Can I put Styrofoam around the terrarium and that could help insulate it would that work?Glad to see you changed the substrate, that looks much more suitable. Again the if you are comfortable they are comfortable rule would apply. That heat mat should be plenty as long as your house is heated. However, if it would help your worries, use a space heater to warm whatever room you have the t in.
Thats not too cold. No external heat source is needed. Tarantulas actually do better in cool temps. The warmer they get, the more they can dehydrate. Also, cooler temps slow their metabolism. They grow slower and possibly live longer. I wouldn't worry about 71°f. Perfect temp. Room temp is PERFECT for all tarantula species EXCEPT for Megaphobema species (they need to be kept cooler at about 60-65°f and do not do too well in warmer temps).At night usually usually gets down to 71