In Vancouver we often experience rain and this is not always the ideal weather for any exterior work.  One of our clients recently asked me, as the construction of her garage was in progress, how the building paper will be affected when you leave it exposed to rain for numerous days.  We couldn’t shingle because rain makes Hardi-shingles soft and they break.  So in the meantime, what will happen to the building paper after long periods of exposure to less than ideal weather?

Building paper is made from Kraft paper (produced from wood pulp and very strong & coarse) which has been saturated with waterproofing asphalt, the same product used in asphalt pavement to bind aggregate together.  It is very resistant to wet environments.  The purpose of building paper is initially to prevent wind from penetrating into a wall and secondly to act as a backup moisture barrier behind cladding.  In wet regions, the second function is very important since moisture in a wall is definitely not desirable.

For any climate, two layers are recommended.  The outer layer of paper can become damp from wet cladding so having two layers means that the inner layer keeps the wood wall separated from direct contact with this dampness.  The outer layer can take the abuse of numerous wet and dry cycles in the wall while the inner layer remains unaffected over time.  Regardless, it is best to cover any moisture barrier as soon as practical since you expect it to perform for many decades.