Wednesday, 12 July 2017

Is it damp under my house (part 3): I wasn't expecting that!

We have had some fantastic summer days during the last couple of weeks, here in Sunny Southern England.

Daytime temperatures have regularly been in the high 20s/low 30s (...and I'm talking Centigrade here not Fahrenheit).

So surely the under-floor area of my house is now bone dry?

Well, no.

I hadn't thought this through when I wrote my last post on the subject, where I was looking forward to the warm, dry days of summer. Currently my humidity sensor reading is around 100%rh!

My wooden floor is nailed down for almost the entire ground floor area, except in the cupboard under the stairs. This is where I have positioned the BME280 that I am now using to measure temperature and humidity. This sensor is far away from each of the air bricks.

The red trace shows the relative humidity. I fitted a temporary extractor fan (extracting from the room to the floor space) so that I could drive down the %rh and prove that the measurement system was still working (hence the drop to about 85%rh and the rise in temperature to just over 20'C).

The green line data I have called Target Humidity. I take the outside temperature & humidity from a weather service, then recalculate what the humidity would be for the current under floor temperature. During the 5 day period displayed, the Target Humidity has exceeded 90% several times, and was bumping into 100% five or six days ago.

The outside air flows under the house via half a dozen air bricks.
The ground stabilises the air temperature in this partially enclosed space

Even though the under floor area is vented to the outside via 6 air bricks, the temperature of this space is usually warmer than the outside air temperature in the winter, and cooler in the summer.

With an outside air temperature and humidity of 30'C & 50%rh, the dew point is 18.45'C. Therefore if the under floor temperature is below 20'C, this space is going to be very damp and my sensor will indicate around 90 - 100%rh.

It looks to me as if all houses with suspended wood floors must be dangerously damp in the summer, unless the house is on stilts! I can't be the only nut in the world that is measuring humidity under a suspended floor. So I'm on the lookout for anyone that can either prove or disprove my findings.

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