Ontario's highest court has nearly doubled the sentence for a man convicted of attempted murder for setting his girlfriend's house on fire and trying to trap her and two of her children inside.

The Court of Appeal for Ontario says the 11-year sentence imposed on Kenneth Kormendy, of Windsor, was inadequate and "demonstrably unfit" given the severity of his crimes.

Instead, the court says Kormendy will face a sentence of 20 years with credit for the time he served in pre-sentence custody.

The appeal court says the trial judge made several errors that affected the sentence, such as failing to properly consider the gravity of the offence and Kormendy's moral blameworthiness.

It says the judge also failed to treat deterrence and denunciation as the primary sentencing objectives, and to refer to the most relevant cases to gauge an appropriate sentence.

Court heard Kormendy moved in with his girlfriend and her three children after a few weeks of dating and quickly became controlling. He set the house and his girlfriend on fire after she ended the relationship and told him to move out.