Indeed, when one speaks of a house "leaking heat" it necessarily means from inside to outside. If a pipe leaks, you don't generally mean that water is coming into it. On the other hand, if a ship leaks, you do mean water is coming into it. So, it all depends upon one's objective. As insulation is generally intended for both comfort and energy saving, the main concern tends to be keeping heat in. As Chris notes, keeping heat out is more important in a hot climate, but I don't think the meaning of house "leaking heat" is likely to change to incorporate that.
The leakiness is not "due to a passoire". "Passoire" is a metaphor for the leakiness.
They went to sea in a Sieve, they did,
In a Sieve they went to sea:
In spite of all their friends could say,
On a winter's morn, on a stormy day,
In a Sieve they went to sea!
And when the Sieve turned round and round,
And every one cried, 'You'll all be drowned!'
They called aloud, 'Our Sieve ain't big,
But we don't care a button! we don't care a fig!
In a Sieve we'll go to sea!'
http://www.nonsenselit.org/Lear/ns/jumblies.html