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The Insulation Lab

Smart Vapor Barriers - Are They Really That Smart?

Jeff Aderholdt - Thursday, June 06, 2013

Recently a friend sent me an e-mail. He is a builder and frequents many of the construction seminars. He had just returned from a conference, and the instructor, a trusted professional in the construction sciences, recommended that, instead of using poly as a vapor barrier, to use Certainteed's MemBrain. My friend was asking what I thought about the product and how much more this would cost. Apparently, the instructor's reasoning was that, because it was a smart vapor barrier, it would also let moisture vapor back through it into the house. Since moisture will get into the structure, this will allow it also to leave. That reasoning seemed a little shaky to me. Let me explain why.

Insulation Techniques and Advancements in Applied Science

There are few areas of construction that have been as greatly affected by advancements in technology as those related to energy conservation. Fifty years ago, energy was cheap. Making a home energy efficient wasn't even thought of. With the oil embargo of the mid 70's came an effort to cut energy costs. People were building their own ethanol stills, experimenting with ways to increase fuel economy on cars, and insulating their homes. Since this was, for the most part, uncharted waters, there was a lot of experimentation, and a lot of mistakes were made. For example, the first high efficiency furnaces didn't last long because the heat exchangers rotted away from the acids created by the condensation. Who knew! But we learned from our mistakes. Better science leads to better technology. The same was true in insulation.

The first step for many was to just add insulation. Soon it was noticed that just adding insulation seemed to create other problems, such as moisture. Then came vapor barriers. This seemed to help, but there were still issues. Then we started plugging holes and making the house more air-tight. This seemed to keep most of the moisture out of the structure, but the indoor moisture content jumped way up. So better indoor air management systems were developed to improve indoor air quality. Guiding us through these innovations were a lot of smart people in lab coats crunching numbers, proposing theories, and experimenting with different techniques. The result is our modern unified thermal envelope method of energy management. We have come a long way in a very short amount on time. Though some still cling to antiquated ideas, like "a house can be too air-tight" or "don't use vapor barriers because a house needs to breath" or "I've been building like this for 40 years", most builders and insulation contractors see the benefits of the newer methods. Thank you technology!

That doesn't mean that everything that is promoted as the latest advancement in technology is an improvement. There is plenty of pseudo-science that is being promoted as science. Accepting every supposed technological advancement as fact is just as extreme as refusing to accept any advancement because "this is how I have built for 40 years". There is a need for balance when evaluating any new technology or technique. Is it really a step in the right direction or just another detour? You need to ask questions like:

1. How does this fit in established scientific fact?
2. Does this product really do what it claims to do?
3. Is there independent research supporting the claims?
4. If it is supposed to solve a problem, is it targeting the problem or just dealing with a symptom?
5. What are the benefits in relationship to the cost? Are there other methods that produce a comparable result, but more cost effectively?

This is by no means meant to be a comprehensive list, but hopefully it does provide someplace to start.

Something to keep in mind as you are evaluating a new product or technique. BE OBJECTIVE! Be careful not to project a preconceived idea or bias. Recently, a contractor asked me about the best way to insulate an unusual area. I looked it over and gave my recommendation. The problem was, he already had a preconceived idea that spray foam would be the best solution. Though I felt that spray foam was an adequate, albeit expensive solution, my recommendation was that a different method would produce a better result. Well, he started calling other professionals, architects and such, asking for their opinions. He finally came across someone who told him spray foam was the best, just what he wanted to hear! In explaining the conversation to me he told me: "he really sounded like he knew what he was talking about". Of course he did! What he told the builder was inline with his preconceived idea. The builder then projected his bias into thinking that he was right all along and thereby lost his objectivity. So be objective. Let's go back to my friend's question and put it to the test.

A Smart Vapor Barrier?

"If you can’t escape the possibility of moisture entering a wall, why not let the moisture in the wall escape?" This is what Certainteed says was behind the development of their MemBrain smart vapor retarder. The stated principle seems to have a teleological simplicity to it. We can't prevent moisture from entering walls, therefore let's have the goal of letting the moisture out. But how does it fit into established fact?

First off, in all fairness to this product, it is being promoted as an exterior wall vapor barrier, not a ceiling vapor barrier, with the possible exception of a hot roof application. This makes sense, because in the standard insulated and vented ceiling application, the ventilation provides the means for any moisture that finds it's way past the vapor / air-tight barrier to be dissipated. So, in a standard vented ceiling application, not using MemBrain is a no brainer. But what about exterior walls?

