Building paper is the stuff that wraps the house. Like wrapping a giant birthday present.
And then you install siding over the wrapping.
Way back when, home 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 lb per roll, respectively. The heavier stuff was made of thicker paper and had more asphalt.
We still use that stuff all the time. It’s still 36″ wide material but now it’s called #15 and #30 paper. Like many other products, over time it gradually became lighter and lighter as the manufacturers used lighter paper and less asphalt. Now a roll of #15 paper weighs about 8 lbs and a roll of #30 paper weighs about 20 lbs. But that’s mainly trivia.
The next innovation that came along in the late 70’s, early 80’s was house wrap. It had some advantages: instead of 36″ it came in longer (higher) widths like 8 and 10 feet requiring less courses, it was much lighter and easier to work with, it could be sealed to itself with proprietary tape, and was eventually promoted as being waterproof but breathable (vapor permeable). The first generations of house wrap demonstrated the importance 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 involved the “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 doesn’t do that great a job keeping rain out. In fact it doesn’t do a very good job at anything obvious. It’s a great building material BECAUSE of its poor performance. 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 builder’s felt certainly isn’t keeping it out, it’s OK. Because the leaky felt also lets air through the building envelope and everything dries out. Rot and mold only occur when things don’t regularly dry out.
“They don’t build ’em like they used to.” And it’s a good thing, because energy is expensive. More to follow…
– Sam
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