I live in El Paso, TX (high radiation) and would like to know what I can do in order to decrease heat transfer from the roof and one side wall.
The most effective thing you can do to block radiant heat from entering the living space of your house is to install a radiant barrier in the attic. Properly installed with normal attic ventilation, a radiant barrier will block almost all of the heat that is transferred by the radiant mode of heat transfer. An example of this is the sun transfers its heat by radiation to the roof materials where it is absorbed, causing the materials to heat up and conduct that heat to the underside of the roof. From there, the roof heat is transferred, again by radiation, across the air space to be absorbed by the attic building materials, especially the mass insulation on the floor. When there is a radiant barrier installed on the underside of the roof rafters, the radiant transfer is interrupted so that those radiant heat waves cannot get to the large attic air space and then be absorbed by the insulation. With a properly installed radiant barrier, the attic air should never get any higher that the outside ambient air temperature.
If, because of a too low roof pitch or too much mass on the floor, it is not possible to apply the radiant barrier to the underside of the roof rafters, it is permissible to lay the radiant barrier on top of the floor mass. Do not staple in place, just overlap each strip 3 or 4 inches, and make sure the radiant barrier is perforated to allow for moisture migration. Also, be aware that dust can accumulate on top of the radiant barrier over time and this will reduce its effectiveness. If your attic has evidence of significant dust, you may not want to use a radiant barrier. In any case, you should check the radiant barrier once or twice a year — it could take many years for enough dust to dramatically reduce the performance of the radiant barrier.
If you have A/C ducts running through the attic and you need to place the radiant barrier on the floor, make sure you cover the ducts with radiant barrier as well. That will be a big energy saver.
As to the wall, if it is exposed, like in the attic, installing a radiant barrier off the face of the studs will block the radiant heat there as well.
If you have an attic roof fan and then install a radiant barrier, it probably won’t operate as the attic air temperature won’t get high enough to activate it. If you don’t have a fan and you install a radiant barrier, don’t waste your money on a new fan.