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We present a study of the continuum polarization over the 400--600 nm rangeof 19 Type Ia SNe obtained with FORS at the VLT. We separate them in those thatshow Na I D lines at the velocity of their hosts and those that do not.Continuum polarization of the sodium sample near maximum light displays a broadrange of values, from extremely polarized cases like SN 2006X to almostunpolarized ones like SN 2011ae. The non--sodium sample shows, typically,smaller polarization values. The continuum polarization of the sodium sample inthe 400--600 nm range is linear with wavelength and can be characterized by themean polarization (P$_{\rm{mean}}$). Its values span a wide range and show alinear correlation with color, color excess, and extinction in the visual band.Larger dispersion correlations were found with the equivalent width of the Na ID and Ca II H & K lines, and also a noisy relation between P$_{\rm{mean}}$ and$R_{V}$, the ratio of total to selective extinction. Redder SNe show strongercontinuum polarization, with larger color excesses and extinctions. We alsoconfirm that high continuum polarization is associated with small values of$R_{V}$.  The correlation between extinction and polarization -- and polarizationangles -- suggest that the dominant fraction of dust polarization is imprintedin interstellar regions of the host galaxies.  We show that Na I D lines from foreground matter in the SN host are usuallyassociated with non-galactic ISM, challenging the typical assumptions inforeground interstellar polarization models.

Continuum Foreground Polarization and Na~I Absorption in Type Ia SNe