Check out some of the stuff from Auralex -- like this reasonably-priced setup: http://www.amazon.com/Auralex-MudGuard-Isolator-Dampening-Charcoal/dp/B004VM5R7Y/ref=sr_1_6?ie=UTF8&qid=1412014187&sr=8-6&keywords=auralex

That could be enough to reduce the ambient sound that gets recorded in your mic. If that's not enough, a few well-placed Studiofoam panels on stands would help too. And it would all be portable if you decide to go record somewhere else.

Please be very careful if you decide to go with lower-cost wall treatment options. The Station Fire wasn't that long ago.