Radiant Barrier versus Fiberglass Batt Insulation

Thermal insulation such as fiberglass, does not usually live up to its R-value unless certain criteria are met. Classic insulation absorbs heat and then re-emits that heat into the structure and or building materials around it. The role of thermal insulation is to SLOW conductive (heat moving through solid objects) heat flow. Think of a spoon at room temperature that is placed into a cup of very hot water; that handle becomes hot very quickly absorbing heat from the water.

How do we determine the expected performance of thermal insulation? Through the R-value rating. R-value defines insulation performance. R-value varies by type of material, thickness of the material and density. The minimum recommended by the FTC is R-13 for homes in the United States.

Fiberglass batt has an R-value of up to 3.2 per inch of thickness. A typical home has wood framing of two by fours creating a space or cavity of about 3.5 inches. This is the perfect fit for R-13 fiberglass batt. Two by six framed walls provide a 5.5-inch cavity where a fiberglass batt of R-21 will easily fit.

What people do not realize is that the conduction in an attic, into thermal insulation reduces the R-value of that product up to 60%! Let that sink in. If you are losing 60% of the R-value, the insulation is barely doing its job.

An interesting study was done with two homes, one had just Radiant Barrier (RB) in the walls and ceiling/attic and the other was a conventional home with R-19 in the walls and R-30 loose fill fiberglass blown into the attic.

The attic temperature of the RB home was never more than a couple of degrees greater than the outside temperature. The conventional home attic was over 145 degrees every day! The experts concluded the lack of thermal insulation was keeping the attic cooler in the RB home. They decided to place a KWH into both homes for each heat pump. The meter registered a decrease of 35% in energy needed for cooling in the RB home and the home was more comfortable overall. This meant the RB home returned 35% savings over conventional insulation.

They then decided to add blown in insulation to the attic in the RB home. A few hours after completion, the attic temperature was measured, and it was hotter than every reading prior. The attic also remained hotter longer after sundown compared to before thermal insulation was added. These are interesting findings and quite the conundrum.

Since Radiant Barrier reflects up to 97% of radiant heat, emitting up to 5% to the air space and materials near it, RB actually helps thermal insulation perform better by providing its true R-value protection because it only has to absorb less than 5% of the heat now versus 100%. The math says 5-10 degrees vs 145 degrees. It is true that RB is more effective in hot climates and specifically zones 1-3 and some areas of 4. Tests show attics with R-10 insulation and Radiant Barrier installed as well, reduced summer ceiling heat anywhere from 16-42%! All of this translates into a more comfortable home and less heat gradient changes in the building materials. The greatest performance and energy cooling savings are when both are used together. The reason is that the “whole-house” heating and cooling load from the ceiling is greater when the thermal insulation is small and conversely RB produces significantly lower energy savings when used with high amounts of thermal insulation.

This is great news! Zones 1-3 and parts of 4 can spend less on the thermal insulation, using lower R-value which, offsets the cost of Radiant Barrier.

Note the chart with R-value needed by zone and placement in the home.

It is becoming more apparent that combining insulations produces optimal results. When building a home, insulation is not the time for cost-cutting, and this is true for foam and fiberglass. A 5.5 cavity we now know that 2 in foam could be placed in the cavity and then an R-13 batt which, will seal the wall with an R-25 value.

For example, in zone 2, we could use R-30 up to R-60 thermal insulation for the attic but realistically, R-60 is overkill. We now know that combining helps reduce the necessary R-value for the application. For walls in most homes, R-13 to R-15 for 2×4 studs and R-19 to R-21 for 2×6 studded walls is the recommendation. Our wall assembly is; siding, air gap, Radiant Barrier, House wrap, studs, R-13 fiberglass batt (not R-15) the radiant barrier will let the thermal insulation perform at its maximum value. Without the RB, the R-value in zone 1-3 in the wall insulation would be 5.5 (let us use the maximum 60% loss). Even if it does not lose 60% of its performance, it is still losing too much to function where it should. Comparing R-13 and R-15 at 60% loss of performance means, the R 13 is working at 5.5 and the R-15 is working at 6.0. This proves the more expensive higher R-value for the wall without RB is only giving you a 0.5 increase in performance.

With the Radiant Barrier, the thermal insulation only needs to absorb at most, 5% of the heat moving through. That means, the R-value is working at the very least 14.25 vs 15 however most Radiant Barrier emit 0.10% to 0.25%. Clearly, Radiant Barrier and thermal insulation are designed to accomplish different results however, they work together to give you the best performance, energy efficiency, savings, and comfort possible. Their sum excels in performance. Again, studies show that RB produces significantly less in energy savings when coupled with high levels of thermal insulation.

Once again, saving on insulation should not be primary, but instead making use of combined products will maximize function, R-value, decrease HVAC use and longevity, increase energy savings, increase comfort in the home and help tighten the building envelope. The various R-value fiberglass batts range from R-6.7, R-13, R-15, R-19, R21, R-30, R-38, R-49. The higher the R-value, the greater capacity at slowing heat gain/absorption.

The question you should ask, do you want to move your house under the shade (radiant barrier)? Or do you want to add an extra blanket to sleep under? How about both! Instead of two blankets, you only need one when under the shade.