Attic Foil Radiant Barrier - Keeping Heat Out

Cathedral Ceilings - Keeping Heat Out

This application is the best way to install radiant barrier in a catherdral ceiling for comfort and results YEAR ROUND.

One of the most frustrating problems is having comfort issues with a particular room in your home. These are rooms that you can never get quite cool enough no matter how hard you try. The usual suspects are: rooms over garages, bonus rooms, media rooms, master bedrooms that stick out from the house, and home offices.

In hot climates the goal is heat rejection. This being the case, a radiant barrier is most effective when placed closest to the outside of the assembly in question.

Typically your layers will go as follows, from the roof down:

Roof deck

Air gap (created manually with spacers)

Radiant barrier foil insulation

Traditional insulation

Sheet rock/drywall

Creating the Air Gap

foam spacers on cathedral ceiling method for website

If you are going to add insulation to the assembly, then the main goal is to ensure the air space is not compromised when the insulation is added.

Measure the distance between each rafter to make sure you know the correct measurement. Make sure you account for enough overhang that the foil can be attached to the sides of the rafters.

If you want, you can pre-cut the foil into strips to fit between each rafter bay to save time.

Cut the foam board into 2-3 inch wide strips - either 4 foot strips or 8 foot strips, whichever is easiest.

Attach the foam strips in between the rafter bays with Liquid Nails. You need to put 3 foam strips between each rafter bay; one on each side and one down the middle. The one in the middle will prevent the radiant barrier from losing the air space when pushed on by the insulation.

Adding the Radiant Barrier Foil

Cathedral Ceiling Method - Stapling Tabs onto Rafters for website

Now cut your AtticFoil to fit the rafter bays: 27-28 inches wide to go on 24" on-center rafters. Your pieces will be 28" x 48" pieces that will go up and down, on top of each other.

Take a straight edge and make a fold the foil over it to create a tab edge on the foil that you will staple to the sides of the rafter.

Place the edge on the side of the rafter and secure it with 3 to 4 staples.

Pull the foil across the foam strips and create the second tab on the other side simply by folding the foil with your fingers.

Staple this side up to the rafter side to secure it in place.

Now you have the following layers in the assembly: (coming from the top, down) roof deck, foam strips (for air space and ventilation channel), and the radiant barrier foil.

Continue on all the rafter bays until you are finished.

Then, use some air conditioning tape and tape the seams where the foil is overlapped, to make sure that none of the air from the air channel you created escapes and gets into the insulation.

Once you finish sealing the seams, you're ready to install your batt insulation.

Adding Traditional Insulation

Adding Insulation to the Cathedral Ceiling Method for website

If you are adding traditonal insulation to the assembly, now is the time to do so.

Typically an R-19 will fit in the rafter bay (you might have to compress it a little because of the foam board.

Then you can sheetrock the wall, or add whatever material you are finishing up with.

Non-conditioned Space

cathedral ceiling install in attic with no insulation in cavity

If you have a non conditioned space (meaning you are not heating and cooling the space) you are finishing out with drywall, you do not have to add traditional insulation. In this case, the installation becomes very simple.

Cut a sheet of radiant barrier to the proper length of the area you are working on.

Start at the bottom and work your way up.

Run the foil across the rafters horizontally and secure the foil to the rafters with 3 to 4 staples per rafter.

Continue this pattern until you have covered the entire assembly from bottom to top, making sure each run of foil overlaps the previous run by at least 2 inches.

Once the wall is completely covered with the foil, simply attach the drywall/Sheetrock/ceiling over the foil.

The foil is able to work in this set up because you have about 6 inches from the deck to the end of the rafter of air space. It is in this space that the radiant heat will meet the foil and then be reflected back towards the roof.


Installing the foam strips to create the air gap.	The foil goes over the foam strips so it's not touching the deck.

The cathedral ceiling method is the same for knee walls.	Use foil HVAC tape to tape the seams in the wall sections.

Keeping the heat out with the foil closest to the deck.	Finished wall installation of the foil.

After the foil add your insulation to the wall/ceiling cavity. Before you finish with drywall, to really make it air tight, add a layer of foam board over the insulation and attach it to the studs.	Finish the wall with Sheetrock over the insulation (or foam board). If you use foam core, you will need longer screws to attach the drywall. Overall the foam adds another layer of sealing and you get some R-value too.

This customer added foil-faced foam board to a baffle vent so he could slide the radiant barrier up into the cathedral ceiling cavities. The foil side (red arrow) is facing the air gap created by the baffle.	This is a cathedral ceiling installation before the insulation is added to the rafter bays. Foam strips are glued to the roof deck and then the 26" wide radiant barrier is installed veritcally in the cavities.

Using the 26" wide radiant barrier foil eliminates the need for multiple cutting and reduces waste of material. 24" on center rafters are the perfect size for this product.	For conditioned spaces you need to add insulation after the radiant barrier. For non-conditioned spaces, you can just go with the radiant barrier alone.

This attic has a truss system so the 26" wide foil was used to easily go in between the trusses before insulation was added.	Here is another example of a cathedral ceiling with lighting installed before the insulation and sheetrock is added.