Is Interior Paint the Real Vapor Barrier?

[hank mavery]

In this area many use poly over studs and joists/rafters before installing drywall. It's intended to be the vapor barrier. I'm questioning whether or not it is actually the interior paint that is serving this purpose of the vapor barrier or retarder.
If the paint weren't stopping the vapor where would the moisture escape to once it was trapped behind the drywall and in front of the plastic barrier? Doesn't it seem like a bad idea to let the moisture be held here? It seems it would be easier to handle the moisture inside of the house if it was obstructed from entering the drywall by the interior paint. Maybe people aren't using the plastic any longer and we just haven't caught on?

Here's an old study by the Forest Products Laboratory done in 1978 in Madison, WI on interior paint as a vapor barrier.

http://www.fpl.fs.fed.us/documnts/fplrp/fplrp319.pdf

Conclusion
"Adding insulation to walls of an older house with no vapor. barrier subject to the climate of Madison, Wis., is likely to result in critical levels of condensation in the walls where mechanical humidification is used to maintain 35 percent relative humidity or higher.
Two coats of interior semigloss acrylic latex, exterior acrylic latex, exterior soya-alkyd resin, or aluminum paint can be effective in keeping the moisture level of wood in the wall cavity below 20 percent even where 35 percent relative humidity is maintained. A I-inch-diameter vent at the top of the wall cavity is less effective than using these paints to keep the wall dry, but more effective than using vents at both top and bottom of the wall cavity.
It would be helpful to those considering insulating walls of existing homes if commercially available paints were labeled with perm ratings."
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[dave_k]

If the paint weren't stopping the vapor where would the moisture escape to once it was trapped behind the drywall and in front of the plastic barrier? Doesn't it seem like a bad idea to let the moisture be held here?

First the air/vapour barrier should be on the warm side of the building envelope well before the air temperature reaches the dew point. no dew point no condensation.

Second if you have a tight, properly detailed a//v barrier you need mechanical ventilation to accommodate air change. If you don't you are in danger of having mold and other rot related problems.

Unless you caulk at the baseboards and seal the window openings paint or drywall won't be an effective a/v barrier

[dgbldr]

Latex paint is indeed a vapor retarder, but as Dave said you must properly detail the envelope to prevent air leaks.

[Martin Holladay]

Hank Mavery,
Requirements for vapor diffusion barriers (or vapor diffusion retarders) in our building codes were based on a misconception, not science. Outward vapor diffusion has never been a significant mechanism for wet-wall problems.

The two biggest causes of wet-wall problems are:

1. Wind driven rain that gets past flashing or the WRB.

2. Air leaks that allow moisture-laden interior air to enter the wall. (The solution to this problem is an air barrier, not a vapor diffusion retarder.)

For more information, see http://www.greenbuildingadvisor.com/...vapor-barriers

[Ted S.]

Hank Mavery,
Requirements for vapor diffusion barriers (or vapor diffusion retarders) in our building codes were based on a misconception, not science. Outward vapor diffusion has never been a significant mechanism for wet-wall problems.

The two biggest causes of wet-wall problems are:

1. Wind driven rain that gets past flashing or the WRB.

2. Air leaks that allow moisture-laden interior air to enter the wall. (The solution to this problem is an air barrier, not a vapor diffusion retarder.)

For more information, see http://www.greenbuildingadvisor.com/...vapor-barriers

I had a building inspector ask a builder why he installed poly on the interior of the wall vs just using the paint. In the residential work I have not much been involved with wall design, thank goodness, with all the opinions from everyone and their brother, so I was happy to find this building inspector who lowered his ego and made sense.

We got rid of the poly in commercial work back in 1999 at least in my part of the world. That was the last I used it. I guess the confusion is then why the IRC still mandates Class I or II Vapor barriers, with Class III only permitted in certain circumstancese such as vented siding.

In my opinion, I believe it is this misconception, and the code is wrong. I agree moisture problems are primarily due to bulk (rain) and air transport, we knew this years go and omitted the vapor barriers in commercial work. We designed the walls not with vapor barriers, but vapor "open" so that they would dry. It's confusing as to why this was so well understood in commercial work yet we have the 2009 IRC still mandating vapor barriers.

[hank mavery]

Regarding ceilings and blown-in cellulose, it has been suggested that a plastic layer should NOT be placed in the attic below the insulation. Is this assuming that the ceiling paint is the vapor barrier?

[Martin Holladay]

Ted,
The requirement in residential codes requiring a Class II vapor retarder in some climate zones can be satisfied with kraft facing or vapor-retarder paint. Polyethylene is not required.

Hank,
When it comes to ceiling assemblies, air tightness is much more important that vapor permeance. Just make sure that everything is as airtight as possible. Ideally, your ceiling will have no recessed can lights, your attic hatch will include weatherstripping, your plumbing vent penetrations and wiring penetrations will be sealed, and you will have caulked the crack between the drywall installed on your partition walls and the partition top plates.

You don't need polyethylene. If your building inspector asks for a vapor retarder, then go ahead and install vapor-retarder paint.

[Ted S.]

I guess my question is Martin, if the codes were based on misconception and outward vapor drive has never been a significant mechanism for moisture (i agree with you) then why are vapor barriers still in the codes?

My opinion is that the codes are wrong.

[davenorthup]

Some interesting reading on the topic starting page 34.

http://cchrc.org/docs/reports/BAAIIClass.pdf

The best performing wall was one with exterior insulation.

We recently did ours - lap siding over fur strips, 4" EPS, Tyvek Drainwrap, 2/6 wall with cellulose or batt insulation and no vapor barrier (gasp). ;)

We did use a ceiling VB to aid in air sealing.

[mrd999]

I'm questioning whether or not it is actually the interior paint that is serving this purpose of the vapor barrier or retarder.

A paint-only solution would be a vapor retarder at best, not a vapor barrier. If you need a vapor barrier, you need poly. Aluminum paint would likely be the most vapor-restrictive paint, but last I recall there were fire hazards associated with its use..?

If the paint weren't stopping the vapor where would the moisture escape to once it was trapped behind the drywall and in front of the plastic barrier? Doesn't it seem like a bad idea to let the moisture be held here?

At the surface of the vapor restrictive plane in the assembly, you find a region of high vapor pressure. If the temperature at this location is cold enough, then it can condense and act like a leaky faucet in there.

Assuming you can't avoid some portion of the assembly being cold, the design goal is then to keep the vapor pressure low in the cold region. Hence, you want the vapor barrier/retarder on the warm side.

A vapor retarder on the surface of drywall vs a vapor barrier underneath the drywall isn't much of a difference of location. Odds are the temperature difference here is minute compared to the other side of your assembly. If condensation were to occur for one, it would probably occur for the other.

Once condensation conditions exist here, I'd expect a different evolution of material conditions - a surface wetting should dry faster than a wetting on the back of the drywall. The drying of either case would likely only occur toward the interior as it returned to less humid conditions.

However the vapor barrier effectively stifles the flow of vapor through the assembly, whereas the vapor retarder still permits vapor to flow. It's conceivable that an assembly with a few weak vapor retarders placed throughout it could have condensation occur at multiple planes inside the assembly. Maybe an assembly with no thoughtfully-placed vapor barriers/retarders acts in such a way, like a sieve for moisture as it passes through.

I think Martin makes an excellent point, air & rain tightness are more deserving of attention. In my opinion, a vapor barrier is worth consideration if you're going to strive for active humidity control, as then you have an energy-cost associated with vapor migration, and you have consistent vapor delta across the envelope assembly.