During the lining refit
of our aluminium boat, and as we discovered non insulated areas, we used "Depron" insulation sheet, originaly produce for home insulation.
The "Depron" is sold in 3 or 6 mm thickness sheets (from 1/8" up to 1/4" thk) that can be glued on planar or slightly curved surface.
The insulation power is said to be the equivalent of twice the "Depron" thickness in wood (the 3 mm - 1/8" sheet insulate as 6 mm of wood, the 6 mm - 1/4" sheet insulate as 12 mm of wood), you can of course put several layers to increase the insulation.
The advantages of the material is that, as "closed cells" one, does not absord humidity, is light and easy to install (cutting, forming...) and really effective.
You can see the application of the product on our boat on our blog (see the link in our signature).
Of course any similar material or product will be Ok as well, just to give an idea of which type of product can fo the job.
Once the bare aluminiuml surfaces covered, we really felt a difference in temperature and, after 8 years did not see any problem with it.