Most of the heat that gets into a house comes through the windows, not through the walls. Tropical-home studies keep showing that glass hit by direct sun can account for a large share of a room's heat load. That's why an upstairs bedroom often feels like an oven by late afternoon: the roof has soaked up heat all day, and the glass faces a sun that's now low and glaring.
In Bekasi, daytime temperatures during the dry season can reach 33 to 36 degrees Celsius. That's the figure outside. Inside an upstairs room with west-facing glass, it can feel hotter still because the heat gets trapped. Most people's first reaction is to set the AC to 18 degrees and leave it running all day. The August power bill is usually the part that stings.
The core issue: dropping the AC temperature only treats the symptom. As long as heat keeps flowing in through the glass, the AC has to keep fighting it. Stopping heat at the point of entry is far cheaper than chasing it out over and over.
Why the Upper Floor Is Always Hotter
Two things stack up. First, heat rises. Warm air from downstairs drifts up and collects on the second floor. Second, the roof. Concrete slab or roof tiles absorb solar radiation all day, then radiate it back into the room below — right around evening, just as people are getting ready to sleep.
On top of those two, there's the window factor. Plain clear glass holds back almost nothing. Sunlight passes through it nearly whole and heats the floor, the mattress, and the wardrobe, which then release that heat slowly until late at night. That's why solutions aimed at the window deliver the most noticeable result for a top-floor bedroom.
First, Understand Two Numbers on Window Film
Before picking an option, two terms are worth knowing so you don't buy the wrong thing. VLT (Visible Light Transmission) is the percentage of visible light let through. VLT 40 percent means the room stays fairly bright; VLT 10 percent is much darker. Heat rejection is the percentage of solar heat turned away. That's the number that matters most for staying cool.
A common trap: a dark film doesn't necessarily reject the most heat. There are clear films with high heat rejection (the spectrally selective kind), and dark films that just dim the light without holding back much heat. So ask for the heat-rejection figure, not just how dark it looks.
Comparing the Options: Temperature, Cost, Trade-offs
No single solution wins on everything. The sensible move is to match the option to the window orientation and your budget. Here's the rough picture. The cost figures here are installed ballparks and will vary with brand, size, and installer.
| Solution | Effect on temperature | Cost range | Trade-offs |
|---|---|---|---|
| Solar / reflective window film | Noticeable, stops heat before it enters | ~Rp 75-250k/m² installed | Permanent, neat, keeps daylight; needs a fitter |
| Solar shades / blinds (roller, cellular) | Moderate, blocks heat after the glass | Varies, from hundreds of thousands per window | Adjustable, easy DIY; blocks only part of the heat |
| Thick blackout curtains | Moderate for light, mild for heat | Varies, fairly affordable | Great for full darkness when sleeping; dark even by day |
| Exterior awning / overhang | Strong, stops heat before it hits the glass | Varies by material | Most effective since it's outside; needs space & facade approval |
| Low-E glass (when building / replacing) | Strong and permanent | Highest upfront | Long-term fix; impractical to retrofit one window |
If you work it out roughly in rupiah per degree of cooling, window film usually lands as the most sensible middle for a finished bedroom: the cost isn't extreme, the result shows, and it stays tidy. Exterior awnings win on raw effectiveness, but not everyone has the space or the approval to install one, especially on the front facade of a cluster house.
How It Ties to the Electricity Bill
The logic is simple. An AC works by moving heat out of a room. The more heat that comes in through the glass, the more often the compressor switches on and the longer it runs to hold the same temperature. Once the incoming heat load is blocked at the window, the AC reaches the set temperature faster and then rests for longer.
The "rests for longer" part is what pulls the bill down. The compressor is the hungriest component in an AC. Cutting how often it has to restart hits the kWh you use directly. The size of the saving can't be promised as a fixed number, since it depends on glass area, orientation, and how often the AC runs. But the direction is consistent: less load, less AC work.
Which Windows Come First
So rather than filming every window at once, it's cheaper to order them by orientation:
- West first. West-facing windows take the harshest afternoon sun at its lowest angle, right when the family starts gathering at home. This is priority number one.
- East next. East windows catch the morning heat. Not as brutal as the afternoon, but they warm the room from the start of the day.
- North and south later. At our latitude these two sides get the least direct sun, so they can wait or make do with ordinary curtains.
- Combine on the hottest spot. For a west room that's genuinely bad, window film plus solar shades together give the most noticeable drop.
Install tips: clean the glass as spotlessly as you can before the film goes on — a single speck of dust makes a bubble. For film, an experienced fitter is worth the cost. For shades and blinds, make sure they cover a little past the window frame so heat can't sneak in from the sides.
Tying It Back to a Home Built to Stay Cool
These methods work best when the house itself already has good bones for airflow and window placement. Take Rumah Emerald 70 at Kingspoint Residence — a two-storey house with 70 m² of building on a 47.25 m² plot, priced in the Rp 700 million range, with installments from Rp 5 million a month. Its window openings can pair with film on the sides facing the afternoon sun without giving up morning daylight.
It sits on Jl. Raya Perjuangan, North Bekasi. If you want to go deeper on how air behaves inside the house, two other reads on this blog round it out: how to arrange cross-ventilation in a 70 m² two-storey house so it stays cool without AC, and a guide to choosing and placing AC for a two-storey house in Bekasi. The three reinforce each other: hold heat at the window, move air with ventilation, then let the AC finish the job with a much lighter workload.
So before the dry season peaks in July and August, it's worth prepping the upstairs rooms now. Start with the west window, and let next month's power bill prove the difference.
Want to see a cool two-storey home in North Bekasi for yourself?
The Kingspoint team can walk you through window layout, orientation, and an installment simulation for Rumah Emerald 70 — so you can plan comfort and budget from the start.
Ask the Kingspoint Team