Bedroom Humidity Against Dust Mites: The Ventilation and Temperature Conditions That Hold a Bedroom Below the Mite Breeding Threshold Through a Winter Night

House dust mites breed where the air stays humid, and the bedroom — warm, occupied all night, and full of the bedding they live in — is the room most worth drying. This paper balances overnight moisture production by two sleepers, a stated 45 grams per hour, against ventilation with winter outdoor air, and computes the steady relative humidity of a 36.4 cubic metre bedroom held at 19 degrees. At the closed-door infiltration rate of 0.1 air changes per hour the room settles at 100.0 percent, deep inside breeding territory; holding it at or below a stated 55 percent target requires 0.27 air changes per hour. The same calculation exposes a winter constraint in the other direction: single glazing condenses once room humidity passes 61.3 percent. The mite threshold and the condensation ceiling define a corridor, and the paper sizes the ventilation that keeps a bedroom inside it. This study is simulated, not measured.