Posted By Dana1 on 30 Jan 2017 03:41 PM
XPS performance will eventually fall to that of EPS. The extra volume of excavation for thicker foam at the same labeled R is cheap.
Polyiso is hygroscopic and can load up with moisture under a cold slab, but it's fine on the interior sides of walls down to slab level. If it's going to rest on the slab the vapor barrier needs to extend up the foundation wall, behind the wall-foam. It's better to use EPS along the slab edge between the foundation wall & slab, with the bottom edge of the polyiso resting on EPS rather than concrete.
Where are you getting your building science from? GBA school of dumb bells? You should be asking more questions rather than giving bad costly advice. As a result homes designed here and GBA will end up with costly mistakes.
Right Side Graphs
Water vapor diffusion depends on a
resistance factor (u-value), vapor diffusions thickness sd-value,
permeability.
1.All
of the foam products have low moisture contents until they see RH 60% + then
they are still low, applicable to walls, or terrestrial vapor pressures/temp @
capillary material air pockets, peak load @ 100% depending on porosity.
2.PISO
has better perm-in retention
than XPS but
not much.
3.POLISO has lower u-value
than EPS/XPS
is it’s free saturation state due to its lower surface diffusion/liquid
transport, as
it takes on water/temp and XPS has lower stable perm-in than EPS/PISO.
Left
Side Graphs
Liquid
Transport Coefficent (DW)
– This is liquid convection treated as diffusion since it relates absorbed
water over an sf area over time. It depends on material and boundary
conditions.
DWs - The liquid transport coefficient
for suction Dws….describes the
capillary uptake of water when the imbibing surface is fully wetted. In the
context of building physics this corresponds to rain on a facade or an
imbibition experiment. The suction transport is dominated by the larger
capillaries, since their lower capillary tension is more than compensated by
their markedly lower flow resistance.
DWW - The liquid transport coefficient
for redistribution - describes
the
spreading of the imbibed water when the wetting is finished, no new water is
taken up any more and the water present in the material begins to redistribute.
In a building component, this corresponds to the moisture migration in the
absence of rain. The redistribution is dominated by the smaller capillaries
since their higher capillary tension draws the water out of the larger
capillaries.
Since the redistribution is a slower
process (taking place in the small capillaries with their high flow
resistance), the corresponding liquid transport coefficient is generally
markedly less than the coefficient for suction.
The foams exhibit no capillary conduction
@ Max Moisture Content above free saturation. Mineral wool board exhibits
better properties.
So if you have a high soil Plastic Index (PI)
XPS due to it’s low perm-inch is a better choice, otherwise, there is not alot of
difference between EPS/POLISO to justify cost, not only perm-inch but Dw.
DANA1: "XPS performance will eventually fall to that of EPS. The extra volume
of excavation for thicker foam at the same labeled R is cheap."
Not - EPS has better perm-in thermal conductivity than XPS. XPS will never "fall" to it's performance in any boundary conditions due to the surface layers @ MMC.
DANA1 : "Polyiso is hygroscopic and can load up with moisture under a cold slab,
but it's fine on the interior sides of walls down to slab level. If it's
going to rest on the slab the vapor barrier needs to extend up the
foundation wall, behind the wall-foam. It's better to use EPS along the
slab edge between the foundation wall & slab, with the bottom edge
of the polyiso resting on EPS rather than concrete. "
and this too is WAY OFF Dana 1,
"POLISO is hygrophobic" It does not have more normalized water content nor Dw than the others.
Plastic Index (PI) test at differeent depths properly designs insulation & vapor management & footing/wall depths, and they are cheap. A soils shear test especially for low PI s/b conducted but can get expensive but so can failed foundations.
Foam compression, deflection, creep are on the
order of 7-10 lower than high perm-inch MWB. If the slab is not suspended properly, along with
low shear & high PI soil the foam/barrier will crack in tension. Once that
happens high pressures/flow rates at the cracks cause lots of issues.