Is it safe to cover a floor heating vent and if so with what?

We have a 2 story house with all the bedrooms upstairs. The smaller bedrooms get too hot when the heater is on, but the master which is the only room with a vaulted ceiling will still be freezing. Is it safe to cover the heating vent with something to keep the small rooms cooler. Someone suggested foil but I... show more We have a 2 story house with all the bedrooms upstairs. The smaller bedrooms get too hot when the heater is on, but the master which is the only room with a vaulted ceiling will still be freezing. Is it safe to cover the heating vent with something to keep the small rooms cooler. Someone suggested foil but I didn't know if that would be safe.
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