Using aluminum flashing as radiant barrier
GBA Editor
| Posted in Energy Efficiency and Durability on
As part of a bathroom remodel I asked the contractor to install a radiant barrier inside the wall behind a radiator. I had envisioned the use of the foil type material, but instead he used a sheet of aluminum flashing. I haven’t yet determined whether he left an air gap behind the flashing, but if he did, will the flashing be at all effective? I assume that without the air gap it would be ineffective.
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Replies
John,
As long as there is an air gap on one side of the radiant barrier — in this case, between the radiant barrier and the radiator — the radiant barrier will be effective.
The shinier the radiant barrier, the more effective it will be. If the aluminum flashing is dull or dusty, it will be less effective.
Martin,
Wouldn't an air gap need to be behind the barrier in order to prevent the material from becoming a conductor?
John,
A radiant barrier does not eliminate heat flow; it just reduces radiant heat transfer. A radiant barrier has a high reflectivity and a low emissivity. It helps reflect much of the radiant heat that strikes it. Of course, being near a radiator, the radiant barrier will generally be at room temperature (when the radiator is off) or warmer (when the radiator is on). Of course it will conduct heat to the cooler drywall behind it.That's why the stud cavities require insulation.
All that said, by reflecting much of the radiant heat, the radiant barrier will help more of the radiator's heat to stay in the room. The effectiveness of the radiant barrier will be greatest if used in a house with very little wall insulation; there will be almost no effect when it is used in a house with thick wall insulation.
John,
What do you mean by "inside the wall"? Is the flashing behind the drywall? If so, is it in contact with the drywall? What's behind (outside of) the flashing - insulation, air?
Robert,
Good catch -- I missed that. Obviously, the radiant barrier should not be installed "inside the wall." It should be installed between the drywall and the radiator.
Robert,
A compact radiator, 19" H x 22" W x 5" D is butted against a 20" x 23" x 2" niche constructed with drywall. Behind the drywall is the flashing, but I don't know whether the flashing has an air gap on none, one, or both of its faces or whether any insulation was used. Before I ask the contractor about the details, it, I'd like to be ready to knowledgeably explain what he may have done wrong, if anything.
My understanding is that, in order to be effective, the radiant barrier should either have an air pocket or insulation on the back side, that is, the side facing towards inside of the wall frame.
John,
If the purpose of the radiant barrier is to reduce heat loss through the overheated wall behind the radiator, then it needs to have an air gap on the outer face. If the purpose is to retain the radiator's heat in the living space and prevent it from penetrating the wall, then the barrier needs to be between the radiator and the drywall with an air gap on both sides. The latter is many times more efficient.
I agree with Robert for a gap on both sides. One gap will work when it is hotter outside and the other will work when it's is warmer inside the home. At least a 3/4" airspace is best.