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Question: Radiant barrier in attic – both under rafters and over insulation?

Question by Tynman: Radiant barrier in attic – both under rafters and over insulation?
Hello, I am thinking about installing Radiant Barrier type product in my attic to save on energy costs. I was reading over the recommended installation methods and most of them say to install it under the rafters in the attic OR over the insulation on the attic floor. Does it make sense to install Radiant Barrier on BOTH the rafters and over the insulation? Or does that create a worse oven that what already exists in an attic without Radiant Barrier?
I’m not really concered about the cost. I’m curious to know if installing under the rafters AND over the insulation is more efficient? Or is it less efficient and should I just stick with one or the other.

Best answer:

Answer by JamesD
It’s just a chance to spend more money. I would put the radiant barriers under the rafters. That way if you have to have something installed that needs to use the attic you don’t have to worry about whom ever stepping through the radiant barrier and then the ceiling.

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One Response to Question: Radiant barrier in attic – both under rafters and over insulation?

  1. pp_314156

    How about no radiant barrier at all? The whole concept is rather dubious and at times borders on being a scam. Pretend for a minute that you can see infrared (heat) and that it was much like regular light. A radiant barrier is supposed to reflect infrared — so if you put it on the rafters it would reflect heat radiating from the ceiling back to the ceiling, and heat from the roof, back to the roof. As a regular conductive insulator, it is almost worthless.

    You would be much better off using regular fiberglass insulation because most of the heat transfer is via conduction and not radiation. Spend your money on sealing air leaks through pot lights and openings in the ceiling, and then add as much regular insulation as you can afford.

    The best place for a reflective coating in the summer would actually be outside the roof — e.g. a reflective roof to keep it cool.

    Many studies have been performed on radiant barriers and they all point to dismal results. Not so much because they don’t work — but because radiation is not the primary method of heat loss/gain. I will add a few links below for you to ponder. If you like doing this kind of things, by all means set up some experiments — but be aware that your savings may be negligible.

    You mention in the first sentence “to save on energy costs” so I assume your primary motivation is monetary. It would be worthwhile to get someone to come in with a thermal imaging camera (sometimes called a thermal audit) and give you some color photos pinpointing where you are losing the most energy. Then you can concentrate your efforts and money on where it is really needed. The last link below has some infrared photos I have taken of ceilings with defects that you should find interesting.

    Good luck with your project.

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