Damp/condensation advice please

[Martin Richard]

Ah I really feel for you with this because older semis can be such a mix of issues too.

From what you have described, a few things stand out.

Black mould usually points much more towards condensation than classic rising damp. Rising damp tends to show white salts and a horizontal tide mark, whereas black mould is typically about warm moist air meeting a cold surface. A cold old stone wall in a 100 plus year old house is a perfect condensation surface, especially in winter.

The block paving being above the original damp level and stopping ventilation is a big red flag though. Suspended timber floors rely on cross ventilation to stay dry. If air bricks have been buried or airflow reduced, moisture can build up under the floor. Under stairs cupboards are notorious for being cold, unventilated little moisture traps, so things going mouldy in there suggests high humidity not really groundwater pushing up through the wall.

Before ripping everything out, I would personally check a few practical things first:

• Are the air bricks visible and clear? If not, that would be top of my list

• Is the paving level higher than the internal floor? If so, lowering it could make a big difference

• Can you safely lift a small board and look underneath to see if the void is damp or just poorly ventilated?

• Get a cheap digital hygrometer and measure humidity in the room and in the cupboard for a couple of weeks you could buy online?

If humidity is regularly above around 65 to 70 percent, you are in condensation territory.

It sounds very possible that this is a combination of lost underfloor ventilation plus a cold solid wall creating condensation, rather than rising damp.

I would be really cautious about replacing the floor before restoring airflow and checking external levels, otherwise you risk sealing in the problem.

Has it worsened since the paving was laid that could tell you a lot.

[Matt F]

that definitely helps thank you might the DPC be visible from the inside too when we lift the boards? If no DPC, what then?

[Michael Watkin]

In an early 1900s house the DPC, if there is one, is often a thin slate layer in the mortar joint. It usually looks like a dark horizontal line running along the wall a couple of bricks above ground level. In stone houses it can be harder to see and sometimes they relied more on drainage and ventilationtoo.

The bigger issue may be high external ground levels and lost ventilation rather than classic rising damp too.

Rather than more ACO channels, what often works better with older houses is lowering the ground right next to the wall and creating a simple gravel trench so water can drain away and the wall can breathe.

Before starting work it might be worth a quick check can you see if you can spot a slate DPC line, compare the outside paving level to the internal floor, check if old air bricks might be buried, if possible lift one board and look into the floor void?

Those few checks will reveal a lot and help confirm what’s actually going on.

Hope that helps?

[Martin Richard]

Again, a clear and amazing answer I think it’s condensation even though it’s low down – no evidence of rising damp currently. It’s an old stone built house so the walls are cold.

There aren’t and haven’t been any ventilation bricks since we moved I. 25 years ago (naive, I know ). I think they must have been blocked during the installation of the paviours. Also there is hardly a step up to the front door. We installed channels years ago as the paving was laid badly and pooling by our front door. These are coming up. Could I please ask the following?

*Would a sort of trench on that outside wall work or say installing more ACO channels, but below the DPC this time? (We are hoping to lay tarmac but unsure how/what that would then butt up against.) it’s quite a narrow parking frontage for a very small car and we’re slightly lower than the footway.

*what is and early 1900 property DPC likely to be/look like?

[James Morris]

If the soil/paving is above the damp proof course that is your primary cause of damp ingress. The soil level needs to be 2 bricks lower(about 8″) than your DPC. You will need to remove 1 or 2 rows of blocks and possibly dig down further. Cover the gap with a grating or fill with coarse gravel about 2-3cm. There is no way round this. You have to prevent water collecting, and rain splash as well.

[Ruth Knox]

naively I don’t even know what a DPC consists of on an old property? The door is probably easy to

Solve but in the front room it on yeh walls behind furniture etc as the walls are so cold. Would you recommend getting a damp surveyor in before laying floors? We really want to do this properly and don’t want to be left with the same issue once the old timber is removed

[James Morris]

It doesn’t matter what you install internally if your DPC is breached. Generally speaking condensation is high up and rising damp starts lower. And you nearly always see condensation on windows in the morning in affected rooms. You can get condensation behind furniture where air flow is restricted too.

Condensation is caused by lack of ventilation. Double glazed windows in older houses are a big cause of condensation. Windows need to be opened regularly, ideally throughout the house to give a through flow. In Germany they do this for 20 minutes each morning.

Looking at this again I notice it’s by a door so you should check that the mastic around the door hasn’t degraded letting water in.

But no matter whether you sort the mould the DPC must be cleared. Otherwise your bricks can never dry properly and regular wetting and freezing will destroy the bricks in time.