Lately I have been seeing & getting numerous questions on the use of WRB’s or Weather Resistive Barriers. For example; do I need it if I am using foam? If I am installing foam, should it be placed over the foam or under it? According to the ZIP system, I don’t need one, is this really a good way to go? Then of course the ever famous should I use tar paper or Tyvek? Needless to say that sounded like a great article idea so here is my take on it.

So what say you?

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Hi Sean, besides the cost issue, I would like the tyek to perform a real function. Sandwiching it between the foam and sheathing seems like it would provide little benefit. If the foam layer is properly taped and we can trust the tape for the life of the house (??) then I see no benefit for covering the exterior of the foam. IMO, the tyvek is best used to protect materials like osb and framing to prevent water damage and of course provide drying to the outside.

Just my 2 cents.


Tyvek on the outside of foam would be the primary water resistant barrier & allows for full integration with the windows assuming they are not inset (though there are ways even if they are)


Tyvek under the foam can serve a few purposes besides being the last line of defense against water - many like it because once it is up the structure is dried in & it doesn't matter when the foam is installed - it also makes for a good bond break between the foam & sheathing as some have had issues with different rates of expansion & contraction


Can you get away with using an approved foam & tape & no Tyvek - yes, just like you can with a Zip-Wall system assuming things were installed properly which gets down to what is the skill level of the crew installing it. Of course I happen to recall an interesting "lessons learned" on Joe L's house where he showed pics of where the foam panels had separated - this is one reason why I suggest two thinner layers (instead of 1 thick one) of foam along with a Tyvek style product - it is pretty cheap insurance

As long as the tape is good I would be confident that most rigid foams would be water barriers. That said, I have a piece of old 2" foam around here somewhere that is now closer to 1 3/4". When I tried to use it I was shocked. It was too thick to be my 1.5" and too thin to be the 2". All I can guess is the sun, but strictly a guess.

Around my windows I still have not concluded a best practice for flashing. I just leave a little extra space and do it twice, not exactly. But taping to tyvek does not work well. My current windows have a wide trim with "J" built in, so they get flashed 6" away from the rough opening. I also like to deliver any water back to the surface as soon as possible rather than allowing it to run down the tyvek to the bottom. I install a piece of flashing up under the bottom edge of the lowest flash and tape it to the top of my vinyl. Still experimenting.

I will add one thing. The old hardboard siding I removed had a terrible tar paper job underneath it, but the plywood was perfect and never say water. In some 8' x 8' areas I removed as many as 20 small pieces of tar paper applied like postage stamps. From 100 yards away it probably looked like a good job.



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