People, animals and cooking give off moisture in a house. Add the water sources like clothes washing, bathrooms (toilet with the lid up- always cover your toothbrush) hand washing, clothes dryer, etc., you get some water vapor that has to go somewhere. The pressure differences (natural and forced) in a house feed the "stack effect" which forces moisture right through holes (light fixtures, outlets, switches) in drywall on the ceiling into wall/joist spaces, through the subfloor- to your new flooring- if the pressure drive is strong enough, humidity inside high enough, etc. http://www.wag-aic.org/1999/WAG_99_baker.pdf
Flooring manufacturers want to warranty their products and know that inside/outside (think wet) air will get to the undersides, sooner or later. Check any house for the RH and watch it change with number of occupants/seasons/weather- rainfall, temperature, % of moisture in the air, etc. http://philipmarshall.net/pdf/tsongas_92_residences.pdf
If any ads are present below my answer or words underlined/colored, I do not condone/support/use the product or services listed/linked to, they are there without my consent.
17,000 dryer fires a year, when did you last clean the inside of the dryer near motor or the exhaust ducting?
Thank you, Gary. I see your point. However, it's all not compelling enough for us to want to put an asphalt product into our bedroom. I also called another respectable flooring company in our town, and they emphatically said we don't need a vapor retarder in our particular situation. Plus, the installation instructions for our bamboo don't call for any underlayment for nail-down installation. So there seem to be a lot of diverging opinions rather than a unanimously accepted directive, and we feel we can take the risk and just use red rosin paper.