Cavity wall insulation and air vents

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We're looking at getting our walls insulated in the house we're buying and we know it's not very expensive (plus extra good for energy bills and the environment). One thing I'm wondering is though, what happens with the air vents? The house has a few in each room and whilst I don't really like them and would be happy to fill them up, I'm not sure if that's allowed? Somebody told me that the 'foam' will come out of them......

Any ideas/experiences?

We've had cavity wall insulation in our house and it certainly didn't come out of the air vents. I don't think you're allowed to fill them up now either, you certainly aren't allowed to fill them up if you have an open fire.

When our next door neighbours had cavity foam insulation installed some came out of one vent - I would think that these are supposed to be kept clear otherwise there could be problems with damp in the house. When you have the work done make sure the workmen know where all the vents are, so that doesn't happen.

I'll make sure they look at them and check, just seemed odd as the whole point of air vents is they let air out/in!

We're having a gas fire put in our living room (plus a combi boiler/new radiators) so I'm not too concerned about keeping the air vents, I just don't like the idea of having foam coming out of them.