• Welcome to the new and improved Building Code Forum. We appreciate you being here and hope that you are getting the information that you need concerning all codes of the building trades. This is a free forum to the public due to the generosity of the Sawhorses, Corporate Supporters and Supporters who have upgraded their accounts. If you would like to have improved access to the forum please upgrade to Sawhorse by first logging in then clicking here: Upgrades

Tyvek

mfichter80

Registered User
Joined
Nov 4, 2016
Messages
36
Location
Virginia
You know, you can't have a dual barrier, somewhere sometime water is going to get inside the wall and needs to dry in or dry out, put sheetrock or plaster on the inside and it has to dry out, Zipwall seals it in.

I have no skin in this game, just looking for different perspectives on this. But my understanding is that tyvek, tar paper, and I assume the zip system do not (or are not intended to) completely seal moisture in. The perforations in tyvek are supposed to allow moisture to evaporate out, but keep water from leaking in. I always thought tar paper made more sense though, and would hold together a lot better.

I would think the only time something similar to tyvek is actually on the inside of a wall would be on the inside of masonry or concrete foundations and crawls, when there are no studs on the wall.

And I have never heard of putting sheathing over tyvek... I guess that would be similar to putting the tyvek around the outside of the studs and putting t-111 over that?

Anyway there is a difference between the "puffy jacket" thing of completely sealing a house with a membrane, foam insulation, etc. vs. just having something like tyvek as a vapor barrier, where it should be able to dry out. In addition to the potential for water damage, for how freaked out everyone is about mold, and considering the real dangers related to radon, I have always thought making a house completely water and air tight is absurd... but for some reason people are doing it, and it's not cheap either.
 

JBI

Registered User
Joined
Oct 17, 2009
Messages
2,788
Location
The Empire State
If tyvek is a "Water-Resistive" barrier which is different than a "Vapor Retarder" that is required under slabs.
What's the difference?
Water resistive barrier prevents liquid water from penetrating into the assembly but is vapor permeable allowing water vapor to escape, a vapor retarder impedes water vapor from passing into the assembly.
 

tmurray

Registered User
Joined
Jun 10, 2011
Messages
2,138
Location
NB, Canada
If tyvek is a "Water-Resistive" barrier which is different than a "Vapor Retarder" that is required under slabs.
What's the difference?
Water resistive barriers do not allow the passage of liquid phase water. Vapour retarders slow the passage of gas phase water (vapour). Important to note that the poly underneath a slab is not there for vapour resistive purposes, but as a capillary break. The slab itself may be considered a vapour barrier. Keep in mind that just because a building material absorbs water, doesn't meant it can't be a vapour barrier. Stud wood is another good example of a material that absorbs water, but is also a vapour barrier.
 

mfichter80

Registered User
Joined
Nov 4, 2016
Messages
36
Location
Virginia
If tyvek is a "Water-Resistive" barrier which is different than a "Vapor Retarder" that is required under slabs.
What's the difference?

I don't know the actual definitions of these terms, it's just like mobile home, manufactured home, industrialized building, trailer, tiny home... other than "industrialized building", all the other terms can mean different things depending on who you are talking to or what you are talking about. But the bottom line is that that tyvek has holes in it, and what you put under your slab doesn't. The holes in tyvek are supposed to be small enough to keep out drops of water, but big enough to allow vapor to evaporate out from behind it, because vapor particles are smaller than water droplets. As far as I can tell tar paper does the same thing, so I don't know why tyvek is supposed to be better.

Usually what goes under a slab is solid plastic. Around foundations and under slabs you want to block water out of any kind, because if water drains under the slab, basement wall, or foundation, it evaporates up onto the floor, or into the crawl or basement. It rises up into the area you are trying to protect.

Under a building, depending on the grade and density of the ground, gravity and dirt will keep water from draining or evaporating away from a structure, but in a wall it's not as hard to direct the vapor away from the structure.
 

tmurray

Registered User
Joined
Jun 10, 2011
Messages
2,138
Location
NB, Canada
I don't know the actual definitions of these terms, it's just like mobile home, manufactured home, industrialized building, trailer, tiny home... other than "industrialized building", all the other terms can mean different things depending on who you are talking to or what you are talking about. But the bottom line is that that tyvek has holes in it, and what you put under your slab doesn't. The holes in tyvek are supposed to be small enough to keep out drops of water, but big enough to allow vapor to evaporate out from behind it, because vapor particles are smaller than water droplets. As far as I can tell tar paper does the same thing, so I don't know why tyvek is supposed to be better.

Just to be clear, Tyvek (and other WRBs) allow vapour to pass in both directions. This is an issue along coastal regions where capillary moisture between the WRB and the siding is pushed into the building envelope when that wall surface gets exposed to the sun.

Tyvek is better because their marketing says so. Tar paper has an advantage in that the permeability rate varies with the amount of moisture, while WRBs mostly have a static permeability rating. Really, neither is better than the other, it all depends on how you design your enclosure. Just different tools for different jobs.
 
  • Like
Reactions: JBI

Yikes

Gold Member
Joined
Nov 2, 2009
Messages
1,903
Location
Southern California
Water resistive barriers do not allow the passage of liquid phase water. Vapour retarders slow the passage of gas phase water (vapour). Important to note that the poly underneath a slab is not there for vapour resistive purposes, but as a capillary break. The slab itself may be considered a vapour barrier. .

