Very interesting, thank you. We have a old coal cellar in our south London victorian flat, and one confusion I have regarding sub-floor areas and wood rot risk is that even when they are well ventilated, the humidity should match the outside air. But in London, this is often well over 60%, which as I understand it is a high risk for mould growth. Yet I'm not aware of any of the multitude of victorian properties nearby having any wood rot issues.
Thank you for your question-this is a fascinating topic and can be a little complex to explain. Here are the key points: 1️⃣ Dew Point vs. Relative Humidity A well-ventilated property will have a matching dew point (a measure of vapour content) between the sub-floor and the outside air-not necessarily matching relative humidity (RH), because RH depends on both temperature and vapour pressure. If the sub-floor air is warmer than the outside air, the RH will be lower even if the dew point matches. 2️⃣ Humidity in the UK In the UK, including London, we have a temperate and humid climate. Outdoor RH often reaches 100% during rain or stays high for prolonged periods. However, the heat escaping from a property should reduce the sub-floor RH, as warmer air holds more vapour without reaching saturation. For a deeper explanation, you can visit damp.ai. 3️⃣ Mould Growth Basics Mould requires high surface RH to thrive: Black mould (most common in homes): Needs 85%-100% RH for at least 6+ hours (but not during surface condensation, as mould also needs oxygen). Brown mould: Needs 80%-85% RH, sustained over days or weeks, and is more common in vacant properties. 4️⃣ External Air Dynamics Outdoor air is constantly moving, which means its relative humidity fluctuates. While mould can grow on exterior walls, it’s relatively rare because the microclimate changes so frequently. Sub-floor areas, however, are more stable, which could allow mould to grow. That said, hygroscopic salts in the soil (often present in Victorian cellars) can inhibit mould growth. 5️⃣ Rot and Sub-Floor Ventilation Rot needs water to grow, which can come from condensation. Proper sub-floor airflow-via front and rear air bricks-and eliminating sources of water (e.g., leaks or condensation) are usually enough to prevent rot. If there’s no water source, even limited airflow can suffice. However, you’ll need to: Monitor sub-floor RH weekly (aim for
@@dampsurveys What an excellent and detailed response - thank you! Our long, narrow coal cellar unfortunately does not have a "through" air flow, as a result (I think) of a 70s kitchen extension. So I've added mechanical ventilation - extraction from the front of the cellar, and positive input at the rear from the flat itself. However I found that the positive input during the summer months, when the dew point is high, was perhaps adding to the problem as the humid air would (I think) condense in the cooler basement (I monitor both the RH and dew point from three places: outside, inside the flat, and the cellar). So my plan is to either focus on increasing the extraction air flow from both ends of the long cellar (a technical challenge), or re-instate the positive input, feeding air from the flat, where I would run my dehumidifier during the humid late summer/early autumn months. The walls of the cellar actually have a low moisture content; the issue is the clay floor which is perpetually moist, as it's just, I think, soil (which in turn could be "tanked" but I fear that may well create more problems!). Complex stuff! Luckily I can visually inspect the woodwork, and cannot see any signs of rot to my to my albeit untrained eye. Sorry for the long post - it's great to be able to share all these ideas.
@@dampsurveys What an excellent detailed response - thank you! In our case the long coal cellar does not have "through" ventilation (due, probably, to a 70s kitchen extension at the rear of the property blocking a prior airflow), so I've installed a vent and mechanical extraction fan at the front of the cellar, and am experimenting with mechanical positive input from the flat itself at the rear. This has improved the cellar environment somewhat, but humidity levels are still higher than I would like. I have three data loggers: inside the flat, outside and in the cellar, measuring both RH and dew points. The walls of the cellar appear to be reasonably dry according to my moisture meter; the moisture source appears to be the clay (soil?) floor of the cellar, which has no moisture barrier (I have considered tanking, but that could cause other problems). Complex stuff! Luckily I can visually inspect the floor timber from the cellar, and to my albeit untrained eye I can't see any signs of rot. Yet!
@@frogtantrum Thank you for your detailed reply! 😊 It sounds like you’re putting a lot of thought into managing the airflow and humidity in your cellar, and I think you’re on the right track. To improve airflow, you could create a circular airflow system by installing an inline fan with a long pipe blowing air in through an air brick. Just make sure the air brick has a metal grate to prevent mice from getting in and causing, well, undesirable surprises! 🐭 It’s great that you’re monitoring the RH and dew points so carefully-it’s the best way to stay ahead of potential issues. Running a dehumidifier during the humid months, particularly if you reintroduce positive input from the flat, sounds like a solid plan. It’s also wise to avoid tanking the clay floor, as that could lead to unintended moisture problems elsewhere. Your regular inspections of the woodwork are crucial. Even without formal training, keeping a close eye on it will help you spot any early signs of trouble. If you ever suspect rot, using a pinless moisture meter on the wood could be helpful for non-invasive checks. For more insights, you might find these videos helpful: 🎥 Mould, Moisture, Mayhem: The Vapour Strikes Back th-cam.com/video/vVFBM-w5V3A/w-d-xo.html 🎥 Sweat in the Cellar: From Poor Ventilation th-cam.com/video/J8wdxehh7GM/w-d-xo.html 🎥 Dampness in Cellar: Two Sources, Poor Ventilation th-cam.com/video/vS8VkyB5sCU/w-d-xo.html 🎥 Rising Damp Treatment: A Cover-Up, Not a Cure th-cam.com/video/iSpecL_3aFE/w-d-xo.html Let me know how your adjustments work out-I’d love to hear about the results! 😊 Do visit damp.ai, which helps me craft these answers.
@@dampsurveys many thanks, and apologies for the confusing double post - I thought I had lost the first as my internet connection dropped out when posting!
