  Temperature fluctuations over long time scales ($\gtrsim 1\,\mathrm{h}$) are
an insidious problem for precision measurements. In optical laboratories, the
primary effect of temperature fluctuations is drifts in optical circuits over
spatial scales of a few meters and temporal scales extending beyond a few
minutes. We present a lab-scale environment temperature control system
approaching $10\, \mathrm{mK}$-level temperature instability across a lab for
integration times above an hour and extending to a few days. This is achieved
by passive isolation of the laboratory space from the building walls using a
circulating air gap and an active control system feeding back to heating coils
at the outlet of the laboratory HVAC unit. The latter achieves 20 dB
suppression of temperature fluctuations across the lab, approaching the limit
set by statistical coherence of the temperature field.
