Appearing in the November 1998 edition of DOORS AND HARDWARE.
As autumn turns to winter, we become increasinglly aware of the impending colder weather. In recent years, economic and environmental concerns have led to energy-conserving building designs and creative means to control heat loss from buildings. Technologies and products have evolved to yield ever-higher performance levels to meet specific demands.
Hollow metal doors and frames offer a wide variety of choices in characteristics and performance: fire resistance, access, egress, security and environmental control (sound, light, heat, radiation and corrosion). To ensure that desired performance is achieved in any given opening, it is important that specifications be clear in their intent as well as their content. This means that the building occupants, architects, specifiers and suppliers must agree on the priority of performance parameters, in the context of related products and their surrounding construction elements. Failure to reach such an agreement frequently results in constructions that do not meet expectations, even though there may be nothing wrong with the individual elements.
Certain commercial hollow metal products have been developed specifically to control thermal transfer, namely insulated steel doors and thermally broken hollow metal frames. Properly supplied and installed, they work quite well. Improper application can lead to some less-than-spectacular results.
Following are three scenarios, all relating to insulated steel doors and thermally broken hollow metal frames. Each poses its own problems, driven by the priority of parameters.
"We have insulated steel doors in thermally broken steel frames on our exterior openings, but whenever the outside temperature drops below freezing, we get condensation or frost buildup on the indoor side. What's wrong?"
If all-out strength and durability are higher priority than thermal resistance, consider using the steel-stiffened door in a standard (non-thermal break) frame, and spending the difference on top-quality weather-stripping.
Careful analysis revealed some contributing factors:
The frames, while thermally broken, were not insulated in any way-cold air could move around freely inside the jambs and head. Furthermore, the frames were not caulked to the adjacent wall, permitting air movement and water infiltration, especially noticeable during high winds.
The doors, while insulated with fiberglass batting, had been specified and supplied as "16 gauge, with continuous vertical steel stiffeners, on 6" centers, spotwelded to both faces on 3' centers.' Strangely, the frost buildup on the doors appeared as vertical ridges spaced about 6" apart.
The weather-stripping was not making consistent contact with the face of the door; the door sweep was missing most of its flexible seal, torn away in repeated freezings to the threshold.
The solution to these problems is dependent on the priority of performance desired.If thermal resistance is a high priority, then the thermally broken frame is a good start. However, proper insulation (such as fiberglass or mineral wool batts, or injected insulating foam), proper anchoring (minimizing thermal transfer) and proper sealing (caulking frame to wall and gasketing door to frame and sill) are essential if the frame is to perform as intended. A thermal-break threshold would also help.
If thermal resistance is a high priority, then the steel-stiffened door is among the worst possible choices: By its design, it is a very robust "thermal bridge," with the stiffeners conducting sufficient thermal energy to defeat the thermal resistance of the insulating filler. A polystyrene or polyurethane foam core door would provide much superior thermal resistance; lighter gauge door face sheets would further reduce the total mass of conductive steel in the opening.
If all-out strength and durability are higher priority than thermal resistance, consider using the steel-stiffened door in a standard (non-thermal break) frame, and spending the difference on top-quality weather-stripping. Silicone-rubber seals perform well over broad temperature ranges and tend to be quite damage-resistant.
Proper adjustment and maintenance are also crucial to the long-term performance of openings.
"We have insulated doors on all of our exterior openings, and most of the time they work very well. But once in a while, some of them seem to seize up and become very difficult to open. What can we do?"
Investigation revealed that these were good quality insulated doors-16-gauge galvanized steel, bonded polyurethane core, watertight flush tops, first quality hardware, installed in thermally broken frames. Some of these doors never exhibited the reported symptoms, while others were only occasionally problematic. The worst offenders, it appeared, were in minimally heated areas (e.g., exit stairs, parking garages), painted a dark color, facing south; the symptoms were most evident on cold, sunny days.
The problem becomes evident: The dark color absorbs large amounts of solar energy fairly rapidly, causing the outside face of the door to expand. The insulating core severely limits the transfer of heat to the inside face. This significant thermal differential causes the door to bow outwards, jamming the latchbolt in the strike, making operation difficult. (One example seen had exerted sufficient force to fracture the cast-metal latchcase of a top-quality exit device.)
The solution comes in several parts, any of which may be sufficient on its own or in combination with others:
Lighter colored paint, or an awning or canopy, could reduce the amount of solar energy exposure on the door.
Lower thermal-resistant door core (e.g. polystyrene or honeycomb or steel stiffeners could lessen the thermal differential through the door (given the locations of some of the doors, insulation was rather pointless.)
Adjusting hardware clearances could render them more tolerant to bowing in the door.
Increasing ambient temperature indoors, where practical, could lessen thermal differential.
"We have large areas of thermally broken hollow metal windows. We're certain that they are properly insulated and sealed, yet some days we have major condensation problems. What's going on?"
Inspection confirmed the assemblies were made of double-rabbetted sections, properly assembled, fitted with 1" thick sealed glass units, fully injection-foam insulated and caulked. The designer of this project had insisted on decorative structural steel muntin bars in every glass opening, welded to the soffits outside the glazed rabbet and the thermal break.
The problem becomes evident: Instead of the normal orientation, with minimal mass of steel and glass exposed to the cold exterior, these units had about five times the normal mass of steel outside the thermal break. The exposed portion also provided about five times the normal "shelf area" outside, on which snow could collect and melt, creating a greater thermal drain on the assemblies. On colder days, the problem was amplified by high relative humidity and poor air circulation within the building.
We're not sure they've found a solution to this one yet, short of removing the existing units and starting again. Reducing the relative humidity and improving warm airflow over the interior side would likely limit some of the condensation, but would not alter the mass of conductive steel outside the thermal break.
We would be glad to tell you more but we have to go now. There's a caller on the line...