· by Michael Blair, Nashville's Windows & Doors
A storm door adds a layer of glass a few inches in front of the entry door. In winter that traps a pocket of still air and reduces drafts. In summer, on a door that gets direct afternoon sun, it traps heat instead, and the space between the two doors can reach temperatures that damage paint, warp fiberglass and delaminate composite skins. Most entry door warranties say so explicitly.
What happens between the doors
Glass lets sunlight through and the dark entry door behind it absorbs the energy. With the storm door closed there is nowhere for that heat to escape, so the air gap heats up far beyond the outdoor temperature. A dark-painted door on a west-facing wall behind a full-view storm door in August is the worst case.
Fiberglass and steel doors with foam cores are the most vulnerable, because the skin heats while the core insulates the other side, and the differential bows the door. Paint and stain finishes blister and fade. Wood doors move less but their finish suffers the same way, and the weatherstripping on any door hardens and cracks early.
Which exposures are the problem
North-facing doors and doors under a deep porch rarely have the problem. East-facing doors get morning sun, which is weaker and shorter. South and west exposures with little overhang are where the damage happens, and in Middle Tennessee the west side of a house in July gets sun until well into the evening.
Door manufacturers commonly exclude damage from a storm door on a sunny exposure from their warranty, and some specify a minimum ventilation or a light door color if a storm door is used at all. Read the warranty for the door you have before adding one.
- Low risk: north-facing, or shaded by a porch roof
- Moderate: east-facing, or south with a decent overhang
- High: south or west with little shade, especially a dark door
When a storm door still makes sense
On a shaded or north-facing door it does what it is meant to do: cuts drafts in winter, keeps rain off the door and its threshold, and with the glass swapped for a screen it lets the house ventilate in spring and fall. Older wood doors in the pre-war neighborhoods benefit from the weather protection.
If the exposure is sunny and you want one anyway, choose a ventilating model whose glass slides to expose a screen, and leave it vented through the hot months. A light-colored entry door absorbs less. Some storm doors are sold with low-E glass to reduce the heat load, which helps at the margin but does not remove the risk on a full-sun west wall.
The alternatives
The draft problem a storm door is bought to solve is usually a weatherstripping and threshold problem on the entry door itself, and fixing that costs less and works better. A quality entry door with intact seals does not need a second door in front of it.
For weather protection, a porch roof or even a small awning over the entry does more than a storm door and keeps the sun off the door as well. For a screened opening in mild weather, a retractable screen door mounts in the same frame and disappears when not in use, with no glass to trap heat.