I have been getting questions asking why storm doors are so important and how do they save energy.
When an engineer calculates the rate of heat loss through a door, each obstacle in the path of the heat flow needs to be evaluated. In the winter heat from inside will flow out to the cold outdoors.
The object is to slow down the heat flow
One of the roadblocks that heat encounters is at the boundary between each material. In the case of a plain solid wood door, the first obstacle that the heat flow hits is the boundary between the indoor air and the wood door.
Every time heat flows from one material (air) into another material (wood) it is slowed down. Each boundary acts like a speed bump to the flow of heat.
The next item is the wood door itself. Wood does not conducts heat slowly so it is a good material for making doors. The thickness of the wood determines how slowly the heat flows through it. The thicker the wood is, the slower heat escapes through it.
The final barrier to heat flow is the boundary between the wood door and the outdoor air.
The total resistance to heat flow is the sum of the three items above. They are the indoor air to wood boundary, the wood, and the wood to outdoor air boundary.
A storm door more than doubles the resistance to heat loss
Adding a storm door to the doorway adds several more “hoops” for the heat to jump through. A description the path that the heat flow takes with a storm door would look like this.
- Indoor air to wood boundary
- Thickness of the wood
- Middle wood to air boundary
- Thickness of the air trapped between the doors
- Air to storm door boundary
- Thickness of the storm door
- Storm door to outside air boundary
Without the storm door, there are only three obstacles to heat flow. With the storm door, there are seven obstacles to heat flow. The heat flow rate is substantially reduced by the storm door.
Many storm doors are made of aluminum. Aluminum offers very little resistance to heat flow but it still works as a storm door because of the additional obstacles to heat flow that it provides.
Wood storm doors do work better but they are heavier and cost more to buy and maintain. The lightweight and low cost make aluminum more popular.
Make sure both your security door and the storm door are in good shape and shut tightly.
Remember the more boundaries there are the slower the heat escapes. The same idea work for people too. That’s why several thin layers of clothing keep us warmer than one heavy layer.
Click here for a free money saving report written by the Energy Boomer titled HOW SAVE MONEY ON YOUR NEXT HEATING BILL