Perm Classifications

[C. Furtaw]

I have a few questions concernig Perm Ratings. WUFI, in their material data base has a perm rating (perm in) for spun bonded polyolefine of 0.392 and an 8" concrete masonry unit (CMU) (pumice aggregate) of 32.30. The Tyvek web site has a perm rating for the commercial Tyvek of 28.
ASHRAE a perm rating for a cored limestone CMU of 2.4 gr/h-ftsq. - in. HG. I thought the conversion factor from the above units to US Perms is 3.4 /1000 ft sq./24 hr. -in- HG. Therefore the limestone CMU would have a perm rating of 3.4*x*2.4 or 8.16 Perms.
First, why is there i big difference between what Tyvec publishes and WUFI uses for its Perm rating?
Second, is my calculation correct for the limestone CMU? In regard to Perm ratings for CMUs, does anyone have any correlation/information for other types and thickness of CMUs?
Will the Perm rating of the pumice CMU be different for a 12" CMU although the face shell is the same thickness only the core is wider?
Thanks,
C. Furtaw

[Thomas]

WUFI, in their material data base has a perm rating (perm in) for spun bonded polyolefine of 0.392 [...] The Tyvek web site has a perm rating for the commercial Tyvek of 28.

Dear Mr. Furtaw,

the number given by WUFI is not a perm rating (measured in perms), but the permeability of the material, measured in perm inch.

The perm rating (a.k.a. permeance) describes how much water vapor will pass through a given layer of a material, having a given thickness. The permeability, on the other hand, describes the vapor diffusion properties of the material, independent of the specimen shape or size. Divide the permeability by the layer thickness to obtain the perm rating.

For example, consider a spun-bonded polyolefin membrane with a thickness of 0.00787 in. The spun-bonded polyolefin material has a permeability of 0.392 perm in. Dividing this by 0.00787 in gives you the perm rating of this specific membrane: 49.8 perm, which seems to agree well with typical Tyvek ratings given on their website.

and an 8" concrete masonry unit (CMU) (pumice aggregate) of 32.30. ASHRAE a perm rating for a cored limestone CMU of 2.4 gr/h-ftsq. - in. HG. I thought the conversion factor from the above units to US Perms is 3.4 /1000 ft sq./24 hr. -in- HG. Therefore the limestone CMU would have a perm rating of 3.4*x*2.4 or 8.16 Perms.

If the "gr" in "gr/h-ftsq. - in. HG" means grains, not grams, then I think those should already be perms. One perm is one grain (avoirdupois) of water vapor per hour flowing through one square foot of a layer, induced by a vapor pressure difference of one inch of mercury across the two surfaces. If this is correct, then the perm rating of the cored limestone CMU is simply 2.4 perm. To find the perm rating of the pumice aggregate CMU, divide the permeance of the concrete material (32.2 perm in) by the thickness of the concrete blocks to find the perm rating of an 8 in thick block of pumice aggregate concrete: 32.2 / 8 = 4.0 perm.

In regard to Perm ratings for CMUs, does anyone have any correlation/information for other types and thickness of CMUs?
Will the Perm rating of the pumice CMU be different for a 12" CMU although the face shell is the same thickness only the core is wider?

All other things being equal, the perm rating of a 12" pumice CMU will be two thirds of an 8" pumice CMU: 32.2 / 12 = 2.7 perm, whereas 32.2 / 8 = 4.0 perm.
Sorry, I don't understand what the 'face shell' and 'core' are about. If the two different sizes of concrete blocks themselves are composed of different layers of concrete with different quality, then the perm ratings of the layers have to be determined individually, their sum will be the perm rating of the complete block

Regards,
Thomas

[C. Furtaw]

Thomas, First let me thank you for such a thorough reply. The gr does mean grains.
I am not sure you are correct about the perm rating of an 8" or 12" CMU. All (almost all) CMUs are hollow. The face shell is the face of the CMU unit and is about 1 1/4" thick. That is the same for the 8" and 12" CMU. Therefore, the only difference between an 8" CMU and 12" is the size of the core or void between the front and the back The CMUs also have two end face shells and a web betwenn the two cores. It dosen't seem correct to divide the perm rating of the 8" CMU by 8 since there is only 2 1/2" of material from the front to the back, the rest is a void. Should the air be taken into accont as far as the perm ratin is calculated?
The thickness of a CMU may vary but the the face is 8" high x 16"long. The thicknes may vary fro 4" to 16"
C. Furtaw

[Thomas]

Okay, if they are hollow, then the perm rating is determined mainly by the two face shells. Furthermore, I was mistaken when I said the perm rating of a block composed of different layers is the sum of the perm ratings of the individual layers; rather, the reciprocal of the perm rating of the block is the sum of the reciprocals of the individual perm ratings (I was fooled by the corresponding SI quantities which are simply added).

