Taper line style

Hi

I often use the yellow line in the picture below. It's our symbology for insulation. But theres often cases where I need to draw that insulation tapered, like the gray line above. Right now I do this by hand but it's time-consuming. Is it possible to achieve that tapered line in a smart way. 

The yellow insulation is created with the line style editor but I can't find a way to taper that line. 

Regards, 

Robert