A fire at a vacant house in Innisfil on the 6th Line is under investigation. (Steve Mansbridge/CTV News)

Fire crews are investigating what caused a blaze to break out at a boarded-up, vacant house in Innisfil.

Officials say crews were first called to the empty house on 6th Line near Emberton Way and Saint John's Road Saturday for a fire.

They say battling the flames was difficult because of all the "hidden voids" in the old construction.

Fire officials say the fire sparked back up on Sunday and was likely a "continuation" from the day prior, adding the construction may have kept a small area of the blaze hidden from the crews.

There is no damage estimate.