The sleep sack label says 2.5 TOG. What does that mean for a chilly nursery, and what should your baby wear under it? For my baby, the onset of winter was a very stressful time. I was constantly checking to see if they were too cold but had no idea how to tell. 

For most winter nurseries, this rating is fine. In a room between 61°F and 68°F (16°C to 20°C), a 2.5 TOG sleep sack over a long-sleeve bodysuit or footed sleeper keeps most babies comfortable. Colder than that, add a clothing layer underneath. Warmer, drop to 1.0 TOG.

The chart below is a neutral reference. It isn’t published by a sleep sack seller, so it isn’t tuned to anyone’s product range.

What Does a TOG Rating Actually Measure?

TOG is a measure of thermal resistance. In plain terms, it tells you how well a fabric holds in heat. According to The Lullaby Trust, a higher TOG number simply means a warmer product.

Thermal Resistance, Not Thickness or Quality

A higher TOG isn’t a better sack. It’s only a warmer one.

If you want the backstory on the scale, this spring and summer TOG guide walks through the basics.

Why a Thicker Sack Isn’t Automatically Warmer

• Warmth comes from trapped air, not bulk, so two equally thick sacks can carry different ratings. Go by the label.

What TOG for Winter? The Room Temperature Chart

Find your nursery’s temperature in the left column, then read across.

How to Read the Chart

• Measure the room temperature instead of relying on your weather app. A heated apartment in January can run warmer than a drafty house in October.
• At 18°C (about 64°F), a 2.5 TOG over a long-sleeve bodysuit or sleeper is the usual fit.
• The recommended room temperature for babies is 16°C to 20°C (61°F to 68°F). That’s exactly where a 2.5 TOG does its best work.
• Even The Lullaby Trust won’t publish a TOG chart, because every fabric, layer, and baby is different. Treat each row as a starting point, not a rule. Some babies naturally run warmer, and some prefer to be cozy. 

Where to Measure the Temperature

• Use a room thermometer. Guessing is unreliable, especially at night.
• Place it near the crib at mattress height, away from windows, vents, and heaters.

How Can I Tell if the Baby is Warm Enough?

I didn’t know that hands and feet are bad indicators of whether the baby is comfortable. They tend to run colder than how they’re actually feeling. Instead, check the back of their neck or place a hand on their chest.

• Baby is  too cold if the skin on their abdomen or the back of their neck feels cold.

• If they’re extra fussy and keep curling up on their tummy and tucking their hands and legs under them.

• Baby’s too hot is when their skin feels hot, sweaty, or clammy. 

• Their cheeks are flushed, and their hair is damp. 

2.5 TOG vs 1.0 TOG: Where Does the Crossover Sit?

Most families only need two sacks for the cold months: a 2.5 and a 1.0. Here’s how they compare.

Why the Overlap Zone Exists

• Between 68°F and 72°F (20°C to 22°C), both sacks can work. The layer underneath tips the balance.
• A 2.5 over a short-sleeve bodysuit and a 1.0 over a footed sleeper end up in a similar place.
• Pick based on which way your room drifts. If it cools off overnight, lean toward the 2.5. If the heat runs high, lean toward the 1.0.

What Counts as a Layer, and What Doesn’t?

Many blogs use the word ‘layer’ over and over again without explaining what it means or what counts as a layer for babies under 12 months. 

Layers That Count

• Bodysuits, footed sleepers, pajamas, and pants worn with a bodysuit
• For a season-by-season breakdown, see how to dress your baby for sleep in summer vs. winter.

Things That Don’t Count, or Shouldn’t Be Added

• A second sleep sack. Two 1.0 sacks don’t equal a 2.0, because the air trapped between them adds extra warmth.
• Loose blankets. U.S. safe sleep guidance keeps blankets out of the crib and suggests a wearable blanket or extra clothing layer instead.
• The fitted sheet. It doesn’t change the sack’s TOG, but it still affects warmth (more on that next).

What Doesn’t TOG Account For?

A TOG number only describes the sack. Your baby’s real warmth comes from everything around them.

One landmark UK study added up babies’ full clothing and bedding. For babies older than about 10 weeks, risk rose sharply once the total passed 10 tog, compared with under 6 tog. That’s why the whole setup matters, not just the label.

