Storm windows and interior panels
Applies to: Assumes single-glazed timber windows retained in place, with a second layer added inside or outside rather than replacing the glass.
Last worked on 2014-10-30
Adding a second layer of glazing keeps the original window and changes the two things people actually complain about: the cold surface and the noise.
Inside or outside
An external storm layer protects the original window from weather as well as adding a still air gap, which extends the life of the joinery. It is visible from the street and it has to be detailed to drain, because any water that gets between the two layers has to leave.
An internal panel is invisible from outside, easier to fit and remove seasonally, and does nothing for the original joinery. It is usually the practical answer where the external appearance is fixed or where access is difficult.
The gap does the work
Most of the improvement comes from the still air between the layers, not from the second sheet itself. Very small gaps let the two surfaces exchange heat directly; very large gaps let the air circulate between them, which also moves heat. The useful range sits between those, and within it the exact figure matters less than keeping the air still.
For noise, the logic runs the other way: a larger gap is better, and the two layers performing differently helps rather than hurts. This is why a panel fitted at the reveal often does more for noise than a sealed unit does.
Condensation moves, it does not vanish
Once a second layer is added, the coldest surface in the assembly is the original glass, which is now on the outside of the still air gap. If room air can reach it, condensation will form there rather than on the surface you can see and wipe.
The internal layer therefore needs to be reasonably well sealed to the room and the cavity between the layers needs some path to the outside air, not to the room. That combination - tight inside, slightly open outside - is what keeps the gap dry.
Fitting an internal panel
- Measure the reveal at three heights and three widths and work to the smallest of each, because old openings are rarely square.
- Decide the fixing method before ordering: magnetic strip, turn button, or a framed panel in its own subframe.
- Seal the panel to the reveal continuously on the room side.
- Leave the original window able to be opened, or accept that it no longer opens and say so before fitting.
- Check after the first cold spell for condensation between the layers, which indicates room air is still reaching the gap.
The strongest argument for a second layer over replacement is reversibility: nothing about the original window is destroyed, and if the approach does not suit the room it comes back out.