Vapor Barriers in Residential Construction

Vapor Barriers in Residential Construction

Ahhhhrrrrrwheeeewwwww….  Ahhhhrrrrrrggghrrrrwwhewww…  those are the snoring sounds from many people when faced with this subject.  To me, however, I take vapor barriers in residential construction very seriously (and vapor retarders).  You should, too, if you are planning on having a new home designed and built soon, if you want it to be a healthy place for your family, and energy efficient.

The way things used to be done and they way they may be done now is probably wrong, in many homes.  Why?  Because it can be a confusing subject, with as many opinions as there are people involved in construction.  Most people in the construction industry do what they are used to doing, what they have reliable cost information on, with dependable subcontractors who perform reliably for a set price, so that the builders can make a living for a reasonable cost.

Even so, there are tried and true solutions that can be used, products and materials that have been around for decades that work just fine and deliver a true value: good quality for reasonable cost.  For instance, most homes these days are wrapped in a big white sheet of material.  It really is not doing what you may have been led to believe, or may be doing it too well, which might cause other problems.











Residential Construction, Vapor Barriers, Vapor Retarders

I recently attended a continuing education seminar with Dow, Certainteed, Cox Lumber, Trex, Huber, Kolbe and other major companies presenting the latest information to licensed architects like me.  Jennings Building Supply was courteous enough to host the event.   What was interesting was that each presenter with products functioning as vapor barriers (and vapor retarders) mentioned Dr. Joseph Lstiburek, PhD, P.E., who is a licensed professional Engineer in mechanical engineering and who is a principal in Building Sciences Corporation.  He is also a member of ASHRAE, serving on the technical committee for Standard 62-1999, Ventilation for Acceptable Indoor Air Quality.  I have also noticed scientifically tested data from the University of Massachusetts, which is in the vicinity of Dr. Lstiburek’s location, and I can only assume that he has shared information with this institution.

I am not an engineer.  I am an architect.  I use the information that I see tested by laboratories and scientists like Dr. Lstiburek, and I use that information to try to make intelligent choices for the systems I use in my designs.  Sometimes I am just plain lucky.  For instance, in my usage over the last several decades of certain vapor barriers (and vapor retarders) and water barriers, particularly for walls.  Vapor barriers block almost all water vapor movement.  Vapor retarders allow more water vapor transmission. 















As defined by ASHRAE Fundamentals 2001, Chapter 23, a Vapor Retarder is the element that is designed and installed in an assembly to retard the movement of water by vapor diffusion.

Dr. Joseph Lstiburek offers these definitions:
Class 1 Vapor Retarder: 0.1 perm or less.
Class 2 Vapor Retarder: 1.0 perm or less and greater than 0.1 perm.
Class 3 Vapor Retarder: 10 perms or less and greater than 1.0 perm.

Test procedure for vapor retarders: ASTM E-96 A.

Vapor Barrier: a Class 1 Vapor Retarder (0.1 perm or less).

Lstiburek proposes the following levels of permeance descriptions:
Vapor Impermeable:                   0.1 perm or less (e.g.: plastic sheets).
Vapor Semi-Impermeable:   1.0 perm or less and greater than 0.1 perm.
Vapor Semi-Permeable:        10 perms or less and greater than 1.0 perm.
Vapor Permeable:                    greater than 10 perms.

Both the Dow representative and the Huber representative cited a perm rating of 10 +/- as being about right for an exterior vapor retarder that is also trying to act as a water barrier.  In other words,  a “Vapor Permeable” “Vapor Retarder.” They mentioned Dr. Lstiburek as a source for that perm rating.  A “perm rating” is the number assigned to a particular material that tells us what its ability to resist water penetration is.  The higher the rating, the more water it lets into the wall or whatever surface it is applied onto. 













Therefore, many of the most used “housewraps” which have perm ratings of 50 to 80, let more water vapor (and some might let in more liquid water) through them than other materials that have lower perm ratings.  What can add to the confusion is that TyVek now has been tested to almost entirely resist liquid water penetration, which can generally be viewed as a good thing.  The problem, in the opinion of vapor barrier/vapor retarder researchers, is the remaining general high water vapor permeability (see later information from the U of Mass. later in this post).

