Image Credit: Energy Vanguard
Image Credit: Energy Vanguard Permeance vs. relative humidity for asphalt-coated kraft paper, showing that it does retard vapor when the relative humidity is low but is vapor permeable when conditions are wet.
Image Credit: Building Science Corporation (buildingscience.com)
More Building Science
If you install fiberglass batt insulation* with a kraft paper vapor retarder in a home, which way do you face the vapor retarder? To the inside of the home or the outside of the home? For many building science questions, the answer is, “It depends.” For this one, however, the answer is clear.
SPOILER ALERT: The answer is in the next paragraph — so if you’d rather wait and find out when you see the movie in the theater, don’t read any further.
The answer is: It doesn’t matter. Nope. You can install the paper facing however you want — as long as the building inspectorlets you, of course.
Why doesn’t it matter?
First, the kraft paper is a vapor retarder meant to reduce the potential for moisture problems caused by diffusion. That sounds like a good idea, but the vast majority of moisture problems are caused by air leakage, not diffusion, even in places like Maine. Do the air sealing; stop worrying so much about vapor retarders.
As you can see, the permeance of the kraft paper rises as the relative humidity rises and hits 10, the point at which we describe a material as vapor permeable, when the RH is 60%. The upshot here is that if you put the kraft paper on the wrong side and it gets wet, it won’t trap moisture. The wetter it gets, the better it dries. If you put it on the right side, where the humidity is, it’s not much of a vapor retarder, because that’s where it becomes vapor-permeable.
Also on the graph is the permeance of polyethylene. As Dr. Joe says in the article, “Plastic vapor barriers really are vapor barriers when things get wet. Not so asphalt-saturated kraft paper. And most walls with asphalt-saturated kraft paper thank the building science gods for the difference.”
Warm in winter
What about the advice to put it on the “warm-in-winter” side? If that really mattered, do you think the U.S. building code would have dropped the requirement to use paper-faced batts? The warm-in-winter suggestion says that if you’re trying to limit the diffusion of water vapor, put the vapor retarder on the humid side of the wall, where … uh … it’s not able to retard much vapor.
In a really, really cold climate, it may matter, but even in Maine and Ontario, vapor retarder paint would be a better way to go. If you want to slow down the vapor diffusion, why not do it before it hits the drywall?
So just relax. If your building inspector wants you to put the kraft paper on the “wrong” side, take another look at the graph above and be comforted that it doesn’t really matter. Then go get yourself some unfaced batts (and do your best to install them to Grade I quality).
~~~~~~~~~~~~~~~
* Speaking of fiberglass batt insulation, Carl Seville (a.k.a. the Green Curmudgeon) wrote another article on poorly installed batts recently, with photos of Knauf fiberglass. Rather than being a bully, like Guardian did to me a couple of years ago, they commented on the article and asked for a dialogue on how to get better installation in the field. Kudos to Knauf!
My friend Abe Kruger likes to say that you should treat building inspectors like wild animals. You have to approach slowly and quietly because they spook easily. In the HERS rater class I taught in Toronto this week, I used that analogy but before I could get to the reason why they’re like wild animals, one of the students said, “You mean we can shoot them?”
________________________________________________________________________
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to the Energy Vanguard newsletter and follow him on LinkedIn.
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25 Comments
Not that I disagree, but
Not that I disagree, but should we not follow the manufacturers installation instructions. I have a hard time finding insulation installed properly as it is. Why complicate it. Personally I am anti fiberglass anyway and would not recommend it unless enclosed on all six sides. Just my 2 cents.
Terry
http://www.jmhomeowner.com/insulation/install/batts.asp
Question
Maybe it does not matter, but why would you install the insulation with paper set to the exterior? What is the advantage?
Ron: i believe that mr
Ron: i believe that mr Allison is trying to say that it does not matter because the paper is pretty much vapor permeable when there is sufficient humidity to push transfer.
I understand that for
I understand that for diffusion transfer, it would make no difference which side the paper were on. But as a practical matter, why would a person put the paper inward? When you say it does not matter, does that mean that an insaller should simply pay no heed to the paper position, maybe installing some bats paper-in, and some paper-out?
And while it may not matter for stopping diffusion, what about stopping airflow? Wouldn't the paper flaps sandwiched between the stud faces and drywall stop more airflow than paper turned inward, and therefore totaly unsealed at each batt edge?
I dont think the facing would
I dont think the facing would have much of an effect on air flow. Air leaks are going to come from around the edges of the drywall, around electrical outlets, etc. Have you ever seen the facing detailed to be an air barrier.
