Become A Duvet Connoisseur — Know Your Tog!

Dr Stuart Woolley
Apr 11 · 6 min read

Togs aren’t just a feel warm factor, they’re a well defined unit of how well you’re not losing valuable sleepy time heat.

Photo by Victoria Borodinova from Pexels

Never Judge A Duvet By Its Cover

Buying the wrong duvet can be extremely disappointing as if you’re losing heat too fast to the bedroom you’ll be chilly and if you’re not losing it fast enough¹ then you’ll be all hot and bothered. Either way it’ll be hard to nod off for forty winks, if you’re not at your personally comfortable temperature.

Duvets come in many shapes and styles, even before you put on your decorative cover of choice, and it’s best (really, bear with me) to take a scientific approach to this exercise. Trust me, I’m a scientist².

You may have come across the tog — a unit designed to convey the effectiveness of your duvet in terms of how warm (or cool, kind of, I mean it’s not an air conditioner or anything but it won’t make you sweat on a hot night is what I mean) it will keep you — but have you ever wondered how it’s defined and what it actually means?

It’s About Your Thermals

Imagine, if you will, that you’d like to come up with a measure of how effective something is of insulating you from the cold. You’d most likely want to know much heat it could prevent you from losing, right?

We’ll really need to think about the following,

  • An area to work with, in metres², over which we’ll measure things.
  • Some way to measure your temperature, we’ll use °K but we could equally use °C as they’re equivalent in terms of absolute temperature.
    Fahrenheit, no. Just no³.
  • And, finally, some way to measure energy.
    Let’s use Watts to measure the heat involved.

And that’s it, the basic SI unit of insulation and it’s known as the thermal resistance denoted by the letter R⁵ and it’s defined by the equation,

R = 1m².K / W

This is a measure of the difference in temperature between the two sides of whatever it is you’re measuring over 1m² of its area given an amount of heat provided in Watts.
The units are Kelvin Square metre per Watt. Remember that, you’ll need it later.

Consider the following duvet situation (I’ve always wanted to say that),

Credit: Author

The equation, if we keep you radiating at say 1W of heat and only consider 1m² of duvet⁶ becomes,

R = 1 . K / W

The only thing that’s varying is K, the temperature.

If the temperature difference is low that means a lot of the heat is getting through the duvet to its outer surface.
This is bad, you’re losing heat.
From the equation, If K is low then R gets low too.

If the temperature difference is high that means the heat is staying in with you, inside the duvet and not heating the room outside.
This is good, you’re keeping heat.
From the equation, if K is high then R gets high too.

Upshot: It’s a good duvet when its R is high.

From Thermals to Togs

No-one wants to see, for example,

Credit: Author (unhealthy interest in both units and duvets at this point)

Well, what happened? Well, back in the early 1900s the Shirley Institute in Manchester, England came up with the tog as an easy to understand measure of the thermal resistance, or as I call it the warmth of textiles in general or duvets in particular⁷.

I say easy to understand as m²K/W wouldn’t be helpful for most people on a duvet box, neither would it’s size. We, as humans, like easy to follow simple measurements that have a nice warm (no pun intended) touchy feely side to them. Marketeers know this too. Everyone could be a winner, and as it turns out they were! I love a story with a happy ending.

So, it was decided that,

1 tog = 0.1 m²K/W

Helpfully, by scaling down the units by an order of magnitude it gives a fairly friendly and easy to the eye scale of the warmth of duvets. Bigger the number, better the warmth.

There’s also, at least I think, a relation to degrees Celsius here in that higher numbers in this range just feel warmer and smaller ones feel colder.
Know what I mean? Yeah, I feel it too.

British duvets, for example, have the following ranges for different seasons of the year,

Duvet Type      Tog Rating
Summer 3.0 - 4.5
Spring/Autumn 7.5 - 10.5
Winter 12.0 - 13.5 or more

Of course you could have rated them after the seasons but giving a tog rating both gives it a more definite figure for personal preference and gives it that scientific gloss that we all know and like.

A Warm Fuzzy Feeling

“I put my faith in science”
- Moira, Overwatch

[1]: I’m in the slightly colder and windier part of Europe so this doesn’t happen often. I appreciate that you may be in a warmer climate, but rather than not having a duvet at all you’d prefer the comfort of some kind of covering at least. It’s kind of nice to feel enclosed when you’re sleeping, I feel, I hope you do too. Try it. I believe weighted blankets are a thing now too for this very reason.

[2]: I have numerous pieces of paper to prove it. Also, my mum told me.

[3]: You could do conversions and things. I won’t here as units I’ve always found to be tedious ever since encountering a rather units obsessed physics teacher in secondary school. Besides Fahrenheit eh, am I right?

[4]: Other units are available, but would you really want to use degree Fahrenheit square-foot hour per British thermal unit (°F.ft².h/BTU) eh?
No, nor me.

[5]: Sometimes R has the suffix SI to denote that we’re using SI units, which we are here, so I’ll just mention it and not put it in everywhere.

[6]: Yes, that’d be a small duvet, perhaps for a cat. If they’d stay under it long enough. But the quantities and measurements all scale up and standards, really no matter how tedious, are actually a Good Thing(TM).

[7]: It’s used for carpet underlay too, as it happens. Bit more niche though, I don’t know too many people with carpet to be honest. It’s all stone floors and the occasional laminate here.

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Dr Stuart Woolley

Written by

Worries about the future. Way too involved with software. Likes coffee, maths, and . Would prefer to be in academia. SpaceX, Twitter, and Overwatch fan.

The Shadow

We publish inspiring stories about different topics for a productive and entertaining life

Dr Stuart Woolley

Written by

Worries about the future. Way too involved with software. Likes coffee, maths, and . Would prefer to be in academia. SpaceX, Twitter, and Overwatch fan.

The Shadow

We publish inspiring stories about different topics for a productive and entertaining life

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