Reflective Radiant Barrier Insulation Joist Space

Before and after reflective radiant barrier installation in a joist space  equipped with Hydronic Radiant heat.

Reflective Radiant Barrier was installed on the bottom of floor joists in a basement. Hydronic Radiant heat is fixed to these floor joists about 1 inch below the subfloor. The heating system is turned on during this experiment with water  flowing through the pipes at a temperature of 135 F.

This experiment shows how radiant barrier can  effectively direct heat where desired for a warm comfortable floor with minimal striping.


This installation is a renovation of an existing home to provide radiant heat to a bathroom above. 1/2 inch pex tubing is installed below the bathroom in the joist space. The heating system is turned on during this experiment with water is flowing through the pipes at a temperature of 135 F. The basement is already heated with electric baseboard to around 70 F. Due to extensive protruding nails, the tubing was installed directly to side of Floor Joist. While r12 fiberglass batt (preferably with reflective facing) is commonly reccomended to direct the heat up, in this installation only reflective foil is used.

Temperatures taken before and after reflective insulation was installed, to show how the reflective barrier stops virtually all unwanted downward heat.




          Before Reflective Foil                      After Reflective Foil     

 Note: Larger number on temperature guage is reading being taken.

Temperature in joistpace 

Note: temporary tinfoil on joistspace to left


Note: "After reflective foil" temperature measurements were taken more than 1 day later to allow temperature to stabilize.

Painted black for accurate temperature measurement



Temperature immediately after tinfoil was removed


temperature at thermostat height

Temperature just below ceiling

Temperature of nearby room at ceiling without radiant heat


Picture of existing bathroom directly above the joistspace installation.

Temperature in between joist.




Temperature on joist.

Less than 4 degrees striping effect.

Barely noticeable with bare feet.

 Temperature of carpet



In this installation over a heated space, reflective foil barrier is very effective at directing the heat where you want it, with minimal downward heat transfer.

Pex tubing installed directly to the side of a floor joist with reflective barrier attached to the bottom of the floor joist, transfers heat very effectively, and has minimal striping effect.

Reflective foil insulation radiant barier made in Canada


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