Enough natural ventilation to create through-draft in cellar?

[MacLaverty]

Need a bit of advice:

We moved into a 1901 house last year with a converted cellar done in 2020, cellar was always dry and never flooded before conversion. Within 2 weeks of moving in we had a washing machine leak so bought a dehumidifier to dry the cellar back out and since then just left the dehumidifier running 4/5 hours a day with windows closed. I'm getting bored now of paying £15 per month to run it and emptying it every 3 days and asking people to empty it when we are away. I believe the previous owners had nothing down there and had no issues but unsure as I don't have their details. The room has no issues currently with damp or mold etc but does suffer high humidity at around 75% when the dehumidifier switches on.

Therefore, looking to utilise natural ventilation instead of mechanical to help shift the moisture/ humidity and prevent mold etc. Just had a small window put in on the external wall and there is a air brick on the other, which leads to a crawl space that then has an air brick to the outside. 1st picture is the air brick by the stairs, no door at the top so opens to the kitchen, and 2nd picture is the window from where the air brick is. I also have a powerful air purifier down there as I work down there and don't want to be breathing in mold spores etc and helps with air circulation.

With the window open or on a latch, would this be enough to create a through-draft from the air brick to the window for ventilation or do I need more? And presume I should leave the window open/ latch 24/7?

 

[MacLaverty]

a

 

[foxhole]

Small fan would cost a few pounds a year to run 24/7 and ensure ventilation.

 

[MacLaverty]

So potentially get a humidity controlled fan, can get 1 installed under the window, and just use that to extract air24/7?

 

[Harry Bloomfield]

With the window open or on a latch, would this be enough to create a through-draft from the air brick to the window for ventilation or do I need more? And presume I should leave the window open/ latch 24/7?

An extract fan would obviously be better, with the cellar drawing air from a low humidity place, such as the main house. Drawing air in from outside, might not be the best idea, because the external humidity can at times reach 100%, so you would need to be selective as to when you draw external air into the space.

 

[Lower]

An extract fan would obviously be better, with the cellar drawing air from a low humidity place, such as the main house. Drawing air in from outside, might not be the best idea, because the external humidity can at times reach 100%, so you would need to be selective as to when you draw external air into the space.

Don't forget any air you pull from the house has got to be replaced from somewhere, and its likely that the outside air will be cooler than inside the house so the humidity will decrease when the air temp increases.

 

[MacLaverty]

Thank you, I think a humidity controlled fan sucking air out of the cellar sounds the way to go as the room is unheated so last thing I want to do is suck in very humid external air which will condense in the cellar. I guess I could heat the room but the temperature hovers around 16/17 degrees without heating all year round so its not unbearable and the room is hardly used so don't want to heat it unnecessarily just to warm the humid air I am dragging in from outside.

 

[Harry Bloomfield]

Don't forget any air you pull from the house has got to be replaced from somewhere, and its likely that the outside air will be cooler than inside the house so the humidity will decrease when the air temp increases.

It needn't be a large volume of air.

 

[Lower]

Thank you, I think a humidity controlled fan sucking air out of the cellar sounds the way to go as the room is unheated so last thing I want to do is suck in very humid external air which will condense in the cellar. I guess I could heat the room but the temperature hovers around 16/17 degrees without heating all year round so its not unbearable and the room is hardly used so don't want to heat it unnecessarily just to warm the humid air I am dragging in from outside.

You won't pull humid air from outside that will condense inside. Humidity is a relative percentage. As air gets warmer it can hold more water without condensing. So if you bring air in from outside into a warmer house, its relative humidity will go down.

Using a fan to pull air in from outside is no different to having a large open window.

 

[Lower]

It needn't be a large volume of air.

Doesn't matter how much air it is, it still has to be replaced from somewhere. If you pull if from other rooms in the house it'll just be replaced with air from outside that leaks in where ever it can.

 

[MacLaverty]

Sorry I'm really confused now. What's the best course of action then?

1) leave it as it is as if it's not broke with the dehumidifier don't fix it?

2) just keep the window slightly open 24/7 and create through draft through the room to the stairs/ air vent.

3) Do purge ventilation a few times a day by opening the window for short periods of time when it's colder outside than inside to control it more?

4) get an exhaust extractor fan installed by window to ensure ventilation across the room from stairs/ air vent and exhausted outside?

But on the window front I've heard opening the window when warmer outside than inside can cause condensation issues so surely if I leave the window open 24/7 I can never guarantee outside will be colder than Inside and may cause issue or is that not really a thing in the UK?

 

[Harry Bloomfield]

What's the best course of action then?

The extract fan..

 

[Lower]

Options 1,2 & 4 are all fine. It’s very unlikely that letting air from outside will cause condensation inside.

 

[foxhole]

So potentially get a humidity controlled fan, can get 1 installed under the window, and just use that to extract air24/7?

Not humidity control or it won’t run 24/7.

 

[Harry Bloomfield]

Not humidity control or it won’t run 24/7.

Many fans which include a humidity sensor, tend to not work that well, so I would suggest a better option would be an external, decent quality one, which includes an adjustment for humidity level.