Insulation, Radiant and Vapor Barriers - Certainteed Membrain and cotton insulation

I have two questions for anyone that may have some information. (Sorry for the long ramble that follows. I will put my questions up front here, so people who don't want to read this, can just read my questions. :) )

1. Does anyone know where I can buy Certainteed Membrain vapor barrier? Any local home improvement stores/online vendors carry them?

2. Has anyone used cotton/denim insulation? For a basement?

The ramble:

I was doing some research on vapor barriers since there are so many opinions on what is correct for basement finishing. Some people say two vapor barriers, some say one against concrete wall, some say one over studs, some say none at all. And to top it off, it all depends on the climate you are in.

So, through some quick research, I found a product by Certainteed called Membrain, which is a 'smart' vapor retardant that changes it's permeability based on the humidity level. Through all of my research, there is one basic point that I think a lot of people don't talk about that much -- You have to let the walls dry, if moisture gets inside of them, whether they dry to the interior or exterior.-- You are never going to have an absolutely perfect seal on any vapor barrier, so the walls need to be able to breathe in case this moisture gets inside the wall and condenses on a cool surface and drips/puddles to the ground. If it is trapped between inpermable vapor barriers, it will have a very hard time evaporated/drying back out. That will cause possible mold, rot, etc.

The Membrain vapor barrier has a permability well less than one during low humidity, acting as a vapor barrier, but it is >1 when there is very high humidity, allowing the walls to dry to the interior/exterior. This can be the case during a midwest climate where the humidity in the walls can change from higher humidity in the summer to lower humidity in the winter. Anyone have experience with this?

Also, the second part of this deals with cotton/denim insulation. I'm thinking about using this for my insulation between metal studs for finishing my basement with the Membrain over it. Anyone have any experience with this? Through quick research, it seems like it would be easy to work with.

Thanks,
Eric

R-19 for example is rated under controlled conditions. Where the temperature inside is 70 degrees and the outside is 32 degrees. R-19 means 1/19th of a BTU travels through this insulation every square feet an hour. However, since this is done under controlled temperatures, when the temperature goes down outside the heat loss is greater. If it goes above 32 degrees, the heat is less. This principle also applies to vapor barriers or to so called smart membranes. As the moisture increases or decreases on either side of the vapor barrier, so does the vapor permeability of the vapor barrier.

The drying out process or moisture expulsion is accomplished by Equilibrium Relative Humidity. Vapor Barriers are used only with insulation because of the effect that temperature has with humidity. Relative Humidity expresses the amount of humidity an object can hold at a certain temperature. For example, a certain volume of air is at 70 degrees with 100 grains of moisture in it and it gives it a Relative Humidity of 20%. If you lower the temperature of the air to 60 degrees, the Relative Humidity goes up to 50%. The 100 grains of moisture did not change, the temperature did. It if this behavior with humidity and temperature for applying vapor barriers to the warm side of insulation. As the heat travels through the insulation, it begins to lose temperature. This temperature drop causes the Relative Humidity to increase. Vapor Barriers impede the moisture flow into the insulation. Which reduces the probability that Dew Point wll be reached as the temperature drops inside the insulation.

Besides Equilibrium Relative Humidity causing all the objects and materials seeking equal amounts of humidity, during the winter heating produces positive pressure inside your home. It is this pressure the a pushes the humidity in the house outwards. This further supports the need for vapor barriers on the warm side of insulation.

As far as mold and mildew caused by vapor barriers installed in basements, this is a misconception. The truth here is the cooler surfaces are usually created by air conditioning and failure to control the amount of moisture being produced in the basement.

This principle also applies to vapor barriers or to so called smart membranes. As the moisture increases or decreases on either side of the vapor barrier, so does the vapor permeability of the vapor barrier.


Resercon,

Thanks for the reply. I have found some tests done on standard poly vapor barrier and it shows that the permeability barely changes, if at all, based on the change in humidity. The test also included Kraft-faced insulation, which also barely changed, but a bit more than the poly, especially as the realtive humidity reached near 100%. However, the Certainteed 'smart vapor retardant' maintained a very low permability at low relative humidity levels, but increased (like an exponential curve) as the humidity reaced the 100% level.

