Monitoring the humidity in my underfloor space during my step-wise retrofit

I am wondering about purchasing or building a system to monitor the humidity in my underfloor space after I insulate it, seal it and close off the air bricks. I’d like to have eight or more humidity (and some temperature) sensors of some sort down there, in order to monitor the various corners which may become stagnant and humid. It seems from what I read on the OpenEnergyMonitor website that I may not be able to use a single eMon system, and our local Hackspace experts are suggesting various ways I could build my own system. Am I correct that I would need two or more eMon systems to do this, and are they correct that it may be more economical to build my own system with their help? I am not a current owner of a emon system, and am only just beginning to become familiar with the concepts, but at the Hackspace they are knowledgeable and extremely helpful. It may be that in future we will be interested in monitoring other parameters of our house, but incident solar radiation is limited, so for my retrofit I am concentrating on fabric first, so majoring on air-tightness and insulation with low-embodied carbon materials.

Sounds like DHT22 sensors might do the trick here.
You could connect them to an emonBase and get some code together to bring the data into emonCMS.

DHT22 sensors need three wires: power, data, gnd. Multiple sensors require their own data line, each connected to a dedicated pin at the destination. 4-wire telephone cable is an economical way to connect two at a time, as I’ve done before.

By coincidance I’m getting ready to connect an existing DHT22-based system with 5 sensors to a rPi. It is also monitoring construction layers. It’s been logging to an Arduino/SD card for years and now I want to access the data more readily.

I don’t know enough about emonTHv2 but perhaps @glyn.hudson can recommend a way to have them powered in your construction layers, so they can be transmitting data without you needing to think about batteries.
Using the emonTHv2 with external power could mean using an emonBase receiving radio packets.

Edit: noticing also the DHT11 option, and the AM2302 (same as DHT22 I think?)

I have connected 8 Dallas One-Wire (which the DHT22 is) temperature sensors to a single WEMOS D1 running Tasmota. Alternative means of measuring temperature Part 3

Needs a bit of Node-Red to ensure 100% reliability but that is what I have at 10s resolution!

The data is easily fed into EmonCMS.

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@borpin now I see where your experience of emonHub not handling quick MQTT requests comes from!
I don’t do enough in node-red, a quick look around and I find this.

@PaulH I’m interested in the kind of construction you’re doing. I wrote my dissertation on passivahus refurbishment and am doing an MSc along these lines now. Are you writing up the build process in a blog?

It’s not emonhub I had an issue with but the emoncms_mqtt input script that missed messages in quick succession.

The node-red is deliberately just the processing not the reading of the data off the sensors so is very straightforward.