Insulation, Radiant and Vapor Barriers - Styrofoam over Tyvek over OSB?

I am building a 3700sf brick home in Houston tx (Hot-Humid Climate). The house is dried-in with Tyvek over OSB. I am preparing for the brick installation. Before installing the brick I am considering adding rigid foam board insulation. I figured this is the last opportunity I will have to add more insulation in this area. I should have enough room for 1/2" foam board while still leaving a 1" air gap between the brick and sheathing. (Brick is 2 1/2" deep on a 5 1/2" brick ledge)

I am considering the following options. Are these doable? Can I have both Tyvek and foam board in the same wall? Am I violating in vapor/moisture barrier rules? Am I just going overboard with this!


Current (planned) Exterior wall configuration
- 1/2 inch drywall
- 2x4 studs w/ fiber glass spray-in insulation (JM Spider)
- 1/2 inch OSB
- Tyvek
- Air Gap
- Brick Veneer


Proposed Wall Configuration 1. Styrofoam over Tyvek over OSB
- 1/2 inch drywall
- 2x4 studs w/ fiber glass spray-in insulation (JM Spider)
- 1/2 OSB
- Tyvek
* 1/2" Dow STYROFOAM Residential Sheathing (R-3)
- 1" Air Gap
- Brick Veneer

Proposed Wall Configuration 2. Tyvek over Styrofoam over OSB
- 1/2 inch drywall
- 2x4 studs w/ fiber glass spray-in insulation (JM Spider)
- 1/2 OSB
* 1/2" Dow STYROFOAM Residential Sheathing (R-3)
- Tyvek
- 1" Air Gap
- Brick Veneer

I like the idea of Proposal 1 since I will just have to hang the foam board on the tyvek. Proposal 2 is not desirable because I will have to rip off the Tyvek and then install the Styrofoam, then reinstall new Tyvek.

I would appreciate in comments or suggestions.

Either will work. Your Tyvek is not a vapor barrier so you don't have to worry about that. You can put foam over it without any problems. The only thing I'd suggest is that you tape the seams of your foam to make it weather resistive.

Either will work. Your Tyvek is not a vapor barrier so you don't have to worry about that. You can put foam over it without any problems. The only thing I'd suggest is that you tape the seams of your foam to make it weather resistive.

I'm sort of wondering the same thing here but my questions is "Would the foam over the tyvek seal in the moisture and cause wood rot?" In other words would the foam restrick the tyvek from doing it's job. It is my understanding that tyvek is a unidirectional membrane to allow moisture to escape.

Stealthorse

Tyvek is not "unidirectional", it is vapor permiable in either direction... it's neutral, so to speak. This simply means that Tyvek is not a vapor barrier, and as such, it will not TRAP water vapor on either side. You can install Tyvek in any direction, it cannot be installed backwards since it does that same thing no matter how it is placed on the wall. Tyvek's main job is to protect sheathing from liquid penetration. It's other properties, such as wind resistance and vapor permeability in either direction (you usually only think of Tyvek as letting vapor out, but it can let vapor in as well!) are other marketing angles that are often over-hyped.

It is possible that if a vapor barrier is applied to both the interior (4mil poly) and exterior (foam sheathing) that there would be the potential for problems due to there being a "double vapor barrier". There is much discussion about whether or not this is a true problem in ALL climates, and from what I've seen and read, it is largely a problem in only the most humid of climates... or perhaps when one or both vapor barriers are poorly sealed- letting moist air into the walls somehow, but not letting it out. This topic could be argued either way. In the original question, there was no interior vapor barrier, the foam sheathing was on the warm side of the home, so no problem with a double vapor barrier.

Water vapour is an incredably fine gas that will find its way into most things.
Tyvek is designed to let water vapour through and to stop water which has a much larger structure.
Warning: Tyvek that is not quickly protected from the sun will die. It will not appear to die, but experience is showing that Tyvek once exposed to several days of sun, will continue to degrade inside the walls and roof.
Water vapour is put into a building by us.
It is our breath and sweat that causes the interior of our homes to have a much higher level of humidity than the outside.
Plasterboard is transparent to water vapour, and as much as one and a half litres of water vapour can and does pass through an 8 by 4 in 24 hours.
It is therefore, very important to stop this water vapour from entering a wood wall.
The usual procedure is to line the internal walls with an almost water vapour proof membrane, (plastic sheet)taking care to seal all holes, before fixing the plasterboard.
You would do better to increase the wood thickness to 6x2 and to install five inches of blown foam into the walls and roof, with the inside of all walls and ceiling being covered with two inches of foam board to avoid the conduction of heat through the fabric of the building.
I will mention that a white or light coloured roof will reflect the suns heat.
A roof covered with five inches of foam and protected by two inches of lightweight concrete, will give you a comfortable (low cost to run) home.
Perry