When the Chumash speaks about a kohen's seeing some tzaraas on a person's body for the first time, it says the kohen "shall defile" (Vayikra 13:3, 13:11, 13:20, 13:25, 13:30, 13:44). Rashi (13:3) says that the kohen states to the other: "You are defiled." In some circumstances, though, the kohen does not declare person defiled; rather, he closes him in his house to await further evidence as to whether he has tzaraas (Rashi, 13:4). Nonetheless, during that closed-up period, the person is m'tame, i.e. he defiles things he comes in contact with (Rambam, Tum'as Tzaraas 10:12).
When it comes to tzaraas on clothing or a house, though, it is never full-blown tzaraas from the outset: The kohen, if he thinks it's tzaraas, closes it up to await further evidence (13:50, 14:38). During that closed-up period, the seeming tzaraas is m'tame (Rashi, 14:36; Rambam 13:13, 16:1).
From what point is the closed-up person, garment, or house m'tame? Specifically, is it from when the kohen sees the tzaraas (and knows it needs to be closed up) or from when he actually closes it up? If the latter, what exactly does the kohen do to effect the defilement? (For example, is it sufficient that he pronounces the house off-limits? Does he need to physically lock it up?)
Especially, does anyone address this question explicitly? Here's why I'm asking:
Yad Hachazaka never addresses the issue explicitly (as far as I see).
Rashi (14:36) writes about a house: "… the kohen will come and see the tzaraas, then it will need to be closed up and everything in it will be defiled". This is unclear: Rashi may mean that everything in it will be defiled when it's closed up, or he may mean everything in it will be defiled as soon as the kohen sees the tzaraas and the house needs to be closed up. The Maharal seems to resolve that ambiguity: "Meaning, when he sees the tzaraas, everything in it will be defiled immediately…." However, even this is not entirely clear, as he goes on to contrast this immediacy with a rule, not applicable here, about how long something can be in a tzaraas-afflicted house before becoming defiled. So he may mean "immediately" only in contrast with that and not as an indication of when the defilement begins. Nonetheless, his language does somewhat seem to me to imply that he holds the defilement begins immediately the kohen sees the tzaraas.
Rabenu Hilel ben Elyakim (commentary to Sifra to 14:36) writes about a house: "… For we didn't learn '[he's not defiled] until he remains [in the afflicted house] the time it takes to eat a half loaf' except about such items as he brings in [to the house]. But such as were there when the kohen arrived are all defiled immediately." Again, like the Maharal, this is ambiguous, but seems somewhat to imply the defilement begins on the kohen's arrival, not on his closing up the house.
Meshech Chochma to 14:36 says very clearly that the pasuk is teaching us there that the house is not defiled before the kohen comes. But he's not as clear about when it is defiled. He writes at the end: "…for at the time he sees the tzaraas, that is closing up, likewise everything in the house will be defiled". He may mean that seeing the tzaraas constitutes closing up, i.e. everything becomes defiled when the kohen sees it; but he may mean that the "seeing" referred to in Vayikra there is not literal seeing but an act of closing up the house.