Doors with a Vertically Stiffened Core have sound deadening fiberglass insulation. Doors with a Mineral Fiberboard Core are non-combustible and provide a heat barrier. These doors are most often found in stairwells.
“Fire-rated” steel doors are insulated and prevent fire from heating through for a time. You choose which level of rating you want to buy. Fire Ratings run from 20 min, 45 min, 60 min, 90 min, up to 3 hours.
Several factors involved in rating the insulation value of a steel door
Ok, let’s wade into these factors and what they really mean to us.
• R-value = thermal resistance. The higher the R-value, the better insulation a steel door provides. R-values range from R – 1 snow to R – 45 for Vacuum insulated panels.
• U-factor = heat transmission. U-factor or U-value is the rate of transfer of heat. The lower the U-factor number, the better the insulation.
• STC rating = sound transmission class. This is a measurement that indicates the reduction in noise level. The higher the STC, the less noise comes through the door.
• Fire Rating = heat transfer time. This rating measures how long it takes for heat to transfer from one face of the door to the other face. Fire Rating range: 20 min, 45 min, 60 min, 90 min, 3 hours. It can be combined with temperature rise ratings.
• Temperature Rise Rating = heat transfer temperature. These ratings are 250°F, 450°F, and 650°F. The door will not exceed that temperature for 30 minutes. The 250°F is best because it allows for the least transfer of heat. Often used in stairwells because of their mineral core.
Now that we have gotten all that information, how do we apply it to our door?
Applying Rating Factors to Insulated Steel Doors
Let’s look at each insulated door and find out which might be the best for your application.
• Polystyrene Core: R-Value: approx. 3.8 to 6.4 – U-Factor: approx. .26 to .16 – STC Rating: approx. 27 (with seals) – Fire Rating: less than 650 degrees in 30 mins
• Polyurethane Core: R-Value: approx. 10+ – U-Factor: approx. .1 to .09 – STC Rating: approx. 22 (with seals) – Fire Rating: less than 650 degrees in 30 mins
• Vertically Stiffened Core: R-Value: approx. 1.6 – U-Factor: approx. .63 – STC Rating: approx. 38 to 41 (with seals) – Fire Rating: less than 650 degrees in 30 mins
• Mineral Fiberboard Core: No R-Value/U-Factor. Interior doors are not thermal tested – STC Rating: approx. 33 (with seals) – Fire Rating: 250 degrees or 450 degrees in 30 mins
It seems steel doors with Polyurethane Cores are the best insulating exterior doors. Doors with a Mineral Fiberboard Core provide the best fire protection. These are great for stairwells and other interior doors.
Many commercial steel doors are not insulated. Reviewing the above information will help you choose the right insulated steel door.