First we need to remember the function of a vapor barrier is to stop moisture from moving through materials, such as drywall and then vaporizing on the other side. In recent years, we have learned that controlling moisture movement though materials is minor when compared with controlling moisture movement around materials. Air tight is more important than vapor tight (The type of insulation used can have a dramatic effect on controlling moisture transferred by air movement. See "Dense Packed Cellulose - Cellulose Reborn"). Now, just because vapor barrier is a minor player doesn't mean we don't at least try to control it. The more effectively a vapor barrier is at stopping moisture from moving through it, the less problem moisture will be to deal with, as well as the loss of energy resulting from the condensation of the water vapor (See" Birds Bees and Energies" for more information). Because of this, building codes set a minimum standard. And this brings us to first area of concern. A product that, by design, will allow moisture through it under certain circumstances is more likely to allow more moisture into, say, a wall cavity that will need to find a way out. This is circular reasoning, "we may be letting more moisture in, but we are giving it an easier way out". This is the same reasoning that came out a few years ago regarding fiberglass as sill box insulation, to use un-faced insulation, because, well moisture will get behind it and the un-faced insulation will let the moisture out easier. The real solution was to stop using fiberglass in the box sill and use a product that didn't let the moisture through. Solve the problem, don't deal with the symptom. So, at least on this point, even though Membrain does meet the minimum standard required by code, it really isn't that smart.

Moisture control in Sealed Wall Cavities


On this, the statement that the possibility, in fact the probability, that moisture will get by the vapor and air-tight barrier is true. And this moisture will need a way out. But this does not need to be, and it should not be, back through our vapor and air-tight barrier. There is usually enough vapor release to the outside through the sheeting. Plus, the more you detail your vapor and air-tight barrier, the less moisture there will be for the structure to deal with. Now there is no perfect air / vapor barrier. Still the closer we get to perfection, the better the performance.





There are some situations where standard construction practices can create a situation where there is insufficient exterior area for moisture to adequately dissipate to the outside. One such situation is where cultured or thin cut stone is installed directly over the exterior sheeting ( Check this out:). The common method is to first install a moisture barrier directly over the wall sheeting, then a lattice and scratch coat, followed by the finished stone (Though this is the "common" method, it is neither code compliant, nor in line with the manufacturer's installation guidelines.). The effect of this is making the exterior of the treated areas vapor tight. This does not necessarily create a problem. If only a portion of the wall, perhaps the lower half, is sealed, the rest is still available for vapor release. But the more wall that is covered, the less trapped moisture can dissipate. If the entire wall is covered, there is no place for the moisture to go.


Apparently, this is the type of situation that the instructor at the conference was trying to address. There are numerous documented cases where the structure has failed in such situations. The OSB Sheeting has literally just rotted away. So, is this a place for a product like MemBrain? Perhaps. But would this be addressing the problem, or just dealing with the symptom? The first step in solving a moisture problem, any moisture problem, is to first stop the moisture from going where we don't want it. This means detailing our interior air-tight / vapor tight barrier, thereby minimizing the amount of moisture that get into the structure. A vapor barrier that allows some moisture to pass through it under certain circumstances just doesn't seem that smart.

It is important to note the reason for the structural failure in such situations. It was not primarily due to living space moisture entering through flaws in the vapor / air barrier. Rather, it was due to improper installation of the cultured or thin stone. These were:

1. The lack of an adequate moisture barrier behind the stone work, and
2. The lack of a weep / venting space between the stonework and the exterior sheeting.

The only thing some installers used was standard paper house wrap. The result was that the moisture in the masonry work would directly transfer into the wood framed structure. Without a vented space, the moisture then had no where to go. The sheeting was pretty much in a perpetual state of dampness and eventually just rotted away. No amount of vapor release could prevent this. The solution to the problem was to install an adequate moisture barrier and an adequate weep / venting space, not to manage the symptom by providing vapor release through a product like MemBrain. So, how should we deal with a situation where the exterior of a wall is completely covered with a masonry veneer? Is it now time to provide a mechanism that allows the moisture to escape to the inside, with a product such as MemBrain?

Let's again identify the problem and not just the symptom. The symptom is moisture trapped and collecting in the wall. The problem is:

1. A situation has been created that allows exterior moisture into the walls.
2. The normal and preferred moisture / vapor release route, through the exterior of the structure, has been blocked..

The solution is to directly address this. Therefore, the logical progression would be:

1. Immediately behind the masonry veneer (as in cultured or cut stone) there should be a water tight barrier. The sooner we stop the water, the less water there will be to deal with.
2. Behind the water barrier, allow a vented airspace to allow and moisture that makes it through the water barrier under the veneer or from the interior through imperfections in the walls vapor / air tight barrier. To do this, there are a number of manufacturers that address this issue with their products,Though I am not endorsing any particular brand or product, here's one:.
3. Between the vented air space and the wall sheeting install a house wrap.
Since we are targeting the problem and do not just deal with the symptom, this is really the smarter choice.


Certainteed at their website says that using MemBrain will add about $500 to the cost of an average size home. Though that's not a lot, is it worth it? When you consider the marginal benefits, if any, wouldn't that money be better spent on other areas, perhaps doing a little extra air sealing?


I understand the concerns that lead the instructor to make this recommendation, but when we weight the facts, there is little, if any advantage to using a product like MemBrain, and there probably are some drawbacks. This isn't to say that there won't be some situations where the characteristics of products like this will be useful. Still, since vapor barrier is really a minor player in controlling moisture (air leakage is the big issue.), if you choose MemBrain instead of poly, I don't see any major problems resulting, if all other things are done correctly. Still, the added cost, as minimal as it is, the smart course would be to spend the money elsewhere.

At Northern Thermal Comfort LLC, we are always working to give you the best insulation product for your money. Give us a call and let us put our expertise to work for you.

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