Indoor humidity check
Your indoor humidity is predictable from outdoor conditions — heating cold air is what dries a house out. No sensor needed for a useful answer.
How this works without a sensor
Absolute humidity is roughly conserved when outdoor air leaks into a house. Heating it does not add water — it raises the air's capacity, so relative humidity falls. Outdoor air at 32°F and 80% becomes about 20% once warmed to 70°F, which is why January is hard on tropical houseplants.
The estimate assumes no humidifier and typical air exchange. A bathroom, a room with a humidifier, or a very tightly sealed house will differ — that is where a $12 sensor earns its money.
Questions
Why is my house so dry in winter?
Cold air holds very little water. When it leaks in and you heat it to room temperature, the amount of water has not changed but the air's capacity has risen sharply — so relative humidity collapses. Outdoor air at 32°F and 80% becomes roughly 20% once warmed to 70°F. That is the whole mechanism, and it is why winter is hard on tropical plants indoors.
What is VPD and why use it instead of humidity?
Vapour pressure deficit is how much more water the air could hold than it currently does — the actual force pulling water out of a leaf. It matters because relative humidity alone is misleading: 50% at 60°F and 50% at 85°F produce completely different transpiration. Commercial growers work in VPD for this reason. Below about 0.4 kPa the air is stagnant and fungal problems start; 0.4–1.2 is comfortable; above 1.6 is strongly drying.
Does misting raise humidity?
For a few minutes, no more. The water evaporates and the room returns to where it was. Grouping plants together works because they transpire into each other's air continuously; a humidifier works because it runs continuously. Misting is the most persistent piece of ineffective plant advice there is.
Is dry air really the cause of brown leaf tips?
Far less often than people think. Dry air crisps whole leaf EDGES on genuinely humidity-hungry species. Brown TIPS, especially with a thin yellow halo, on a dracaena, spider plant, calathea, peace lily or palm are usually fluoride and salts in tap water. Checking your humidity number is how you tell the two apart.