Cork insulation; a complete illustrated textbook on cork insulation ...

476 CORK INSULATION
that for each one per cent moisture absorbed the heat conduction
increases four to forty per cent, depending upon the material used.
In most branches of engineering progress is being made by de-
grees only. When, over thirty years ago, corkboard first came into
use, it was being protected against air and moisture by means of
waterproof insulating paper on both sides. Also the pure corkboard
used to be more dense; it weighed fully fourteen pounds per cubic
foot as against nine or eight pounds now. The lighter board is a
better insulator, but is, of course, not so strong and durable as the
dense board. Formerly the finish for floors, walls and ceilings used
to be one or two layers of %-in. tongued and grooved sheathing,
nailed with galvanized nails against furring strips imbedded between
the corkboards. This construction when varnished looked well, but
was rather expensive. It was not fire resistive, and at low tempera-
ture the boards dried out, shrank, and allowed moisture to enter the
cracks, which caused rotting to take place.
The next step was the erection of corkboard in hot asphalt. Walls
on the inside have to be pointed up with cement mortar so as to be
reasonably flat, to avoid hollow spaces behind the corkboard because
these would hold moisture and might freeze, forcing off the insula-
tion. The wall was mopped with hot asphalt, although with the
rapid chilling on the bare wall the resulting surface could not be
air-tight. The corkboard also was mopped over with hot asphalt
and thus cemented against the wall. The second layer of corkboard
was again dipped in hot asphalt, and wooden skewers used to pin
the various slabs together. The exposed face of <strong>insulation</strong> also
might have been mopped over with hot asphalt, but this was not
done because fire insurance companies prefer a Portland cement
plaster finish, at least j4-in. thick, the ^-in. thick coat of asphalt
being incapable of withstanding abuse and contact with trucks and
packages, although wood fenders might be erected for this purpose.
Insulation asphalt necessarily has to be odorless so as to impart
no foreign odor to the products stored. It is difficult to employ hot
asphalt in ceiling work due to hot drippings. The heating of the
asphalt by maintaining fires in the building is a nuisance, and so the
<strong>insulation</strong> contractors soon avoided the use of asphalt and recommended
Portland cement mortar and plaster in nearly all cases.
Simple home-made boxes of wood were designed facilitating the rapid
coating of cork slabs with a uniform thickness of cement mortar.
Thus mortar exclusively was used next to the wall, between courses,
and cement plaster finish in two %-\n. coats, applied to the erected
<strong>insulation</strong>.
The moisture in cement mortar occupies a certain amount of
space, one cubic foot for every 62.4 pounds of water. In due time
all this moisture disappears and causes shrinkage cracks. When the
air is saturated, moisture re-enters every pore in the plaster, and
the open cracks especially. In an eflfort to hide these cracks plas-
tered surfaces are usually marked off in 3-foot squares or scores, the