QOD March 31/06

[Claude Lawrenson]

This ones about "tar paper".

[Claude Lawrenson]

Although asphalt paper can act as a vapor barrier, the fact that it is placed on the outside of sheathing precludes answer "C", and also answer "D". It does add a little to the thermal resistance but its primary purpose is to prevent any water that seeps behind the siding from getting into the structure. It also helps prevent air infiltration.

Therefore "B" is the correct response.

[Jay Moge]

damn. trick question. rule # 1 in test taking, if one choise is "all of the above" than most likely that's the answer. guess you didn't read that huh Claude. good question.

[Roy D. Cooke, Sr]

Originally Posted by clawrenson

Although asphalt paper can act as a vapor barrier, the fact that it is placed on the outside of sheathing precludes answer "C", and also answer "D". It does add a little to the thermal resistance but its primary purpose is to prevent any water that seeps behind the siding from getting into the structure. It also helps prevent air infiltration.

Therefore "B" is the correct response.

That was not nice ,I love it and I failed. Roy sr

[Marcel R. Cyr]

Roy;

Do not feel bad, I got it wrong too, according to Claude's description.

I do have an expressed opinion as to why we were wrong.

Claude;

Here is why I answered your question wrong.



Comparing specs on weather barriers Though barely two decades old, plastic housewraps have pulled even with felt and building paper as the preferred weather barrier on the nation's homes, and new wraps appear on the market with regularity. Yet many builders and some building science experts remain skeptical of their value, and manufacturers disagree on optimal performance specifications. Despite the limited consensus on the issues, there are enough hard facts to make some comparisons.
Getting the right wrap depends in part on what you want it to do. Housewraps serve two functions: as breathable moisture barriers and as barriers to air movement through walls. Black felt and paper are good moisture barriers, but make poor air barriers due to their narrow widths and the difficulty of sealing seams.
U.S. codes don't require air barriers, and generally require moisture barriers only behind porous material such as stucco. Many builders install moisture barriers as a matter of course anyway, if only for temporary weather protection. Those who install housewrap don't always take its air barrier function seriously, however, since many neglect to tape the seams.
How important is air leakage? According to the U.S. Department of Energy, up to 40% of the energy consumed to heat or cool a building is lost to air leakage. As a whole-house problem, leakage requires solutions such as careful gasketing and sealing, but studies show sealed wraps can prevent the air movement that occurs even across tight-fitting sheathing, including OSB and t&g polystyrene. A DuPont study put the payback time for its housewrap in energy savings at 1.6 years.
Air leakage is also a frequently neglected source of moisture—and thus mold—in wall cavities. A 3/4-inch hole can transport up to 30 quarts of moisture into a wall over a heating season.

Black felt and paper
The two original moisture barriers were No. 15 felt (it no longer weighs 15 pounds per square, so 15 is just for reference) and Grade D kraft-based building paper. The latter is stronger than felt, and is identified by a "minute" rating depending on its water hold-out performance—generally 10 to 60 minutes.
Both felt and building paper exhibit good bulk water holdout, better than some plastic wraps but worse than others. In practical terms, the asphalt tends to seal around nails and staples, but water can also find paths into tears and holes left in this weaker material. They also lose strength and even rot when wet.
But these materials have some advantages as well. Because they are absorbent, they can effectively wick liquid water out of wall cavities much more quickly than plastic wraps, which are permeable only to moisture in vapor form. Their permeance is also variable, allowing more vapor through when wet. Both these factors make felt and paper more forgiving of some kinds of moisture intrusion.
In terms of material costs, felt or paper can save a builder about $200 to $300 over housewrap on the average home,

Marcel