Good day all.

Over last 5 years , i’ve been renovating and improving a listed property, (old barn with attached cottage), in the cottage section, there are 3 parts , original cottage and a 1930’s extension, added coal store (now included in cottage) again 1o30’s.
This has a mix of solid and suspended floors. Lots of alterations over its history. When floors were lifted, lots of rubbish and debris beneath, all cleared out. Evidence of previous damp / rot. But once cleared this all dried out over a year or so. There were only air bricks in the main extension, 6 more were added and ducted under solid floors where needed. A mix of insulation methods as it was all reinstated. All’s well and good most of year. But in hot humid periods ( i’m half mile from coast) i get a musty smell, that’s seemingly coming from under the floor.

My thoughts are it’s moist air condensing under the floor sufficiently to allow a bit of mould/damp. The void space is mostly about a metre deep, cool and has a lean mix blinding ( this was there already). Whilst i’d tried to make sure that there was good airflow patently it’s not been enough.

I’m considering adding some forced ventilation using a constantly running fan to draw air out via an airbrick that’s ducted to what i believe is the problem area. The air being drawn in via the other airbricks.

Is this a sensible way forward or am I risking just drawing more moisture into the area and adding to the problem? Is constant low volume flow any better or worse that going for periodic high flow (downside of background noise) either way the fans will be on a voltage controller so that extraction rate can be varied.

I expect there’ll ne the suggestion of adding a humidity sensor to the problem area to control the fan. That is’nt going to be practical now that this section of the house is now finished. I’d rather add the fan and see if it makes a difference then consider if the amount of work to add a sensor is worth any likely improvement ( especially if a low volume fan works).
Fan i’m considering has a max air flow of 180 m3 /hr.
Lastly whilst my intention is to extract, would it be better to force air into the suspected area instead?

Many thanks.

The principle is, when outside air is warmer than in the underfloor, and/or has higher RH, or combination of, then drawing it in or even allowing it in passively, only brings new moisture-laden air in, which condenses. When such warm/high RH air is cooled in the underfloor, its RH increases (without adding any more moisture mass into it), likely to 100% RH (saturated), when it condenses on the nearest coldest surface. Times of warmish, high RH outside air, when the underfloor is cool, are spring and autumn, so best to prevent outside air ingress at those times. In high summer, air may be warmer still but possibly lower RH, so may be OK to ventilate. In winter outside air may be colder than the underfloor, so even if of high RH, will reduce in RH when warmed in the underfloor, so generally useful to ventilate in winter.

Next, when it condenses in the underfloor, does it actually do any harm? It'll only condense on the coldest surfaces i.e. hopefully remote from your timbers.

Thanks all, when i first bought the place i removed a large extractor fan and grill to get as much air into the place as possible to help dry it out (it was vacant prior to my buying it and the metal thieves had got in and stripped it but water had’ t been turned off, everything became sodden, did get a pretty good discount with added bonus of it bringing down very poor lathe and plaster i’d otherwise have been expected to save).
Barn and cottage are oriented north south and it makes for some pretty effective mitigation of overheating to the extent that it felt considerably colder at times in the unheated barn. This led to having warm air blown in through the fan hole condensing on the internal wall it hit to the extent it’d run down the wall.

The musty smell does coincide with warm spells, had it last 2 years as well but only for brief periods this year it’s been much more noticeable. Heating doesn’t need topping up and once i’d replaced a leaky incoming water main, there’s no discernible movement on water meter ( the weepy water main was easily seen on meter), so pretty sure condensation is my problem.

Foster Tom i’d expect any condensation to be in corners of the dividing walls and where a fireplace had been i’d dug down another 600mm or so clearing it, there’s was lots of bits of wood from previous changes in the place. It had been an oasthouse at one point and i believe it was where the hearth had been. I’ll look at making up some blanks for the air bricks and see if that makes a difference, though won’t be able to seal too well as the 1930ks extension has a cavity that is open plate level and vents into the underfloor void (though that’s only 8” deep in the extension).

From WIAs comment trying to raise the temp under the floor by ventilating would seem sensible. BUT a quick look at dew points for eg 30 c and 80% humidity the net tells me dew point is 26.2 c, for what is a pretty cool crawl space that’s quite a temp to achieve.
With hindsight, filling the void with foamglass would probably have been a solution though expensive. (Though perhaps i could have just filled what i feel is the problem area (has no clear cross flow).

Plenty to consider , thanks you.

Mike1, yes, that seems sensible, but i’ve just had a look and every airbrick that is in the region of the problem area is “periscoped” so not practical. However i could drill a hole in the floor of an understair cupboard and drop a sensor in there. Thanks.

Posted By: ArtiglioEach is dropped in around 70cm so suspended about 20cm from the hard oversight ( some sort of concrete/lime).

It doesn't sound as though the readings are concerning at the moment, and it's good that that the fan has dispersed the smell. I'd pull the one under the stairs back to directly under the floor, where the airflow is likely to be lowest & RH potentially highest, though it probably won't make a huge difference to the readout - the time of day when you read them may well be the biggest factor.

Posted By: ArtiglioThese are cheap chinese units so results are more for comparison than hard data.

It's possible to calibrate them, but not essential for what you're doing. On those I've calibrated the temperature was within around 1°C and the RH within about 4% of reality, to give you some idea of what you might find if you did.

Posted By: ArtiglioPlan is to leave fan on for a month and see how readings settle, then turn it off and monitor for a couple of weeks. Weather is now cooler than over the summer and humidity much reduced so may well be something i’ll need to repeat tnext summer if problem recurrs.

Sounds like a good idea. I'd check them every 2 or 3 weeks over the winter too, when you could be hitting dew point (which is why I'd leave the other sensor where it is, to be less affected by warming from the radiator tail).

Just checking - how do you see PVP being determined? a combination of local air temp (higher temp > higher PVP) with local absolute concentration of H2O vapour molecules (higher concentration > higher PVP)? i.e. not by local RH?

So broadly, PVP correlates with air temp, unless distorted by some sort of pump-input of H2O vapour molecules as well, into a particular location.

So, agreed, all the gas molecules that make up warm house air, like O2, Ag, N, CO2, have higher PVP than cooler underfloor air molecules, and colder air molecules within subsoil have lower still PVP.

So why do we agree that H2O vapour molecules in particular are diffusing, through and relative to the other gas molecules that make up air, without any bulk movement of all the air molecules, from the house into the underfloor and on into the subsoil, 'downhill' from high PVP to lower PVP?

Why do you say that's

Posted By: tonycounter intuitive and the majority believe moisture moves into the opposite direction

you mean majority think moisture moves from subsoil to house interior? Do they really? I must have lost touch with 'the majority'- ivory towers?

Excellent Will - much corrected and elaborated understanding there.

Posted By: WillInAberdeenFor gases that are nowhere close to liquid form (N2, O2, CO2, Ar) there is no temperature effect

Why is that? Steepness of a gradient? Is it a bit like Carnot, the ratio of evaporation temp minus present temp compared to present temp minus absolute temp? I've given up trying understand algebra like
https://en.wikipedia.org/wiki/Clausius%E2%80%93Clapeyron_relation
Late in life I'm relieved to discover that 'visual thinkers'
https://www.penguin.co.uk/books/443322/visual-thinking-by-grandin-temple/9781473590601
don't have the brain to understand that type of paper abstraction - which was true at A-level but surprises me, as I have no trouble with mechanics and what the shape of graphs indicate.