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[EnergyPlus_Support] CTF calculation convergence error

This is a repost from an earlier post, "Modeling bermed structures". I
changed the heading thinking that this may apply to other design types:
I simulated a simple 160m2 zone without windows. I assigned the
ceiling and wall fabrics a SOIL object with the following thermal

conductivity (k) = 0.5857 W/mK
density = 1750 kg/m3
specific heat (c) = 1004 J/kgK

I gradually increased the thickness of the soil object with the intent
to simulate a 6 month internal temperature lag. When I go beyond a
thickness of 1.2 m, I get the error "CTF calculation convergence
problem for SOIL" where soil is the aforementioned SOIL object (wall
and ceiling fabric).
Tweaking k and c allows for limited increase of soil wall/ceiling
thickness but the thermal lag of the zone decreases. The error occurs
using 1 hr or 6 hr timesteps. Can the wall thickness limitation be


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