Water vapour-excluding effectiveness of finishes

[JohnPW]

The following are from the United States Department of Agriculture Forest Service Forest Products Laboratory.

http://www.fpl.fs.fed.us/documnts/fplrp/fplrp462.pdf
http://www.fpl.fs.fed.us/documnts/techl ... midity.pdf
http://www.woodworks.org/wp-content/upl ... eriors.pdf

From p 22 of the third link:

Moisture-excluding effectiveness of three coats of various finishes
(ponderosa pine sapwood was initially conditioned to 80°F (25 degrees C?) and 30 percent relative
humidity and then exposed to 80 °F and 90 percent relative humidity for 14 days)

One-hundred percent = perfect protection or no absorption of water vapor; 0 = no protection as with unfinished wood

I've picked out a few:

Paraffin wax-dipped - 95%
Epoxy finish—clear (2-component) - 91%
Polyurethane paint–gloss(2-component) - 74%
Paraffin wax–brushed - 69%
Polyurethane finish–clear(2-component) - 66%
Enamel paint–gloss (soya alkyd; interior) - 59%
Shellac - 42%
Oil-based house paint - 41%
Spar varnish (soya alkyd) - 30%
Nitrocellulose lacquer - 10%
Tung oil - 2%
Linseed oil - 0%
furniture wax - 0%
Unfinished wood - 0%

Linseed oil gets zero as a water vapour barrier!

 

[custard]

Good post. In a contest between finishes and water vapour then water vapour will always win. That makes sense and underlines the fact that trying to insulate timber from the environment is a fools errand. Finishes certainly have their place and perform many useful functions, but delivering wood stability is way down that list.

 

[AJB Temple]

Dipped paraffin wax may be a bit unfair, as it appears to suggest immersion coating rather than face coating. Snag with water vapour is it gets everywhere!

Interesting post. Thanks for that.

 

[Chrispy]

Also you have to consider that atmospheric air pressure is always changing, so air carried water vapour will be be moving into and out of any air containing material, even the very best finish won't stop changes in air pressure.

 

[ED65]

Thanks for posting that John, I didn't spot this had been posted yesterday when I responded to custard in the other thread a few hours ago. I had completely forgotten about the FPL data until just a few minutes ago and logged in to post it.