Keeping the Changing Room Warm: Insulating Next to the New Bath (Matsugasaki, Part 4)

2026年10月28日 2026年10月28日

tsujikenzai

Location: Matsugasaki, Kyoto | Work: Insulation of the changing room beside the bathroom | Series: Matsugasaki Machiya Renovation, Part 4

1. The Space

Next to the new unit bath (see Part 3) is the changing room, called datsuijo in Japanese: the small space where you undress before the bath, with the washbasin and often the washing machine. Because this whole area was rebuilt from the framing up, we were able to plan the changing room properly: new wall framing, a new window for light and ventilation, and new hot and cold water lines for the washbasin.

[IMAGE 1: The changing room framed out beside the wrapped unit bath, with the new window and plumbing in place]

2. Why This Room Matters More Than It Looks

In Japan there is a well-known problem called heat shock: moving from a warm living room into a freezing changing room, and then into a hot bath, puts sudden stress on the heart and blood pressure. It is a serious concern, especially for older people, and it happens mostly in older, uninsulated houses exactly like this one. A warm changing room is not a luxury in a Kyoto winter; it is one of the most practical safety improvements you can make.

A room with a window on an exterior wall is also one of the fastest to lose heat, so this was the place to be thorough.

3. What We Did

Framed the new walls of the changing room while the adjacent bathroom work was under way, so both spaces were done together.
Installed a new window, then fitted glass wool insulation in every bay of the exterior wall, cutting it carefully to fill the spaces above, beside and below the window frame where cold air would otherwise enter.
Ran the water supply for the washbasin through the insulated wall, then closed the walls with new boards ready for finishing.

[IMAGE 2: Glass wool insulation fitted around the new window, with the washbasin hot and cold water lines below]

4. A Note on the Window

Since the window here was being replaced anyway, we fitted a new one rather than keeping an old single-pane aluminum sash. If you are renovating a changing room but keeping the original window, an interior window (a second window fitted on the room side) is the next best step, and it can qualify for Japan's window insulation subsidy depending on the glass used. We cover how we check whether an interior window fits in this article.

FAQ

Is it worth insulating such a small room?

Yes. Because the room is small, the cost is low, and because it is used every day right before and after bathing, the comfort and safety benefit is immediate.

Should the bathroom itself be insulated too?

A unit bath comes with its own insulated panels, so the bathroom is already far warmer than a traditional tiled bath. The changing room is usually the colder of the two.