If a seller did not know that his house was affected by a latent defect, can he still be responsible towards his purchaser?

Yes, under the provisions of the Civil Code of Quebec, the seller of an immovable property remains responsible for "latent" defects... thus unknown.

The seller is naturally responsible for defects which he knew but omitted to declare to the purchaser. He is also responsible for defects which he could not ignore.

For example, if the seller has often noticed excrement near the roof of the house and saw small brown creatures flying around, he could not ignore that his roof sheltered a colony of bats, even if he has never been up there to confirm it personally!

However, as it was evoked above, the seller is not required to guarantee the defects which he has brought to the attention of the purchaser by speaking to him about those or by showing them to him, or even the defects which are apparent, that is to say those that the purchaser could have found if he had made a careful and diligent examination of the house and that, without having to resort to the services of an expert.