Here's what we're using for a dielectric: 4 lb/ft3 white closed cell crosslinked polyethylene foam tape that is 0.062 -0.009/+0.018 in. thick. Not recommended for UV exposure, which means it doesn't have UV protectants in it. It's 3M (http://www.3m.com/) tape # 4466.
We can calculate the Er of the foam tape by estimating the density and Er of polyethylene in general.
Low density (LD), high density (HD) and ultra-high molecular weight (UHMW) polyethylene seems to have all about the same density, and about the same Er according to multiple sources on the internet.
Here's my choice: Er = 2.3 with a density of 0.0330 lb/in3 (57.0 lb/ft3). I picked the middle Er value, and the middle of the LDPE values. The reason for the LDPE is that I don't think they start with the HDPE for foam - so it's the "closest" to the foam in this table.
4 lb/ft^3 / 57.0 lb/ft^3 = 0.070 = 7% PE, 93% air
(93% * 1.001) + (7% * 2.3) = 0.931 + 0.161 = 1.092
Spencer Webb found the Er very close to 1.0.