tyvek or polystyrene on old house?

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  #1  
Old 05-24-03, 07:10 PM
ahlala
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tyvek or polystyrene on old house?

Hello,

I am planning to redo the exterior of my house. It is an old house (beginning of the century) in a cold climate (Morin-Heights, north of Montreal, Canada). The walls have been insulated from the inside with fiberglass wool bats (3.5inches). The house is not airtight at all.

There are two options:

1) Either I just put a Tyvek-wrap over the existing wood shingles, and then install the new facing. Will this make a real difference? Does anyone have experience with this kind of work?

or

2) I take the old shingles off, install blue foamboard (1inch), tape em, and then install the new facing. The question here would be, what are the risks of condensation within the wall related to the new blue polysterene board (it works like a vapor-barrier, and the interior vapor barrier is far from perfect from what I can see)?

Thanks for any help,

MM
 
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  #2  
Old 05-24-03, 09:52 PM
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http://www.eere.energy.gov/consumeri...riefs/bd4.html This is a U.S. Dept. of Energy Brief on Vapor barriers and air barriers. The brief discusses several of your concerns. The first is "Thermal Moisture Dynamic". It explains that the majority of moisture related problems concerning insulation is not due to heat transported moisture but due to air transported moisture. Tyvek, similar materials and air sealing industry were created based on this conclusion.

The second is "Perm Ratings". All materials have a perm rating, wood, building felt, Tyvek, siding materials, etc. To qualify as a vapor barrier, the material must have a perm rating of 1 or less. Most paint will have a perm rating near 1. So besides the vapor barrier you installed, the paint you applied to the wall will act as a vapor barrier. The brief gives a rule of thumb for materials applied after the vapor barrier in which the heat travels. This is 5 to 1. What this means is any materials applied to the outside of the vapor barrier must have a perm rating 5 times greater than the vapor barrier. This is why most professionals use a vapor barrier that has a perm rating of .1 or less. Almost all common materials applied to structures will have a 5 times greater perm rating than .1, including your pink board.

The brief also discusses air barriers and most of it is about Tyvek. The true advantage to Tyvek is wind driven rain. Tyvek is specifically designed to have a very high perm rating and will not trap moisture inside your walls. One of the biggest problems with Tyvek is applying certain wood and manufactured siding products over it. For example, cedar has a natural occurring substance in it known as LIGNIN, which is a detergent. As with all detergents, it reduces surface tension. When the cedar is hit with wind driven rain, the Lignin in the cedar degrades the water repelling properties of the Tyvek. This usually results in moisture problems inside your walls.

This brief is one of the better ones the DOE has written, however, I would have written it differently or would have stressed the purpose of air barriers more. I would have not included Tyvek and similar materials/products as an air barrier. Though many of my collegues would disagree with me. In my opinion the air barrier should be applied with the vapor barrier. The brief discusses this in the air/vapor barrier section. Again in my opinion this is poorly written and in conjunction with the other topics it either misleads or confuses the reader.

If this was my house, I would choose the Tyvek because of its ability to prevent the adverse effects caused by wind driven rain. I would still use it even if the siding contained Lignin, except that I would apply building felt over the Tyvek. Thereby prohibiting the Lignin for degrading the water repelling properties of the Tyvek. I think prohibiting the effects of wind driven rain to the structure is that important. Furthermore, the pink board is a poor substrate for any type of siding, it just doesn't provide a good nailing and secure surface even with battens. Then I would turn my attention to the inside of the house for any wall penetrations like electrical outlets and switches, the casements around windows and doors. The air barrier does not stop air exchange in the home, it directs the air exchange where it could do no damage or harm to the occupants of the home, unlike inside your walls, floors and ceilings.
 
  #3  
Old 05-25-03, 03:15 PM
ahlala
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thanks for your ideas and the url.

But when you say " I would have not included Tyvek and similar materials/products as an air barrier. Though many of my collegues would disagree with me. In my opinion the air barrier should be applied with the vapor barrier. The brief discusses this in the air/vapor barrier section. "
How would that work in a cold climate? Wouldn't you get tons of condensation within the wall???

Otherwise, I think I'll go for the tyvek option and keep the existing shingles underneath. These shingles are cedar but have been painted. Is there still a lignine problem in relation to the tyvek in this case??
 
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Old 05-25-03, 08:28 PM
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Relative Humidity (RH%)

The Lignin would only come out of the cedar if it got wet and was wind driven into the Tyvek. So if the cedar is under the Tyvek and let's say vynal siding is put over the Tyvek, the Tyvek would prohibit the cedar from getting wet. The Lignin in the cedar in this application could not get to the Tyvek to degrade its water repelling properties.

