Faced insulation, plastic & drywall??


Old 02-24-06, 07:09 AM
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Join Date: Jul 2004
Posts: 53
Faced insulation, plastic & drywall??

I'm installing insulation and new drywall in our house, and I had a question. Our house is 150 years old, and after ripping off all the old plaster & lathe, we noticed that there are some places in the wall where you can see light sneaking in around the old exterior siding (a layer of of pine shingles covered with a layer of the asbestos tile). There's no exterior insulation or plywood, which we'll address when we decide to re-side the house (after we find a way to remove the asbestos shingles that won't cost us an arm & a leg).

I re-framed the walls to be plumb & flush, put insulation in the walls (faced batts with the vapor barrier on the interior), and then put up a large sheet of 6 mm plastic to help keep the cold out until we're ready to put up the drywall.

Is there any problem leaving the plastic up and putting the drywall up right over it? Any chance of getting moisture trapped between the plastic & the drywall?? Does it create a better seal to leave the plastic up??

Any input??

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Old 02-24-06, 10:07 AM
Join Date: Nov 2001
Location: USA
Posts: 1,873

This site discusses vapor barriers also known as "Vapor Diffusion Retarders" (VDR's).

Your applcation is quite common among insulating contractors that incorporate air sealing. While we hear a lot about dual vapor barriers and the 5 to 1 rule, installing plastic sheets directly over faced insulation does not cause a moisture problem if installed correctly. The reason for this has to do with Relative Humidity (RH%) and the fact that moisture flow during the winter is pressure induced.

RH% reflects how moisture behaves at different temperatures in objects. For example air (an object) has 50RH% at 60 degrees Fahrenheit will increase in RH% as the temperature of this air drops and will decrease in RH% as the temperature of the air increases. In your application because both the faced insulation and plastic sheets (objects) are installed on the warm side of the wall/ceiling the temperature of those objects will be very close to the temperature inside the house. Even though both objects impede moisture flow, it is highly unlikely that "Dew Point Temperature" will ever be reached on those two (2) objects.

Equilibrium Relative Humidty (ErH%) uses a simple rule found throughout nature known as "High to Low". This states that an object of higher humidity will give humidity to an object of lower humidity it comes in contact with until the humidity levels in both objects are equal. In other words the object of higher humidity will lose humidity by giving it to an object of lower humidity it is in contact with. The lower humidity object will increase in humidity as it absorbs that humidity coming from the object of higher humidity. Temperature in objects behave the same way.

Pressure induced moisture flow is an exception to the rule High to Low. In other words Pressure Induced Moisture Flow can give moisture from an object of lower humidity to objects of higher humidity. This pressure comes from the heating of the house during the winter. As the air inside the house is heated, it expands. Unlike a balloon that would get bigger as the air expanded inside it, the house cannot get bigger. Though the pressure created here is quite slight, as little as 2 or 3 Pascals, this along with heat flow that follows the rule high to low influences moisture flow inside your home during the winter. Vapor barrier resolve this problem by impeding moisture flow. A Moisture Problem under these circumstances is defined as the introduction of moisture to the objects that exceeds the ability of those objects to asborb and expel the moisture. In other words vapor barriers provide the time needed for objects inside wall and ceiling cavities to absorb and expel the moisture being introduced.

This brings us to the purpose of the plastic sheets. The expansion of the air inside your home during the winter caused by heating can force warm air into your wall and ceiling cavities. In effect it can by-pass your vapor barrier. So the plastic sheets purpose is not as used as much as a vapor barrier as it is used as a "Air Barrier". The industry that uses this application is known as "Air Sealing".

One of the misconceptions to Air Sealing is that it is designed to reduce air exchange in homes. NO! Air Sealing is designed to direct where the air exchange occurs. You do not want the air exchange to occur inside your wall and ceiling cavities, where a moisture problem can arise if it did. In fact the average home today has their roof life expectancies cut in half due to air leakage. Especially when these plywood and particle boards delaminate from moisture. So air sealing has more to do with preserving the integrety of the structure and "Sustaining a Healthy Indoor Environment" than it has to do with reduction of air exchange. Though air sealing does reduce air exchange, that is not its goal or purpose.

It is good to note that Minnesota leads this country in the Air Sealng industry. For example the Minneapolis Blower Door is the standard in this industry. Though there are other manufacturers of this product it is still referred to as it. Just like Xerox is to copiers. Besides the fact that Minnesota's practice towards "National Energy Code" far exceeds every state in the nation. In fact the phrase "Sustainable Healthy Indoor Environments" came from their university. Who has a ton of research and seminars on the topic of "Air Sealing". As an Energy Conservationist at one time my concern was the reduction of one's energy costs. Today my decisions are influenced more about that health and well-being of the occupants of dwellings than cost reduction. Most of this attitude is a result of what and is happening in Minnesota. For those interested, you may want to go to their University's website.

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