Condensation may develop in walls during cold weather and the 
more efficient the insulation the more likely condensation is to occur. 
Most building materials, such as plaster, wood, "brick, and concrete, 
are permeable to vapor. Condensation takes place when outside tempera- 
tures are below the dewpoint of the inside atmosphere and the moisture 
passing as vapor through the plaster and insulation reaches some sur- 
face that is below the dewpoint. The same principles of vapor movement 
apply to unventilated attics. The relative humidity in a ho\ise of 
normal occupancy and without air conditioning will average about 20 per- 
cent when the outside temperature is zero. The dewpoint for 1 r J°Y- and 
20 percent humidity is 2g°F« , and laboratory tests on insulated walls 
show that the inside face of the sheathing will be considerably below 
28° during periods of zero weather and possibly even with temperatures 
as high as 15° above. Conditions are then favorable for condensation. 
Obviously, the higher the inside humidity the higher the dewpoint tem- 
perature, and the greater the amount of water that will condense in the 
wall in cold weather. 
Attics can usually be protected against condensation by ample 
ventilation. Protection against condensation in side walls is obtained 
by increasing the resistance of inner wall surfaces to vapor transmis- 
sion. For new houses, the use of asphalt-coated sheathing paper applied 
over the inside face of the studs before plastering is recommended. 
For buildings already constructed, suitable paint coatings on the 
plaster or interior finish should give ample protection for most cases. 
Aluminum paint followed by other finishing paints gives excellent pro- 
tection over plaster, primer and sealer paints also make good base 
coats. Glue size, kalsomine, and cold water paints offer practically 
no protection. For further details on the subject, see the publication 
"Condensation problems in Modern Buildings", which may be obtained from 
the Forest products Laboratory upon request. 
If a wall containing sawdust or shaving insulation become thor- 
oughly damp it would take a long time for it to dry out, and considera- 
ble trouble might result. The insulation should, of course, be thor- 
oughly dry when placed in the walls, the walls should be dry and the 
construction should be such that they will remain dry. If this can be 
accomplished no disadvantage from the standpoint of decay should result 
from the use of untreated sawdust or planer shaving insulation. 
A wall properly filled with sawdust or planer shavings is more 
resistant to flame spread up through the wall space than a similar 
hollow wall, for the insulation prevents drafts and thus retards the 
spread of flames. A smouldering fire in a sawdust-filled wall might 
be difficult to get at and extinguish but this disadvantage should be 
more than offset by the very slow rate of burning which should allow 
plenty of time for extinguishing. If one side is burned away from a 
sawdust -filled wall, of course, loose sawdust will run out. 
K1092 _2- 
