Vapor Barriers for Dummies (and rocket scientists!)

[John Raabe]

This fun read on the 2nd law of thermodynamics explains:
• Where (not) to put the vapor barrier
• Why kraft faced batts may be better in your climate than a plastic vapor barrier.
• Why the space shuttle kills people
• Why mold comes to visit upscale motels and FEMA trailers

http://www.buildingscience.com/documents/insights/bsi-021-thermodynamics-its-not-rocket-science

[ScottA]

Nice find John.

[MountainDon]

That is a wonderful "must read" article. Thanks

[poppy]

Amen to that article, thanks for linking it John.

That example is part of the reason that I like the book Timber-Framed House by Jack A. Sobon.  Here is a quote that Jack used in the book before writing about vapor barriers:

"The architect should also have a knowledge of the study of medicine on account of the question of climate, air, the healthines and unhealthiness of sites, and the use of different waters.  For without these considerations, the healthiness of a dwelling cannot be assured."  Marcus Vitruvius Pollio, 100 BC

[Don & Ginger Lundgren]

The link supplied has changed but I did find the new location for it.

http://www.buildingscience.com/documents/insights/bsi-021-thermodynamics-its-not-rocket-science

[John Raabe]

Thanks for the update.

[waggin]

Fascinating!  Well written article that educates us on a topic for building and other applications.  Thanks for posting that.  Since I'm no rocket surgeon myself, I'm trying to apply this concept to a mostly unheated building in a predominantly cool, damp climate that I'd like to insulate.  Am I correct in assuming less is more for permeability on the inside of the building?  If I use Kraft-faced insulation, will that be appropriate for something that may have cool, damp air that I rapidly heat on occasion?  Note:  The exterior will likely be some form of Hardi, either plank or shingle, sheathed with plywood, and the building will be wrapped with builder's felt/tar paper (hope that's the correct terminology.)  This also makes me wonder if for a building like this, I should have some special venting for interior space while it's closed up, not heated, and going through exterior temperature swings. 

[bayview]

   Following site and table may be of some use . . .

http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11810

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[John Raabe]

Good chart. Especially for the SE.

In my neck of the woods (interior side) the standard technique is to seal the drywall (for an air barrier) and then use a two coat PVA primer and latex paint for the vapor retarder - gives a perm rating under 1.0.

[OlJarhead]

Good chart. Especially for the SE.

In my neck of the woods (interior side) the standard technique is to seal the drywall (for an air barrier) and then use a two coat PVA primer and latex paint for the vapor retarder - gives a perm rating under 1.0.

Interesting stuff -- I always put in vapor barrier but also live in the NW.

Our cabin however, is in the Okanogan so this article helped a little.  However it seems to me that a poly vapor barrier is still the way to go up here?

Am I wrong?  One thing i will admit is that I didn't know not to use Kraft faced bats AND plastic...I'll not do that again now.  Thanks!

[John Raabe]

I have not had any problems in the many energy efficient custom houses I've designed in the past for Western and Eastern WA. Most of those have a Poly VB on the Warm-in-Winter side. Most also had wood siding and were insulated with unfaced batts or BIBS (but no problems with kraft faced batts either). If you can do a good job on airsealing you don't really need the poly. Air leakage is the primary mover of moisture.