Germinating Basil

In the past I have had difficulty getting basil to germinate when starting it indoors. I have decided that bottom heat may be necessary. What should the soil temperature be to ensure germination of sweet basil and purple ruffles basil?

I suspect that you have not kept the seed bed warm enough. Literature I consulted says that sweet basil likes days 20-30 degrees Celsius and nights 15-25 degrees Celsius. Purple Ruffles likes nights at 18-20 degrees Celsius.
Basils do not thrive if the nighttime temperature falls below 10 degrees Celsius.