I am curious about the attic spray on barrier being pushed at big box home stores. Do these work and are they worth the cost? I live in central Texas so it gets very hot here for about five months. My house is ten years old and the attic and upstairs room are very hot. Might it be better to install an attic fan?
I’m not familiar with a specific brand of spray on barrier you see in the box store, but I do know that the best spray on products generally don’t have the reflective performance of a metallized radiant barrier that typically has reflectivity of 95%. You might try looking up the brand name of the spray on and see what the manufacturer claims are.
As to an attic roof fan, they do not provide any energy savings — in fact they are an energy added cost. That’s because the attic air does not get its temperature from the hot roof as one might suspect. Instead the attic air temperature is controlled by the temperature of the insulation blanket on the floor. The roof temperature is transferred by radiant heat waves across the air space without adding temperature to the air. It then strikes the mass on the floor and is converted to conductive heat. The attic air is then heated by upward convective heat loss off the floor. What you want to do is get rid of the heat before it gets to the floor and that’s what a radiant does so effectively. In fact, if you had a roof fan and added a radiant barrier, the fan would never turn on because the attic air would not get any higher than the outside ambient air temperature. So, an attic fan isn’t the answer.
My suggestion would be to get a radiant barrier material and install it by attaching to the roof rafters. If it isn’t possible to attach the radiant barrier to the rafters, you may choose to lay it on top of the insulation on the floor. There is, however, a potential for dust to accumulate on the top surface of the radiant barrier, so you’ll need to assess that. Make sure you use a radiant barrier with metallized film on both sides and one that is perforated to allow any moisture in the mass on the floor to escape.