Building paper is the stuff that wraps the house. Like wrapping a huge birthday present. And then you install siding over the wrapping.
Way back when, building paper was also called building “felt”, and it was merely asphalt-impregnated construction paper. Black in color, it came in two strengths, 15lb. and 30lb. because it weighed 15 and 30 pounds per roll, respectively. The heavier stuff was made of thicker paper and had more asphalt.
We still use that stuff all the time. It is still 36 “wide material but now it is called # 15 and # 30 paper. about 8 pounds and a roll of # 30 paper weighs about 20 pounds. But that is mainly trivia.
The next innovation that came along in the late 70s, early 80s was house wrap. It had some advantages: instead of 36 “it came in longer (higher) widths like 8 and 10 feet. However, the first generations of house wrap demonstrated the importance of permeability because most of them were, in fact, not vapor permeable.
It is ironic that the whole concept of permeability and the performance of the building envelope developed around the forensic study of building defects that embodied these “new” house wraps. Only after a myriad of problems appeared did building “scientists” start studying the movement of water vapor, its different concentrations inside and outside a building, its ability to condense (dew point) inside a wall, and how it behaves as a function of heat and humidity. The building paper plays a crucial role in all of these things.
And you thought it was just there to keep the rain out!
An old house covered with building felt is a leaky house. The felt does not do that great a job keeping rain out. In fact it does not do a very good job at anything obvious. It is a great building material because of its poor performance. Why? Because it breathes. It lets water vapor out. Where does all that steam go? All the moisture your body gives off? It heads toward the cold, condenses and then dries to the outside. Similarly, when your poorly-maintained paint job lets outside water into and behind the siding, and then into the framing (because the building felt certainly is not keeping it out), it is OK. Because the leaky felt also lets air through the building envelope, everything dries out. Rot and mold only occurs when things do not regularly dry out.
“They do not build 'em like they used to.” And it is a good thing, because energy is expensive. With dwindling fossil fuel supplies and rising global temperatures, we need to be using more energy efficient building technologies. But that is a topic for the next article.