Measuring noise floor in an impulse response
I can suggest you take a look at the supporting information for Dirac software, where they describe the process of INR (IR to Noise Ratio) calculation. 
The simplest approach is to estimate the noise level based on the beginning of the IR before the main peak. For that purpose, you can also do the tail of the Impulse response.

Yet another article supporting that estimation is by Lundeby et al.: "Uncertainties of Measurements in Room Acoustics". Their algorithm is an iterative procedure which tries to estimate the background noise level and the slope:



You could also try to verify that the noise floor doesn't have any distortions, by fitting the linear regression and making sure that the slope is as close to 0 as possible.