Myth Busting Dust and Radiant Barriers

From the Levins and Hall dust tests performed in the late eighties, their conclusions show that although dust degrades performance, it is not devastating. Dusted radiant barrier is still effective in reducing attic heat flows AND total house cooling loads. These tests were done in TN and it is my assumption the radiant barrier was not double-sided foil since the article does not state this. Also, we know more now and double-sided would be used for a horizontal attic floor application. From Measured Effects of Dust on the Performance of Radiant Barriers Installed on Top of Attic Insulation, W.P. Levins and J.A. Hall.

The rate of dust accumulation and the effect of this accumulation on a horizontal radiant barrier are of interest because the horizontal application may perform better in both summer and winter than a truss radiant barrier and it is much easier to install for retrofits in existing homes, and requires less radiant barrier material than a truss/rafter application. However, horizontal application has two important potential disadvantages, condensation during winter which, might cause structural damage and, dust accumulation. These both could significantly degrade thermal performance of the radiant barrier. At the time this study was done, it is highly likely the product was one sided and not perforated as radiant barrier is today with accompanying vapor or permeability ratings.

The effect of dust on a horizontal radiant barrier can be divided into three areas of concern: (1) the rate of dust accumulation in actual homes, (2) the effect of dust on RB emissivity, and (3) the effect of dust on actual radiant barrier performance.

The results indicated that dust accumulation may not be a prohibitive problem for horizontal attic applications. However, the researchers believed further study of this subject was needed. A national laboratory conducted tests on dust horizontal radiant barrier applications at three research houses near Oak Ridge, Tennessee, during the summer of 1988. Two dust loadings were tested: 0.34 mg/cm2 (E = 0.125) and O. 74 mg/cm2 (E = 0.185). Results showed that the lighter dust loading increased total house cooling loads, compared to a clean horizontal barrier, by 2.3%. The heavier dust loading increased house cooling loads by 8.4%, again compared to clean. However, horizontal radiant barriers with these dust loadings still decreased house cooling loads by 7% when compared to the same house with no radiant barrier.

It was found that a complete covering of Arizona dust on a horizontal radiant barrier did not significantly degrade its performance. This surprising result led to more extensive tests during the summer of 1987. The purpose of this test was to determine the effect that horizontal radiant barrier dust accumulation has on the ceiling heat flux. The cooling load of House 1 (heavy dust loading on the barrier) was reduced 14.3%. House 2 was reduced only 9.5% by the clean barrier. The corresponding A/C electrical inputs to houses 1 and 2 were reduced by 9.3% and 10.0%, respectively. House 1 showed a cooling load reduction of 7.1% with a dust loading of 0.74 mg/cm2 (e = 0.185) compared to house with no horizontal radiant barrier, while the cooling load in house 2, with a dust load of 0.34 mg/cm2 (e= 0.125), was reduced by 7.4%, compared to no horizontal radiant barrier. The corresponding A/C electrical inputs were reduced by 4.8% and 8.5%, respectively. The effect of dust relative to a clean horizontal radiant barrier shows that a heavier dust loading in house 1 increased the total cooling load by 8.4% compared to a clean barrier. The lighter horizontal radiant barrier (HRB) dust loading in house 2 increased the total house cooling load by 2.3% compared to a clean barrier. The corresponding effect on air conditioner electrical input was an increase of 4.9% in house 1 and an increase of 1.7% in house 2, again, compared to clean radiant barriers.

A clean barrier reduces the net attic heat flux by 46.6% in house 1 and 41.4% in house 2. The heavier dust loading in House 1 only reduces the attic heat flux by 31.5%, compared to house 1 with no HRB, or increases it by 28.4% when compared to that of house 1 with a clean HRB. The attic heat flow in house 2 with the lighter dust loading is only decreased by 34.1% compared to house 2 with no HRB, or it is increased by 12.6% when compared to that of house 2 with a clean HRB.

Test results showed that a clean horizontal radiant barrier reduced the net ceiling heat flux by 58% compared with no radiant barrier, while the dirtiest horizontal radiant barrier reduced the net ceiling heat flux by 19%. These tests showed that dust accumulation does not totally eliminate the effectiveness of a horizontal radiant barrier and that dust may not be a prohibitive problem for these types of installations.

Both sets of tests show that although dust degrades the performance of a horizontal radiant barrier in summertime, the degradation is not devastating. Dusty barriers are still effective in reducing both attic heat flows and total house cooling loads. Long-term testing should be done to verify the results obtained in these tests as well as to ensure that natural dust does not behave in a more severe manner than Arizona dust.

Thirty years later, we have great results with truss/rafter double-sided radiant barrier applications and very negligible dust issues. With the double-sided radiant barrier for a horizontal application, installed correctly, all the dust in the world will not have a significant impact on the radiant barrier performance. The bottom side working off the airspace between the insulation and foil (unless it is literally squished into the insulation), will still block 97% of the heat and only pass the low emissivity. Today most foil is double-sided and for the horizontal application, must be perforated and if in the cold climates, the permeability rating should be higher than what would go into a home in the Sun Belt. Radiant barrier is never a waste of money unless the few cardinal rules regarding installation are ignored. Air space is always key and if you can manage air on both sides, radiant barrier works perfectly.