Proper vapor barrier?

[Scott Falvey]

Can you use Tyvek for a moisture barrier on the ground in a crawl space?

Isn't Tyvek a retarder?, although it is a little nicer to crawl on then plastic.

[David P. Valley]

Originally Posted by sfalvey

Can you use Tyvek for a moisture barrier on the ground in a crawl space?

Isn't Tyvek a retarder?, although it is a little nicer to crawl on then plastic.

Scott,

Tyvek® is not a vapor barrier. It is a breathable fabric which allows moisture and vapor to pass right through it. It is wrong. You want to see 6 mil (minimum) sheets of polyethylene throughout the entire floor area. I've got a great web page describing how to properly cover a crawl space dirt floor.

HERE

[Scott Falvey]

Thanks Dave,

That's what I thought, but wanted a second opinion.

[Scott Falvey]

How about being attached to the floor joists in a crawl space? Do you want that as a barrier or a retarder?

[Richard A. Hetzel]

You don't want Tyvek there at all.

[Scott Falvey]

What do you think of this?
http://apps1.eere.energy.gov/consume.../mytopic=11480

Go to the bottom of the page and read #3.

[David P. Valley]

Originally Posted by sfalvey

What do you think of this?
http://apps1.eere.energy.gov/consume.../mytopic=11480

Go to the bottom of the page and read #3.

Though it is true that many people in the construction industry will use these two terms synonymously, there is a very big difference between the two.

A vapor barrier is a material (typically plastic), that resists passage of moisture through wall, ceiling and floor assemblies of buildings. It is used to separate an environment which is at a high vapor pressure from an adjacent one at a lower vapor pressure.

A vapor retarder is the material on the outer side of insulation. It helps control the amount of moisture passing through the insulation and collecting on exterior walls, ceilings and floors. In my area, this retarder goes on the warm side of the building. In hotter climates, this material goes on the most outer side of a building.

[Carl Brown]

Isn't Tyvek a retarder?,

Nope just RETARDED!

IMO

[Richard A. Hetzel]

Originally Posted by sfalvey

What do you think of this?
http://apps1.eere.energy.gov/consume.../mytopic=11480

Go to the bottom of the page and read #3.

Then click on the highlighted words, and read the document that follows. The wording of that #3 is a little unclear and inaccurate. House wrap, or "Tyvek", is not a vapor retarder, although it is a moisture barrier. It stops water, but not vapor. A "vapor retarder" stops both.

[Carl Brown]

Tyvek properly installed on a wall to duponts specs can also trap water in the wall .

imo

[Brian A. MacNeish]

Originally Posted by dvalley

Though it is true that many people in the construction industry will use these two terms synonymously, there is a very big difference between the two.
HUH???

A vapor barrier is a material (typically plastic), that resists passage of moisture through wall, ceiling and floor assemblies of buildings. It is used to separate an environment which is at a high vapor pressure from an adjacent one at a lower vapor pressure.

A vapor retarder is the material on the outer side of insulation.?????? It helps control the amount of moisture passing through the insulation and collecting on exterior walls, ceilings and floors. In my area, this retarder goes on the warm side of the building. In hotter climates, this material goes on the most outer side of a building.

If it is on the outer side of the insulation, how can it be on the warm side of the building?

Yes, the above terms are used interchangeably but somewhat correctly; "vapour retarder" is actually the better term to use unless you have the perfect vapour "barrier" that has a "ZERO" perm rating....... very thick foil or sheet metal would be in this category.....which is a very rare occurrence.

The term being used for housewraps, building papers, trowel-on and some peel & stick products installed over exterior sheathing to shed bulk water, allow any moisture in a wall cavity to diffuse to the exterior and possibly provide an exterior air barrier is "sheathing membrane". See: http://www.buildingscience.com/gloss...glossary_terms

From Pathnet: http://rehabadvisor.pathnet.org/sp.asp?id=10291

Vapor Barriers or Vapor Retarders
A vapor retarder is a specially treated paper, thin plastic sheeting, or low permeance paint that prevents condensation of water vapor inside wall or ceiling materials. This trapped moisture can cause damage to the wallboard and paint as well as structural deterioration.
The vapor retarder must have a vapor permeance (the rate that moisture can pass through materials) of not more than 1.0 perm (tested in accordance with ASTM Standard E96-80).
The term "vapor barrier", which is also commonly used, is somewhat misleading since it does not completely bar the transmission of water vapor. A vapor barrier is actually a vapor-resistant membrane, and is more properly called a "vapor retarder".

Other sites:
http://www.daviddarling.info/encyclo..._retarder.html
http://www.insulate.org/tb6.pdf
http://www.cmhc-schl.gc.ca/en/inpr/b...codemo_049.cfm


This has some interesting research mentioned about not using vapour retarders (barriers) at all!!!!!!?????? Anyone remember my question of a few weeks ago on the IR thread??
http://www.coler.com/pdf/Vapor%20Barrier%20Info.pdf

From the DOE site- Energy Efficiency and Renewable Energy (EERE):
http://apps1.eere.energy.gov/consume.../mytopic=11810

"Effective moisture control in these areas and throughout a home includes air sealing gaps in the structure, not just the use of a vapor diffusion retarder."
and.......
"Most paint-like coatings also retard vapor diffusion. While it was once believed that only coatings with low perm ratings constituted the only effective vapor diffusion retarders, it is now believed that any paint or coating is effective at restricting most water vapor diffusion in milder climates."


