Formalin in vapor form is a good disinfectant. As a matter of fact, it's being used in isolation rooms of some human hospitals that I've been to.
Here are the materials needed to come up with formalin gas:
-16 liters formalin
-8 kg potassium permanganate
-8 liters water
For poultry houses, it's used as final disinfectant. Final means that the house has already been cleaned of debris and fecal materials and has been disinfected once using a quaternary ammonium compound disinfectant or any other commercial disinfectant.
To generate vapor, place the pottassium permanganate powder on a can. Add the formalin and water and then run!
The quantities enumerated above can be used to fumigate a building for 3,000 heads broiler.