Let's try again with an 8" CMU. With an assumed permeability of 32 perm inch for the concrete, each face shell has a perm rating of 32/1.25 = 25.6 perms. The air space is 5 1/2" thick, the method used in WUFI for estimating the perm rating of air layers gives 273 perms for such an air layer. The perm rating of the complete CMU is then

1/( 1/25.6 + 1/273 + 1/25.6) = 1/0.0818 = 12.2 perm.

Ignoring the contribution of the air layer, we have

1/( 1/25.6 + 1/25.6 ) = 12.8 perm.

So for many applications and considering that the permeability of the concrete is also known only within some uncertainty, the air layer may be ignored.

For the 12" thick CMU, the result would be practically the same.

Of course, this calculation is only valid for a cross-section of the CMU that includes the air space. A cross-section close to an end where there is only concrete would have a much lower perm rating, an the permeance of the wall as a whole would be some kind of weighted average of these two cases.
So which number to use in a one-dimensional calculation depends on what you are interested in. If you want to look at the moisture conditions at a spot that sits in the middle of a CMU face, the numbers for the cross-section including the air space should be used. For a spot sitting on a mortar joint, a number representative for pure concrete or mortar would be more appropriate (possibly slightly adjusted to account for the influence of the near-by air space). If the details of the wall structure are not important and the wall just acts as a general resistance to vapor trying to get into or out of the wall, an appropriately weighted average of wall parts with and without air spaces would be representative for the vapor permeance of the wall as a whole.

Regards,
Thomas

[C. Furtaw]

Thomas,
I agree and a great answer. I therfore think that the perm rating of the CMU would be the sum of the percentages that are contributed by the cored areas and the two end shells and the center web.
How does this sound?
Regards,
Charley

[Thomas]

I therfore think that the perm rating of the CMU would be the sum of the percentages that are contributed by the cored areas and the two end shells and the center web.
How does this sound?

As a first estimate, this sounds quite reasonable. If the total area of the CMU is A, the area of the core is A1 and the area of the end shells plus web is A2 (A = A1 + A2) then the vapor fluxes through the respective areas are

G1 = A1 * P1 * dp
G2 = A2 * P2 * dp,

where P1 and P2 are the respective perm ratings and dp is the vapor pressure difference across the CMU. The total vapor flux is then

G = G1 + G2 = (A1 * P1 + A2 * P2) * dp = A * (A1*P1 + A2*P2)/A * dp
= A * P * dp,

where P = (A1*P1 + A2*P2)/A is the effective perm rating of the whole CMU.

Strictly speaking, this is not entirely correct, since part of the vapor flux that enters the end shells from one side will not pass through the entire end shell to exit the CMU on the other side. It will take the easier way and pass from the end shell (or the center web) into the core and from there to the outside. The flux coming out from the end shells and center web will therefore be somewhat less and the flux coming out from the core will be somewhat more than computed above, changing the effective perm rating. This is basically a two-dimensional effect, depending on the geometry of the competing paths and the difference in the respective perm ratings. The size of the effect could be determined by a two-dimensional hygrothermal simulation, I don't know a simpler and generally applicable method. However, considering that the material data used in calculations will always differ more or less from the properties of the actual batch of material used on-site, the uncertainty introduced by the one-dimensional estimate for the effective perm rating described above will probably vanish among the other uncertainties connected with real-life material data.

Regards,
Thomas

[C. Furtaw]

Thomas,
Thanks, I think the topic is now well covered.

Regards,
Charles Furtaw

[C. Furtaw]

Thomas,
One last question (I think). What would be a value for the pressure difference, dp?
Thanks,
Charley

[Thomas]

What would be a value for the pressure difference, dp?

This is the difference between the water vapor partial pressures on the two surfaces of the component. At a given temperature, the partial pressure cannot be greater than the saturation vapor pressure, for example 0.694 in Hg at 68°F, or 0.180 in Hg at 32°F. So these would also be the maximum possible pressure differences at the respective temperatures, assuming saturated air on one side of the component and completely dry air on the other side.

A more realistic example would be air at 68°F and 50% relative humidity on one side and air at 32°F and 90% RH on the other side, resulting in a pressure difference of (0.5*0.694) - (0.9*0.180) = 0.347 - 0.162 = 0.185 in Hg.

Regards,
Thomas