Fri, 04 Sep 2026 14:36:14 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311316#Comment_311316 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311316#Comment_311316 Thu, 03 Sep 2026 13:33:20 +0100 Artiglio
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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311320#Comment_311320 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311320#Comment_311320 Thu, 03 Sep 2026 13:57:21 +0100 WillInAberdeen
Normally not a problem in UK because the subsoil temperature is not cool enough and the air is not hot or humid enough to cause condensation.

But this year the weather has been completely messed up ofc

First thought is that blowing more hot humid summer air in will just cause more condensation. Maybe try ventilation in the spring and autumn to gently warm up the subsoil without bringing in high humidity?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311322#Comment_311322 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311322#Comment_311322 Thu, 03 Sep 2026 14:43:24 +0100 fostertom
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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311324#Comment_311324 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311324#Comment_311324 Thu, 03 Sep 2026 15:16:55 +0100 WillInAberdeen
Summer is when the musty-ness is smelled so likely that's when the outside air dewpoint is warmer than the subsoil, which tends to stay pretty constant T year round.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311328#Comment_311328 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311328#Comment_311328 Thu, 03 Sep 2026 17:42:48 +0100 Artiglio 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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311329#Comment_311329 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311329#Comment_311329 Thu, 03 Sep 2026 19:18:02 +0100 Mike1 Posted By: ArtiglioThe 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.I wonder if there's a connection with the wind speed too - i.e. lower wind speed when it's hot = less ventilation. But maybe you get enough sea breeze for that not to be a factor (though it may be less at ground level).

I'd probably make a hole big enough to get some sensors under the floor to measure the RH & temperature - maybe temporarily remove an airbrick to do that. Amazon sell cheap ones that connect over WiFi with batteries that will last a few months - and if you keep them attached to a string / bamboo cane you could retrieve them later. With them in place you could also compare what happens if you add some forced ventilation.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311338#Comment_311338 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18473&Focus=311338#Comment_311338 Fri, 04 Sep 2026 10:21:51 +0100 Artiglio