An entry door insulates with a polyurethane foam core between two skins, roughly R-6.5 per inch of foam. Steel and fiberglass doors share that core; they differ at the edges, where steel conducts cold around the perimeter unless the door has a thermal break. The glass lite, the weatherstrip, the threshold and sweep, and the frame-to-wall gap decide how much of that insulation you feel.
If you are deciding between fiberglass and steel as a purchase, that comparison is already written: jmbbuildersmn.com/blog/fiberglass-vs-steel-entry-doors. This guide is only about the insulation: what is inside the slab, why some doors frost, how door glass is rated, and what the install has to get right in a Twin Cities winter.
What is inside an insulated entry door?
Take a modern entry door apart and you find two thin skins, steel or fiberglass, glued to a perimeter frame of wood or composite stiles and rails, with the cavity filled by polyurethane foam injected under pressure. The foam does the insulating. It runs roughly R-6.5 per inch, and a standard entry slab is thick enough that manufacturers rate complete fiberglass doors somewhere in the R-5 to R-7 range, with premium units above that.
The old wood panel door had none of this. Solid wood is a poor insulator compared with foam, and a raised-panel door has thin panels in the field. If your house still has one, the slab itself is the biggest loss, before you get to weatherstripping.
The stiles and rails matter too. Wood edges rot at the bottom where water sits; composite edges do not. Neither is a thermal problem by itself. The thermal problem is what the skins do at those edges.
Why does a steel door frost on the inside?
Because steel conducts. On a plain steel door the outer skin and inner skin wrap around the edge of the slab, or connect through a steel edge channel, so there is a continuous metal path from the cold side to the warm side. In subzero weather that path carries the cold straight to the interior face, and the moisture in your entry air condenses and freezes on it. You see frost along the edges and around the lockset first, then on the whole face on the worst nights.
A thermal break is what stops it. Better steel doors separate the two skins at the edge with a non-conductive material, a composite or wood edge that the skins stop short of, so the foam core is the only thing between outside and inside. Door literature calls this a thermally broken edge or thermal break construction. It is the single spec to look for on a steel door in Minnesota, and it is why two steel doors with the same foam core behave so differently in January.
A metal frame without a break does the same thing at the jambs. Most residential entry frames are wood or composite so this is less common on houses, but on a steel-framed garage service door or commercial-style unit it is the reason the frame frosts.
How is a fiberglass door different at the edges?
The fiberglass skin is a polymer, and polymers do not conduct heat well. So the edge of a fiberglass door does not need a separate thermal break to keep the inside face warm; the skin itself is the break. That is why a fiberglass door's real-world performance sits closer to what the foam core alone would predict, and why fiberglass doors do not frost inside the way an unbroken steel door does.
Fiberglass also does not dent, and dents in a steel skin can crush the foam behind them. Neither material insulates better in the core, and both are sold with the same glass packages. On insulation alone, the honest summary is: a steel door with a real thermal break and a fiberglass door of the same thickness are close; a steel door without one is not in the same class in a Minnesota winter.
| Point | Steel | Fiberglass |
|---|---|---|
| Core | Polyurethane foam | Polyurethane foam |
| Edge conduction | High unless thermally broken | Low; skin does not conduct |
| Interior frost risk | Real on doors with no thermal break | Rare |
| Dents | Can crush foam locally | Does not dent |
| Glass options | Same lites and Low-E packages | Same lites and Low-E packages |
| What to check | "Thermal break" or "thermally broken edge" on the spec | Slab thickness and composite edges |
How much insulation does door glass cost you?
A lot, relative to the slab. Foam is an excellent insulator and glass is not, so every square inch of lite replaces foam with a much weaker material. ENERGY STAR reflects this by rating doors in tiers by glass area. Under Version 7.0, an opaque door qualifies at a U-factor of 0.17 or lower; a door with a half-lite or less qualifies at 0.23 or lower; a door with more than half its area in glass qualifies at 0.26 or lower. The more glass, the higher the allowed U-factor, because the standard knows glass is the weak spot.
So the glass package in a door lite is worth reading. Look for double-pane insulated glass at minimum, a Low-E coating on the inside face of the outer pane, argon fill, and a warm-edge spacer so the perimeter of the lite does not frost. Triple-pane door glass exists and is the right call on a full-lite or three-quarter-lite door facing north or into the wind. Decorative lites with internal caming are still insulated units; ask for the Low-E and gas fill on those too.
If your current door slab is fine but the lite is single glazed or fogged, the glass can be swapped without replacing the door: jmbbuildersmn.com/services/doors/exterior-door-glass-insert-replacement.
What seals the door: weatherstrip, sweep and threshold
The slab insulates; the perimeter seals. On a prehung entry door the seal is a set of replaceable parts:
- Compression weatherstrip. A foam-filled bulb in a kerf (a slot) in the jamb stop, on the two jambs and the head. The door closes against it and compresses it. Foam-core kerf strips are standard; magnetic strips on steel doors and silicone bulbs are upgrades that recover their shape longer.
