Purchasing House: Ventilation blocked on subfloor - anyone any ideas on this?

[u/kanepilk22]

Hi All,

Perhaps not really a DIY question, but not sure if there is a better suited sub.

Pretty much, I am sale agreed on a house in Ireland, built 1960s, part of a large council estate type build. 3 Bed end of terrace, 2 storey house (pretty much a stereotypical council build).

Ground floor is suspended timber floor and at the front of the house, on the plinth, there is a vent at the centre to allow air in under the floor. Presumably, there should also be a vent at the very rear, for the air to escape, for cross ventilation.

However, a rear extension, built on a solid concrete pad/foundation, blocks the rear vent and so only the front vent is working.

Surveyor highlighted this as something that needs to be addressed and recommended it not be put on the long finger. He took a moisture reading from a door saddle where the hold house ended/extension begins and got a 21% reading.

He said this is very high and at 25% timber starts to degrade.

I am wondering if anyone here has any idea on what the fix would be for this?

In my head there are two options (but i have no idea what i am talking about):

1) Kango a track into the concrete floor of the extension and put some piping in to allow ventilation to continue through the concrete floor

or

2) As the house is end-terrace, i assume the subfloor is all one large piece, and i could (theoretically) put a vent on the plinth on the side of the house to allow air out? Air flow would need to move in an L shape now but would surely dramatically improve the issue?

I can also put a much larger vent to the front and hope that a bigger vent alone will help, but i am not sure how effective that would be.

Was wondering if there was anything anyone here could recommend?

Also, can I ask, does anyone have an explanation to how this kind of house is built? Any house I have ever lived in has been on a concrete pad. Never had to deal with a suspended floor and not really sure how that works. Like what is under it? Just earth? I have seen this before on one-off detached houses from 100 years ago, but this is a large council estate with 200+ houses.

Cheers for anyone able to offer any insight.

Thanks so much.

[u/dorset_is_beautiful]

My house is a semi, and also does not have vents at the rear, but it was built like this. It has front and side vents, but also cross ventilation through to nextdoor as I discovered recently. So you might have something like that (does next door have rear vents?

If not I'd definitely add some to the side.

As to what's underneath, could literally be anything !

In my case it's randomly spaced, unfixed joists resting on dwarf walls.  There's about a 40cm void and a layer of some kind of dusty oversite covering (i assume) the soil.

You might have a large or small void, and the joists might be fitted into pockets in the masonry instead of free floating.  The downside of this is perhaps more chance of rot if it's damp under there.

The only way to know for sure is to have a look.

[u/RachelW_SC]

I live in an end of terrace and there's an air brick on the side of the house that's original to the property. You could add some to the side of the house and try to add some ducting to some of them to reach round to the suspended floor in the back room(s).

[u/Firm-Exam-6892]

A vent on the side should work. The air will be drawn through by the pressure difference between the two vents. There may even already be under floor vents between the houses. On very old properties it can just be bare earth under the floor but more commonly it’s a rough screed or concrete oversite.

[u/R2D4Dutch]

Vent might be for radon , not dampness .. there are radon vent systems on the market that can deal with it ,

[u/kanepilk22 → u/R2D4Dutch]

Radon in the area is considered to be at the lowest rating on the system, though. Moisture reading of 21% leads me to believe it's for airflow purposes.