Humidity in subfloor void

[Kaymo]

I am wondering if anyone has any views/knowledge on ideal relative humidity in the subfloor void.

I have just replaced joists in a 200 year old house which were resting on the ground and rotted through, with shallower treated timber joists with a shorter span to allow for a gap between the joists and the ground. Having done all this work, insulated between joists, fitted new flooring etc I don't want it all to start rotting again due to inadequate ventilation.

Ventilation is not ideal due to sheltered location of the vents and although I have improved this by uncovering one that had been rendered over, its difficult to know how to measure whether or not this is effective, other than waiting to see if it starts rotting! I thought I was being clever by leaving a humidity sensor under the floor, but now that the floor has been closed up I have realised that I have no idea what RH reading would indicate that the ventilation is sufficient.

My first reading since closing up the floor was 94% which seemed high so I did a bit of a search and found this site which suggests it needs to be below 85% to avoid mould etc. I then tried positioning a fan to see if mechanical ventilation would improve it, but it didn't really budge. It then occurred to me to check the ambient RH outside and according to the BBC it was 95%+ today so the subfloor void is just reflecting the ambient conditions.

Going back to the claim on the site mentioned above that <85% RH is the aim, I checked the average RH in the UK and it rarely drops below 85%, which makes the claim on the site above a bit suspect. Seems to me that the subfloor RH can never be any lower than the ambient RH.

Sorry for the long winded background, but the bottom line is, does anyone know what the subfloor RH should be or any other way to measure whether or not subfloor ventilation is sufficient?

 

[JohnD]

Please draw a plan of your house, showing where the damp is, and the airbricks.

Also show where the drains are, and the external stopcock, and the internal stopcock. Draw a line between them indicating the probable route of the old water pipe.

 

[Kaymo]

Hi John,
I am not asking about damp, I'm asking what the RH in a subfloor void should be.

 

[noseall]

Hi John,
I am not asking about damp, I'm asking what the RH in a subfloor void should be.

It's underground - it will be damp/humid. Just make sure their is air-flow, regardless. Throw that meter in the bin.

 

[Murdochcat]

And during the winter with more rain, the water table will rise increasing damp

I suspect that more rot won’t be your problem so don’t worry about it unless you’ve used cheap inappropriate wood

 

[Kaymo]

Thanks for the replies. The wood is C24 treated so it should last a good while even if the conditions are poor. The vents are clear and unobstructed so there is air flow, but its never going to be a strong through flow of air because the vents are on the north and east due to solid floors on the adjacent internal rooms, and the ones on the East are in a passageway so very sheltered. This means that the prevailing winds will never face the vents.

I know that you can buy subfloor ventilation fans if ventilation isn't sufficient, but I suppose its just a question of how much airflow is good airflow, and a humidity sensor is probably the only way to measure that because if it isn't sufficient then moisture from the solum and footings wont be carried away and RH will rise. The thing I didn't think through is that in our climate the humidity is always high, so while in theory humidity of less than 85% is desirable, its probably not realistic most of the time.

I did a bit of reading on this last night, found some building science papers online, and most of the subfloors measured had a lower humidity, and the only one with a mean of over 95% had visual mould growth. That being said, I don't know yet if mine will have a mean in the 90's over the course of the year. The other thing of interest is that subfloor RH tends to be lower in winter because the subfloor is usually warmer than the ambient temp, while in summer warm moist air is entering a colder void pushing the RH up.

The only reason I am doing any of this is to determine whether I need to improve ventilation, possibly with a fan, but I can monitor the RH and if it goes up and down in line with the outside air then that is probably a good sign.

 

[Harry Bloomfield]

but its never going to be a strong through flow of air because the vents are on the north and east due to solid floors on the adjacent internal rooms, and the ones on the East are in a passageway so very sheltered. This means that the prevailing winds will never face the vents.

I would want to attend to that, as a priority....

For preference, air should enter on one side, and exit on a directly opposite wall, allowing the wind, and air pressure differences to do what it does well. In most of the UK, west and east facing is best. Sheltered, is not good. Failing that, add some mechanical ventilation, such as a low power, slow running fan.

 

[Kaymo]

I would want to attend to that, as a priority....

For preference, air should enter on one side, and exit on a directly opposite wall, allowing the wind, and air pressure differences to do what it does well. In most of the UK, west and east facing is best. Sheltered, is not good. Failing that, add some mechanical ventilation, such as a low power, slow running fan.