This is what I am questioning. It makes a lot of sense to me to be able to expel any moisture from within the walls before any damage can be done. Does this make sense or am I still missing a basic point here?

Thanks for the help,
Eric

Eric,

You do have a farily good understanding of Vapor Diffuser Retarders (VDR), now all you have to do is relate it to the other processes. Impedence or Perm Ratings explicitly imply moisture flow. It does not stop it, it slows it down. And in both directions.

Let's assume you have good drainage around the perimeter of the basement and inside the basement the normal things that generate moisture is fairly controlled. Meaning to say there are no things either inside or outside the basement that would produce an abnormal amount of moisture inside the basement. The first process you have to relate to the vapor barriers is how the materials and objects in the basement become moist. It is accomplished through a process known as Equilibrium Relative Humidity (ErH%). This states that an object of lower humidity will absorb humidity from an object(s) of higher humidity until the humidity levels in all the objects are equal. So regardless of the Perm Rating of the materials or objects, the Relative Humidity (RH%) of these objects will seek and obtain equilibrium. The Perm Rating of those objects influences the time it takes for equilibrium. The only time condensation could occur under these circumstances is when there is an abnormal amount of moisture present and we already excluded that for this explanation.

Take the same situation and include Relative Humidity (RH%). This describes the behavior of moisture in materials and objects under different temperature conditions. This states that as the temperature of an object increases, the RH% of the object decreases. As the temperature of the same object decreases the RH% of the object or material increases. But the amount of moisture inside the object or material does not. In other words, if the temperature of the object or material keeps on dropping, eventually Dew Point Temperature (DPT) will be reached without adding anymore moisture to the object or material. On the other hand, if the temperature of the object is rising, the only way condensation can occur is by adding more moisture until Saturation Point is reached in the object or material. Again, we have excluded an abnormal amount of moisture present for this explanation.

For this explanation we are considering the effects of vapor barriers applied to the warm side of insulation on basement walls. Though the processes remain the same, the difference is below grade temperatures are usually 55 degrees Fahrenheit most of the time. Not true with above grade applications. Of course there are other factors but they will not be discussed for this explanation.

During the winter all the materials and objects in the basement seek and obtain equal amounts of humidity (ErH%). As the heat travels from inside the basement through the insulation, it begins to lose temperature. We know that as the temperature drops of an object, the RH% increases. However, the vapor barrier impedes moisture flow. Meaning to say it takes more time for the moisture to permeate the vapor barrier. This reduces the amount of moisture inside the insulation as the heat travels through it. There is a rule here; a material or object must possess more than 20% moisture content, not RH%, for condensation to occur at Dew Point Temperature. In other words, even if Dew Point Temperature is reached inside the insulation, if the moisture content of the insulation is less than 20% condensation would not occur at Dew Point Temperature and the insulation would retain the moisture. However, ErH% still applies. The lowering of the temperature inside the insulation increases the RH% of the insulation, regardless of its moisture content. This increased or higher RH% is absorbed by the lower RH% of the masonry wall. Clearly the vapor barrier plays an important role in the extraction of moisture from the home during the winter and avoiding condensation from occurring inside the walls.

If we apply this during the summer, what effect does the vapor barrier on the interior has with moisture inside objects or materials? The answer is none. The reason for this is the ambient temperature of the masonry wall, which is usually 55 degrees Fahrenheit. The temperature in the basement and the vapor barrier is going to be higher than 55 degrees. If this is true, then condensation cannot occur on either side of the vapor barrier because as I raise the temperature of the object, I lower its RH%. The only time condensation could occur under these circumstances is if I add moisture to the object or lower the temperature inside the basement below 55 degrees Fahrenheit.

There are a lot of factors that contribute to the propagation of mold and mildew in basements. Vapor barriers may be a contributing factor but is certainly not a source of it. And having a vapor barrier that performs differently under different humidity conditions is certainly not a viable solution nor preventative measure.

If we apply this during the summer, what effect does the vapor barrier on the interior has with moisture inside objects or materials? The answer is none. The reason for this is the ambient temperature of the masonry wall, which is usually 55 degrees Fahrenheit. The temperature in the basement and the vapor barrier is going to be higher than 55 degrees. If this is true, then condensation cannot occur on either side of the vapor barrier because as I raise the temperature of the object, I lower its RH%. The only time condensation could occur under these circumstances is if I add moisture to the object or lower the temperature inside the basement below 55 degrees Fahrenheit.