Interpretation
This is the conclusion I am left with: The paper will not stop airflow or diffusion, so there is no point in using paper faced batts. Since there is no point in using paper faced batts, it makes no difference where the paper is located. Moreover, since there is no point in using paper faced batts, one should use un-faced batts. That seems to be the point of the last sentence which is:
“So just relax. If your building inspector wants you to put the kraft paper on the “wrong” side, take another look at the graph above and be comforted that it doesn't really matter. Then go get yourself some unfaced batts (and do your best to install them to Grade I quality).”
And that sentence raises this question:
How can the building inspector want you place the paper on the “wrong” side if there is no wrong side?
And secondarily, if it makes no difference, why would you be concerned about which side the inspector wants the paper placed?
HOWEVER: There is another way to look at this. The paper barrier is intended to retard both diffusion and airflow. As a vapor retarder, the paper may be placed at some point midway in the insulation layer. However, as an air barrier, it should be on the warm side of the insulation layer. So it seems to me, that it does indeed matter whether the paper is in or out.
Response to Ron Keagle
Ron,
In Comment #4, you asked, "But as a practical matter, why would a person put the paper inward?"
A. Because that is how fiberglass batt manufacturers recommend that the batts should be installed.
Q. "While it may not matter for stopping diffusion, what about stopping airflow?"
A. I agree with Robert Hronek: kraft facing is not an air barrier and will do almost nothing to reduce air leakage.
Q. "How can the building inspector want you place the paper on the “wrong” side if there is no wrong side?"
A. The reason that Allison put "wrong" in quotes is that Allison doesn't believe that there is a wrong side. However, some batt manufacturers may insist there is a "right" and a "wrong" side. In the real world, however, some installations are head-scratchers -- especially hot-climate walls subject to inward solar vapor drive, or ceilings of damp crawl spaces. If the installation instructions say, "do it this way," and the builder happens to be smarter than the manufacturer and prefers to do it some other way, Allison advises us, "Relax. It makes no difference, so don't fight the inspector. Better yet, buy unfaced batts -- or use some different type of insulation altogether."
Q. "If it makes no difference, why would you be concerned about which side the inspector wants the paper placed?"
A. Because having a fight with a building inspector often leads to consequences that aren't good for the builder. That's why you might be concerned. It pays to listen to the inspector if you like to avoid fights. So, in this case, the building inspector is always right.
Q. "So it seems to me, that it does indeed matter whether the paper is in or out."
A. You didn't provide any logic to support this conclusion. I suggest you re-read Allison's article, because it really doesn't matter where the kraft paper goes.
Martin,
I understand your
Martin,
I understand your points. Just to clarify, when I said, "But as a practical matter, why would a person put the paper inward?", by “inward,” I meant inward moving away from the room side of the wall, but I understand that my use of the term inward can be interpreted to mean the room side of the cavity.
I had never heard of anyone wanting to install the paper toward the exterior side of the cavity, but I can see how that might be the preference when considering summertime inward vapor drive to be the main issue.
Just for clarification, is it your position that the paper does nothing to retard airflow if the paper is on the room side and sandwiched between the stud faces and the drywall? I realize that the sandwiching might have defects, and that the tops and bottoms of the batts will not be sealed off, so retarding airflow will be compromised. But are you saying that the paper can do nothing whatsoever to retard airflow?
Confining this question to climates where summertime inward vapor drive is inconsequential, and where no other interior air barrier is used; if the paper can retard airflow to some extent, wouldn’t there be a preference to placing the paper to the room side?
From the blog piece: “Second, if you install it the wrong way, it's unlikely that you'll have a problem.”
I understand the qualifying points following that quote about being able to dry if it does get wet. But why would a person install it the “wrong” way under the assurance that a problem is “unlikely,” as opposed to installing it on the manufacturer’s recommended room side where a problem will be impossible?
Response to Ron Keagle
Ron,
I'm not advocating that you ignore the manufacturer's instructions. You should follow the manufacturer's instructions. In fact, all building codes require that building materials must be installed according to manufacturers' instructions.
I'm sure that blower-door contractors will back me up when I say that the kraft facing on fiberglass batts does nothing to retard air leakage. If you want an air barrier -- and I recommend that you include one -- then follow the airtight drywall approach (ADA).
