Subfloor Ventilation

[Al1234]

Hi

I have recently moved into a house, originally built in the 1930's and extended in 1991/92.

Having been under the floor to check on some wiring I am concerned that the subfloor area is damp. The flooring is a mixture of bare earth and in places concrete but is noticeable damp, certainly in comparison to previous houses. The timber floors however appear fine and there is no noticeable signs of mould or damp on them.

Should I be worried?

The house is on a slope and the sub floor is stooping height at the front of the property decreasing to maybe 8-10 inches (below joists) at the back of the original house. The extension is the full width of the back of the house and as a result all the original air bricks are blocked, the extension having a solid floor. The only ventilation at the rear is through a single 4 inch pipe running out to an air brick.

There are some air bricks in one side of the house but not the other and these would run contrary to the sleeper walls which are designed to flow front to back.

I hope this all makes sense and if there is cause for concern you might be able to offer some advice.

Thanks Andrew

 

[vinn]

If you scan a rough hand drawn plan view of the original house & extension showing all air bricks and sleeper walls it should help with devising a way of possibly introducing more ventilation?

Your location on a slope might account for the damp sub surface being in the path of ground water going downhill.
Membranes (DPM) spread over the sub area and lapped up the walls by, say, 100mm would really help with possible sub area condensation issues.
Are the knee walls honeycombed?

After twenty odd years since the extn. any fungal infection to the suspended flooring should be showing so perhaps there's more ventilation than it seems?
Check both sides of the wall that meets the extn for damp signs at skirting level.
Probe all joist tails sitting in wall pockets.

 

[Al1234]

Thank Vinn

Attached is a plan, solid walls are solid, dashed walls are the sleeper walls. Hope this makes sense.

Sorry I don't understand what you mean by knee walls and honeycombed.

Do you think that the moisture on the floor could just be down to the slope? This hadn't occurred to me, my other houses have all been on the level, there has been a lot of rain recently.

could it just be that water naturally makes its way to the surface but actually the ventilation works reasonably well, ie. no signs of moisture/mould on the suspended floor?

Regards Andrew

 

[vinn]

Sleeper walls aka knee walls.
Honeycombed means regular gapping in the brickwork. Its necessary to provide through ventilation - no dead areas.
Water is "naturally making its way to the surface" - its called gravity.
On the garage side add air bricks in any accessible brick areas below the DPC.
ACO or French drains could possibly be installed before the extension but doubtful how useful they would be - it would need a site decision.