Too-high temperatures are a bigger problem IME.
When the temp is below optimum it just slows things down. I'm not sure what temp a sub has to drop to for all growth to stop, but at 60 you're still well-above that but the growth will be slow enough where dehydration of the sub can become an issue.
Without using supplemental heating, you can boost the substrate temp by insulating whatever they're growing in to contain the heat they generate. Self-generated heating won't effect the subs's temp until there's enough mycelium present (which depends on the size of the sub) but eventually you'll see the temp start to rise.
Incubating/fruiting your grow as high up in the room as is practical can help, plus it also has the added benefit of reducing your sub's exposure to contaminants (since ~90% of airborne contams are within 12" of the floor).
Edited by TVCasualty, 08 September 2019 - 04:39 PM.