Updated 11 March 2026
Most parents choose a waterproof sheet to protect against mould. That's a good reason. Now science has since added another.
For a long time, the case for waterproofing was simple: milk and moisture seep into mattresses, mould grows, and the mattress needs replacing. A waterproof layer keeps fluids at the surface. Practical, obvious, done.
What research has since shown is that the waterproof layer also affects how CO₂ behaves in the sleep environment. It's a quieter benefit, and one that wasn't part of the original conversation. It is now.
THEN: PROTECTING THE MATTRESS FROM MOULD
Babies spit up. It's one of the most predictable features of early infancy. Even a small amount of fluid soaks through a sheet and into the mattress, mould growth can begin surprisingly quickly — sometimes within 24 to 48 hours.
Mould spreads through microscopic spores that thrive in damp, warm environments. Once established inside a mattress it can't be reliably removed at home and the mattress needs to be replaced.
Mould inside a mattress also affects air quality in the sleep space. Developing lungs are particularly sensitive to airborne spores, a key reason to keep the sleep environment clean and dry.
A waterproof layer keeps fluids from reaching the mattress so the sleep surface stays clean, dry and hygienic. That job hasn't changed.
NOW: WATERPROOF SHEETS AND CO₂ REBREATHING
Research into infant sleep environments has identified CO₂ rebreathing as a possible contributing factor in some sleep-related infant deaths. When a baby exhales, small amounts of CO₂ can remain near the face if the surrounding surface absorbs that air. Young infants also have a limited ability to reposition themselves in response.[1,2]
Mechanical studies have measured CO₂ levels below 5% on waterproof mattress surfaces, compared with above 25% on soft, porous materials such as sheepskins.[2,3] An impermeable waterproof layer stops exhaled CO₂ from being absorbed into the mattress at all. It stays at the surface, where normal room air disperses it.
This works together with mattress firmness. A firm mattress keeps the sleep surface flat so a baby's face doesn't sink into the bedding, while the waterproof layer prevents the mattress from absorbing exhaled air.
This applies to both waterproof cot sheets and waterproof bassinet sheets. The sleep surface principles are the same regardless of product type.
Most parents choose waterproof sheets for hygiene. As it turns out, that same choice also supports the breathing environment around your baby — making the safest option the simplest one.
Standards Australia has since developed AS 5407.3, a CO₂ mitigation standard for infant sleep surfaces. The mandatory 2026 infant sleep standard reflects where the evidence and the industry are heading.[1]
WHAT MAKES A WATERPROOF COT SHEET ACTUALLY WORK
A sheet with a PUL (polyurethane laminate) membrane creates a waterproof barrier between your baby and the mattress. The same technology is used in modern cloth nappies and medical-grade textiles.
Protectors that rely on quilted or padded fabrics without a waterproof membrane can absorb spills, but moisture can still reach the mattress. Thicker padding also softens the sleep surface. Australian safe sleep guidelines recommend a firm, flat surface with tightly fitted bedding — bulky protectors compromise that.
A low-profile waterproof membrane with absorbent cotton on top gives you both: hygiene protection and a surface that stays firm.
Regular sheet on the outside. Superhero on the inside.
What to look for on the label
- Built-in waterproof layer, not a separate underlay
- PUL membrane, independently tested for CO₂ rebreathing safety
- Oeko-Tex 100 certified membrane, free from harmful substances
- GOTS-certified organic cotton on the sleep surface
- Hot-washable to 60°C and dryer-safe on delicate
HOW TO GET THE MOST FROM YOUR WATERPROOF COT SHEET
Buy two or three and layer them
Layer two waterproof sheets directly on the mattress. A 2am change means pulling off the top sheet rather than remaking the cot from scratch. One of those small things that makes a real difference.
Change bedding promptly
Damp patches against skin can cause irritation. Wash at a minimum of 40°C on a full cycle. Quality PUL sheets go in with the rest of the laundry; no soaking or special treatment needed.
A quality waterproof sheet should be silent, soft, and invisible to the touch. If it crinkles, it's an older waterproofing approach.
FREQUENTLY ASKED QUESTIONS
Do babies need a waterproof mattress protector?
Most nurseries use a waterproof layer to protect the mattress from spills, leaks and spit-up. This can be either a waterproof mattress protector or a fitted sheet with a built-in waterproof layer. The important thing is that the sleep surface remains firm, flat and tightly fitted, in line with Australian safe sleep guidance.
