⚡ The short answer
A printed polyester curtain, like our SpongeBob curtains at €42.90, is a light-filtering curtain: it filters light and blocks views, but it doesn't plunge a room into darkness. For true blackout, you need to add a blackout lining behind the decorative curtain — this is the simplest and cheapest solution. And 80% of residual light does not pass through the fabric: it enters through the sides and top. A rod placed 15 cm above the window frame and extending 20 cm on each side does more for darkness than any fabric.
📑 In this article
- Sheer, light-filtering, blackout: three levels, three uses
- Measuring your window: the three dimensions to take
- Fullness: why 1.5 to 2 times the width
- Light leaks: where it all really comes down to
- Adding a blackout lining behind a printed curtain
- Grommets, pencil pleat, or rod pocket
- Washing, ironing, and colorfastness
- FAQ
There are two ways to mess up a bedroom curtain. The first is to buy a beautiful visual and find out that at seven in the morning in June, the room is as bright as at noon. The second is to buy a technically perfect, mouse-gray blackout curtain and end up with a child's bedroom that looks like a meeting room.
The good news is that it's not a choice. A decorative curtain and actual blackout are two distinct functions, carried out by two different layers of fabric, and there's no need to ask the same product to do both. This is exactly how hotel bedroom windows are set up: a sheer curtain, a decorative curtain, a technical lining, each with its own role.
This article explains how to achieve a truly dark bedroom while keeping a Bikini Bottom curtain on the wall. We start by clarifying the vocabulary, often used indiscriminately in product descriptions, then move on to measurements, fullness, and especially light leaks, which explain most disappointments. If you're primarily looking for standard dimensions, our guide how to choose curtain size covers that aspect, and the article on SpongeBob shower curtain material addresses the specific case of the bathroom.
Sheer, light-filtering, blackout: three levels, three uses
Commercial vocabulary mixes everything up. Here are the three real categories, and what to expect from them.
Sheer curtains
Light fabric, 40 to 80 g/m², transparent when backlit. It diffuses light, softens the ambiance, and prevents a clear view of the interior from the street during the day — but not in the evening, with the lights on. It has no darkening function.
Light-filtering curtains
This is the category for almost all printed curtains, including ours: a woven polyester of 100 to 150 g/m², opaque to the eye, which allows attenuated light to pass through. In a north-facing bedroom, this is often sufficient for sleeping. Facing south in summer, no. This level is suitable for a teenager's room, a living room, or a home office, and it's the one that best renders printed colors.
Blackout curtains
They feature an additional layer: either a coating applied to the back or a triple-weave fabric. The grammage increases to 250-300 g/m², and the fabric blocks 90 to 95% of incident light. We never talk about 100%: even a completely opaque fabric leaves the room in a dim light, not in absolute darkness, because light bypasses the curtain.
Why print and blackout don't mix well
A blackout coating is a layer of resin applied to the back of the fabric. It stiffens the drape, dulls colors, and doesn't hold high-definition prints well. This is the technical reason why curtains with detailed visuals, those where you can distinguish character expressions, are almost always light-filtering. Our article on printing SpongeBob products details this compromise between image sharpness and fabric thickness.
💡 Did you know?
A lined curtain also acts as insulation. The still air layer trapped between the two fabrics limits heat loss through the window, which represents a significant portion of a room's heat loss. Closed from dusk in winter, a lined curtain improves comfort near the window.
Measuring your window: the three dimensions to take
Never measure the curtain you are replacing: it may already have been the wrong size. Start with the window and the rod.
Rod width
This is the reference dimension, not the window width. Measure from one end of the rod to the other, excluding finials. If the rod is not yet installed, plan for the width of the frame plus 20 cm on each side.
Drop height
From the bottom of the rings or grommets to the desired finishing point. Three possible finishes: 15 to 20 cm below the windowsill for a short curtain, 1 to 2 cm above the floor for a neat ceiling-to-floor drop, or 2 to 3 cm too long for a pooling effect on the floor. In a child's room, arriving 1 cm from the floor is the safest: nothing drags, nothing gets stepped on.
Side clearance
This is the wall space available on either side of the window. It determines how far you can extend beyond the window, and thus the quality of the blackout. A radiator or a wall corner 5 cm from the frame severely limits possibilities: in this case, an indoor blind becomes the logical complement to the curtain.
Our SpongeBob curtains for the bedroom
Four visuals in printed polyester, to be lined for complete blackout.
