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.