While walls can be easily filled with batts of insulation, thresholds are a tough area of the house to prevent the escape of heat. Insulation has to be built into the door’s construction in the form of a “core” that fills the hollow inside the door’s frame. For this reason, the core of your door plays an especially important role in preserving a building’s energy.
Weatherstripping doors and windows, caulking cracks and insulating the attic and sills are great ways to start improving a house’s insulation. These techniques help prevent cold air from infiltrating a warm house. But the materials that already exist in a building’s construction play a major part in temperature control as well. This boils down to the mechanics of heat transfer.
Heat transfer is a type of conduction. Using less conductive materials keeps your house better insulated and prevents heat from going to waste.
To understand how an object’s insulative properties are identified, we first have to look at a measure called “R-value.”
R is calculated as R = 1/U, U being the coefficient of heat conductivity. R-value quantifies the effectiveness of a barrier at blocking the conductive flow of heat. A higher R indicates a barrier that is more effective at stopping a building’s heating system from getting through.
The degree of heat transfer through a barrier is determined by the differential in temperature between each side as well as the material’s resistance to heat conduction. This differential can be divided by the R-value and multiplied by the total surface area of the barrier to give the total BTUs per hour in heat transfer that occurs through the barrier.
When it comes to exterior applications, hollow metal doors are going to be the best choice for just about any application for R-value and weather resistance. But if this isn’t an option (metal doors aren’t exactly known for their cozy, Victorian-esque aesthetic), the next consideration is the door’s core composition.
Solid doors are heavy and involve a lot of material to produce. For this reason, most modern doors are “hollow” on the inside save for a filler material composed of lightweight insular material. While solid core doors are still made, they are typically designed for purposes where the additional expense is worth it.
So what kind of filler materials are inside hollow doors? Door cores have several jobs that they need to fulfill: Adding stiffness to the door, supporting weather resistance, and increasing the door’s overall R-value by fighting heat conduction. All this has to be done with a material that is ideal as lightweight as possible.
Polyurethane is a relatively high-density core material that provides superior resistance to temperature change. It is applied in the form of a liquid spray that fills the hollow of the door before expanding and drying. This has the benefit of creating a bond between the frame and polyurethane core which adds strength to the door and leaves little or no gaps inside. Polyurethane cores can also be pre-formed and installed as rigid boards more like their less dense cousin, polystyrene. Polyurethane cores are excellent for exterior doors, especially in cold climates, as they provide excellent insulation. With an R-value typically falling somewhere between R-6 and R-8, polyurethane is going to be the best commonly available choice for insulation.
Polyurethane is not unlike typical fiberglass batts of insulation in that it consists of small pockets of gas which collectively impede the progress of heat conduction. However, instead of air, polyurethane contains hydrochlorofluorocarbon. And while hydrochlorofluorocarbon may sound like an ingredient you would encounter at a particularly dicey potluck, it is actually a low-conductivity gas that effectively increases R-value.
Unfortunately, this gas will inevitably leach from the polyurethane into the surrounding air. Outgassing is most prevalent during the first two years after manufacturing, during which time the door’s R-value will decrease slightly. After this initial period, outgassing will taper off and R-value will remain relatively stable.
Expanded polystyrene (EPS) is a less dense material than polyurethane and is less insular. EPS provide a good balance of providing sufficient insulation while keeping costs low. If you’ve ever stopped at a gas station for coffee during a late-night drive and picked up a disposable foam cup, then you’re already familiar with EPS. The material provides some stiffness to the door frame and works as a barrier that prevents moisture from working its way inside the hollow of the door and stagnating. Coming in with an R-value of around R-3 to R-5, EPS insulates less than polyurethane even on its best day, but still comes in as a solid middle-of-the-road choice when it comes to insulating a door.
Honeycomb cores come in the form of a hexagonal lattice made out of cardboard. (Just in case you thought EPS sounded like a cheap material…). In this case, the cardboard is coated in resin to increase the stiffness of the material. The honeycomb structure serves to maximize the overall structural rigidity of the door while remaining as lightweight as possible.
This core type is ideal for interior applications, since it doesn’t provide much in the way of insulation but is cost-effective and lightweight. It is also good at scattering soundwaves and results in a good degree of sound dampening. With a low R-value, the doors have good stability but aren’t appropriate for exteriors.
EPS and polyurethane core doors provide sufficient insulation for most exterior applications, but if you live in an area with colder weather or are looking to maximize your home’s efficiency to conserve as much as possible, polyurethane is going to be your best bet.