Question for Sam Glass
Sam,
Can you provide a quantification of how much of a role diffusion plays? Some say that diffusion is so small as to be inconsequential. This is claimed to have been proven by the example that a 4’ X 8’ sheet of drywall will offer diffusion only to the extent that it will pass 1/3 quart of water during the entire wintertime heating season. It is detailed with full parameters on page 17 of this reference: http://dc243.4shared.com/doc/RD5UTvRw/preview.html
Response to Sam Glass
Thanks for your comments, Sam. Let me dig into this and see what I can find. I'll also ask Joe about the permeance vs. RH graph and the air leakage vs diffusion results. 70 quarts is a lot more than 1/3 of a quart!
Response by
Thanks Sam and Allison. It is a big discrepancy. I look forward to the explanation.
Actually it can matter
Allison,
I generally find your articles informative, entertaining, and well worth reading, but this one needed some fact checking. I disagree with your blanket statement that it doesn't matter which direction the kraft paper faces. I'm challenging both of your reasons for concluding that it doesn't matter.
First, I agree that air leakage control is a higher priority than vapor diffusion control. However, it is illogical to conclude that vapor diffusion control is unimportant. The basic principle behind interior vapor diffusion control is to reduce the vulnerability of your assembly to moisture accumulation caused by vapor diffusion. If a wall with wood-based exterior sheathing gets loaded with moisture from vapor diffusion, it will have less capacity to survive air leakage. Back to building science basics: it depends. The need for interior vapor diffusion control depends on the climate, the indoor humidity levels, and a number of factors particular to the building enclosure assembly (e.g. type of exterior sheathing, type of cladding, whether the cladding is ventilated, whether there is exterior insulation). I would agree that interior vapor control in Atlanta is unnecessary. However, different story in cold climates. I realize that you end up saying "it may matter" in cold climates, but the take home message of your article is that it doesn't matter from a scientific perspective. This is incorrect.
Second, the graph from Joe Lstiburek's article showing vapor permeance of asphalt-coated kraft paper versus relative humidity appears to be incorrect. The values are much higher than literature data that I've compiled. See my attached file, which has historical data from the ASHRAE Handbook of Fundamentals as well as relatively recent measurements by the National Institute of Standards and Technology and Oak Ridge National Lab. The data in my graph generally show the same trend of increasing vapor permeance with increasing RH, but the values are lower than those in Joe's graph. It's possible that Joe's graph is based on asphalt-saturated felt (building paper typically used as an exterior water-resistive barrier) rather than asphalt-coated kraft paper. I totally agree with Joe's argument in "Mind the Gap, Eh!" regarding walls that can dry in both directions. I'm just questioning the source of his vapor permeance curve and your statement that it's not much of a vapor retarder.
Finally, your statement, "If you put it on the right side, where the humidity is, it's not much of a vapor retarder, because that’s where it becomes vapor-permeable" is sufficiently vague that it could be misunderstood. "Humidity" can mean a number of things. Vapor permeance of hygroscopic materials such as wood and paper generally is a function of RH. Vapor diffusion depends on the gradient in vapor pressure. The key point about cold climates is that indoor RH in winter is generally less than 50% while indoor vapor pressure generally exceeds outdoor vapor pressure in winter. This is why asphalt-coated kraft paper works fine for diffusion control in cold climates. Same thing for "smart" vapor retarders. Both have the added benefit of allowing drying to the interior if things get wet.
Sam Glass
USDA Forest Products Laboratory
Response to Ron Keagle on diffusion
I'm familiar with the BSC reference but never bothered to check the math. The rate of diffusion is the product of the vapor permeance, the area, and the difference in vapor pressure across the assembly. Let's take Chicago as a cold climate example. Assume the interior is 70 F and 40% RH same as the BSC reference. The vapor permeance of uncoated gypsum drywall at this condition is around 50 US perms (ASHRAE Handbook). To do a simple vapor diffusion calculation, you have to assume a particular time frame and assume the conditions on each side don't change. A convenient method is to use monthly average outdoor conditions, then add up the monthly flows. The vapor flow through a 4 ft x 8 ft board of gypsum drywall over the whole heating season (Oct - Apr) under these assumptions would be about 70 quarts (after mathematically converting a mass of water vapor to a volume of liquid water). In order to arrive at 1/3 quart for Chicago, I'd have to assume the vapor permeance was about 0.25 US perm. Anyone (Allison??) care to check my calculations?
Sam Glass
USDA Forest Products Laboratory
Deep Thoughts
"If trees could scream, would we be so cavalier about cutting them down? We might, if they screamed all the time, for no good reason."
