Nuclear modification factor $R_{\rm pPb}$ for muons from W$^-$ (top) and W$^+$ (bottom) decays with $\pt^\mu >$ 10 GeV/$c$ in p-Pb collisions at $\sqrt{s_{\rm NN}}=8.16$ TeV. The measurements are compared with predictions from pQCD calculations with several nPDF sets. The horizontal bars correspond to the width of the rapidity bins. The vertical bars and boxes indicate the statistical and systematic uncertainties respectively. The grey bands indicate the uncertainty on the pp production cross section. The data points are placed at the centres of the rapidity intervals while the theory points are horizontally shifted for better visibility.