Wasps in your loft pose more of a problem than those that are nesting outside. Typically, the wasps that choose to nest inside will be paper wasps, either European or their domestic cousins. Paper wasps build exposed nests with open cells on the bottom, typically resembling an umbrella. European paper wasps have much larger colonies than their domestic cousins, which may make it more difficult to eliminate. Both species can sting repeatedly and normally only do so when the nest is disturbed. European paper wasps, unlike domestic wasps, also leave a chemical in their stings which attracts the rest of the colony to the disturbance, in this case, you.
Cool your loft as much as possible. During the summer, this may be difficult, but wasps become sluggish and don't fly well when temperatures are below 50 degrees F.
Spray the nest with an aerosol "freeze" spray at night, often called "wasp and hornet spray." The ingredients in these products knock down the wasps immediately on contact. During the night, the wasps will be in the nest. If you have a very large European paper wasp nest, use extreme caution. In the fall, most wasps will be dying, but during the summer it is best to get to the nest around June. In June and early summer, the queen is still building her colony and the nest is small.
Quickly place the plastic bag over the nest and rip it down.
Sweep up the knocked-down wasps with a vacuum cleaner.