IMO, in these matters, some codes, practices, and general on-the-street knowledge lag the research by 15-40+ years!!!!


From a Canada Mortgage and Housing Corporation 1977 document titled "The Conservation of Energy in Housing":

"In order to control the formation of condensation one must underastand the processes that cause it. The old theory was that condensation was produced by entirely by the diffusion of water vapour......

The problem with this theory is that it is incomplete: it does not take into account the larger causal factors. As a result, the preventative measures are often ineffective. Many dwellings constructed on the principles outlined above have experienced severe rot from excessive condensation...... The main contributor to excessive condensation, that is massively greater in effect than is vapour diffusion, is the movement of air.

It is now understood that the excessive formation of condensation is largely caused by the flow of warm indoor air into the insulated enclosure. This air carries with it a load of water vapour, which condenses when it comes into contact with the cold outer surfaces of the enclosure. Vent spaces in insulated walls and some flat roofs and vent openings placed in the exterior of the building envelope can contribute to the condensation process by promoting more movement of air from the interior through the envelope. This effect is exactly opposite to the purpose that such vents are intended to serve.........

In existing housing, it is a matter of experience that fully insulated walls without vapour barriers or venting remain dry and sound given that interior humidity levels are controlled"

Selected quotes from a May 1994 issue of an Energy Design Update article titled ORDINARY PAINT AS REPLACEMENT FOR POLY VAPOR BARRIER:


"New research evidence and field experience are leadng to the conclusion, that even in cold regions, poly vapor barriers are probably overkill and that ordinary paint should provide adequate resistance to moisture difusion through walls and ceilings."

"These assertions are not new. Canadian building scientist Gus Handegord (a Joe Lstiburek mentor) published similar findings as early as 1950."

"Three extensive field studies by George Tsongas, professer of mechanical engineering at Portland State Univesity, found virtually no realtionship between vapor retarder presence and moisture content in walls. A fourth study conducted last year did find a presence between air leakage sites in walls and moisture"
Note: I have pictures of sheathing rot occurring only behind interior mounted switch and receptacle boxes on exterior walls.

[Brian A. MacNeish]

Originally Posted by cbrown1

Tyvek properly installed on a wall to duponts specs can also trap water in the wall .

imo

Can you expand on your theory, Carl?

In 1994/5, I worked on some of the first TYVEK "failures" found here....rotting wood sheathing and trim (Halifax, Nova Scotia where we have horizontal wind driven rains several times a year). The first home found won the architect an award in 1989 but was severely rotting 5 years later (IMHO, we should take the award away from him!!!). Phd building scientists (even one from Dupont) were flown in as more exterior sheathing rot was found.

The first day that the young engineer from my department came back from the field speaking increduously about the rot found in this architect's very dry interior home (only 2 occupants in 2,500+ sq ft), I brought out my $4 booklet purchased from Canada Mortgage and Housing Corporation (CMHC) in 1985 that described the "rainscreen principle" and why it is needed. (That's when I was asked to participate.) It's finally a code requirement here now, but only since 2004 even though I tried to get an architect friend of mine (he adopts/manages the code for the province) to change the code about this element in 1996/7..........so we have another 7 years of built housing of which some that may be/will be experiencing wall rot!!! SAD!! SAD!!! SAD!!!

Anyways......IMO, it was not a TYVEK problem here. It was an architect/builder problem for simply accepting a new product without doing a bit of research or simply calling/dropping into a CMHC office on a regular basis to check for new publications. This same type of rot was occuring with old style tar/building paper.....it only took longer!!

Constant re-education must be a requirement for the building profession!!!

[Carl Brown]

No way should a horizontal joint be taped on any moisture barrier!

When they install their flex wrap in the sills of openings it is reverse lapped on the sides.

All moisture barriers need to over lap all flashings to exit the water to the exterior of the cladding.

[Brian A. MacNeish]

Originally Posted by cbrown1

No way should a horizontal joint be taped on any moisture barrier!
What if the top piece of sheathing membrane overlaps the lower one properly?

There is a detail I have where there is a horizontal slit is made in the housewrap over the window and the vertical leg of the drip cap/window Z flashing is inserted up behind the wrap and then the flashing/wrap joint is taped horizontally

When they install their flex wrap in the sills of openings it is reverse lapped on the sides.

All moisture barriers need to over lap all flashings to exit the water to the exterior of the cladding.

Where are the details that you mention located?

[Richard A. Hetzel]

If it is on the outer side of the insulation, how can it be on the warm side of the building?

The sentence is badly wriiten, but I think they mean the "outer surface" of the insulation itself, which is, of course, to be installed on the winter warm side of the wall or assembly.