-Jack Handey
"The authority of those who teach is often an obstacle to those who want to learn."
-Marcus Tullius Cicero
"...And no one dared
Disturb the sound of silence."
-Paul Simon
"If you're not careful, the more you write, the more you're wrong."
-BS Whisperer
Deep Thought experiment of the day
1. Pour yourself a drink.
2. Accept the fact that gypsum board is on the high end of the vapor permeance scale relative to many other building materials. (If you question this, good; then go find some data and draw your own conclusion.)
3. Accept the fact that building materials get wet in many ways (bulk water, capillary action, air leakage, vapor diffusion).
4. Accept the fact that vapor diffusion is an important mechanism for drying (not the only mechanism, just an important one).
5. Consider the implications for drying if vapor diffusion through gypsum board were really as slow as the caricature suggests. (For the caricature, see Figure 4 in the document link given in Comment 11 above.)
6. Consider how long it would take materials to dry out that are less vapor permeable than gypsum board (wood for example).
7. Imagine the economic implications for folks who make a living by drying out and restoring buildings that get wet. Hard times for sure if vapor diffusion is truly inconsequential.
8. Finish your drink.
9. Decide whether or not you believe the caricature. Is that the world you live in?
10. Have another drink and brainstorm steps 11 and 12.
The Next Step
Step 11 is easy. Re-check the calculation to determine whether the total diffusion is 1/3 quart according to BSC, or 70 quarts according to Sam Glass.
Step 12
Check your drink. If you've been drinking Kool-Aid, your judgment could have been influenced. Choose another drink and go back to Step 1.
vapor barrier for shower
Hello;
One wall of my shower is to an outside wall. The shower will have Durock on top of the 2x6 wall. On the Durock (Concrete Board) I will have tile. My walls are insulated with Roxul Insulation. My outside sheathing is OSB Board. My question is, do I need a vapor barrier (6mil. Plastic) between the studs and Durock? Is this a good solution? I do have house wrap and the siding will be 1" thick Vertical Cedar siding I Milled. I live in East Central Nebraska. They just change the requirement to use a vapor barrier on exterior walls. (warm side of wall).
Thanks for your Professional Advice!
Response to Curt Shinn
Curt,
As I wrote in my Fine Homebuilding article on tile backerboard:
"[Some] types of backerboard, including cement backerboard and fiber-cement backerboard, are vapor-permeable. The permeance of HardieBacker ranges from 1.75 perms to 2.84 perms, depending on thickness, making it fairly permeable to water vapor. Although manufacturers of cement backerboard have not had their products tested for vapor permeance, it’s safe to say that cement backerboard is highly permeable.
"Manufacturers of cement backerboard generally recommend that a moisture barrier of some sort (WonderBoard calls for #15 felt or 4-mil polyethylene sheeting) be installed behind the backerboard when used in a wet location.
"According to some tile contractors, however, this is bad advice. “Plastic is a bad idea because you are nailing it on and putting holes in it,” says Tom Meehan. “When there is plastic, I’ve found mold behind the plastic. It locks any moisture behind it, and the moisture can’t dry.” If you want to waterproof the wall, Meehan recommends the use of a liquid-applied membrane such as Laticrete Hydro Ban or Mapei Mapelastic AquaDefense on top of the backerboard. For a steam shower, he prefers a sheet membrane from Schlüter or Noble."
Follow-up to post #12 ???
Regarding post #12, is there any word yet on the discrepancy between 1/3 quart and 70 quarts?
Has anybody run the numbers to confirm that the 1/3 quart result requires a drywall perm rating of .25, as Sam Glass stated in post #12?
It strikes me that the discrepancy is rather important to resolve because the 1/3 quart result is the underpinning of a lot of theory about vapor management in buildings. And 210 times difference can’t be within the margin of error.
Response to Ron Keagle
Sorry, Ron. I've been on vacation and am just back to work today trying to get caught up. Yes, this is an important discrepancy to understand, and I'll get a response by the end of this week.
I know this thread is old, but data doesn't age out unless replaced by better data, so I hope the logic is still applicable.
I work in a lot of conditioned attic spaces that have faced fiberglass between the rafters. Sometimes it's not in budget to do fully replace; sometimes I'm talking the owner into making any improvement at all. By the logic here, it should be fine to layer polyiso or rockwool under the rafters with the old insulation in place without worrying about the paper facing creating a dew point within the assembly, right? This is in DC, so a winter heating climate with hot, very humid summers.
