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What Is Hydrostatic Head And Why Is It Important When Camping?

Don’t you like waking up after a cold and wet night refreshed and ready to go? But what if the tent gets soggy and starts to leak in the middle of the night? This could make for a miserable morning.

Why are some tents better at keeping water out than others? To find the answer we must take a look at a property known as hydrostatic head. Let’s investigate.

Hydrostatic head is the pressure measurement at which water first penetrates a fabric like a tent. Measured in millimeters of liquid, the higher the number, the greater the hydrostatic head rating. Therefore, the higher the rating, the greater your tent’s water resistance, and a better chance of keeping you dry when in your tent.

Let’s take a closer look at hydrostatic head from a practical standpoint.

Why is hydrostatic head important when backpacking?

One of the most important things when backpacking is getting a good night’s sleep. Yes, you need to eat and stay hydrated, but after a couple of bad nights sleeping and you are dragging on the trail.

Backpacking tents have come a long way since the canvas ones that your grandparents used. Canvas tents were heavy, hard to set up, usually moldy, and not breathable. Worst of all they leaked. It was only time before the tent started to get that awful drip, drip, drip.

Fortunately, today’s tent materials are much lighter, compact, durable, breathable and you got it, water-resistant, and even waterproof.

But how do we know how good a tent is before purchasing it? Or if it is the right tent for “my type of camping”? Well, the answer lies in hydrostatic head ratings. The hydrostatic head provides us with an objective measurement for comparison.

For example, if we live in a wet part of the country, like New England, water resistance is very important to us. Also, living in areas where it snows a lot. Oh, yeah, New England again. We need a higher hydrostatic head rating for our tent material.

What is a good hydrostatic head measurement/ waterproof rating?

Depending on the tent that you hope to purchase for your first or fiftieth backpacking trip, you need to take into account the hydrostatic head. Like I mentioned earlier, you need to take into account the area of the country where you live and if you expect lots of rain, wind, or snow during your trip.

Hydrostatic head measurements for tents range between 1,000 mm and 5,000 mm in general. After reviewing several manufacturers, the hydrostatic head varies greatly between them.

So, what else do we need to look at in order to better understand which tent to buy?

What other properties are important in a tent? and why?

While the hydrostatic head is important in measuring the water-resistance of tent material, other factors also play a stake in comfort on a backpacking trip. They include tent weight, ease of setup, waterproof properties such as seams, rainfly, and openings, and ventilation.

Tent weight

We have a great article on how much a backpacking tent weighs. You can find it HERE. But why does tent weight matter anyway?

When embarking on a backpacking adventure it is important to carry as much equipment as you need at the lowest weight possible.

Years ago, I came across two guys who were traveling around the world. They had large canvas duffel bag backpacks. The packs must have weighed 80 pounds. It was just ridiculous how heavy these were. I remember thinking to myself, if I have to carry that much weight, I will never go backpacking…ever again.

Fortunately, for me, tents are much lighter, and because I am a minimalist at heart, I carry as little as possible without leaving anything of importance out.

Bottom line: The lighter the backpack, the better and more comfortable the trip. Remember, when traveling with others you can split the backpack with your mate. For example, you take the tent and tarp and they take the stakes, poles, and footprint. You decide.

Waterproof Seams Repel Water and Keep You Dry in Your Tent

You might think that all seams are sealed well and do a great job repelling water. Unfortunately, that is not the case. The manufacturer does not generally treat the tent seams at the factory. So, they require some attention from us when we buy a tent.

There are many seam sealers out on the market. We have included a few here from the Amazon collection:

  • Gear aid fast seam gear sealant: Gear aid provides waterproof protection with 1 to 2 uses per bottle. It dries clear and restores old UV protection.
  • Kiwi Camp Dry Heavy Duty Water Repellant: Kiwi Camp Dry provides a tough water barrier while still allowing the tent to breathe. Good for boots and backpacks as well.
  • Coleman 2000016520 2 oz Waterproof Seam Sealer: Coleman Waterproof Seam Sealer is fast drying and provides breathable laminate protection.

5 Steps to Waterproofing Your Tent from MSR

It is important to read the labels and instructions before you begin. You will need the seam sealer, wet rag, dry cotton rag, and well-ventilated space.

  1. Set up your ten fully
  2. To get started attach the rainfly in an upside-down position; stretch it as you would normally and attach it to the tent.
  3. Seal the seams: remember to seal all of the seams, lifting seams and completely sealing both the top and bottom of the seam.
  4. Remember to avoid adding sealant to the zippers. This will make them sticky and hard to operate.
  5. Once you complete the first pass wait twenty minutes and repeat the process.

When the sealer dries completely, it is bomb-proof. Also, remember to seal about once per year. If you use your tent often, then seal more often.

For these instructions in video format check this one out by MSR.

How to seal your tent by MSR

What is the difference between waterproof and water-resistant?

We all have gotten caught in a rainstorm without the proper jacket. Within minutes we know whether our clothes are waterproof (we stay dry) or water-resistant (we get wet).

By definition, waterproof material is impenetrable. We see this in the big blue polypropylene (poly) tarps. While poly tarps don’t let in any water and have water flow off of them like water off a duck’s back, they are not breathable, and therefore not a great idea to put directly over your tent in a rainstorm.

On the other hand, water-resistant means that your tent fabric is resistant to the penetration of water into the tent. Also, it does this most of the time, but not all.