The technical support reps at Stego Wrap say that the air gaps between aggregate underneath the poly is the capillary break, and the poly is the vapor barrier, and concrete has tiny pores that can make it act like a sponge.
With our buildings being sealed so efficiently these days, you can actually get a water vapor pressure differential via your HVAC system. Turn on your air conditioner, and the room is dehumidified. Meanwhile, the soil under the foundation has moisture in it, and in vapor form it eventually gets sucked up into the slab towards the "low pressure weather system" that is your room interior. If you have VCT, some of that water lingers on the underside of the tiles, and today's water-based glues can't stick when the slab stays moist. The next thing you know the building owner is yelling at the contractor that their building tiles are popping up. I've seen this happen in buildings in the Arizona desert in summer!

To prevent this:
1. Use aggregate, not dirt or sand, below the slab to create a robust capillary air break.
2. Put the poly vapor barrier right on top of the aggregate. (This where are reinforced barrier will help prevent tears.) Seal all edges and penetrations; they make special tapes and adhesives for this.
3. Put the floor rebar system on top of the barrier. Do not add a layer of protective sand (explanation to follow below*). A good reinforced vapor barrier will help prevent punctures during construction.
4. Pour the concrete. Keep only as moist as necessary, but don't over-water (e.g. don't run a sprinkler 24/7 on top of the slab).

*The reason you don't want a protective sand layer on top of a well-sealed vapor barrier is that during the concrete pour, the sand will hold a lot of moisture form the concrete mix, but it can't drain downward (because congratulations, you did such a good job of sealing the barrier). It is too easy to over-water the slab during its curing period. If the sand gets soaked in that situation, it may take 12-18 months to dry out enough to glue down flooring.
 

tmurray

Registered User
Joined
Jun 10, 2011
Messages
2,138
Location
NB, Canada
The technical support reps at Stego Wrap say that the air gaps between aggregate underneath the poly is the capillary break, and the poly is the vapor barrier, and concrete has tiny pores that can make it act like a sponge.
With our buildings being sealed so efficiently these days, you can actually get a water vapor pressure differential via your HVAC system. Turn on your air conditioner, and the room is dehumidified. Meanwhile, the soil under the foundation has moisture in it, and in vapor form it eventually gets sucked up into the slab towards the "low pressure weather system" that is your room interior. If you have VCT, some of that water lingers on the underside of the tiles, and today's water-based glues can't stick when the slab stays moist. The next thing you know the building owner is yelling at the contractor that their building tiles are popping up. I've seen this happen in buildings in the Arizona desert in summer!

To prevent this:
1. Use aggregate, not dirt or sand, below the slab to create a robust capillary air break.
2. Put the poly vapor barrier right on top of the aggregate. (This where are reinforced barrier will help prevent tears.) Seal all edges and penetrations; they make special tapes and adhesives for this.
3. Put the floor rebar system on top of the barrier. Do not add a layer of protective sand (explanation to follow below*). A good reinforced vapor barrier will help prevent punctures during construction.
4. Pour the concrete. Keep only as moist as necessary, but don't over-water (e.g. don't run a sprinkler 24/7 on top of the slab).

*The reason you don't want a protective sand layer on top of a well-sealed vapor barrier is that during the concrete pour, the sand will hold a lot of moisture form the concrete mix, but it can't drain downward (because congratulations, you did such a good job of sealing the barrier). It is too easy to over-water the slab during its curing period. If the sand gets soaked in that situation, it may take 12-18 months to dry out enough to glue down flooring.

Be careful not to confuse moisture moving through assemblies in different phases. Just because a product prevents the passage of moisture doesn't mean it won't absorb water. concrete and lumber both have permeability rates less than 1 perm (60ng/(s*Pa*m^2) in Canada, man I wish I could measure permeability like you folks), but both are hygroscopic and will readily absorb water. This makes a concrete foundation ideal to prevent soil gas infiltration into most buildings, but a capillary break is required below the concrete. The major issue is that most contractors do not use a capillary break beneath the footing, so unless unless there is another capillary break, say between the footing and the wall and slab, you've just created a capillary bridge. Water will travel about 10 kilometers high in concrete, so it can travel quite a distance.
 

ADAguy

Sawhorse
Joined
Sep 11, 2013
Messages
5,985
Location
California
There is strange language in the code stating that the WRB can be omitted: . 3) Under paperbacked stucco lath when the paper, Paperbacked stucco lath is two layers of Grade D paper so it is an approved water-resistive barrier. Because of the problem you reference I go to great lengths and expense to eliminate the plywood sheathing, including getting the engineer to approve putting the diaphragm plywood on the interior rather than the exterior and it some cases installing a full steel frame. The last home I built I installed a full steel frame because in our seismic zone the engineers require so much steel that it really isn't a larger expense. In a prior house I had $30,000 worth of Simpson metal which probably cost me $60,000 to install and there were still $30,000 worth of steel moment frames, on the last house I did a full steel framed for $120,000 with probably $10,000 worth of Simpson and labor. Most affluent owners will pay the extra cost to get a better building. I then use old fashioned line-wire and 4 coat stucco so there is no sheathing to rot out and seal up the walls.

The fact is that walls have to breathe to dry out from the inevitable moisture that is eventually going to get in, plywood seals the exterior and sheetrock seals the interior, if you use plywood sheathing you are creating a dual barrier sealing moisture in. In the old days before plywood we used 1x8 boards for sheathing, and that is still in the prescriptive code, we used low grade 1x8 full of knots and green, when the knots fell out and the boards dried out there was plenty of room for the moisture to dry out the exterior.

Thank you for reminding me how "old" I am as I know of what you speak. So much is unknown/forgotten by old and young tradesmen alike. Something to be said for institutional memory loss/retention. You are appreciated for this (smiling).
 
Top