48:20 +0100 lineweight Posted By: tony'I can't think of many/any traditional UK building methods that use a rainscreen - let alone one added post-hoc'

Lime wash has been used as a protective layer and can act as a rain screen, often applied every autumn.

In the normal definition, a rainscreen has a drained cavity behind it. Limewash I'd call a surface treatment.]]>