The vapor thing is sooo confusing, help?
I’ve read the articles on vapor migration and understand that vapor travels through a wall / roof until it hits something that is impermiable and when this happens it condenses (if the dew point is reached) and soaks down whatever is next to it. I also think I understand that being in a hot & humid COASTAL environment (IRC 2a TX) you do not want a vapor barrier next to the inner drywall. Unfortunately, the articles I read did not say if there should be a vapor barrier or retarder on the outside of the wall sheathing (like house wrap or poly, etc) in the hot & humid environment. It seems that most of the authors were northeners, (not that there’s anything wrong with that as Jerry used to say).
At any rate, it never freezes here but we do get 1 or 2 hard frosts a year. In the old days, we put building felt on the outside sheathing (up north) and that was that.
So what’s a fella to do in this hot & humid environmet? Vapor barrier or retarder or not? And if yes, where ? If no, why?
My house was built in the late ’70’s and so it needs a definite upgrade in insulation both attic & walls cause my summer cooling bills are $350 to $400 a month and I need to put a stop to it.
Typical daytime humidity here runs a minium of 60% and sometimes goes right to the 90% level with the upper “normal” being in the mid 70’s%. Day temps usually are bumping 90 degrees with a few triple digit days thrown in for good measure.
I have Charlie Wing’s Visual building book and he mentions it but doesn’t elaborate what to do in Hot & Humid either, unless, of course, I was temporarily blinded by the light when I read that section.
Buddy, can you spare the time?
Ken
Replies
Dis ees de place Boss
http://www.buildingscience.com/
Joe H
Housewrap is not a vapor retarder.
Assuming you're running A/C
you need to be aware that the dewpoint might be on the back of your drywall when it's hot and humid outside.
The right thing to do, though, is to find out what the best practices are for your area. I would absolutely install either housewrap or felt under your siding, to keep water off your sheathing, and if you are concerned about vapor migratng from the exterior then felt might be the right choice.
But whadda I know, I live way up here. It's totally different.
http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers/?topic=/doctypes/digest
Understanding Vapor Barriers
By Joseph Lstiburek, (BSD106)
Building Science.com was indeed the place for me to get the issue cleared up. The paper referenced above does an excellent job of sorting through wall construction by various climatic areas and where to put what & why.
For those who would like to know, the answer to my question for hot & humid environments is to build a standard stud wall and put the vapor barrier (class 1 vapor retarder) air barrier and drainage plane on the outside of the exterior sheathing. This can be overlayed with rigid insulation that will form an air barrier if taped properly. Since I will be using Hardi panel (4x8 & 4x10) with a stucco look finish as siding, leaving a drained cavity between the panel and the rigid insulation provides a drainage path for any moisture that does form on the outside of the vapor barrier. With no vapor retarder or barrier on the inside wall surface, any moisture trapped on the inside of the vapor barrier on the outside of the sheathing will be able to dry to the inside.
Thanks for tipping me off to building science.com I was not previously aware of it.
Thanks to all for your replies!
Ken