Our living room and kitchen are one big open space, and the kitchen has a “kattegutchi” (a back/service door directly to the outside). Cold air pours in through it in winter. To cut that out, I built an interior storm window over the back door.
The cheap, standard DIY approach in Japan is to use twin-wall (hollow) polycarbonate sheet, but for this build I had two extra constraints:
・Don’t reduce daylight through the door
・It’s visible from the living room, so it has to look reasonable
So instead of twin-wall poly, I went with
a clear acrylic pane in a wood frame.
This write-up is for anyone who wants to build their own interior storm window and is willing to spend a bit more than the absolute cheapest option for the sake of how it looks. (The acrylic pane I bought was about 7,000 yen.)
For readers outside Japan: a kattegutchi is a small secondary door — typically off the kitchen — that opens directly to the outside, often used for taking out the trash or as a tradesperson’s entrance. They tend to be poorly insulated. In our case the door is essentially never used, so sealing it off behind a fixed interior storm window is a fine solution.
Before and after
The back door — before
This is the kitchen back door ↓
Open it and you’re outside, so cold air comes straight through in winter. We don’t actually use it as a door at all.
That said: I want to keep the daylight, and the door is in the living room’s line of sight, so the appearance of whatever I install over it matters too.
The storm window — after
Skipping ahead: here it is finished ↓
The acrylic pane fills most of the opening, so the daylight is essentially unaffected.
Building it
Basic structure
The plan: a simple wooden frame with a groove milled into the inner edge to capture the acrylic pane.
I don’t need this to open and close, so I’m skipping any sliding mechanism — it’s a fixed pane.
What if you don’t have a router?
The design above mills a groove into the frame so the pane drops into a slot. If you don’t own a palm router (or just don’t want to bother milling grooves), the structure below works just as well.
●No-groove version: capture the acrylic with two strips of trim instead
[Update]
I later learned that “grooved” lumber exists — it’s milled at the factory with a slot already running down its length. (The DIY YouTuber Kume Mari was using it in one of her videos.) That would be another good option.
Materials
For the frame I used 30 × 40 mm pine sticks (red pine “noubuchi KD”), 1985 mm long, 498 yen each at the home center — 3 of them.
The thinner stick at the top of the photo is a 12 × 12 mm hinoki (Japanese cypress) hobby stick, 128 yen. Strictly speaking I shouldn’t have needed it, but my acrylic pane wasn’t quite long enough, so I used it to extend the frame on one side (more on this below).
The acrylic pane is 2 mm thick, also from the home center.
It came as a single 930 × 1860 mm sheet for about 7,000 yen.
It wouldn’t fit in my car, so I borrowed the home center’s loaner kei-truck to get it home.
Cutting and assembly
Cutting the frame stock
First, the frame stock.
I marked the cut positions by holding each stick directly against the actual door opening.
Mistake
I’d regret this later, but I dimensioned the frame to fit absolutely tight to the opening with no slack.
Result: the assembled frame turned out a hair too big and wouldn’t fit, and I had to trim it after the fact…
Cut on the marks with a circular saw.
Milling the groove
Then I milled the groove for the acrylic with a palm router.
3 mm-diameter bit, 5 mm-deep groove.
I used the router’s edge guide so the groove ran parallel to the inside edge of the stick.
(The router opens up a lot of options for DIY, but the noise is the catch — it’s loud, mildly intimidating to use, and you worry about being a noise nuisance to the neighbors.)
Cutting the acrylic to size
I cut the acrylic to size next.
The home center’s acrylic counter sells a dedicated acrylic-scoring cutter for about 200 yen — and “dedicated” really does mean dedicated; it works well.
Run the cutter along a straightedge several times to score a groove, then snap the sheet over an edge — it breaks cleanly along the score.
I left the protective film on the acrylic during cutting to keep it clean.
Dry-fit time: I tested the cut acrylic against the frame (no fasteners yet).
The top half of the window seated perfectly.
The bottom half — the acrylic was too short.
To make up the missing length on the bottom, I used the 12 × 12 mm hinoki hobby stick. As in the next photo, I bonded it to the bottom rail of the frame to extend the captured edge inward.
Wood-glue (Titebond), clamp, leave overnight.
※ If you don’t want clamp marks on your wood, put a sacrificial offcut between the clamp and the workpiece. I didn’t bother and clamped directly.
Onto finishing.
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