Fullness: why 1.5 to 2 times the width
This is the most often ignored rule, and the one that makes the difference between a curtain that drapes well and a curtain that looks too small even though it's the right size.
The calculation
The cumulative width of the panels should be 1.5 to 2 times the width of the rod. For a 180 cm rod, you therefore need 270 to 360 cm of fabric in total, or two panels of 140 to 180 cm. A single 180 cm panel, stretched flat, will not create any folds and will let light through over the entire surface.
What fullness adds to blackout
Folds create extra thickness. Where the fabric folds, light passes through two or three layers instead of one. A light-filtering curtain installed with a fullness of 2 darkens significantly better than the same fabric stretched flat. It's free, you just need to order the right amount of fabric.
The case of a shared room
In a bedroom with two beds, fullness also allows for modulation: you can close one side completely and let a sliver of daylight in on the other. To furnish this type of room, our article on SpongeBob bedroom decor offers complete combinations, and the student room guide addresses small spaces.
Light leaks: where it all really comes down to
Here's the point most buyers discover after the fact: a perfectly opaque curtain in a poorly equipped room remains ineffective. Light doesn't pass through, it goes around.
From above
This is the number one leak. Between the top of the curtain and the ceiling, a band of light passes through and reflects off the white ceiling, illuminating the entire room. Remedy: install the rod 15 cm above the window frame, not just directly on it. The curtain also appears higher, which visually enlarges the room.
From the sides
Second source. There must be a lateral overlap of at least 15 to 20 cm beyond the frame on each side. A rod that stops at the edge of the window guarantees two vertical bands of light, exactly at eye level when lying down.
From below
Less serious, unless a radiator is blowing under the curtain and causing it to gape. In this case, a curtain that stops 15 cm below the sill is better than a long curtain that rests on the radiator.
At the central junction
Two panels that meet edge-to-edge leave a vertical gap. They should overlap by 10 cm in the center. This is one argument for a fullness of 2: you have the necessary fabric for this overlap.
🧠 Tip
To diagnose a room without buying anything, go in at noon, close the existing curtains, and wait two minutes for your eyes to adjust. The leaks will become obvious: you'll see exactly where the light is coming in. Nine out of ten times, it's the top and sides, not the fabric. The solution then costs the price of a longer rod, not a technical curtain.
Adding a blackout lining behind a printed curtain
This is the method that allows you to keep a Bikini Bottom visual and achieve true darkness. Three possible installations, from the simplest to the most elaborate.
Hook-on lining, on the same rod
A plain blackout panel is hooked behind the printed curtain, on the same rings or on a second set of hooks. Reversible installation, no DIY required, the lining can be removed in spring. This is the preferred option for rented accommodation.
Double rod
Two parallel rods: the blackout lining at the back, the printed curtain at the front. They are manipulated independently, which is practical for a daytime nap. This is the most comfortable setup to use, and it allows for pairing a short curtain with a long lining.
Blackout roller blind
Installed within the window recess, it covers the glass surface, and the printed curtain remains purely decorative on either side. This is the setup used in hotel rooms, and the most effective for high and lateral light leaks — provided the blind is surface-mounted and not installed between the window frames.
How much difference it makes
By switching from a single light-filtering curtain to a lined light-filtering curtain, you gain the equivalent of a full step: a room where you could clearly distinguish furniture at six in the morning becomes a room where you can only make out shapes. This step makes all the difference for a child who wakes up with the daylight.
Three versatile visuals
Patterns sober enough for an adult's bedroom, fun enough for a child's bedroom.
Grommets, pencil pleat, or rod pocket
The hanging system influences the appearance of the pleats, ease of sliding, and, indirectly, blackout effectiveness.
Grommets
Metal rings integrated into the top of the panel, through which the rod passes directly. Regular and deep pleats, very smooth sliding, installed in ten seconds. Weak point for blackout: the waves formed around the rod let in some light at the top. This is the most common system for printed curtains.
Pencil pleat tape
A gathering tape sewn to the back, into which hooks are inserted. Fullness is adjusted by pulling the cords. More work to install, but full control over the final width, and the top of the curtain remains flat against the wall, thus more light-tight.
