What is in this section
This branch is the part of the site you look things up in rather than read through. The numbers, the method and the vocabulary sit here so the rest of the pages can refer to them instead of repeating them.
Three of them are worth reading once, properly, before you use anything else. The humidity chart gives the target for each room and each outdoor temperature in one place. The measurement guide covers where to put a hygrometer, how long to leave it, and how to check a cheap one against a known reference — because a reading taken in the wrong place is worse than no reading at all. The glossary untangles relative humidity, absolute humidity, dew point and grains per pound, which are four ways of describing the same water and are constantly confused.
The other two answer questions that come up on almost every page: where the water in your house is actually coming from, and the point at which a damp problem stops being a job for a homeowner.
The four references everything else leans on
The chart
Target humidity by room, and the winter ceiling by outdoor temperature and window type. It is the one page worth printing and putting inside a closet door.
The method
Where to place a hygrometer, how long to wait, when in the day to read it, and the salt test that tells you how far a cheap unit has drifted. Costs nothing and makes every other number on the site trustworthy.
The vocabulary
Relative humidity moves when you change the temperature; absolute humidity and dew point do not. Almost every confusing argument about indoor air comes down to two people using one word for both.
The sources of the water
People, cooking, washing, drying, unvented appliances, ground moisture. Quantified in pints per day, so that the size of a fix can be matched to the size of the problem instead of guessed at.
Look it up
Humidity chart
Every number this site uses, on one page: by season, by outdoor temperature, by room and by climate.
How to measure it
A cheap hygrometer in the wrong place, read at the wrong time, produces a number that means nothing. Four rules fix that.
Glossary
Plain definitions, in the sense the terms are actually used when someone is trying to fix a damp house.
Where the water comes from
The full accounting: what each activity releases per day, ranked. Most damp problems are on this list.
When to call a pro
The lines worth knowing before you spend money: what is yours to fix, what is not, and who to call for which.
Common questions
Which page should I read first if I know nothing about this?
The humidity chart, then the measurement guide. The chart tells you what number you are aiming at in each room and each season; the measurement guide makes sure the number you are reading is real. Everything else on the site assumes those two.
How accurate are cheap hygrometers?
Typically within about 5% when new, and they drift. That is good enough for every decision on this site, provided you check the offset once with the salt test and subtract it thereafter. What matters far more than absolute accuracy is consistency — the same unit, the same place, the same time of day, so that changes are real rather than artifacts.
Do I need an infrared thermometer as well?
Not to start with, but it is the best twenty dollars in this subject after the hygrometer. It turns the mold question from a guess into a measurement: point it at the coldest corner, compare with the dew point of the room, and you know within seconds whether that surface is at risk.