Underpinning of a lot of theory?
Ron,
I'd be curious to hear how the BSC figure and the 1/3 quart result is the "underpinning of a lot of theory about vapor management in buildings." It strikes me that the point of the figure is simply to illustrate that air leakage is more important than diffusion. Does anyone actually use the figure for more than that? By the way, for air leakage, I calculate much more than 30 quarts; I get over 200 quarts carried by air leakage over a heating season.
This article should get you closer to an understanding of vapor diffusion:
http://www.ornl.gov/sci/roofs+walls/staff/papers/143.pdf
Sam Glass
USDA Forest Products Laboratory
Follow-up on #12: how to do the calculation
Ron,
The calculation is fairly straightforward if you’d like to check it yourself. The number 12 keeps coming up in this thread, so I’ll lay out the calculation procedure in 12 steps.
1) This is a rough “back-of-the-envelope” calculation (or is it a “back-of-the-enclosure” calculation?). We’re not seeking a high degree of precision when the issue is entirely hypothetical (when was the last time you saw an assembly constructed entirely out of gypsum board?). So we neglect things like boundary layer mass transfer resistance and the effect of climate change on the outdoor conditions. A convenient method is to take monthly average values and use a spreadsheet.
2) Diffusion is analogous to conductive heat transfer. The rate of diffusion is the product of the vapor permeance, the area, and the difference in vapor pressure across the assembly. This is similar to the rate of heat conduction being the product of the thermal transmittance (U) times the area (A) times the difference in temperature (ΔT).
3) The vapor permeance of uncoated gypsum drywall at 40% RH is around 50 US perms (ASHRAE Handbook).
4) 1 US perm = 1 grain per square foot per hour per inch of mercury (in Hg) difference in vapor pressure.
5) 1 pound(mass) = 7000 grains.
6) 1 quart of liquid water has a mass of about 2.08 pounds.
7) The area is 4 ft x 8 ft.
8) The interior vapor pressure (at 70 F and 40% RH) is 0.297 in Hg. Check this using an online psychrometric calculator such as this:
http://www.vaisala.com/humiditycalculator/vaisala_humidity_calculator.html
9) The outdoor vapor pressures for Chicago can be calculated from outdoor dew point temperatures. Here are the monthly mean dew point temperatures for Chicago (degrees F):
Oct: 43.6
Nov: 33.2
Dec: 23.1
Jan: 18.9
Feb: 21.8
Mar: 29.8
Apr: 37.9
If you’d like to check these, see page 3 of the “2012 LOCAL CLIMATOLOGICAL DATA ANNUAL SUMMARY WITH COMPARATIVE DATA” for Chicago compiled by the National Climatic Data Center:
http://www.ncdc.noaa.gov/IPS/lcd/lcd.html
10) Convert dew point temperatures to vapor pressures using a psychrometric calculator (see above). Here are the monthly mean vapor pressures for Chicago (in Hg):
Oct: 0.286
Nov: 0.190
Dec: 0.126
Jan: 0.105
Feb: 0.119
Mar: 0.166
Apr: 0.229
11) Calculate the difference between indoor and outdoor vapor pressure for each month.
12) The rate of diffusion for each month is the product of the vapor permeance, the area, and the difference in vapor pressure. The amount of water transferred for each month is the diffusion rate times the number of hours in the month. The amount of water for the whole heating season is the sum of the amounts over all the months.
Make sense?
Sam Glass
USDA Forest Products Laboratory
Response to Sam Glass
Sam,
Thanks for your detailed response. It is very useful to know your method of obtaining the 70 quart result for the vapor transfer scenario that BSC claims will result in 1/3 quart. I will see if I can work through your formula.
Regarding my comment about the BSC diffusion calculation being the underpinning of theory about vapor management:
I agree that the information serves to show that vapor transferred by airflow is much more effective than vapor transferred by diffusion. However, the 1/3 quart of diffusion also leads to the commonly stated conclusion that the diffusion mechanism is so small that it need not be addressed (unless there is a source of unusually high vapor pressure in the building). So there are two basic conclusions conveyed by the information. One is that air leaks matter more than diffusion; and the other is that diffusion does not matter.
This information has become the underpinning of the conclusion that a warm-side vapor barrier is not necessary.
In the past, I have wondered if the 1/3 quart figure was calculated, modeled, or measured, but other than that, I had no reason to question it. However, your conclusion that the total diffusion transfer would be 70 quarts rather than 1/3 quart raises doubt about the 1/3 quart result.
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