Resercon,

Excellent explanations. Exactly the kind of information that I love to read. I'm the kind of person that likes explanation on why things work, not just someone stating that something works or it doesn't.

I fully understand relative humidity and the need of a vapor barrier in the winter, if heating the basement. What I needed more clarification on was what I quoted from you above:

I didn't know that the masonry would stay about 55 F during the summer. That clears things up a lot for me and I appreciate all of the time that you have taken to reply to me. I have one last question based upon your quote. Maybe just a little last piece of clarification.
You said:

"[For summer] The only time condensation could occur under these circumstances is if I add moisture to the object or lower the temperature inside the basement below 55 degrees Fahrenheit."

Say it's 95 F outside temperature, with a very high relative humidity, such as 85%. Say that my house is air conditioned and the house's RH is much lower (or maybe this is where my logic is flawed...can the RH inside the house be lower?)

Anyway, say that the RH inside the house is lower. Couldn't the higher RH outside, reach the basement masonry wall, permeate through the cement, and now have a much higher RH because of the temperature drop? If that is true, then you would have a good chance that the Dew Point temperature could be reached inside the wall, and since the vapor barrier is on the inside of the wall facing the interior, that moisture now cannot escape or dry to the interior.

Is my logic way off here?

Thanks in advance. I'm here to learn as much as possible. :)

Eric

Oversizing of air conditioners is probably the number one cause for many of the Indoor Air Quality (IAQ) problems that exists today. Your concern is certainly warranted but it falls under abnormal circumstances.

If you have central air conditioning then you should be familar with condensation drains/pipes attached to the evaporator coil. This is how humidity problems are avoided inside homes during the summer when such applicances are used to drop the temperature inside the home. In fact manufacturers use National data to design their evaporator coils to extract humidity from household air as it passes through the evaporator coil. It is known as the "Sensible Heat Factor" (SHF). For example. if you lived in Arizona the SHF would be quite high so it would extract as little moisture as the household air passed through the evaporator coil because the air is already dry in Arizona. Whereas if you lived in an area that experienced very high humidity during the summer, the SHF would be quite low so it would extract as much humidity from household air as it passed through the evaporator coil. This extraction results in the lowering of RH% throughout the house, including the basement. Because ErH% still apllies.

The reason why your concern falls under abnormal circumstances is because evaporator coils need a considerable more time to extract humidity from air than it does to drop the temperature of the same air. Oversizing drops the temperature of the air quickly but does not extract enough humidity from that air. The result is the moisture problem you are referring to, regardless of the SHF.

So undersizing your air conditioner is actually better than proper sizing it according to ASHREA. Because it removes more humidity from the house. The system must run longer in order to reach your desired temperature. And the longer it runs, the more humidity the unit will extract.

Given that cicumstance, though, even though it may be abnormal, wouldn't it be detrimental to have a vapor barrier installed?

I live in Chicago where the summers can get very hot and humid and I don't want to be trapping any moisture inside the basement wall.

So would you vote be to use a vapor barrier on the inside of the wall (facing the interior)?

Thanks,
Eric

I would vote for the vapor barrier because it would not be the source of the moisture problem during the summer and the benefits it has for the winter. If heat and humidity outside during the summer is the source of a possible moisture problem, then what you do inside your home during the summer determines the probability of a moisture problem arising. For example, what would be the probability of a moisture problem arising if you didn't use air conditioning? The answer is NIL. How about if the unit is oversized? VERY LIKELY. Properly sized according to ASHREA? UNLIKELY. Undersized? VERY UNLIKELY. Which one of these addresses or produces your desired results? Does the application of a vapor barrier have anything to do with addressing the source of the moisture during the summer or producing the results you desire?

After what we discussed here, if you know someone who had a moisture problem and was told it was due to the vapor barrier, do you honestly think that the removal of the vapor barrier resolves their situation?

Resercon,

Thanks for all of your help and time. I truly appreciate it.

Eric