Where to put subfloor ventilation fans?

[ChrisKingston]

Sorry, but your comments in this thread demonstrate a poor understanding of how floor ventilation works. And you really need to be careful not to create the environment and the problems that you are trying to prevent, by applying the wrong way of thinking

It's not a case of insulating it, sealing gaps, stopping air movement between the habitable room and void and its definitely not the case of 'warm damp air from house coming down into the void' - heat rises! The void is not a room to be sealed. Do not seal the gap between the house walls and any sleeper wall or timber.

And you don't want to be pumping external air (whether by input or forced extract) into the void and in doing so create an artificial environment ripe for humidity and mould issues.

Floor void ventilation works by pressure differential and not by wind blowing through the void. This gentle movement of air prevents mould growth - even in high humidity air. That needs to be understood, before going down a route of elaborate schemes trying to stop something that might never happen in the first place. Secondly, it works by a state of near equilibrium, whereby the air in the void is slightly warmer than external air, and the surface materials slightly warmer too, so reducing the risk of air condensing. So that's why you don't want fast air changes and colder air being introduced by a fan.

Above all if there is no problem now, despite not being textbook correct, then what makes you think that there will be a problem in the future. Has the presence of the extension over several seasons caused an issue?

If you are dealing with a true ventilation issue, then you should get advice from someone experienced in that. That someone won't be the one that just quotes standard textbook stuff for non-textbook situations.

Thanks, yes this is first time I have tackled this.

Pretty sure there is a problem with the sub-floor ventilation, just based on the stale smell and signs of previous dry rot in the centre of the house. The extension is new, but there was no better ventilation out of that side of the house before. It's that stale dry rot smell and timber softness rather than proper damp from water ingress.

I'm insulating and sealing mainly for heat in the living area. (Although, like Lower I found when I did that in another part of the house it made surprisingly little difference). Unless there is anyway I could make the subfloor worse by doing that?

My plan is to push air out of the stillest part of the subfloor and out of an airbrick - hoping to mimic best what would it would be like if there was a decent cross-flow. If I do this with a standard quiet bathroom fan, I can fit that under an accessible airtight hatch under the cupboard - which will also give me some idea how it smells/looks down in the subfloor.

Although a fan would be faster air movement than most airbricks, it's going out not in. And I'd put it on its lowest trickle setting, maybe just for a couple of hours a day. I'll have to scope the ducting run but I will use rigid stuff with no bends as it's about 4m. I can see the risk of bringing in lots of cold air quickly into a slightly warmer void, but I'd hope a weak fan with a long ducting run from the dankest part of the house would be quite gentle.

Or how about as a compromise: insulate+seal for warmth, then put in the ducting (while the floor is up) and an access hatch then see how it goes for a winter, fitting a fan next year in the hatch if the musty/dry rot smell is still there?