''The stone walls are to be imsulated as wel with 300mm Rockwool batts on the inside and will monitor those as well.''

What did WUFI say about that? That sounds like a very cold wall!

Posted By: djhPS Those are interesting looking sensors. I wish they'd been around when I installed mine!

I have just got some (BME280), not got around to wiring any up yet. Got to work out how to get them to sit on the same I2C GPIO as the real time clock

Does WUFI show any areas of low temperature or high RH?
If it does, then fit the sensors there.

Alternatively, place them in random locations, then model from there. Or you could try and work out areas that may pose a problem i.e. junctions, interfaces, high and low airflow areas, north and south facing etc.

Diverting slightly, I think you can write an address to the 280's and then read them individually. What I am looking at later (after I have done my running around)
https://stackoverflow.com/questions/17687132/multiple-sensors-on-raspberry-pi-gpio

http://www.robot-electronics.co.uk/i2c-tutorial

Agreed, one address with SDO high, the other with it low. It might not be obvious how to set the SDO pin, though, depending on the breakout board in use. For the Adafruit board you just ground SDO to get the other address. For the ones Steamy has (which appear to be the same as the ones in the vAir COâ‚‚ monitor if you select the option of having a pressure sensor) it might not be so clear - would need to trace the connections out.

That multiplexer looks neat but you could avoid the extra hardware by just bit banging other GPIO pins to give more I²C buses. High data rates are hardly critical to monitoring temperatures and humidities in walls.

Steamy's considering using SPI as it has a chip-select line which you could run off other GPIO pins. It occurred to me today that you could maybe actually use the SDO pin as a chip-select line for I²C. Ground it for the device you want to talk to then address that using the alternative address, leaving all the others on the bus at the default address. Assumes the SDO pin is dynamic - it doesn't just sense its address on startup then stick with it.

I suggest ensuring an outside sensor (sheltered) and one or more inside sensors - without that data running in parallel with the 'in fabric' sensors your have no real idea where the moisture is coming form - or how long it takes to get there or what is the best level that could be achieved. Also before installation have all the sensors in one place and calibrate them with each other under various conditions.

I wager that so long as the vapour barrier is relatively intact that....
a) the absolute humidity of a sensor just outboard of the vapour barrier will closely track the outside absolute humidity and
b) the absolute humidity just inboard of the vapour barrier will closely track the internal absolute humidity (provided there is an air gap between the 20mm timber board and the VCL - and the 20mm board is not designed as an air barrier)

(derive absolute humidity from the RH and Temp - in any units you like (grams/M3 is my favourite it makes for easy calcs for the effect of ventilation)).

Posted By: nigelmWhat is the relevence of absolute and relative humidity in relation to the building fabric?

Absolute humidity is the density of water molecules in the air (or actually in the space that the nitrogen, oxygen etc also occupies). It has physical significance and has pretty strong conservation properties. In particular, if you take air from outside and bring it inside, it will still have the same absolute humidity, but its relative humidity is likely to be be quite different.

Relative humidity is to do with the relative rates of evaporation and condensation at a particular temperature and pressure. It's a much more nebulous concept and more complicated to deal with. But it is of great importance because it is largely relative humidity that matters to living things. How comfortable you feel, how well fungus grows (whether mushrooms or rot in wood), how comfortable mites feel, how well bacteria grow and spread all depend on what the relative humidity is. As does the moisture content of wood etc etc.

https://en.wikipedia.org/wiki/Humidity has some more info.

The bme280 sensors use 3.6uA at 1hz. I believe the idle current is even lower so hopefully local heating of the sensor will be minimal.

I have been thinking about read frequency, I am guessing that once every 30 minutes would be sufficient.