Rod pocket
A simple hem through which the rod slides. Very good light seal at the top, but the curtain slides poorly: it must be moved by hand. Suitable for a curtain that is rarely opened, or for a fixed decorative panel.
| Configuration | Light blocked | Relative cost | Installation | Recommended for |
|---|---|---|---|---|
| Sheer only | Low | € | Immediate | Living room, daytime privacy |
| Printed curtain only | Medium | €€ | Immediate | North-facing bedroom |
| Printed + lining | High | €€€ | Hooks to add | Children's room, naps |
| Printed + roller blind | Very high | €€€€ | Drilling required | South-facing window |
Washing, ironing and color fastness
Bedroom curtains should be washed once or twice a year. More often if there's a pet, smoker, or open kitchen nearby.
Washing
Machine wash at 30 °C, synthetic or delicate cycle, spin cycle limited to 600 rpm. Use a mild liquid detergent, no bleach, no fabric softener: fabric softener leaves a film that dulls prints and makes the drape heavy. Remove hooks beforehand, they can pierce the drum and the fabric.
Drying
Absolutely no tumble dryer for printed polyester: heat combined with friction dulls colors and can create shiny marks. The right method is to take the curtain out while still damp and hang it directly on its rod. It will uncrease as it dries under its own weight, and ironing becomes unnecessary.
If you must iron
Iron on synthetic setting, on the reverse side, without direct steam on the print. A damp cloth in between prevents any risk of glazing. The same caution applies to all printed household linen, as explained in our SpongeBob product care guide.
Fading in the sun
This is the real enemy of a curtain. A south-facing window gradually discolors the exposed edge. The trick is to reverse the two panels once a year, left to right: the wear is distributed instead of concentrated.
⚠️ Mistake to avoid
Never fix the curtain rod directly to the window frame to "save space." The curtain will then bump against the handle, wear out at that specific spot, and the window will no longer open completely. Allow a minimum of 8 cm between the wall and the axis of the rod to allow for the handle and the opening clearance of the window.
Delivery and lead times
Curtains are shipped folded in a compact package: €5.90 shipping costs in France and the European Union, free delivery for purchases over €60 — a threshold that a pair of curtains almost always exceeds — and €7 outside the European Union. Allow 7 to 14 business days for delivery.
Upon receipt, the curtain will have pronounced packing creases. Do not try to iron them immediately: hang it up and let it hang for 48 hours; most creases will disappear on their own. To create a cohesive look, our bathroom guide and the article apartment decoration for adults show how to incorporate the theme without saturating a room.
Frequently Asked Questions
Are SpongeBob curtains blackout?
They are light-filtering curtains: an opaque printed polyester that significantly reduces light without plunging the room into darkness. For complete blackout, add a plain blackout lining behind, on the same rod or on a second rod.
What width curtain for a 120 cm window?
Plan for a curtain rod of approximately 160 cm, meaning 20 cm overhang on each side, and a total fabric width of 240 to 320 cm, or two panels of 120 to 160 cm each.
How high should I install the curtain rod?
About 15 cm above the window frame. This eliminates light leakage from the top and gives the impression of a higher ceiling. If the ceiling height allows, you can go up to midway between the frame and the ceiling.
Can I put a printed curtain in a baby's room?
Yes, with two precautions: a drop that stops at least 20 cm above the floor or above the windowsill, and no cord accessible from the bed. The same safety principle detailed in our article on baby plush toys.
Does the curtain shrink when washed?
A well-finished polyester hardly shrinks at 30 °C. The risk appears beyond 40 °C and especially in the dryer, where the fiber retracts and the hems pull.
How to avoid the light gap in the center?
Overlap the two panels by about 10 cm in the middle. This assumes you have ordered enough fullness: this is one of the reasons for the rule of 1.5 to 2 times the rod width.
Does a curtain improve thermal insulation?
Yes, moderately but genuinely, especially when lined. The air gap between the fabric and the windowpane slows down heat exchange. Closed at night in winter and during the day in summer on the south side, it significantly improves perceived comfort near the window.
What's the difference from a shower curtain with the same pattern?
Everything: the shower curtain is made of water-repellent polyester or PEVA, with reinforced eyelets and a weighted hem, designed for humidity. It doesn't drape like a window curtain. This topic is covered in our dedicated article on the material of shower curtains.
In summary
Darkening a bedroom is not a matter of magic fabric, it's a matter of installation. A printed SpongeBob curtain provides the visual and a first level of filtering; the rod, installed high and wide, eliminates light leaks which represent most of the residual light; an added blackout lining behind provides the final touch. All three together cost less than a high-end technical curtain, and allow the room to look good. Take your three measurements, apply the fullness rule, and the result will be good from the first installation.
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