I am installing vinyl siding and intend to install radiant barrier on the plywood prior to installing the siding. Will the air space that will be created by the siding itself (which appears to be about 5/16 to 3/8 of an inch) be sufficient for the radiant barrier to be effective? How much air space is required and is there an optimal amount of air space?
There are several concerns relating to your application.
- Wrapping a house with a radiant barrier material puts a vapor barrier on the outside and could trap moisture in the walls. Even with perforated products, often the perforations aren’t sufficient to meet the requirements for breathability of a house wrap.
- There is a concern that the radiant barrier could trap heat behind the vinyl siding and cause it to degrade — in especially hot direct sun conditions.
- The optimal air space for a reflective surface is 3/4″ — so your application would fall far below that. Although it would provide some thermal benefit, it might not be enough to make much difference in energy savings.
All that said, I installed a radiant barrier in the form of a reflective insulation material on the outside of my old farm house about 20 years ago. I then installed cedar siding on top of firing strips that were only 1/4″. I’ve had no problems, however, it has to be noted that the house is very old – about 120 years – and there are lots of gaps in the construction. Also, it is surrounded by trees and foliage that protects most of it from direct sunlight. I would suggest you approach your application at your own risk.