Now then, the novice’s immediate reaction would be to say: “Hey, I don’t want any water to penetrate my walls and other surfaces, so, I want a perm rating of 0.  Would you keep out all of the water?  Probably most of it, if all of your seams were sealed.

Importance of both sides of a wall and desired permeability

Only, a wall is a 2-sided coin.  Water vapor also can come from the interior.  Really?  From what?  You, your laundry, your cooking, transient moisture in the air already inside your home, including from all the cracks and crevices that exist around doors, windows, framing, ducts, vents, plumbing and a host of other moisture sources like living plants, pets, your daily shower and baths, sink water running, toilets, sweating plumbing pipes inside walls that are not covered with pipe insulation, and other sources.  And what about the rain that fell on your house while it was being built?  Where did all that absorbed moisture go?  The answer: if it hasn’t yet dried out of the framing, it might still be in there.



















So what needs to happen with all that interior moisture?  One might think that a good air-conditioning system will help, and dehumidifiers, and proper ground water redirection to keep water out of your home, and proper gutters and downspouts and underground piping connected to the downspouts to keep water away from your house.  All of these techniques help and we put these requirements into our projects to keep them healthy and dry.  However, you still could have windows and doors leaking that could be introducing water into your walls, as well as from other sources.

The main concern of Dr. Lstiburek: you do not want to trap any moisture that gets into your walls inside your walls.  There needs to be some way to allow the wall interior to dry, while keeping out gross amounts of newly introduced moisture.  This means that the membranes we use to block moisture from coming into our houses from the exterior, and the interior vapor retarders as well, that hold warmer moist air inside our homes (such as in winter for our comfort), need to also have some degree of permeability to water vapor to allow the moisture inside the wall itself to escape and hopefully help prevent the growth of mold inside your walls.

Sounds impossible.  How can we find a water barrier/retarder for the exterior of our houses that block nearly all of the water from coming into our walls from rain, snow and other moisture-laden air, yet will also allow moisture in the form of vapor already inside the wall to exit, and not allow water vapor from outside to come inside (or at least have a physical characteristic to remove it if it enters)?  Also, how to have a water vapor retarder on the inside face of the wall that will hold in most of the winter moister, warmer interior air inside the house, yet allow what is inside the wall to vent and remove the moisture trapped inside the wall to dry out?

The reason that we humans find this hard to understand is that we are used to seeing things in our everyday lives like plastic baggies that we use to preserve food in, and we see how water cannot get inside that.  We see paper and how, if water hits it, it almost immediately disintegrates.  We tend to deal in absolutes: This keeps all water out; That lets all water through.

The reality is that there is a sliding scale of permeability with a variety of materials, particularly in the building material industry, where scientists working for and with large corporations purposely create materials and membranes to have various degrees of water vapor transmission characteristics.   Why would anyone want a water vapor membrane with anything but a “0” perm rating?  Because while that would be excellent at keeping out water (like polyethylene plastic roll sheets), they trap water inside the wall.  And once you have trapped moisture inside a wall, mold growth inside the wall is inevitable.

So, one wonders: “What would this magic, wonder-material be?”  Let’s consider the Exterior Water Barrier-Vapor Barrier/Retarder first.  If Dow and the other companies are correct in their information, that a perm rating of about 10+/- is ideal for the exterior water-vapor retarder (for mixed humid climates), then this would be a material that keeps out nearly 100% of wind-driven rain water, yet allows any water vapor inside the wall to escape through it, or though its seams.

Desired Permeability of your Exterior Water Barrier/Retarder

Does such a wonder material exist?  It does, in several forms.  One is a new pink plasticized sheet from Dow with a perm rating of 6.8.  (that is probably close enough to the 10 perm rating desired, although that is an assumption that both Dow and I make).  It sounds like this new exterior vapor barrier would do a great job for mixed humid environments.