The Sheet and Mattress

• A synthetic sheet or a plastic-backed mattress protector can hold heat against your baby’s back.
• A breathable cotton sheet lets that heat escape. This look at breathable crib sheets for warm climates explains what to check, and the same logic applies to a heated winter nursery.

Airflow, Humidity and Heat Sources

• Keep the crib away from radiators, space heaters and vents, as The Lullaby Trust advises.
• Humid air makes it harder for the body to shed heat, so lean lighter in a damp or steamy room.

What Are the Most Common Winter TOG Mistakes?

I’ve made a couple of these myself. 

Sizing Up “to Last the Winter”

• A roomy sack can let your baby slip down inside it.
• Most sacks are sized by weight, not age, and should fit snugly at the neck and shoulders.

Putting a Swaddle Under a 2.5 TOG

• A swaddle is a layer, and a warm one. Stacking it under a 2.5 can push your baby well past what the room needs.
• Swaddling should also stop once your baby starts to roll, usually around 3 months.
• If your baby still needs a swaddle, choose one rated for the room on its own.

Using the Night TOG for Daytime Naps

• Daytime rooms often run warmer from sunlight and household heat.
• Check the thermometer before naps too. You may need the 1.0, or the same 2.5 over a lighter layer.

What If the Room Temperature Swings Overnight?

Many homes cool several degrees by dawn. One TOG can still cover the night if you plan for the range.

• Check the thermometer at your baby’s bedtime and again before you go to bed.
• Both readings in one chart row? You’re set. Dipping into a colder row? Add a clothing layer at bedtime, not a blanket later.
• If you leave the heat on, keep it low. The Lullaby Trust suggests no higher than 20°C (68°F) and notes an extra layer usually does the job instead.
• Do a quick chest check at the late-night feed. Hot or sweaty skin means one layer comes off. Cool hands and feet are normal.

Building a Winter Sleep Setup That Stays Just Right

Winter sleep doesn’t need a drawer full of sacks. A 2.5 TOG, a 1.0 TOG, a room thermometer, and a few base layers cover almost every night.

Match the sack to the room, adjust with clothing, and trust the chest or neck check over any chart, including this one.

Why can a chart like this stay neutral? Joey & Joan doesn’t sell sleep sacks. What Joey & Joan makes is the layer under everything else: fitted sheets in 100% jersey cotton, OEKO-TEX certified, cut to fit specific bassinet, crib, mini crib, and playard models so they lie flat instead of bunching into warm folds. Find the sheet that fits your baby’s sleep space.

Frequently Asked Questions

Is a 3.5 TOG sleep sack ever a good idea?

Only in genuinely cold rooms, within the maker’s stated range. In most heated homes, a 2.5 plus an extra clothing layer is enough.

Can my baby wear a sleep sack in the car seat?

No. The AAP advises against bulky layers under harness straps, so buckle your baby in thin clothes and lay a blanket over the straps instead.

Are weighted sleep sacks safe?

No. Safe sleep guidance lists weighted blankets and swaddles among the items to keep away from a sleeping baby.

Should my baby wear a hat to bed on cold nights?

No. Indoor hats can make babies too hot, so take them off inside, including for sleep.

Does a bassinet need a different TOG than a crib?

No. TOG follows the room temperature, not the sleep space. Just make sure the fitted sheet matches that exact mattress.

When can my baby stop using a sleep sack?

There’s no set age. Many toddlers keep wearing one well past their first birthday.

Sources

1. Room Temperature (The Lullaby Trust) – The charity’s recommended nursery temperature range and advice on overnight heating and heat sources.
2. How to Dress Your Baby for Sleep (The Lullaby Trust) – Guidance on sleep bag fit, layering, and why the charity does not publish a TOG chart.
3. Ways to Reduce Baby’s Risk (NICHD Safe to Sleep) – U.S. safe sleep recommendations on overheating, bedding, swaddling, hats, and weighted products.
4. Interaction Between Bedding and Sleeping Position in SIDS (BMJ, via PubMed) – A population-based UK study linking total clothing and bedding insulation to sudden infant death risk.
5. Winter Car Seat Safety Tips (HealthyChildren.org, AAP) – AAP advice on keeping babies warm in car seats without bulky layers.