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Inside Exterior Walls – Hot/Mixed Climates

keeping the heat out of exterior walls

This method also keeps heat in during cold months.

Installing a radiant barrier in the walls is done just like the cathedral ceiling method. When you are trying to block heat from coming in, it is best to have the foil closest to the outside layer as possible, while still maintaining an air space on one side of the foil. If you are remodeling a wall that shares exterior sheathing, read below for the typical instructions on a retrofit/remodel application.


If you are doing NEW construction, you should really consider utilizing a radiant barrier house wrap OUTSIDE the wall, between the brick/stone/siding and exterior sheathing. For more information on house wrap see: Radiant Barrier as a House Wrap.

The exterior of a west facing wall can reach over 150º on a hot-sunny afternoon. Typically the wall absorbs the radiant heat from the sun and heats up. Then, this heat travels by conduction through a wall made of wood and insulation. The problem exists in that traditional insulation only slows down heat flow by conduction, it does not stop heat.


The goal is to force the heat to CONVERT to radiant form (radiant heat), so that you can REFLECT it back to where it came from. The best way to make the heat convert to radiant heat is to make it cross an air space. So, we will use they same method as the Cathedral Ceiling Method except the airspace will be “dead air” instead of ventilated.


To make this simple, think in terms of LAYERS of your wall and how the heat will flow through the layers from the outside to the inside. You want the radiant barrier to be the first line of defense against radiant heat and the regular insulation to be the second line of defense against conductive heat flow.


The Layers of the Wall

Here are the layers (from the exterior to the interior) as they should be installed:


  • Outside air
  • Brick/stone/siding
  • Exterior sheathing
  • AIR SPACE – created by using foam strips to separate foil from exterior sheathing. You MUST have an airspace for a radiant barrier to work.
  • AtticFoil™ Radiant Barrier Foil – Perforated, not solid
  • Standard insulation with R-value to reduce conductive heat flow. This insulation can be fiberglass, cellulose, foam board or spray foam.
  • Interior Sheetrock/drywall
  • Inside of the home/building


One optional method we like is to put a layer of either ½” or ¾” foam board BETWEEN the studs and the sheetrock. You can use roofing nails with a large head to hold the foam board in place before you finish with your sheetrock. Basically you will install the foam board like sheetrock and then install the sheetrock using longer drywall screws so you can make it through the foam layer too.


For Added Performance of the Wall

For an extra air-tight wall, we recommend putting a bead of caulk on the face of the studs before the foam layer (or before the sheetrock layer). Be sure to caulk between the bottom plate of the wall to either the concrete slab or the subfloor to reduce air leakage under the wall as well.


Adding the foam board to the assembly does several things:


  • adds R-value
  • helps air seal the wall
  • reduces thermal bypass
  • sound/noise reduction


You will be amazed at how little heat will get through a wall built using this method.


For similar installation instructions, see Installing Radiant Barrier in a Cathedral Ceiling to Keep the Heat Out.

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