I understand what you are saying, but in my OP I mentioned that I tried positioning a temporary fan in front of one of the vents and the RH didn't drop. As far as I understand it the only purpose to adding a fan and increasing airflow would be to reduce the RH.

Below is a quick sketch of the vents. The green one is actually coming in at the East wall and runs through a metal pipe under the solid floor of the adjacent room. This pipe looks very old, may be original from when the suspended floor was first fitted in the 1880's and has obviously been in an attempt to provide through ventilation in combination with the red one. I am fairly certain the blue one is more recent, i.e. in the last 30 years or so and I have a hunch it may have been added because they didn't know the green one was there- when I first lifted the floor I thought the green one was a disused drain pipe until I traced it back and cleared off the render to uncover the vent. The blue one has an alloy vent and is run under/through a fireplace recess so has almost certainly been added after the fireplace went out of use.

I have actually considered closing the blue one because I think it could be counterproductive. Air will take the easiest path and maybe will short circuit the preferred path which is between red and green. I might be able to improve the natural airflow into the green one by adding a 90 bend to the outside end of the pipe, so that the opening faces the wind coming down the side of the house from the south.

 

[Harry Bloomfield]

I understand what you are saying, but in my OP I mentioned that I tried positioning a temporary fan in front of one of the vents and the RH didn't drop. As far as I understand it the only purpose to adding a fan and increasing airflow would be to reduce the RH.

The RH, if ventilated from outdoor, can never be lower than the outdoor RH, but the airflow will still be effective - it will still prevent rot.

 

[Kaymo]

The RH, if ventilated from outdoor, can never be lower than the outdoor RH, but the airflow will still be effective - it will still prevent rot.

Can you explain that?

See my post above, RH will be lower under the floor than outside if the underfloor temp is higher than outside, and higher relative to outside when the under floor temp is lower than outside.

 

[JohnD]

Hi John,
I am not asking about damp, I'm asking what the RH in a subfloor void should be.

The source of water will, of course, be relevant to the damp.

 

[Harry Bloomfield]

Can you explain that?

See my post above, RH will be lower under the floor than outside if the underfloor temp is higher than outside, and higher relative to outside when the under floor temp is lower than outside.

If you hang wet washing out. even if the humidity outdoors is high, it will still dry. It just takes a little longer, though much less, if there is airflow. The same applies to the timber under your floor. The under floor measured underfloor RH, is never the entire story.

 

[Kaymo]

The source of water will, of course, be relevant to the damp.

I'm not sure what damp you are referring to. I've said in my OP that the original joists were resting on the soil, so they would have been sucking moisture directly from the ground. I've rectified that, my question now is as a belt and braces, ensuring that the humidity in the void is under control.

 

[Kaymo]

If you hang wet washing out. even if the humidity outdoors is high, it will still dry. It just takes a little longer, though much less, if there is airflow. The same applies to the timber under your floor. The under floor measured underfloor RH, is never the entire story.

I like the analogy, makes sense, just not sure its quite the same thing. The water evaporates into the air because the RH is less than 100%, if it was at 100% the washing would never dry. I think that the reason that wind helps is that it ensures the RH immediately next to the wet washing is never able to increase, so evaporation rate is constant, because the wind in an open space is constantly refreshing the air to keep it below 100%.

I don't pretend to be an expert in the physics of this, but I think if you carry the above analogy into a partially closed void, if there is moisture there, then the carrying away of moisture in the void by the wind would have to reduce the RH in the void, if it isn't carried away then the RH would increase, relative to outside. Also, the wood isn't wet to begin with. As you say, other than from the effect of temp differences, the void RH can't be better than the outside air coming into it.

Maybe I am answering my own question here, and as long as the RH in the void tracks the outside RH, maybe that's an indication that ventilation is sufficient.

 

[Harry Bloomfield]

The water evaporates into the air because the RH is less than 100%, if it was at 100% the washing would never dry.

I agree, but rarely does the RH hit 100%, except when it is very wet, so generally, it works.

Maybe I am answering my own question here, and as long as the RH in the void tracks the outside RH, maybe that's an indication that ventilation is sufficient.

The closer it tracks the outdoor RH, the better must the ventilation be working - and so yes, it does provide a bit of a clue. A thing to avoid, is corners, or areas which might be missed, by the route of the airflow.