Radiant Barriers

sparkyaclown

sparkyaclown

Active Member
My attic is extremely hot to the point where it is having a serious impact on how efficiently my upstairs HVAC can cool. My house currently has no attic fans and I was considering getting one installed until I saw a video on radiant barriers. Anyone have any experience with these? Also are there any local contractors that install it?
 
PeoplesElbow

PeoplesElbow

Well-Known Member
You need better ventilation and more fiberglass batt or blown in insulation on top of your ceiling.

I just can't see the radiant barrier working that well under the roof. Pretty sure most of the heat transferred through your singles and roof decking is from conduction through the plywood and then convection off the bottom of the plywood.

Your attic is going to get hot in the summer, think 130-150 degrees F. A common problem is that the insulation is blocking the airflow up from the soffit. My attic has gable vents that are supposed to be the out for the air, but the mesh screening to keep the bugs out was filled with what looked like drier lint. I got up there in the winter with a scrub brush and got this out and the next summer it was cooler up there.
 
H

Humbled

Member
No expert here, but I would think that a radiant barrier under the rafters would help, as long as there were good airflow between the eave and peak vents. Attic area should still be vented.
An exhaust fan works, but you must power it, and in a poorly vented attic, with a leaky furnace and ducts, it will pull cool air from the system and house.
 
sparkyaclown

sparkyaclown

Active Member
PeoplesElbow said:
You need better ventilation and more fiberglass batt or blown in insulation on top of your ceiling.

I just can't see the radiant barrier working that well under the roof. Pretty sure most of the heat transferred through your singles and roof decking is from conduction through the plywood and then convection off the bottom of the plywood.

Your attic is going to get hot in the summer, think 130-150 degrees F. A common problem is that the insulation is blocking the airflow up from the soffit. My attic has gable vents that are supposed to be the out for the air, but the mesh screening to keep the bugs out was filled with what looked like drier lint. I got up there in the winter with a scrub brush and got this out and the next summer it was cooler up there.
Click to expand...

Most of the heat is transferred via radiant heat in an attic, convection would not transfer much heat. The natural convection currents would flow along the roof to the ridge vent, while eventually it would heat it up, it would take a long time as there is nothing forcing that heat into the dead space of the attic. I have considered ventilation as I am thinking there is not enough area to allow the heat to escape. That was my original thought for considering adding an attic fan or maybe even just a gable vent. The soffit has plenty of surface area to allow air flow in but I'm not sure the ridge vent can keep up. My air handler is in the attic of my garage so I am considering trying a radiant barrier up there as it is a pretty small area without all of the obstacles of a regular attic. I was just curious as to whether anyone else had tried one out.
 
Clem72

Clem72

Well-Known Member
I considered putting in one of those solar fans (the 30-40 watt ones, not the 5-10watt). Problem is my roof is too damn high and too steep for me to do it myself, and paying someone else for an experiment like that doesn't sit right.
 
PeoplesElbow

PeoplesElbow

Well-Known Member
sparkyaclown said:
Most of the heat is transferred via radiant heat in an attic,
Click to expand...

Not true, the heat transferred to the top of the roof would be by radiation from the sun, but once the roof heats up convection and conduction rule.

To prove this to yourself get two pieces of plywood and go outside in the sun, prop both up in a lean-to fashion. Place a radiant barrier under one of them like you would in the attic. Now go lay under each one, I can tell you that you will not be any cooler under one than the other. This is because the shingles and plywood block the radiant heat from going through much like a tree shades you.

The only radiation heat transfer would be due to the temp difference from the heat of the plywood and the heat of the top of the insulation on top of your ceiling. For most heat transfer problems radiation can be ignored because the value is so small unless one of the bodies is hot enough that it glows (ie the sun). But the two bodies in question here are the plywood bottom and the insulation top.
 
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