So, if waterproof blocks all water and water-resistant lets some water in why wouldn’t all tents use waterproof materials. The answer is in a trade-off.

Just like you don’t drive an armored car every time you drive, you don’t need a waterproof tarp every time you camp.

Here are a few good reasons why nearly all tents are water-resistant and not waterproof.

  1. The trade-off with weight: water-resistant materials are lighter and therefore less stress carrying in your pack.
  2. Breathability: waterproof materials are not breathable. Water-resistant fabrics let the tent breathe and therefore allow for a modicum of ventilation.
  3. Quick-drying: a water-resistant tent only needs to be resistant to the expected weather. Meaning, if it doesn’t normally rain heavily in your area, you don’t need the higher resistance and generally heavier materials. More importantly, water-resistant materials, when breathable dry quickly.

Comparing Water-Resistance of 3-Season and 4-Season Tents

With so much talk about hydrostatic head, what should you look for when buying a tent? Before you buy your tent look at the hydrostatic head ratings and think about how you will use your tent.

Do you intend on backpacking in a rainy area like the Northwest or an arid climate like the Southwest? Will you camp in the summer, winter, alone or with a partner or group?

We have provided a few different options to look at. We compare seasonality: 3-season and 4-season tents, and ultralight and lightweight tents.

In addition we provided a comparison table for your review.

3-Season Lightweight Tents:

Kelty Late Start 2 Lightweight Tent

It surprises me that the hydrostatic head measurement varies wildly between tents. These tents range from 1800 to 5000 mm of resistance. I did include links to each of the tents below. (yes, most are affiliate links)

3-Season Ultralight Tents

Big Agnes Copper Ultralight Tent

As the weight gets lighter it appears to get less protection as well. While they are lighter, manufacturers note that they provide an outer coating to help improve protection.

4-Season Dome Tents

The North Face 2 Meter Dome 4 Season Tent
The North Face 2 Meter Dome 4 Season Tent

I just love the look of the 4 Season tents; Especially the dome tents. It reminds me of hiking big mountains. These tents are bomb-proof, well almost; The 2 Meter Dome provides excellent protection against the wind, rain, and snow.

They have double-wall construction and fewer vents. The Tents require fewer vents as it is generally less humid at altitude and in the cold. Also, fewer vents mean less chance of letting in the cold air.

How do manufacturers measure hydrostatic head?

In the past, manufacturers used an actual column of water to measure the height of water that first penetrated the material. However, as materials improved, the columns became too cumbersome.

Think of it this way. If a material has a hydrostatic head of 5000 mm, that column would be over 16 feet tall. Certainly, labs do not want to have extremely high ceilings, and the cost to heat that lab, astronomical!

Scientists introduced the hydrostatic head testing machine several years ago. This device measures the relative water-resistance of materials and fabrics.

The below video demonstrates how the hydrostatic head tester/machine works.

  1. Water is dripped slowly into a basin below. The water must ‘overflow’ the basin to ensure that it is full. 
  2. The material to be tested is placed gently over the water. The researcher takes care to remove any air bubbles between the fabric and the water.
  3. The device is lowered and locked into place.
  4. The device then applies slow, steady pressure. 
  5. Water droplets emerge on the top of the fabric when the hydrostatic head reaches its proper rate.   
  6. The researcher completes the test when he sees a third droplet emerge. He then records the pressure reading. 
  7. The researcher measures a final reading in millimeters of fluid.

Hydrostatic Head Tester for textiles

Check out this really cool (if you are a geek like me!) video from the Gester Company.

Gester Hydrostatic Head Tester

How does touching the inside of the tent affect hydrostatic head?

Whenever tell a kid to not do something, invariably, they what? Do something! I don’t know how many times I told my kids (when they were little) to not touch the inside of the tent when it was raining. They did and then giggled when the water dripped down their hands. This always called for a noogie!

If water can easily break through a tent just by touching the side of the tent, then what kept it out in the first place. The USGS explains surface tension.

“Common tent materials are somewhat rainproof in that the surface tension of water will bridge the pores in the finely woven material. But if you touch the tent material with your finger, you break the surface tension, and the rain will drip through.” USGS.

Does condensation inside your tent change the hydrostatic head rating?

Condensation forms when the air inside of your tent is warmer than the air outside of the tent. When the water vapor from the warm inside air contacts the cool tent wall, water droplets form at the surface.

But does this change the hydrostatic head rating?

The hydrostatic head rating is gathered when the tent wall contacts water from one side; the researcher adds pressure until water penetrates the fabric. Therefore, if it is raining, water droplets form on the outside of the tent. So, what happens if the water is on both sides, as in the case of condensation?

In this case, the water droplets on the inside of the tent act like you are touching the tent with your finger. The water droplets lower the surface tension and meet with the droplets from the outside of the tent. In conclusion, you are stuck with a soggy tent for the night.

If you do experience a rainy summer night when camping, it is best to eliminate as much moisture inside your tent as possible. Meaning, socks, boots, clothes you wore hiking, and any other wet materials should remain outside of the tent.

Also, opening windows and doors to ventilate the tent helps too. This reduces the likelihood of condensation. Most likely, the tent will need a good airing out and drying in the morning.

Final Words

Now that you understand everything you need to know about hydrostatic head, you can safely venture into the wild without fear of spending a soggy night in the tent. Also, you will know more than the salesperson at REI, EMS, or Kittery Trading Post up in Maine.

Have a great next trip!