Very interesting, thank you. We have a old coal cellar in our south London victorian flat, and one confusion I have regarding sub-floor areas and wood rot risk is that even when they are well ventilated, the humidity should match the outside air. But in London, this is often well over 60%, which as I understand it is a high risk for mould growth. Yet I'm not aware of any of the multitude of victorian properties nearby having any wood rot issues.
Thank you for your question-this is a fascinating topic and can be a little complex to explain. Here are the key points:
1️⃣ Dew Point vs. Relative Humidity
A well-ventilated property will have a matching dew point (a measure of vapour content) between the sub-floor and the outside air-not necessarily matching relative humidity (RH), because RH depends on both temperature and vapour pressure. If the sub-floor air is warmer than the outside air, the RH will be lower even if the dew point matches.
2️⃣ Humidity in the UK
In the UK, including London, we have a temperate and humid climate. Outdoor RH often reaches 100% during rain or stays high for prolonged periods. However, the heat escaping from a property should reduce the sub-floor RH, as warmer air holds more vapour without reaching saturation. For a deeper explanation, you can visit damp.ai.
3️⃣ Mould Growth Basics
Mould requires high surface RH to thrive:
Black mould (most common in homes): Needs 85%-100% RH for at least 6+ hours (but not during surface condensation, as mould also needs oxygen).
Brown mould: Needs 80%-85% RH, sustained over days or weeks, and is more common in vacant properties.
4️⃣ External Air Dynamics
Outdoor air is constantly moving, which means its relative humidity fluctuates. While mould can grow on exterior walls, it’s relatively rare because the microclimate changes so frequently. Sub-floor areas, however, are more stable, which could allow mould to grow. That said, hygroscopic salts in the soil (often present in Victorian cellars) can inhibit mould growth.
5️⃣ Rot and Sub-Floor Ventilation
Rot needs water to grow, which can come from condensation. Proper sub-floor airflow-via front and rear air bricks-and eliminating sources of water (e.g., leaks or condensation) are usually enough to prevent rot.
If there’s no water source, even limited airflow can suffice. However, you’ll need to:
Monitor sub-floor RH weekly (aim for
@@dampsurveys What an excellent and detailed response - thank you! Our long, narrow coal cellar unfortunately does not have a "through" air flow, as a result (I think) of a 70s kitchen extension. So I've added mechanical ventilation - extraction from the front of the cellar, and positive input at the rear from the flat itself. However I found that the positive input during the summer months, when the dew point is high, was perhaps adding to the problem as the humid air would (I think) condense in the cooler basement (I monitor both the RH and dew point from three places: outside, inside the flat, and the cellar). So my plan is to either focus on increasing the extraction air flow from both ends of the long cellar (a technical challenge), or re-instate the positive input, feeding air from the flat, where I would run my dehumidifier during the humid late summer/early autumn months. The walls of the cellar actually have a low moisture content; the issue is the clay floor which is perpetually moist, as it's just, I think, soil (which in turn could be "tanked" but I fear that may well create more problems!). Complex stuff! Luckily I can visually inspect the woodwork, and cannot see any signs of rot to my to my albeit untrained eye. Sorry for the long post - it's great to be able to share all these ideas.
@@dampsurveys What an excellent detailed response - thank you! In our case the long coal cellar does not have "through" ventilation (due, probably, to a 70s kitchen extension at the rear of the property blocking a prior airflow), so I've installed a vent and mechanical extraction fan at the front of the cellar, and am experimenting with mechanical positive input from the flat itself at the rear. This has improved the cellar environment somewhat, but humidity levels are still higher than I would like. I have three data loggers: inside the flat, outside and in the cellar, measuring both RH and dew points. The walls of the cellar appear to be reasonably dry according to my moisture meter; the moisture source appears to be the clay (soil?) floor of the cellar, which has no moisture barrier (I have considered tanking, but that could cause other problems). Complex stuff! Luckily I can visually inspect the floor timber from the cellar, and to my albeit untrained eye I can't see any signs of rot. Yet!
@@frogtantrum Thank you for your detailed reply! 😊 It sounds like you’re putting a lot of thought into managing the airflow and humidity in your cellar, and I think you’re on the right track.
To improve airflow, you could create a circular airflow system by installing an inline fan with a long pipe blowing air in through an air brick. Just make sure the air brick has a metal grate to prevent mice from getting in and causing, well, undesirable surprises! 🐭
It’s great that you’re monitoring the RH and dew points so carefully-it’s the best way to stay ahead of potential issues. Running a dehumidifier during the humid months, particularly if you reintroduce positive input from the flat, sounds like a solid plan. It’s also wise to avoid tanking the clay floor, as that could lead to unintended moisture problems elsewhere.
Your regular inspections of the woodwork are crucial. Even without formal training, keeping a close eye on it will help you spot any early signs of trouble. If you ever suspect rot, using a pinless moisture meter on the wood could be helpful for non-invasive checks.
For more insights, you might find these videos helpful:
🎥 Mould, Moisture, Mayhem: The Vapour Strikes Back
th-cam.com/video/vVFBM-w5V3A/w-d-xo.html
🎥 Sweat in the Cellar: From Poor Ventilation
th-cam.com/video/J8wdxehh7GM/w-d-xo.html
🎥 Dampness in Cellar: Two Sources, Poor Ventilation
th-cam.com/video/vS8VkyB5sCU/w-d-xo.html
🎥 Rising Damp Treatment: A Cover-Up, Not a Cure
th-cam.com/video/iSpecL_3aFE/w-d-xo.html
Let me know how your adjustments work out-I’d love to hear about the results! 😊
Do visit damp.ai, which helps me craft these answers.
@@dampsurveys many thanks, and apologies for the confusing double post - I thought I had lost the first as my internet connection dropped out when posting!