# Vapor permeance of EPS not dependent on thickness?

| Posted in General Questions on

I came across this little blurb from Quick-Therm that essentially states the thickness of EPS does not affect its permeance.

They actually say:
“So, a slab of insulation subject to a vapor pressure gradient will exhibit the same permeability regardless of thickness.”
Which is no doubt correct because they used the word permeability…

but then they go on to say, “Making EPS insulation thicker does not change the rate of flow – it simply increases the distance water vapor can travel into the EPS insulation slab.”

Similar language found here:
https://quiktherm.com/how-insulation-works/

In the second link they say:
Making the insulation thicker doesn’t make it any less permeable – it simply increases the distance and time the water vapor has to travel.”

Maybe I have late-winter mashed potato brain going on, but wouldn’t increasing the distance and time slow the rate?

It seems like all other reading on this subject (at least in the building science world) implies that increasing thickness of EPS will lower its permeance. (But yes permeability— an intrinsic property—stays the same.)

## Join the leading community of building science experts

### Replies

1. | | #1

Another bit of context. They say:
"A 1” thick piece of material with a Perm rating of 1 will reduce to 0.5 Perm when the
thickness of the material is increased to 2”. The process however is not a static; it’s dynamic. "

So the claim is that, yeah, permeance is typically cut in half (or so) if the thickness is doubled, but that's only for 'static' situations. EPS is 'dynamic' ? This smells funny to me.

Oh man, I hadn't even looked at their coffee filter analogy. Doesn't make any sense and uses contradictory language.

2. Expert Member
| | #2

maine_tyler,

"Permeance indicates that water vapor transmission rate over the course of one hour through one square foot of a material of a given thickness at a specified vapor pressure, expressed in perms (gr/hr●ft2●inHg)."

"“Making the insulation thicker doesn’t make it any less permeable – it simply increases the distance and time the water vapor has to travel.”

Or in other words decreases its permeance.

3. Expert Member
| | #3

I'm with Malcolm on this one. If you double the thickness of the material, you'll reduce the flow thruogh it for a given pressure. Double the thickness gives you half the flow, quadruple the thickness gives you a quarter as much flow, etc.

You can easily demonstrate this with coffee filters. Pour some water into a paper coffee filter. Gravity provides the pressure. Measure how much water flows through the filter in a given amount of time. Now put two coffee filters together and try again. You should see somewhere around half as much flow through the double stack of filters in the same amount of time. This is the same basic principle that applies to EPS.

Bill

1. Expert Member
| | #4

"Multiple layers

How do you calculate the permeance of two layers of material? The formula is:

Total permeance = 1/ [1/(permeance of layer 1) + 1/(permeance of layer 2)]

A simple example is when a builder installs two layers of the same material — say, asphalt felt. In this simple example, the permeance of two layers of asphalt felt is half of the permeance of one layer of asphalt felt."

4. | | #5

It's funny you should bring up coffee filters Bill, because they use that as an example in the second link, but claim the following (see attachment):
"The permeance hasn't changed because moisture still moves through both coffee filters as easily as one. However, the permeability has decreased because it takes longer for the water to move through both."

Ok, so they just can't keep their story straight. They clearly have permeance and permeability mixed up here, whereas in the first link they have it right.

Ultimately, their statement that the rate is not changed by adding thickness but that it just 'takes longer' is downright silly.

I just can't tell if its some attempt at obfuscation for marketing purposes or if it's genuine confusion on their part...

There was a part of me that wondered if increasing thickness of foam acted differently than, say, doubling up on membranes. I suspect at a high level, there may be a bit of a difference if the transport mechanism varies. But so far as I can tell, the permeance value of a piece of EPS will still more or less track (inversely) proportional with thickness, unless there is data to show otherwise.

1. Expert Member
Deleted | | #6

Deleted

2. Expert Member
Deleted | | #7

Deleted

3. Expert Member
| | #8

"The permeance hasn't changed because moisture still moves through both coffee filters as easily as one. However, the permeability has decreased because it takes longer for the water to move through both."

