I 
think a lot about vapor barriers and dew points. Irs an occupational hazard For guidance in these matters I pore over arcane \"Olumes of DOE conference proceedings and muddle through the ASHRAE HandOOOk 01 Fundamental$. When puzzled. I talk to the expertS in per­son. And to fInd out what the real world is doing. I talk to builders around the coon­try. At times, irs rather confusing. 
(h'er the phone and at recent confer­enas. f ,'e heard builders and ~enask many of the same questions I've wrestled witn. Few of these questions have defini­tive answers.. Often good research is lack· ing or the theory. research, and anecdotes fail 10 oonfinn each other. In many of the case studies reported. the full story is not known. What was the relative humidity in the house with the rotting rafters? 
But aU the research and analysis has not been in vain. For the major issues. c0nsis­tent findings are emeflPn8. Better news still is thaI there is little (:aU5e for alarm. The energy.cffidenl housing stock does not seem to be rotting beneath our feet. Here are some of the frequent questions on the fundamentals of vapor barTien and oonden­sation. with attempts at brief answer$. Next month ....e11 look at more specific: materials andapplications. 
Q; When does condensotion occur? 
A:. When warm moist air hits a cold sur­fcw:e, or wben vapor flow through a wall gets dammed up and sufficiently cooJed. To lind the dew point for a given air temper, ature and relative humidity (RH). you need a chart or graph. The'e's a good one in Solar Age, 1/84, p. 47. 
Q: Where does condensolion occur? 
A: On ooId interior window surfaces and within walls and roofs. In walb, condensa­tion OCCUB generally on the inside of the sheatI1in8 or on the bac!I. side of the siding. In summer, with air conditioning. the situa­tion may be. reveJ$l!d in very humid cli­mates and condensation can occur 011 slab BooB and Wlder wallpaper. It is pos:sibte lor condensation to drip and coUect on wall pIaIes or under windows. 
Steve Bliss is an .mociaJe ediJeH at Solar
.... 
Q: Is amdcTlSlIliOfl a serious problem? 
A: Mildew and wood-staining fungi ....,ll grow well on a wetted $urface at 60°F and 60 percent RH. For wood-eating fungi to grow, though. wood fibers must be. saturated (about 3!J.percent moisture con· tent) and warm. These decay<ausing fungi grow fastest at SOOf" to 6O"F, but $Orne ex· perts say they can do their thing as low 115 32"F. You need consistently high humidities and cold temperatures in combination with an inadequate vapor retarder to produce this level of concealed condensation. 
Q: HOUJ does the moisture get into the wall? 
A: Water vapor generated in the home in ....i nter mo\'es outside by passing direct· Iy through permeable materials (by a p~ cess called diffusion) and by leaking out doors and windows and gaps in the building shell (carried by the air it is a part of). In most cases. air leakage moves far more ....·ater vapor than does diIfus.ion, which is a relatively weak process. DiItusion is the movement of water vapor from areas of higher to lower concentration, independent of air movement. 
Q: How can I predicl whats goiJIg to happen? 
A: It's tricky. Even if you can do tbe math, many of the variables such 115 perm ratings and air leakage rates will not be pr~knovm. For example, doubling the thickness of materials more than halves the penn rating, Still. a simple model is useful for design pu!'JlO5I'S. One approach is to plot the temperarures tllroogh the wall on a graph and to overlay a plot of the dew point temperatures. Wherever tbe actual temperature falls below tile dew point temperarure. condensation may occur. This method is detailed in ASHRAE Handbook of Fundamentals and in the National Bureau of Standards Report BMS 63. 
Q: fIou; common is condensatioll ill ill­sulated 2x4 and 2x6 walls? 
A: Researchers think that small amounts of liquid or frozen condensation (frost) 0c­cur normally. 
Q: 50 why hoven ~ more IJJOOd.Irame houses rotted away? 
FEBRUARY 1984