Want to know what else happens to have a perm rating of about 5 to 7?  Good old-fashioned 15# asphaltic felt.  Or, “tar paper.”  I learned about this perm rating about 15 years ago. And here is an excellent research article from the University of Massachusetts Amherst (by Paul Fisette, 2001) about this subject:

Asphaltic Felt – Univ Mass Article 2001 .
It is interesting that the author indicates that building codes recognize asphaltic felt as a Grade D building paper meeting the definition of an exterior vapor retarder, which has a minimum value of 5, which it has when it is dry.

Mr. Fisette goes on to mention that the perm rating of “tar paper” can vary from 5 perms to over 60 perms when it is exposed to RH over 95%.  So, its permeability varies depending on weather conditions, which may be a good thing.  At certain points in University of Massachusetts Amherst’s testing of TyVek, R-Wrap and Asphaltic felt, it might appear that TyVek’s latest formulation out-performs felt, because it appears to have been made more water -resistive, while retaining its high permeability.  However, I am still concerned about too Much permeability, and when you read on in Fisette’s research and testing article, he admits that he has felt on his own home and if he had it to do over again, he would still prefer the Asphaltic felt over the housewrap.  Why?  Here is what Fisette said in his article, when directly comparing housewrap versus Asphaltic felt:

As it happens, I have felt paper on my own home, and if I could choose between felt and housewrap and do it over again, I’d still choose felt.  That’s because that under certain circumstances, felt outperforms housewrap.  For example, an ice dam or roof leak may allow liquid water to get behind the felt or housewrap.  It’s also possible for the sun’s heat to drive water vapor through the housewrap from the outside, where it can condense on the sheathing (my underline).   In either of these cases, you now have liquid water on the wrong side of the wrap.  Under these conditions, the liquid water would be trapped by the housewrap, which is permeable only to water vapor.  Felt, on the other hand, will absorb the water and more quickly dry to the outside.”

Fisette also recommends that wood siding be backprimed with water resistive wood preservative coatings, including all edges and ends.

I have been specifying asphaltic felt as the main exterior wall vapor retarder and water barrier on my house projects all of my life, since my first house I designed by myself in 1975 (the Thomas House in Sanibel island, Florida) which is still standing today.  There are certain specifics as to how I have it installed that are proprietary to my practice, including special asphaltic modified flashing tapes to use around doors and windows that will adhere to it.

Why did I start using that so long ago?  Because that’s what crotchety older architects for whom I worked said to use, because it had always worked for them and they never had any problems.  In other words, almost mere luck, but with a good dose of trial and error over decades of experience by myself and those previous architects upon whom I relied while learning my practice decades ago.  And now, about 40 years later, scientists are discovering that the ideal perm rating is about the value of what I have been specifying for all of my professional life.  So I am greatly relieved, that modern scientific tests have confirmed that what I have been using all along works well to keep out water, yet has just enough permeability to allow any water vapor within the wall itself to escape.  Okay.

UPDATE 1/2013: regarding the trend of the IECC (International Energy Conservation Code) to require air barriers behind attic knee walls in house construction, and the various State’s requirements to tinker with and improve upon the IECC for their various jurisdictions, it is only a question of time before the ENTIRE exterior of a house will be required to have an Air Barrier.  Thinking that this might be easily achieved by taping the joints of the Smart Vapor/Water Barrier discussed just above, Rand Soellner contact Dr. Joseph Lstiburek at Building Science Corporation and asked him if it could be this simple.  He agreed.  Therefore, if you use some flashing tape (like WR Grace VyCor) that has been specially modified for use with asphaltic felt (and subject to their warrantees), you should he able to functionally achieve an air barrier using asphaltic felt with taped joints.  Great!  So now we have a Smart Vapor/Water Barrier that also can be our Air Barrier.  This is not intended to be dispensing gratis advice to everyone out there.  Contact this Architect or others to verify the circumstances for your project and jurisdictional authorities’ acceptance of such.

