RH Numbers Rising
09-10-2011, 11:14 AM
RE: RH Numbers Rising
(09-10-2011 07:47 AM)CC Solutions Wrote: Now how can moisture get back in the slab when there is a proper vapor retarder under the slab as dictated by F-710????
You should add "proper and functioning" to that. I never see anything from adhesives manufacturers stating that.
FLOOR SLAB LAID DIRECTLY ON THE GROUND
The above diagrams and correction factors can also be
used for a floor slab laid directly on the ground if the
actual slab thickness is doubled. This presupposes that no
drying occurs downwards. This is the case, for instance,
if the slab is underlain by a layer of extruded
polystyrene. If expanded polystyrene is used, some
drying may take place through this, and the drying times
will therefore be a little shorter than those obtained from
When a floor slab laid on the ground is underlain by
ca 5 em mineral wool insulation, drying can in most
cases occur downwards. Nilsson /10/ has stated that,
because of this drying, the drying times in the above
diagrams (with doubled thickness) can be multiplied by
0.6-0.7, provided that the width of the building is less
than about 10m.
Blomberg /19/ has calculated the temperatures for the
case when the construction is completed in the summer
when the ground has warmed up. The results of these
calculations show that, even during a normal summer in
Sweden, drying takes place downwards through the
mineral wool. It is however shown by the calculations
that if the summer had been exceptionally warm, the
temperature difference across the insulation which is a
prerequisite for drying downwards does not occur around
the outside of the slab. In such a case the factor
0.6-0.7 should not be used, and it should be assumed that
drying occurs from one side only.
If the mineral wool is very wet when the concrete is
poured, it is probable that drying downwards is not
possible, and the factor 0.6-0.7 cannot therefore be used.
Nor can the correction factor 0.6-0.7 be used in
calculating the drying time when there is a plastics foil
between the mineral wool insulation and the concrete. In
principle, this case is the same as when drying takes
place from one side only, i.e. the case with expanded
polystyrene underneath the slab.
WARNING. If a floor slab is dried during the construction by heating, this may also
cause the ground to heat up. Moisture can then be transferred from the ground to the
concrete when heating stops.
Stephen Perrera dba
Top Floor Installation Co.
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RE: RH Numbers Rising - Ernesto - 09-10-2011 11:14 AM