I still get this question all the time. “Can you explain why an air space is needed for a radiant barrier to work?” Watch this video for a complete explanation.
Basically it works this way. Radiant heat is heat that is transferred across either an air space or a vacuum. This is how the heat from the sun reaches the earth. Radiant heat acts similar to a sound wave. By definition, you MUST have either an air space or a void for radiant heat to even exist.
If you don’t have an air space then you basically have a solid. Heat can essentially only move through a solid by conduction. This is by direct contact. This is how an egg cooks on a hot skillet.
So, without an air space, you cannot have radiant heat. Without radiant heat there is no way to have a “Radiant Barrier”. In fact, because of the conductive nature of pure aluminum, if you install radiant barrier foil WITHOUT the required air space it will actually work AGAINST you and INCREASE Heat flow.
The bottom line is you MUST have an air space on one side of a radiant barrier for it to work.
Looking for more videos on this topic? Check out my posts below.
Tags: conductive heat, radiant barrier, radiant barrier foil, radiant heat, radiant heat energy
Ed
I live in California. I am accepting bids from roofing contractors to replace my old cedar shake roof with a stone coated metal roof system on battens. The existing roof has solid plywood sheathing. I’ve been studying radiant barriers and your explanation was excellent. I have several contractors that want to use a radiant barrier system you described with a 1/4″ insulation pad sandwiched between two sheets of aluminum foil.
A couple of the contractors want to put the radiant barrier UNDERNEATH the underlayment on top of the plywood with NO air gap. Doesn’t work as a radiant barrier, right? Also,virtually no insulating properties, right? Is there any value to doing it this way. Can your product go over the underlayment with a 3/4 inch air gap above the the bottom of the metal roofing?
Thanks So much.
Where can I purchase your product?
Gerald Freeman
Gerald,
You have two questions: 1) Does insulation (1/4″ foil/foam) under a metal roof do any good? and 2) Does foil without an airspace on one side have any value?
Answers –
1) Putting insulation at the roof line will ONLY help if the roof is a cathedral type ceiling. If there is ventilated air (an attic) below the insulation then it looses virtually all it’s value. Just like your front door. It has insulating value until it’s swung wide open. Putting insulation at the roof line for a vented attic helps some with heat transfer, but you really only need a radiant barrier to the outside (under the metal roof on top of the decking) and put the additional insulation in the attic on the attic floor.
2) You are correct. You MUST have an air space on one side of the foil for a radiant barrier to exist. By DEFINITION radiant heat is heat transfer WITHOUT contact – or in a waveform ACROSS a void. If you “sandwich” foil between two other products, then all three products become ONE solid with different conductive qualities. (heat can only move through a solid by conduction) In fact, aluminum is the MOST conductive of the three. So, in reality, putting aluminum foil (radiant barrier) in the assembly would actually cause MORE heat flow then without. It would actually work against what you are trying to achieve.
Yes, you can put radiant barrier foil under metal roofs. Here is the page for this with a video and pictures: http://www.atticfoil.com/radia.....ingles.htm
You can order online or via phone: AtticFoil.com
So, the current technology in Texas appears to be plywood/osb decking with one-side radiant barrier nailed to the roof joists, foil-side down. Then (possibly) roof felt directly onto the osb decking and then shingles or metal roof over that (directly screwed into the decking).
In this case, the only “void” or space is the attic, underneath the radiant barrier sheet glued to the osb 4X8′s.
Am I right that this means no radiant barrier? In other words, does the void/space have to be on top of the radiant barrier, i.e. foil?
Based on your explanation of the physics, I would assume so because once the radiation/waves enter the attic below the shingle/felt/osb/foil sandwich, the chances of reflecting back the radiation are gone, no?
And if this is correct, there are a large number of roofs being installed in Texas advertised as “radiant barrier” that are not barriers at all.
Thoughts?
Paul,
Foil backed decking materials have the airspace facing the attic – so, yes they do work and have the required airspace. These product like TechShield and PolarPly actually work off the EMISSIVITY quality of the foil in the Summer to reduce heat gain. When the foil is on the “cool” side it keeps the decking from “emitting” as much heat – it is technically NOT reflecting the heat back in the Summer. In the Winter, it WOULD be technically a “Radiant Barrier” since the foil faces the “hot” side or heat coming from the home.
Either way, foil backed decking is a good option if you have the opportunity to replace the deck. It’s not quiet as effective as stapling AtticFoil to the bottom of the roof rafters, but since there is no additional labor it’s a good “bang for the buck”.
I am about to construct a new home with Insulated Concrete Forms and foam sealed attic. The house is to have composition shingles. Am I correct that to get the air space I would need to put a layer of decking with a radiant barrier (with the foil side up), laths and another layer of decking, or is there another way? What would be your recommendation?
This would work great – in fact I did a similar system to this on my own house. You can achieve this with TechShield, or with AtticFoil attached onto plywood. You can do it like this: foil faced plywood facing up, wooden furring strips, foil faced plywood with foil facing down (toward wood strips) then felt and then shingles. Send me some photos of the install if you can!