Wood with good thermal insulation

Are all types of wood similar with respect to their ability to conduct heat or are some types better than other?

I'm looking for a go-to, cheap type of wood to act as the barrier in windows around window air conditioners; most importantly around unused A/C's left installed during the winter.

It's pretty trivial to come up with solutions involving multiple layers of various materials but I'd really like to stay with the simplicity and ease of a single piece of wood.

1/2" plywood doesn't really do a good job of keeping heat in and cold out, at least not compared to double-paned glass.

The A/Cs themselves are a good insulator and all gaps are well-sealed.

The answer depends on which wood species are available to you. With that being said, you can easily calculate it yourself with a formula and the wood database.

Theory

You can calculate the R value of any specific wood; you need to know the moisture content (MC) and the specific gravity (S).

R = 1 / (S * (1.39 + 0.028 * MC) + 0.165)

Example

Eastern White Pine has an S = 0.4 @ 12% MC, so you get:

R = 1 / (0.4 * (1.39 + 0.028 * 12) + 0.165) = 1.17

Hard Maple has an S = 0.71 @ 12% MC, so you get:

R = 1 / (0.71 * (1.39 + 0.028 * 12) + 0.165) = 0.72

In comparison, air has an R of 6.25 and concrete of 0.13. (Bigger is the better insulator)

In Short

You just have to find which one has the lowest specific gravity and make sure it stays as dry as possible once installed.

Source: Understanding Wood by R. Bruce Hoadley, pp.103-104

• Another tip: make an air gap between two pieces of wood for added insulation, or fill that gap with fiberglass or foam insulation. Not sure what thickness you're looking at, so this may not be feasible. Feb 11, 2016 at 14:46
• FWIW, the lowest specific gravity award, not suprisingly, goes to Balsa with S = 0.15 @ 12% MC, R = 2.36. Paulownia (? Looks like it's native to China) and Northern White Cedar take a distant second and third. Feb 12, 2016 at 18:30
• In short: use balsa wood! :-) Feb 12, 2016 at 19:40

The reality is that compared to other materials, wood is not an ideal insulator. XPS foam, for example, has an R rating of R5 per inch and also acts as an air and vapor barrier. XPS foam however should not be left exposed due to fire hazards, so perhaps the best of both worlds would be a layer of XPS foam with 1/4" plywood on both sides to give it a nice appearance which will accept a finish.