THis web site also has some references
http://www.bhsi.org/cooling.htm
References
We recommend at least two good articles on helmet cooling. Unfortunately, neither is available on the Web.
Andrew Ellis has done his PhD research on bike helmet cooling. It was published in the January-February 2001 issue of the Australian magazine Bicycling Australia. You can find the magazine on the Web but not the article.
Jennifer Reid and Eric Wang have published "A System for Quantifying the Cooling Effectiveness of Bicycle Helmets" in the August, 2000 issue of the Journal of Biomechanical Engineering. You can read the abstract, or perhaps get to the article if you subscribe.
And some older references
Burns, Richard. "How Hot the Hat?" Bike World, September, 1975
This was the first report we had seen of a scientific test of helmet coolness. Dick Burns put sensors on his head and rode around with his Bell Biker on. Results indicated a cooler head (in sun) with helmet than without. The article preceded any other published article on this subject by at least ten years. Bike World Magazine flourished for a short time in the mid-1970's.
Zahradnik, Fred "Helmet Buyer's Guide" Bicycling Magazine, Vol XXX, No. 4, May, 1989. This article reported on Bicycling's innovative ventilation testing with an instrumented headform heated internally and a cooling fan. Has a list of helmets available on the U.S. market for 1989, including their ranking in the ventilation test. Bicycling published a similar article in May of 1990 and May of 1991. Then they apparently retired their instrumented headform and stopped giving helmets ratings for ventilation. We don't know if the ratings were accurate or not, but these were ground-breaking articles superior to anything anyone publishes today on helmets in the US market.
again I think it is up to the rider to choose for their own reason , this just got me to thinking about the heat/cooling affects of helmets.