To the editor:

A traveling salesman is offering free dinners and selling heat shields to homeowners in our vicinity.

The premise is that NASA uses something like it to shield astronauts and spacecraft from the heat of the sun and the extreme cold of space, so the same product lying atop the insulation in your attic will make your home cooler in the summer and warmer in the winter. Nonsense.

In the salesman’s demonstration, two heat lamps irradiated two thick pieces of insulation, one covered with the product and one without. After a few minutes, the temperature of the protected piece rose 9 degrees, versus 61 degrees without protection. Very impressive, but irrelevant.

Space, a vacuum, transmits only radiant heat. The salesman’s model transmits heat by both radiation and convection (movement of air molecules), and the shield blocks only the radiation.

Does the sun shine on the insulation in your attic? Of course not. It irradiates your roof, which conducts heat to the air in your attic, which convects it to your insulation, which conducts it to your ceiling and then to your living space. Putting this product or anything else on top of your insulation does very little to stop this flow. Putting it on your roof would help, until the first windstorm.

In winter, heat flows in the opposite direction. Once it goes through your ceiling, it no longer heats your living space. Anything on top of your insulation is too little, too late.

What does work? An attic exhaust fan in summer and downward-directed ceiling fans in winter. Both make convection work for you instead of against you. Also, double-paned windows with shades or reflective panes, but even then, more heat will be gained and lost through your windows than through the insulation in your walls and ceiling.

Traveling salesmen offer delicious temptations but still sell snake oil.

James Bruner

Oak Harbor