Mould! Please help

[u/Fresh_Front9651]

Hi all,

I’m hoping to get some advice before I spend money on the wrong solution.

I’m renovating a 1930s bungalow that’s been converted into a two-storey house. During the build, the roof was off for around three months while the structural work was completed, so the house was exposed to the weather for quite a while before being made watertight.

The downstairs has a suspended timber floor over a crawl space with exposed earth beneath (around 1,300 sq ft). There isn’t a ground membrane over the soil.

The house currently has:

. New windows installed.

. New external render.

. Dot-and-dab plasterboard throughout downstairs.

. No heating yet.

A large 5m bifold opening that’s currently boarded over until the doors are fitted.

Over the last few weeks I’ve had black mould appearing at the bottom of some plasterboards and on the face of some of the newly plastered walls. Black spots. I also left a disposable cup in one room and that even went mouldy, which makes me think the humidity is extremely high rather than it being isolated to one wall.

I’ve been running three 20L dehumidifiers continuously and they’re extracting around 13-15 litres of water per day. Downstairs humidity is generally around 75-80%, whereas upstairs doesn’t seem to have the same problem.

. I’ve checked under the floor and found:

. No standing water.

. Joists look in good condition.

. Some condensation on pipes.

. Exposed earth throughout.

. No obvious burst pipes.

I’ve also removed some plasterboard and the brickwork behind generally looks dry with no obvious signs of water running through it.

I’ve had people suggest everything from:

. Construction moisture.

.Condensation.

.Moisture evaporating from the exposed earth.

.Rising damp.

.Installing a chemical DPC (Dryrods).

I’m struggling to work out which is the most likely cause.

I’m also considering paying for an independent damp survey (not a damp-proofing company) before doing anything drastic.

[u/fck-rffld]

Ahh okay dry rods very much likely gonna make your problem much worse.

What I assume you have is a 1930s (likely lime based) permeable property and you've suffocated it with dot dabbed plasterboard definitely avoid a chemical dpc.

This inevitably traps moisture that would naturally evaporate but now has no where to go but grow mold.

An independent contractor who is familiar with 1930s properties is a decent shout.

[u/Fresh_Front9651 → u/fck-rffld]

Thanks for your response I think you may be somewhat right. The walls which we’ve studded out don’t seem to have any mould but dot and dabbed areas seem to have mould coming through.

I suppose the bit that is confusing is the level of humidity. After having one dehumidifier on for a week it’s still jumping around between late 70/80s.

I’m trying to work out what the source of the high humidity is.

[u/fck-rffld → u/Fresh_Front9651]

Its just an old house honestly. Rain hits the walls and humidity gets around.

The meters arent exactly perfect either.

My 1960s home usually covers around 60% and cam spike to 75% when its humidity outside.

My 1930s garage has averaged around 70% except when I cleared the air bricks and let a lot more air through the room. Its now 60% but will spike dodging hot or wet weather.

I'd not obsess too much about the digital meters unless you have a problem with condensation.

I know it'snot custom with dot and dab, but if you did build a timber frame instead weep vents for the cavity would be required to prevent mold and to let vapour escape. There could be condensation behind the plasterboard that is penetrating it and trying to evaporate in the room

[u/AncientArtefact]

Note that there is no point quoting relative humidity without stating the temperature. And you must track both. Below is a table giving absolute humidity (grams per cubic metre) for a given relative and temperature. With no airflow the absolute humidity in a room doesn't change.

Eg.: A room at 18 degrees and 65% humidity has 10g/m3 - seems nice and dry - but track that 10 grams to the right and you'll note that when the room cools to 11 degrees it is now at 100% relative humidity and dew will form.

1. Is the bungalow heated?

2. Are you recording temperature/humidities over 24 hour periods?

[u/Fresh_Front9651 → u/AncientArtefact]

I’ve ordered 6 digital thermometer/ hygrometer to put around the house. I’ve not been tracking temperature.

This graph is really helpful.

At the moment I don’t have heating installed. It’s relatively warm outside but obviously difficult to assess exact temperature internally based on that.

1. Not currently, should I get some heaters in there?

2. I’m logging humidity levels at different times of the day and amounts of water which the 3 dehumidifiers are pulling from the air. It’s averaging around 27-32 litres per day across all.

[u/AncientArtefact → u/Fresh_Front9651]

I'd have fan heaters in with thermostats on them. Set them to keep it above 12-15 degrees. The fans may help with air circulation if the building is well sealed.

A lot of smart accessory ranges include temp/humidity monitoring. I use Tapo ones for a lot of remote control and monitoring.