If heat retention is your primary goal in a fully* conditioned building, you want to get your BlueTex™ closest to the inside of the building, with the proper air gap on the foil side. If your building is not fully conditioned (meaning non-conditioned or semi-conditioned, see the install guide here). If heat rejection is your primary goal, see the install page here.
*Fully conditioned means you are heating/cooling the space 24/7 to maintain a constant temp at all times.
This method is most effective at keeping heat inside a building and though it can help keep heat out, it's not the best method for that goal. Unlike the other method, adding radiant barrier for heat retention involves getting the foil layer closest to the inside of the building, after the R-value insulation, with the proper air space. This is different than a hot/mixed climate that's positioning the foil closest to the outside of the building.
If you're in a mixed climate (wanting to both keep heat out in hot seasons, but also keep hot inside during cold seasons), then you should install according to the instructions on this page: Insulate to get Year-Round Results for a Conditioned Building
How It Works
In a building that is being heated, your heating system is putting heat into the building. The radiant heat will heat up the interior of the building and typically (without being insulated), the heat will hit the metal walls and roofline and absorb into the metal, and then dissipate outside. When the metal is cold, this can cause condensation to occur on the inside of the metal, leading to dripping or "raining" inside the building. But don't worry, you can fix this! More information on condensation control here: Solving Problems with Moisture in Metal Buildings
Consider Your Interior Finish
There are two common ways to install a radiant barrier to the interior of a building. The first is to leave the foil exposed and the second is to cover the foil with something like sheetrock or plywood.
By leaving the foil exposed to the inside of the building, the heat coming from your heat source will have to convert to a radiant form of heat to “jump” across the airspace inside the building and it will head toward the walls/roofline as usual. However, once the radiant heat meets the foil layer, the foil will reflect it back toward the interior/the source of the heat. This prevents heat from escaping (less heat loss) and makes the outside layer of traditional insulation more effective.
Alternatively, if you don't want to have the foil exposed to the interior because of functionality or looks, you can put up an interior finish, just make sure you use furring strips first, so you maintain an air space behind the interior finish between it and the foil layer. This means your walls will now have dead air spaces (that's ok) and on the roofline, this would create either a ventilated channel (if you have venting) or a non-ventilated/dead air space as well. Remember, the foil doesn't need moving/vented air to work, just an air space.
How you layer the foil inside a wall cavity or on the roof matters! Especially when you're trying to address potential condensation concerns too. It's helpful to consider the layers you have and work to make them air-tight.
Basically, you want to create a well-insulated box that is air tight, and you want to line the inside of 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, helping to slow the conductive heat loss.
There are two common ways to install a radiant barrier to the interior of a building. The first is to leave the foil exposed and the second is to cover the foil with something like sheetrock or plywood (mentioned above).
Your layers will look something similar to this:
- Exterior Metal Skin
- R-value (foam board, spray foam, batt insulation, etc.) - right up against the inside-facing foil layer
- Foil Surface
*If you're finishing the interior, then continue on:
- Air Gap (created by furring strips of any material)
- Wall/Ceiling Finish (this can be a layer of foam board, Sheetrock, OSB, peg board, sheet metal, etc.)
Remember, for a radiant barrier to work the foil MUST face an airspace on one side - there are NO exceptions to this rule in a metal building! Also, when trying to hold the heat inside your building, you want the foil layer closest to the INSIDE of the space (the building) so you reflect the heat back inside FIRST, then use your R-value to slow down the remaining heat trying to escape. This is the most effective method to minimize radiant heat loss.
One more tip! Go the extra mile to tape your seams and make sure your R-value is nice and snug inside the cavities. Creating an air tight cavity will go a long way toward preventing the wasting of energy and toward reducing the potential risk of moisture problems. When done correctly, your building will hold heat in more easily and stay dry inside.