Borrowdale becomes Paulo Sousa's first signing as the new manager at Loftus Road and could make his debut on Saturday against former club Crystal Palace, who he left to join the Sky Blues in the summer of 2007.
"I'm really pleased to be here - it's a great move for me," the 23-year-old told his new club's official website.
"Things have gone a bit stale for me since the change in management at Coventry, but I've got every belief in my own ability and I'm confident I'll be a success here.
"It's a fresh start for me and I'm looking forward to the challenge that lies ahead."
Trinidad & Tobago international Birchall has also ended his Ricoh nightmare by joining Carlisle until the end of 2008.
The 24-year-old was told he does not figure in Chris Coleman's future plans and caretaker boss Greg Abbott told BBC Radio Cumbria: "We watched Chris last season, so he is a player that we know all about.
"He'll provide a bit of strength and competition and we think that we have got ourselves a good player."
Coventry last week