- Corner pads. Small foam blocks at the bottom corners where the jamb weatherstrip meets the threshold. They fall off and nobody notices, and the corners are where the light shows.
- Door bottom sweep. A fin or bulb strip on the bottom edge of the slab that seals to the threshold. It wears from dragging on the threshold and from the sun, and the fins split.
- Adjustable threshold. The sill has a cap that raises or lowers with screws so it meets the sweep evenly across the width. If the sweep touches at one end and not the other, the threshold needs adjusting before anything is replaced.
What about the gap between the frame and the wall?
The best slab in the best frame still leaks if the rough-opening gap around the frame is empty or stuffed with fiberglass. Behind the interior casing there is a gap on both jambs and the head, and under the threshold there is a joint to the subfloor. Air moves through all of it.
A proper install handles it in this order. A sill pan under the threshold, with a back dam, so water from boots and driven rain drains out instead of into the subfloor. Low-expansion foam or backer rod in the jamb and head gap, without bowing the jambs. A continuous sealant bead between the frame and the drywall or casing inside, which is the actual air seal. Sealant at the exterior jambs and head to the siding, with the sill left to drain, and a drip cap over the head.
When the frame gap has been leaking for years, the evidence is at the bottom of the jambs: soft wood, peeling paint, a threshold that flexes. That is a frame problem, and it can often be repaired without a new door: jmbbuildersmn.com/services/doors/exterior-door-jamb-bottom-rot and jmbbuildersmn.com/services/doors/exterior-threshold-replacement.
What JMB Builders does
JMB Builders is a licensed Minnesota residential building contractor (BC809005) based in Zimmerman, installing across the Twin Cities metro with one crew. We install entry doors from any manufacturer, set them on a sill pan, foam and seal the frame gap, and adjust the threshold and strike so the slab compresses the weatherstrip evenly. Photos get you a written quote within one business day; a measure happens only when the job needs one. Paid by check or direct deposit.
- Labor-only entry door installation of a door you bought — jmbbuildersmn.com/services/finish-carpentry/door-installation-labor-only
- Therma-Tru fiberglass entry door installation, labor only — jmbbuildersmn.com/services/labor-only/therma-tru-entry-door-install-labor-only
- Door sweep and exterior door weatherstripping replacement — jmbbuildersmn.com/services/doors/door-sweep-and-weatherstripping
- Entry door glass insert replacement — jmbbuildersmn.com/services/doors/exterior-door-glass-insert-replacement
- Related reading: fiberglass vs steel as a buying decision — jmbbuildersmn.com/blog/fiberglass-vs-steel-entry-doors
Frequently asked questions
Are steel or fiberglass entry doors better insulated?
Both use the same polyurethane foam core, so the core insulation is similar. The difference is at the edges: steel skins conduct cold around the perimeter unless the door has a thermal break, while fiberglass skins do not conduct. A thermally broken steel door and a fiberglass door of the same thickness perform close to each other; a steel door without a thermal break frosts inside in a Minnesota winter.
Why is there frost on the inside of my front door?
The door's edge or frame is conducting cold to the inside face, and moisture in the entry air is freezing on it. It is most common on steel doors without a thermal break and on metal frames. Frost along the edges and around the lockset points to the slab; frost on the jambs points to the frame. Weatherstripping does not fix it, because the cold is coming through the material, not around it.
What is a thermal break in a steel door?
A non-conductive material, usually a composite or wood edge, that separates the outer and inner steel skins at the perimeter of the slab so there is no continuous metal path from outside to inside. Without it the skins carry cold around the edge and the inside face gets cold enough to frost. Look for "thermal break" or "thermally broken edge" on the door specification.
Does glass in a front door make it less energy efficient?
Yes. Glass insulates far less than the foam core it replaces, which is why ENERGY STAR sets a lower (stricter) U-factor for opaque doors than for half-lite doors, and a higher allowed U-factor again for doors that are more than half glass. Ordering the lite with Low-E, argon fill and a warm-edge spacer, or triple pane on a full-lite door, narrows the gap considerably.
How do I know if my door weatherstripping needs replacing?
Close the door on a strip of paper at several points around the perimeter; if it pulls out without resistance, the seal is not compressing there. At night, look for light around the edges from outside or feel for moving air. Foam kerf weatherstrip that has flattened, corner pads that are missing and a sweep with split fins are all replaceable in place; check the threshold adjustment before replacing the sweep.
What is an adjustable threshold on an entry door?
The sill of a prehung exterior door has a cap that raises or lowers on screws so it meets the door bottom sweep evenly across the width. Over time a house settles and the door bottom sweep contacts at one end only; adjusting the threshold restores the seal without replacing parts. Too high and the door drags and the sweep wears out fast; too low and light shows under the door.