Can't I just use a separate mattress protector underneath?
Yes. You can use a waterproof mattress protector placed over the mattress with a fitted sheet on top. The important thing is that the sleep surface remains flat, firm and tightly fitted, with no bunching or loose fabric.
An all-in-one waterproof fitted sheet combines both functions in a single layer, which simplifies bedding changes and keeps the sleep surface smooth.
Do I need both a waterproof sheet AND a mattress protector?
No. A fitted waterproof sheet with a built-in PUL membrane already provides mattress protection, so a separate protector is not necessary.
The advantage of an all-in-one waterproof fitted sheet is that one layer performs both jobs, helping maintain a firm, uncluttered sleep surface.
Is a waterproof sheet safe for breathing?
Yes. A properly designed waterproof sheet works together with a firm mattress to maintain a stable sleep surface.
The waterproof membrane prevents the mattress from absorbing exhaled air, allowing it to disperse at the surface instead. Current Australian guidance also emphasises a firm, flat sleep surface with tightly fitted bedding.
The term breathable has no standardised definition in Australian infant sleep regulation and is not used as a regulated safety claim.[4]
Will a waterproof sheet make the cot too hot?
A PUL-based sheet allows air movement, so it doesn't trap heat the way older plastic-backed options did. If you're unsure whether your baby is too warm, check the back of their neck.
What should I look for in a waterproof cot sheet?
Look for four key features:
- A built-in PUL waterproof membrane rather than absorbent quilting alone
- Independent testing for CO₂ rebreathing performance
- OEKO-TEX® Standard 100 certification for harmful substances
- High-quality cotton fabric that provides absorbency without bulky padding
These features indicate a sheet designed specifically for infant sleep surfaces rather than general waterproof bedding.
What bedding should a baby sleep on?
Australian safe sleep guidance recommends a firm, flat mattress [5] with a tightly fitted sheet and no loose bedding in the sleep space. Pillows, doonas, bumpers, soft toys and loose blankets should not be placed in the cot. Keeping the sleep surface clear reduces the risk of anything covering a baby’s face during sleep.[1,6]
How tight should a cot sheet be?
A fitted cot sheet should pull tightly over all four corners with no loose fabric on the sleep surface. If it bunches or slips off the corners, it's the wrong size.
Is one waterproof sheet enough?
No. You need at least two. When a sheet gets wet or dirty, you need a clean replacement immediately — often in the middle of the night. Many parents prefer three so there’s always one on the cot, one in the wash, and one ready to go.
How often should I replace a waterproof cot sheet?
A quality waterproof sheet holds its integrity across hundreds of wash cycles. If liquid starts seeping through the fabric or the membrane shows signs of wear, it's time to replace.
THE BOTTOM LINE
Waterproof sheets were always the right call for hygiene. The CO₂ research confirms they're doing more than that.
Look for a fitted sheet with a built-in PUL membrane, independent CO₂ rebreathing testing, Oeko-Tex 100 certification, and organic cotton on the sleep surface. Those four markers mean it's been made properly.
After that, the hardest decision should be choosing a colour.
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About the author
Kellee Eriksson is an Emergency Nurse with 15+ years of experience and an INPAA Baby Safety Ambassador. She participated in the development of Australia's mandatory 2026 infant sleep surface safety standards and founded Little Human Linens to translate safe sleep science into products parents can actually use.
References
[1] Moon RY; American Academy of Pediatrics. Sleep-related infant deaths: Updated 2022 recommendations for reducing infant deaths in the sleep environment. Pediatrics. 2022;150(1):e2022057990.
[2] Kemp JS, Thach BT. Quantifying the potential of infant bedding to limit CO₂ dispersal and factors affecting rebreathing in bedding. J Appl Physiol. 1995;78(2):740–745.
[3] Kemp JS, Livne M, White DK, Arfken CL. Soft bedding and rebreathing potential during sleep. Pediatrics. 1998;102(3):e29.
[4] Australian Competition and Consumer Commission (ACCC). Consultation on Infant Sleep Products and Breathability Claims. Product Safety Australia. productsafety.gov.au
[5] Hauck FR, Herman SM, Donovan M, et al. Sleep environment and the risk of sudden infant death syndrome in an urban population: The Chicago Infant Mortality Study. Pediatrics. 2003;111(5):1207–1214.
[6] Red Nose Australia. Safe Sleep: What is a Safe Mattress? rednose.org.au
