The $700 billion financial rescue plan signed by President Bush contains a tax exemption for idle-reduction systems, such as auxiliary power units and cab insulation.

The exemption relieves buyers from the heavy truck tax on devices or systems "designed to provide ... services (such as heat, air conditioning, or electricity) that would otherwise require the operation of the main drive engine" while the truck is parked.

To be eligible for the exemption, the device will have to be approved by the Environmental Protection Agency, in consultation with the Departments of Energy and Transportation. Insulation would have to have an R value of at least R35 per inch.

The exemption became effective Oct. 3 when Bush signed the Emergency Economic Stabilization Act of 2008. It had been passed by the House earlier this year but was stalled in the Senate until it was attached to the rescue bill.