Interior Vapor Retarder in Wall Construction

Now on to the interior vapor retarder.  Once again, there are many opinions as to what, if anything should be used as an interior vapor retarder.  Dr. Lstiburek, for instance, believes that for projects in Canada, there should be an interior vapor retarder, due to their nearly constant winter.  He feels that the United States, which is south of this nearly polar zone doesn’t really need an interior vapor retarder, because he feels that the bigger issue is stopping the more humid air (during warmer weather) that is outside from coming into the walls.  First and foremost, I agree with Dr. Lstiburek that we need to first stop exterior water from coming into the wall.  While I can understand this requirement, I personally happen to live at an elevation of about 3,500′ ASL in the Blue Ridge Mountains of North Carolina and we get some harsh, extremely cold winters.  And how about Indiana, Chicago, and a huge portion of America where the winters are very cold indeed, which includes much of what is called “Mixed Humid Climates”?  These can be mixed climates, with high-humidity during scorching summer temperatures of 100*F and higher, and yet be -28*F during winter with 3′ high snow drifts (as in South Bend, Indiana where I grew up), which in my opinion needs an exterior vapor barrier/retarder in warmer months and an interior vapor retarder in colder months.  For instance, I lived in Florida for over 30 years, and recall snow during some winters there, ice, and freezing temperatures. “Smart” vapor materials that adapt to these extremes are an excellent solution for much of the USA.

There is also the subject of building codes, which require the usage of  an interior vapor retarder on the “warm side of the wall in winter.”  That, of course, would be the interior side.  So what do we do now?  How do we avoid the problem of trapping water vapor inside the wall if it gets blocked from the interior side?  Well, as it so happens, Dr. Lstiburek has discovered something quite interesting about another time-tested material that has been around for decades.  Would that be polyethylene?  No!  Never use a sheet of plastic as an interior vapor barrier, because this will trap water inside your walls, and any warm moist air coming through your wall from the exterior will hit that membrane of near total impermeability and condense on the exterior face of the interior barrier, growing mold inside your walls.  That is the same reason that no one, particularly in warm, humid climates, should ever install vinyl wall covering on the inside finish face of exterior walls.  I have personally been called in to design the fix for several sick commercial buildings (that I did not design originally), that committed this sin.

Okay, so what is the right choice for the interior vapor retarder for the wall, if you do use one there (as code appears to require)?  Dr. Lstiburek indicates in his 12/2001 HPAC Engineering article, that good old fashioned kraft paper backings provided by most batt insulation manufacturers as their integral interior vapor retarder appear to meet the code required perm rating of 1 or less, however, this is only when the RH (Relative Humidity) the kraft paper sees is low: between 25% to 30%.  Lstiburek goes on to state:

“What is interesting is that this kraft facing becomes vapor permeable as the RH it is exposed to goes up.  In air-conditioned buildings, when the humidity is 50%– not atypical — the kraft facing is 10 to 20 perms.  It’s a “smart” vapor barrier.  It retards moisture in the winter and lets the wall “breathe” in the summer.  That dumb plastic barrier has a perm rating of 0.1 perms — all of the time (he refers to a Figure 1).  We’re talking two orders of magnitude in difference.  Stay away from plastic-film vapor barriers, foil-faced fibrous cavity insulation, and especially foil-backed gypsum sheathing on the interior.”
The above quote is from Dr. Lstiburek’s article from the 12/2001 issue of HPAC Engineering.

In other words, if you do use an interior vapor retarder (as construction codes require, depending on your location), good old-fashioned cheap kraft paper backing on most fiberglass insulation batts can do a pretty good job of keeping in your moisture inside your house during the winter, when you need some moisture in your air for your comfort, while allowing moisture to escape during the summer, when you don’t want it inside your house.

And what do I do as an architect of custom houses all over the USA? Come visit me and let’s have a talk about it.  I will certainly use helpful information from sources such as the above and continue to monitor new data.  It appears that I have done fairly well over the last 4 decades, using time-tested materials that continue to work well, as tested and proven by the latest scientific studies.


Rand Soellner Architect 1 . 828 . 269 . 9046


Your nose can serve you well, also.  My houses never seem to smell of mold, no matter what their age.  When you walk into many houses and smell that telltale moldy odor, you know instinctively that this is not a healthy place in which to live and that water is getting in somewhere and staying in and growing mold.  Make sure that you engage a highly experienced residential architect to design your next residence.

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