Every year, sometime in late May, I open the Japan Meteorological Agency’s forecast and check the same thing: when does tsuyu start? Because from that moment, a three-week countdown begins for our Tokyo properties, and there’s a lot to get done.

Japan’s rainy season isn’t just inconvenient for guests — it’s genuinely risky for properties. If you manage short-term rentals in Japan and haven’t built a pre-tsuyu routine yet, this post is for you.

TL;DR

  • Japan’s rainy season (梅雨, tsuyu) typically runs from early June to mid-July across the Tokyo, Osaka, and Kyoto belt
  • Sustained humidity above 70–80% triggers mold growth within days in poorly ventilated spaces
  • Pre-season checklist: service dehumidifiers, apply anti-mold treatment, test ventilation fans, inspect window seals
  • Update your in-room guest guide with dehumidifier instructions before June
  • Linen turnovers take significantly longer without outdoor drying — plan your staffing or laundry logistics now

What Is Tsuyu and Why Does It Matter for STR Operators?

Japan’s rainy season is less about heavy downpours and more about persistent, oppressive humidity. In Tokyo, relative humidity regularly sits at 75–90% for six to eight weeks running. That sustained humidity — not the rain itself — is what creates problems for rental properties.

For a short-term rental, the stakes are different from a regular long-term apartment. You have new guests cycling through every few nights who don’t know how the ventilation works, who may leave windows shut when they shouldn’t, or who won’t notice a mold patch forming behind the washing machine. By the time a review mentions a “musty smell,” the damage is often already there.

What Does High Humidity Do to a Rental Property?

High humidity without adequate ventilation causes three main problems in Japanese rental properties.

Mold. The bathroom is the most common culprit — grout lines, silicone seals around the tub, the back of the toilet. But closed storage is equally at risk; if guests stuff damp umbrellas or clothing into a closet, mold can take hold within 48 hours. Tatami rooms need particular attention. A moldy tatami mat is expensive to replace and hard to disguise.

Musty odour. Even before visible mold appears, fabrics, tatami, and timber absorb moisture and develop a smell guests notice immediately on arrival. In our experience, “smells musty” reviews cluster almost entirely in July and August and are nearly always traceable to inadequate dehumidification during the preceding tsuyu weeks.

Long-term structural damage. Window frame rust, wallpaper peeling at the edges, condensation staining around air conditioning units. These aren’t tsuyu emergencies individually, but they compound year on year if humidity is never controlled.

Your Pre-Tsuyu Maintenance Checklist

Run through this before the end of May.

Dehumidifiers. Test every unit. If you don’t have one per room, buy before June — stock sells out fast in electronics stores around this time. For a standard 6-tatami room, a unit with 6–10 litres/day capacity is adequate. For properties without openable windows (common in newer Tokyo apartments), treat the dehumidifier as essential rather than optional.

Anti-mold treatment. Spray bathroom grout, silicone seals, and tiled surfaces with a dedicated anti-mold product. Products like Kabi Killer (カビキラー) remove existing mold; anything labelled 防カビ prevents new growth. Do both — treat existing patches first, then apply a preventive coat.

Ventilation fans. Turn on every bathroom and kitchen fan and confirm that air is actually moving. A clogged or failing fan undermines every other measure. If a fan sounds laboured or weak, replace it now before contractor schedules fill in June.

Window seals. Check that rubber seals around windows and sliding doors are intact. Gaps let humid air in and allow condensation to pool in the frame over weeks.

Closets and storage. Place moisture absorber packs (除湿剤, such as the Oshiire Doka Doka range) inside every closed storage space. They’re inexpensive, discreet, and guests almost never disturb them.

How to Update Your Guest Instructions for the Season

The best maintenance routine in the world doesn’t help if guests undo it by sealing the apartment shut for days. Your in-room guide needs a brief humidity section in place before June arrives.

What to tell guests:

  • “Please use the dehumidifier when you’re home, especially after showering.”
  • “Run the bathroom ventilation fan for at least 30 minutes after each shower.”
  • “If you open windows, please close them when you leave.”
  • “Store umbrellas in the entrance area, not in the closet.”

Keep it short — guests don’t read long instructions. Two or three bullet points in the house manual, plus a sticky note on the dehumidifier itself, covers most scenarios.

Linen Turnovers in the Rain

This is the operational headache that catches operators off-guard every year. During tsuyu, high indoor humidity means laundry dries slowly even with a machine dryer running. Turnovers that normally take two hours can stretch to three or four when drying is part of the workflow.

A few ways to handle it:

Check your dryer capacity first. Many Japanese washer-dryer combos have underpowered drying cycles. Run a full linen load now and time it — don’t discover the limitation mid-season.

Laundry pickup services. Several Tokyo-based wash-and-fold operations collect, launder, and return within 24 hours. Worth having one on standby for properties without reliable drying capacity.

Staggered check-in/out times. A 12pm checkout and 4pm check-in gives you a much more comfortable buffer than 10am/2pm during the rainy season. Consider adjusting only for June and July.

Getting Maintenance Work Done Before June

If your property needs anti-mold treatment applied to walls, a ventilation fan replaced, or any damp-related work done, get quotes now. Tokyo contractors book up fast in May as property managers across the city run the same seasonal checklist simultaneously.

We use Aimitsu (aimitsu.benstay.jp) to collect and compare maintenance quotes across our properties — useful when you need to coordinate multiple units and don’t want to ring contractors individually for each one. Aim to have all work signed off and scheduled before May 31st if you can.

FAQ

Q: When exactly does Japan’s rainy season start and end?

Tsuyu timing varies by region and year. In Tokyo and most of Honshu, it typically begins in early June and ends around mid-July. Okinawa enters tsuyu earlier (mid-May) and exits earlier too. Hokkaido has no defined tsuyu season, which partly explains its appeal as a summer travel destination. The Japan Meteorological Agency (気象庁) announces official start and end dates each year — worth bookmarking and checking annually since timing shifts meaningfully from year to year.

Q: What’s the most common rainy-season guest complaint in short-term rentals?

Based on our review history: musty or stale smell, followed by visible bathroom mold. Both are preventable with consistent dehumidification and the ventilation steps described above. A ¥15,000 dehumidifier and a monthly anti-mold spray routine have essentially eliminated tsuyu-related smell complaints across our Tokyo properties.

Q: Do I need to do anything differently for Kyoto or Osaka properties?

The core checklist is the same. Kyoto properties with older construction — machiya-style townhouses in particular — face more severe moisture issues due to building age and materials. If you manage older properties, consider a professional damp assessment every two to three years. Osaka humidity patterns are similar to Tokyo; the main difference is that post-tsuyu temperatures rise faster, so the window for catching any damage before peak summer bookings is shorter.