They have just reversed the terms. It's the permeability that doesn't change. It's a characteristic of the material.

1. | | #12

They did, but that's not the only statement that is fallacy.

I think my biggest gripe is really their underlined statement:
“Making EPS insulation thicker does not change the rate of flow – it simply increases the distance water vapor can travel into the EPS insulation slab.”

Not to lean the nose into the stone too hard here, but there are articles on GBA that get this stuff wrong, and they apparently didn't see fit to correct them after it was pointed out they were in error. I am being a stickler on something that I'm sure some will argue is too 'in the weeds' but there's just too much confusion on this subject to not get a little frustration by published misinformation.

1. Expert Member
| | #13

maine_tyler,

I don't think your point is too far into the weeds at all. It's not too much to ask that an article that is going to use the two terms uses them correctly. From what you posted the Quick-therm stuff seems to be a mess, and just muddies the waters.

5. | | #9

I think there's a lot of caution that need be considered here. There's multiple scenarios that are slightly different, and easy to have misconceptions about.

One is about water vapor passing through a material at constant temperature. This would be the case if you had one or two layers of EPS inside, measure in a lab. where the temperature throughout each was the same, with only a vapor pressure gradient across them.

Another case is a temperature gradient, as well as a vapor pressure gradient across the material, as in the case of insulation where the inner surface is at a different temperature than the outer surface. That temperature gradient can essentially enhance or stall the vapor transmission mid material.

Edit:

The permenance or permeability reported are all derived form the ASTM E96 test. Those conditions maintain a 50% RH on either side of the dry or wet cup method. As a result, the values reported are useful in comparison between materials, but mostly useless for predicting how well they will perform in an environment given the state of (in/ex)ternal RH and the same for temperatures.

1. Expert Member
| | #10

Kyle,

I'm sure you are right, but I'm left trying to think of a situation we commonly encounter choosing or installing assemblies where it matters much. I think the rule of thumb that permeance halves when the material's thickness doubles works fine. Sometimes we wander off into the weeds on these things.

1. | | #11

All I was getting at is that using a single value for the permenance is misleading, but probably not harmful. Much like the static R values of insulation - we know that it changes, but it's not enough for most people to care about.

2. Expert Member
| | #15

The only place I can think of is subslab insulation, or possibly exterior insulation below grade on a foundation wall -- places where the insulation is essentially immersed in moisture, or even liquid water, for extended periods of time. In an above grade wall, the amount of moisture that will make it through EPS is going to be less as the EPS gets thicker. In an application like subslab insulation, time is essentially unlimited, so how much water can make it into the EPS AT ALL becomes more of an issue, since the EPS could be soaking for years in such an application.

As is often the case, what matters most depends on the application the product is being used for. It is exactly this reason that I prefer XPS for subslab applications, but I like EPS for things like rim joists that need a little bit of drying ability. Different applications require different products in these situations.

Earlier today I was hastily covering my just deliverd 7 foot high stack of polyiso with poly sheet to protect it from the freezing rain we're getting. Polyiso is especially bad about moisture absorption (compared to EPS and XPS), so I'm much more concerned with protecting it. It's going to be installed on the exterior of attic-facing studwalls though, where it will mostly get baked in a dry enviornment, where it works great.

Bill

2. | | #14

"Another case is a temperature gradient, as well as a vapor pressure gradient across the material, as in the case of insulation where the inner surface is at a different temperature than the outer surface. That temperature gradient can essentially enhance or stall the vapor transmission mid material."

I think trying to separate the vapor pressure variable from the temperature variable can lead to confusion. They never work in opposition, meaning vapor pressure will always move from higher to lower regardless of temperature. It's just that temperature has a direct relationship with VP. A lower temp yields a lower vapor pressure-- so a temperature gradient can almost always be equated with a VP gradient.
If the two gradients are in opposition, it simply means the vapor pressure is higher on the low temp side (a possible but not typical case).

1. Expert Member
| | #16

The most common exception is with basements and crawl spaces, where it's often the case that the side that is colder is also wetter.

• |
• |
• |
• |