If heat rejection is your primary goal in a fully* conditioned building, you want to get your BlueTex™ closest to the outside of the building, with the proper air gap on the foil side. This method is most effective at keeping heat out, and it also works very well for heat retention in the building during colder seasons too. If heat retention is your primary goal, see the install page here.
If your building is not fully conditioned (meaning non-conditioned or semi-conditioned, see the install guide here).
*Fully conditioned means you are heating/cooling the space 24/7 to maintain a constant temp at all times. Common applications for this install method include container living conversions, barndominiums/shouses, recreational/game rooms, and office space.
How It Works
In a typical building, the outside “skin” can easily reach over 150ºF in direct sunlight. Once the skin heats up, the heat flows via direct conduction through the walls and ceiling. When you create an airspace between the skin and the foil surface, this FORCES most of the heat coming in to the building to CONVERT to radiant heat, in order to cross the airspace. Radiant heat is actually EASIER to control than conductive heat, since it can be REFLECTED just like light*! Once the heat is converted to radiant heat, we OWN IT! We can reflect the energy away from the regular insulation with the foil, which makes the regular insulation more effective.
Simply put, think of your building like a refrigerator sitting in the direct sun. Do we agree that the refrigerator would be more efficient if we could move it from under the sun and put it in the shade? Yes! In this same way, radiant barrier basically acts like shade, making any traditional insulation more effective by reducing the heat load before it ever reaches the insulation inside the walls and ceilings.
*Radiant heat/ infrared radiation is actually the invisible part of the light spectrum.
When dealing with conditioned buildings (air conditioned/ heated) think in terms of layers and air-tightness.
Basically, you want to create a well-insulated box that is air tight, and you want to wrap that box with radiant barrier. The radiant barrier will act as the first line of defense against radiant heat, while the standard insulation (with R-Value) will act as the second line of defense against conductive heat flow. Your layers will look something similar to this:
- Exterior Metal Skin
- Air Gap (created by furring strips of any material)
- RADIANT BARRIER Foil
- R-value (foam board, spray foam, batt insulation, etc.) - right up against the inside-facing foil layer
- Wall/Ceiling Finish (this can be a layer of foam board, Sheetrock, OSB, peg board, sheet metal, or even another layer of BlueTex™, as seen below!)
Remember, for a radiant barrier to work the foil MUST face an airspace - there are NO exceptions to this rule in a metal building! Also, when trying to stop heat gain, you want the foil layer closest to the OUTSIDE of the shell (the building) so you block the heat FIRST, then use your R-value to slow down the remaining heat.
Air flow and ventilation are great for moving the heat out of an area and keeping things dry in general. Remember that in a wall cavity and in some cathedral ceilings, there's not really any air flow and that's ok! If you have an air-tight layer in the cavity, this air won't hurt anything and you don't have to vent it. So, the foil only needs an air space to reflect radiant heat, and that air doesn't necessarily have to be vented.
When you install radiant barrier in a metal building this way, and add your R-value too, you'll have a system that works better than anything else to help the building stay comfortable and efficient in the hot summer months.