What is an 'R-Value' and Why is it Important?

Back in the day, the biggest obstacle for a good night's sleep in the bush was creating comfort on the cold hard ground.
Throughout the night, the cold ground can rob your body of heat, which means a rough night's sleep and waking up tired and with little energy to tackle the next day.
Sure, you want the thickest and plushest mattress money can buy, but perhaps the most important function of your sleeping mattress is its ability to resist the cold. 
We measure this with an R-Value.

What does R-value on my sleeping mat mean?

Simply put, an R-value is the measurement of a certain material's thermal resistance, or how well it resists the transfer of heat across it.
The higher the R-value, the more insulation it will provide you, and the warmer you'll sleep.

R-value isn't just used in the outdoor industry. It's also commonly used in construction, more specifically, the insulation used in the ceilings and walls of homes all have an R-value.

When camping in summer, the R-value isn't something you need to be too concerned about as the ground doesn't get very cold before it warms back up  the next day again in the summer sun.

If you are camping in warmer conditions and find yourself using a mattress with a higher R-value, it won't make you hot unlike when you are using a sleeping bag with a much warmer rating than is needed.
It just means you will have an excellent thermal barrier between you and the ground and you'll be less likely to lose heat.

However, when you find yourself setting up camp on wet or snowy ground and the temperature drops, the R-value becomes a very important consideration.

So how is an R-value measured?

Back in 2016 a new standard of testing the R-Value was developed in order to create a consistent reading across brands and models.
This standard is known as ASTM F3340-18 and many high profile brands have adopted this standard across their sleeping mat ranges including Sea to Summit, Therm-a-Rest, Exped, Black Wolf, and more...

As a result of this new standard some sleeping mats have a new R-value. This doesn't mean the mat is warmer or colder than it used to be, it just means it can be more accurately compared to other mats as they are using the same universal scale to measure their product.

Note: Some brands may not test their products to the ASTM standard, or will choose to do so later. It's best to do your research here and double-check on the product page in-store on the packaging that they are using the ASTM standard if you are looking to compare mats or want the most accurate information.

How is R-Value tested?

R-value is measured by placing a hot plate (representing your body) and a cold plate (representing the ground) on either sides of a mat. Electronic sensors then determine the energy required to keep the hot plate at a consistent body heat.


A Therm-a-rest sleeping mat undergoing R-value testing

So what temperature is my mat rated to?

It's hard to say.
An R-Value is a measure of the transfer of temperature from one side of a material to the other. These numbers can't be translated to a temperature as there are many other factors (Are you a warm or cold sleeper? Do you know the ground temperature? And the air temperature? What is your shelter? Sleeping Bag? Other thermal clothing? etc.)
There are some guides for which R-values are suitable for each season and some brands even give an indication of the temperature rating, however this is usually used only as a guide.


So what R-Value do I need in my sleeping mat?

As a rough guide, here is what we would recommend when looking at values for sleeping mats based on the seasons you'll be expecting to use the mat in.

Summer

For sleeping in warm conditions an R-value of 0-2 will usually be suitable.
However, if you are using a mat with a higher R-value, it won't make you hot. It will just insulate you from the ground temperature underneath you. So if you find yourself in warmer conditions and you have a mat that is rated higher than you need, don't stress, you'll be fine.

The exception to this rule is any mat with a reflective layer inside that radiates heat. These may feel hotter in summer, however it's always better to be on the safe side.

Spring/Autumn

In mild weather conditions, a sleeping mat with an R-value of 2-4 will suit most trips. This is the most common range for general use mattresses.

Winter

If you anticipate to be spending a lot of time in cold weather on your camping and hiking trips, you may need a mat with an R-value of 4 or more.
Again, if you choose a mattress with a higher R-value than needed you won't overheat, but you'll just be more protected from the cold ground.


Camping and hiking over winter requires adequate insulation as the ground doesn't have a chance to warm up during the cooler days.

Alpine Conditions

For alpine conditions, you'll want a mat with an R-value of at least 6 and  potentially higher than that.
You'll also want to pair your mat up with a sleeping bag capable of keeping you warm in what can become extreme conditions as well as appropriate clothing.

If you're hiking you will likely be looking at keeping the weight down and looking at a lightweight, compact mattress. These ultralight models can have an R-value ranging from 0.7 for a non-insulated design, right up to 5.9 for a down-filled model.

Tip: The lighter the mat, the lower the R-value tends to be as there is less insulating material present to keep the weight down. (Foam, Synthetic Fibres, Duck/Goose Down)

If you are planning on hiking and sleeping in alpine conditions and are struggling to find a mat warm enough, you may consider stacking a few mats on top of one another to achieve a higher R-value to better insulate you from the icy ground.
As an exercise, if you placed a mat with an R-value of 4 on top of a lightweight closed-cell foam mat with an R-value of 1-2, you will achieve the insulation you need without the extra bulk and weight. This will also allow you to take part of the sleeping setup out of the pack when you are in warmer conditions, making your setup more versatile.


For Alpine conditions, you may even consider doubling up and using two sleeping mats to insulate you from the potentially icy ground underneath.
Most mats on the market will have an R-value between 0 and 9.5. An example of the highest rating mats is the Exped MegaMat which has an 9.5 R-value. This particular self-inlating foam mat is too heavy and packs away a bit larger to be considered for hiking, but it is a great option for car camping.
A guide of which R-Values are suitable for different seasons.

 

My mat doesn't have an R-Value, is that bad?

Plenty of high-quality mats will provide decent thermal resistance but don't list an R-value, this doesn't necessarily mean that the mat is inferior, it could simply be due to the fact that testing for the R-value can be an expensive process for the manufacturer.
However, if you are planning to use your mat on technical expeditions you might want to confirm the thermal capabilities of the mat, or even consider playing it safe and choosing a mat that has been tested.

How can I increase the R-value of my current sleep system?

If you're planning to venture somewhere that is colder than your current setup can handle, there are ways to improve the thermal resistance of your setup.

A simple way to achieve this is to add a thin foam mat underneath your mattress, or even use an emergency survival blanket to reflect heat back into the mattress or your body.
Essentially, you are adding more layers between you and the cold ground. 

Hopefully this has cleared up some things about R-Values and what they all mean to you and getting a good, warm night's sleep.
This should help you be able to make a better decision on sleeping gear to keep you properly insulated on future camping and hiking trips!

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