Today I want to talk about radiant barriers.
I often get questions from builders about radiant barriers.
If I already have a radiant barrier in place or if I am looking to have a radiant barrier installed, can I also use spray foam insulation? Do they work together? How do they work together? Does it make sense to do both?
To answer this, you have to understand what a radiant barrier is and what it does.
A radiant barrier like, Thermoply or Techshield, these are some brand names of radiant barriers, or some of the spray applied radiant barrier paints on the market, are low emissivity materials.
That is a technical way of saying that they allow less heat to emit, or radiate, from the exposed surface of the roof to the inside of the attic space.t
By applying a radiant barrier to the underside of the roof you can change the radiant heat load in the attic space.
In a typical attic, when the roof system is a 150 degrees Fahrenheit at the plywood, heat is coming off the plywood into the attic, heating up the attic space.
This is why attics can get hot, 120 or 130-degrees Fahrenheit, when the outdoor temperature is only 90 degrees Fahrenheit.
By putting a radiant barrier on the underside of the roof, the roof releases or radiates less heat into the attic, which results in a lower attic temperature. It is common for radiant barriers to lower attic temperatures in the 10-to-20-degree range.
For radiant barriers to work properly the radiant barrier must be exposed to and facing the air space on the inside, such as the attic space between the ceiling plane and the underside of the roof, because the radiant barrier is reducing the radiant heat that is emitted into that airspace.
If spray foam is applied directly to the underside of the roof deck, in contact with the radiant barrier, the radiant barrier will no longer function the way it is intended.
This occurs because when a solid material is in contact with the radiant barrier surface, the key method of heat transfer is no longer radiation, it is now conduction.
So, NO, you cannot apply spray foam directly to a radiant barrier and get the benefits of both.
In many situations contractors will go ahead and install spray foam over the radiant barrier because a fully encapsulated attic with spray foam is going to outperform a radiant barrier only system, but the installation of the spray foam is nullifying the radiant barrier.
With a complete spray foam system, you get the benefits of reducing conduction at the roof deck, of course this is the traditional insulation properties, you get the benefit of reducing convection heat transfer by providing an air barrier, and you change the interior radiant surface, so rather than the radiant surface being the roof deck, now the surface of the spray foam becomes the exposed radiative surface.
Because the spray foam is an insulation material, the interior temperature of the spray foam is going to be much lower than the interior temperature of the roof deck, so the radiant load coming off the spray foam is much lower than the radiant load that would come off the roof deck.
So, you can achieve a radiant barrier-like benefit by putting spray foam on the underside of the roof deck, even though spray foam is not defined as a radiant barrier material.
To use both spray foam and radiant barrier in conjunction with each other, for example if you go to a project to bid spray foam and it already has a radiant barrier installed on the underside of the roof deck, you will need to install a vent baffle the underside of the roof deck.
The vent baffles should be connected from the soffit all the way up to the ridge vent, to make sure the roof is still being vented.
By installing a vent baffle, you maintain an air space of one to two inches on the underside of the roof system.
In this case, the low emissivity coating will work, you can get the benefits of the low emissivity coating and you can install spray foam to the underside of the vent baffle to create an unvented conditioned attic space and achieve the benefits of both technologies.
This has been Robert Naini, your Spray Foam Advisor, check us out on some more videos.