Again the brief is not well written and it has a tendancy to confuse or mislead the reader. In cold climates the air/vapor barrier should be applied to the warm side of the wall (inside). First reason is air from the outside removes humidity from the structure which is good. This is because the cold air can hold less humidity than warm air. For example, air at a temperature of 32 degrees with a RH% of 82%, when this air temperature is raised to 72 degrees Fahrenheit the RH% drops to 17%. This explains why condensation occurs when air temperature is dropping and not rising. In order for condensation to occur the air must be at 100% Humidity or absolute humidity. This cannot be accomplished with air temperature rising, it can only be done with air temperature dropping. Assuming no other source of humidity is available.

If we apply this concept to a structure in a cold climate, what exactly does happen? Cold air from the outside infiltrates the wall. The wall is warmer than the cold air causing the cold air temperature to rise. Can condensation occur under these conditions?

On the other side of the wall, inside the home, is warm air that infiltrates the wall. The temperature inside the wall is cooler than the warm air inside the home and as this air infiltrates the wall it causes the air temperature to drop. Can condensation occur under these conditions?

I base my position on applying the air/vapor barrier to the inside and the warm side of walls in cold climates on this. What further supports my position is Equilibrium Relative Humidity (ErH%). This states that air or objects at a lower humidity level will extract humidity from other air or objects at a higher humidity level until the humidity levels in the air and objects are equal. And vice-versa.

If we apply this to the air infiltration examples mentioned above air temperature rising reduces RH% of the air giving it the ability to extract humidity from the wall. Whereas air temperature dropping increases RH% of the air giving it the ability to add humidity to the wall.

To simplify this is cold air removes humidity from the structure, warm air adds humidity to it. Tyvek stops cold air from infiltrating the wall, does it stop the warm air inside the home from infiltrating the wall? The air barrier in cold climates should be applied to the inside or warm side of the wall.
 
  #5  
Old 05-27-03, 07:12 AM
ahlala
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Now I better understand what you mean by applying air/vapor membrane on the warm side.

This is how, I gather, the vapor barrier in the attic is conceptualized and this is how it functions too. No one puts tyvek over the insulation in the attic.

However, with an old house where there are ex-exterior walls that are encased witin the enveloppe of the present house (because of additions, meaning all kinds of strange joints and hard to reach spaces) and where there are electrical outlets from previous electrical systems hidden within the walls, it is very hard to be thorough from the inside. While from the outside, it is in my case easier to apply a (air barrier) membrane that will direct moisture-charged air away from the insulation.

Also, do you know of a site that would give R-values of different materials like shingles, and plywood. What is the R-value of three inches of wood would be my question.
 
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Old 05-27-03, 12:16 PM
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Again I'd like to stress the brief is poorly written even though it is one of the better ones the DOE puts out.

I would not have put Tyvek and similar products in this brief and I would have avoided naming it a air barrier. The true value of Tyvek is that it protects the structure from the affects of wind driven rain. The brief misleads or confuses the reader because of the controversy over vapor barrier application in different climates. It suggests that Tyvek as an alternative to applying vapor barriers on the outside of walls in warm humid climate areas.

My position is based on logic. What causes the warm air in the home in the winter into your walls is the heating of this air. As you heat air in a confined space like your home, this air expands and if allowed will enter your walls. The probability of condensation occurring is very high. Hence the need to prohibit that from occurring, air barrier. On the other hand, the warm air outside during the summer does too expand, but it is not confined. Because the air outside is not confined, the expansion does not force the warm moist air into your walls, unlike inside your home during the heating months.

The other side argues that air pressure effects are dwarfted by Tyvek and similar products and as such effectively describes these products as air barriers. In my opinion, this is a bunch of BULL! The air barrier industry, along with ASHREA, Oak Ridge, Berkley, TVA, DOE, EPA and everyone else for that matter in this or related industries know that moisture related problems in structures today have a direct link to ineffective air barriers of the homes that led to condensation and the growth of toxic mold.

There is no data or research that can support that wind pressure either in the winter or summer on a home that resulted in condensation. So in essence describing Tyvek and similar products as air barriers is manipulating the meaning of the term for what it is intended for. That is the prevention of condensation inside structual components of the home.

What further supports my position is your acknowlgement that Tyvek is not applied to attic and you will not see them in floors either. The reason for this is your attic is protected by your roof and the floor is protected by either a basement, crawl space or on a slab. This results in either your attic or floor being unaffected by wind pressure. However, your walls are affected by wind pressure and Tyvek is applied there.

The thermal boundary parallels the air boundary. Since insulation is installed in walls, attics and floors that separate conditioned spaces from unconditioned spaces, therefore the air barrier include walls, attics and floors.

Tyvek and similar products have their usefulness and do have considerably value to the user. However, do not be misled or confused that this is an effective air barrier. In my opinion, it is not an air barrier rather a better term would be a wind screen.

Wood siding has a little bit more than R-1.5 per inch. Siding or other members of the structure is not considered when it comes to building code. So regardless of the R-value the siding may have if code requires R-19 in walls, it pertains to the insulation only. The only time the R-values of materials of structures are used are in heat load/gain calculations. In most cases these are defaulted.
 
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